REACTIVE POWER COMPENSATION USING ...

REACTIVE POWER COMPENSATION USING STATCOM

By 1. Muhammad Shuaib Panhwer 2. Bilawal Rattar

(Group Leader)

10EL68

(Assistant Group Leader)

10EL59

3. Zeeshan Anjum Memon

10EL10

4. Sarfaraz Khatti

10EL32

5. Ahmed Maki Utero

10EL51

6. Yasir Memon

10EL16

Supervised by

Prof. Dr. Mukhtiar Ahmed Mahar & Co-supervised by

Engr. Ms Mokhi Maan Department of Electrical Engineering Mehran University of Engineering & Technology, Jamshoro Submitted in partial fulfillment of the requirement for the Degree of Bachelor of Electrical Engineering January2014

In the Name of

Allah The Beneficent, The Merciful.

Certificate

This is to certify that the work presented in this thesis on REACTIVE POWER COMPENSATION USING STATCOM is completed by the following students of department of electrical engineering under the supervision of Dr.Mukhtiar Ahmed Mahar and in co-supervision of Engr. Ms Mokhi Maan

Name of Students

1. Muhammad Shuaib Panhwer 2. Bilawal Rattar

Roll Numbers

(Group Leader)

10EL68

(Assistant Group Leader)

10EL59

3. Zeeshan Anjum Memon

10EL10

4. Sarfaraz Khatti

10EL32

5. Ahmed Maki Utero

10EL51

6. Yasir Memon

10EL16

Signature of Thesis Supervisor

Signature of External Examiner/ Examination Committee

Signature of Thesis Co-supervisor

Signature of Chairman Department of Electrical Engineering

Date: ________________

iii

Dedication

This Humble effort is Dedicated to Our

BELOVED PARENTS & Dr. Narain G. Hingorani who pioneered the concepts of Flexible AC Transmission Systems (FACTS) and Custom Power

iv

Acknowledgements First of all we would like to thank Almighty Allah, the most gracious and the most merciful. It was His grace and blessings that we managed to work on this thesis and completed it. We would like to thank Prof. Dr.Mukhtiar Ahmed Mahar, Department of Electrical Engineering, our Project Supervisor, for his guidance, support, motivation and encouragement throughout the period this work was carried out. His readiness for consultation at all times, his educative comments, his concern and assistance have been invaluable. We are also very thankful to our co-supervisor, Engr. Ms Mokhi Maan, assistant professor, Department of Electrical Engineering, for providing valuable suggestions, guidance and his knowledge during the writing of this thesis. We are also grateful to Dr. Abdul Sattar Larik, Professor and Head, Department of Electrical Engineering, for providing the necessary facilities in the department. In the end, we would like to offer our sincere thanks to our family members and well-wishers whose prayers and moral support played a significant role in motivating us to work sincerely on the thesis.

v

Abstract The increased electric power consumption causes transmission lines to be driven close to or even beyond their transfer capacities resulting in overloaded lines and congestions. FACTS devices provide an opportunity to resolve congestions by controlling active and reactive power flows as well as voltages. FACTS devices can be connected to a transmission line in various ways, such as in series, shunt, or a combination of series and shunt. Shunt FACTS devices are used for controlling transmission voltages, power flow, reducing reactive losses, and damping of power system oscillations for high power transfer levels. The SVC and STATCOM are two important shunt FACTS devices. In this thesis STATCOM is used for reactive power compensation on electric transmission line. A static synchronous compensator (STATCOM) is a regulating device used on alternating current electricity transmission networks. It is based on a power electronics voltage-source converter and can act as either a source or sink of reactive AC power to an electricity network. Usually a STATCOM is installed to support electricity networks that have a poor power factor and often poor voltage regulation. The major attributes of STATCOM are quick response time, less space requirement, optimum voltage platform, higher operational flexibility and excellent dynamic characteristics under various operating conditions.

This thesis explains conventional way of compensation and compensation using STATCOM. Operating principle, circuit configuration, switching technologies and implementation of STATCOM at various places are defined in thesis. The MATLAB simulation results show the relative performance of STATCOM.

vi

Table of Contents

Acknowledgements .................................................................................................................. v Abstract .................................................................................................................................... vi List of Figures .......................................................................................................................... xi List of Tables ......................................................................................................................... xiv List of Abbreviations ............................................................................................................. xv

1. Introduction .......................................................................................................................... 2 1.1 Introduction to thesis title................................................................................................. 2 1.2 History .............................................................................................................................. 3 1.3 Scope of thesis .................................................................................................................. 5 1.4 Outline of thesis ............................................................................................................... 6

2. Preliminary theory of Reactive power compensation....................................................... 8 2.1 Transmission lines ............................................................................................................ 8 2.1.1 Purpose of Transmission ........................................................................................... 9 2.1.2 Choice of voltage ..................................................................................................... 10 2.1.3 Choice of Conductors Size ...................................................................................... 11 2.1.4 Loadability Characteristics ...................................................................................... 11 2.1.5 Effect of using bundled conductors ......................................................................... 12 2.2 Factors that effects the performance of transmission system......................................... 12 2.3 Blackout and its impacts ................................................................................................ 16 2.4 Reactive power and voltage control ............................................................................... 18 2.5 Production and Absorption of Reactive Power .............................................................. 20 2.5.1 Synchronous Generators .......................................................................................... 20 2.5.2 Transmission line .................................................................................................... 21 vii

2.5.3 Transformers ............................................................................................................ 21 2.5.4 Loads ....................................................................................................................... 21 2.6 Reactive Power Compensation....................................................................................... 22 2.6.1 Need of Reactive Power Compensation .................................................................. 22 2.6.2 Compensator requirements ...................................................................................... 23 2.7 Compensation Techniques ............................................................................................. 23 2.7.1 Shunt compensation ............................................................................................... 23 2.7.2 Series Compensation ............................................................................................... 25 2.8 Method of Voltage Control ............................................................................................ 26 2.8.1 Shunt Reactors ......................................................................................................... 27 2.8.2 Shunt capacitors ....................................................................................................... 27 2.8.3 Series Capacitors ..................................................................................................... 29 2.8.4 Synchronous Compensators .................................................................................... 30 2.8.5 Regulating Transformer ........................................................................................... 30 2.9 Reactive Power Limitations ........................................................................................... 33

3. Static synchronous compensator (STATCOM) .............................................................. 37 3.1 Introduction to FACTS .................................................................................................. 37 3.1.1 Basic definitions ...................................................................................................... 38 3.1.2 Objectives of FACTS .............................................................................................. 38 3.1.3 Benefits and cost ...................................................................................................... 39 3.1.4 Generation of reactive power compensation [15].................................................... 41 3.1.5 Classification of FACTS devices ............................................................................ 42 3.1.6 FACTS Application ................................................................................................. 45 3.2 Introduction to static synchronous compensators (STATCOM).................................... 50 3.2.1 Definition ................................................................................................................. 51 3.2.2 Basic Circuit Configuration of STATCOM ............................................................ 51 viii

3.2.3 The major attributes of STATCOM ........................................................................ 52 3.2.4 STATCOM voltage sources .................................................................................... 53 3.2.5 Power industries ...................................................................................................... 58 3.3 Working principle of STATCOM .................................................................................. 58 3.4 STATCOM characteristics ............................................................................................. 62 3.4.1 STATCOM operating characteristics ...................................................................... 62 3.4.2 Transient response ................................................................................................... 63 3.4.3 Harmonics ................................................................................................................ 65 3.4.4 Source voltage ripple ............................................................................................... 65 3.5 Snubber circuit ............................................................................................................... 66 3.6 STATCOM control ........................................................................................................ 67 3.7 Solid-state switching devices ......................................................................................... 71 3.8 STATCOM topologies and configurations .................................................................... 71 3.8.1 Pulse width modulation (PWM) convertor.............................................................. 72 3.8.2 Multi-pulse convertor .............................................................................................. 75 3.8.3 Multi-level convertor ............................................................................................... 76 3.9 Optimal Location of STATCOM ................................................................................... 79 3.10 STATCOM losses ........................................................................................................ 79 3.11 Capability of STATCOM to exchange real power....................................................... 81 3.12 Physical size and installation ....................................................................................... 82 3.13 STATCOM applications .............................................................................................. 83 3.13.1 Transmission applications ..................................................................................... 83 3.13.2 Other Applications of STATCOM ........................................................................ 88

4. Simulation results and discussion ..................................................................................... 91 4.1 Circuit Description ......................................................................................................... 91 4.2 STATCOM Components................................................................................................ 92 ix

4.2.1 48-Pulse three-Level Inverter .................................................................................. 92 4.2.2 STATCOM Control System .................................................................................... 93 4.3 Steady-State and Dynamic Performance of the STATCOM ......................................... 95 4.3.1 System voltage equals to reference voltage ............................................................. 96 4.3.2 System voltage less than the reference voltage ....................................................... 97 4.3.3 System voltage greater than the reference voltage .................................................. 99

5. Conclusions and recommendations ................................................................................ 102 5.1 Conclusion.................................................................................................................... 102 5.2 Future recommendations .............................................................................................. 103

References ............................................................................................................................. 104 Appendix ............................................................................................................................... 106

x

List of Figures Figure 2.1: 500 kV power system Network of WAPDA ........................................................... 9 Figure 2.2: Trends in transmission voltages in 60 years .......................................................... 11 Figure 2.3: single Phase symmetrical bundle conductor circuit .............................................. 12 Figure 2.4: Power System without compensation.................................................................... 24 Figure 2.5: Power System with shunt compensation ............................................................... 25 Figure 2.6: Power System with series compensation............................................................... 26 Figure 2.7: EHV line connected to a weak system .................................................................. 27 Figure 2. 8: Voltage Phasor Diagram for a feeder circuit of lagging power factor: (a) and (c) without and (d) with shunt capacitor ....................................................................................... 28 Figure 2.9: Voltage Phasor Diagram for a feeder circuit of lagging power factor: (a) and (c) without and (d) with series capacitor ....................................................................................... 29 Figure 2.10: Adjustable Synchronous Condenser .................................................................... 30 Figure 2.11: Off-load tap changer ............................................................................................ 31 Figure 2.12: On-load tap changer ............................................................................................ 32 Figure 2.13: Boosters or Regulating Transformer ................................................................... 33

Figure 3.1: Overview of conventional and FACTS devices .................................................... 42 Figure 3.2: Classification of FACTS devices .......................................................................... 43 Figure 3.3 Use of Power Electronics in High-Voltage Systems - “Ranking” of the Controllers .................................................................................................................................................. 45 Figure 3.4: HVDC and FACTS worldwide by Siemens .......................................................... 49 Figure 3.5: Connection of STATCOM with AC bus ............................................................... 52 Figure 3.6: Current sourced convertor CSC ............................................................................ 53 Figure 3.7: power doubling converter arrangement ................................................................. 54 Figure 3.8: Capacitors in a STATCOM (photo courtesy of ABB) .......................................... 55 Figure 3.9: (a) voltage source converter (b) current source converter ..................................... 56 Figure 3.10: STATCOM substation and close-up of the converter valves (photo courtesy of ABB) ........................................................................................................................................ 57 Figure 3.11: Synchronous condensor ....................................................................................... 58 Figure 3.12: A synchronous condenser .................................................................................... 59 Figure 3.13: A single phase STATCOM ................................................................................. 60 xi

Figure 3.14: The waveform of VPN .......................................................................................... 61 Figure 3.15: Reactive power compensation SC & controlled voltage source switching convertor .................................................................................................................................. 62 Figure 3.16: Control characteristics of a STATCOM.............................................................. 63 Figure 3.17: Response of a STATCOM to a system voltage changes ..................................... 64 Figure 3.18: Response of a STATCOM to a depression system voltage ................................ 64 Figure 3.19: Converter output voltage and current waveforms and dc capacitor current and voltage during var generation and absorption .......................................................................... 66 Figure 3.20: Typical snubber circuit arrangement for GTO in a STATCOM ......................... 67 Figure 3.21: 48-pulse STATCOM diagram ............................................................................. 68 Figure 3.22: Control diagram ................................................................................................... 69 Figure 3.23: STATCOM 48-pulse voltage and compensating current .................................... 70 Figure 3.24: VSC using IGBT-based PWM inverters ............................................................. 72 Figure 3.25: Single converter valve unit in a STATCOM ....................................................... 74 Figure 3.26: Two level six pulses VSC Bridge and its AC (Phase & Line) voltage output wave form in square wave mode ............................................................................................. 75 Figure 3.27: Magnetics of 48-pulse, two-level ± 80 MVA STATCOM and its output AC waveform ................................................................................................................................. 76 Figure 3.28: Single Phase of a three-level and four levels NPC converter.............................. 77 Figure 3.29: A view on technology by Siemens: STATCOM ................................................. 78 Figure 3.30: Typical loss curve for a STATCOM ................................................................... 80 Figure 3.31: Typical loss curves for STATCOM applications ................................................ 80 Figure 3.32: Footprints of SVC and STATCOM..................................................................... 82 Figure 3.33: ±80 Mvar in Japan ............................................................................................... 84 Figure 3.34: The ±100 MVAr STATCOM at Sullivan Substation .......................................... 85 Figure 3.35: +225/-52 Mvar SVC including ±75 Mvar STATCOM in England .................... 86 Figure 3.36: STATCOM substation in East Claydon, United Kingdom ................................. 87 Figure 3.37: Various applications of STATCOM.................................................................... 89

Figure 4.1: Detailed model of STATCOM .............................................................................. 91 Figure 4.2: Voltage Source Converter ..................................................................................... 92 Figure 4.3: STATCOM Controller .......................................................................................... 94 Figure 4.4: Waveforms Illustrating STATCOM Response to System Voltage ....................... 95 xii

Figure 4.5: STATCOM Current is zero, System voltage and STATCOM voltage are in Phase .................................................................................................................................................. 96 Figure 4.6: System voltage less than the reference voltage. .................................................... 97 Figure 4.7: STATCOM Current leading the System voltage and system voltage and STATCOM voltages are in Phase ............................................................................................ 97 Figure 4.8: STATCOM produces reactive power .................................................................... 98 Figure 4.9: System voltage greater than reference voltage ...................................................... 99 Figure 4.10: STATCOM Current lagging the System voltage and System voltage and STATCOM voltages are in Phase .......................................................................................... 100 Figure 4.11: STATCOM absorbing reactive power .............................................................. 100

xiii

List of Tables Table 1.1: Partial list of utility scale of STATCOM .................................................................. 4

Table 3.1: Comparison of Basic Types of Compensators........................................................ 39 Table 3.2: Cost comparison of different FACTS controllers ................................................... 41

Table 4.1: Transmission line parameters ................................................................................. 92

xiv

List of Abbreviations AC

Alternating current

BFO

Bacterial Foraging Optimization

CSC

Current Source Convertor

DC

Direct Current

D-STATCOM Distribution Static Synchronous Compensator EPRI

Electric Power Research Institute

FACTS

Flexible AC Transmission System

GTO

Gate Turn Off

HVDC

High Voltage Direct Current

IEEE

Institute Of Electrical And Electronics Engineers

IGBT

Insulated Gate Bipolar Transistor

IGCT

Integrated Gate Commutated Thyristor

IPFC

Interline Power Flow Controller

ISO

Independent System Operator

LMP

Locational Marginal Price

MCT

MOS Controlled Thyristor

NEC

National Electric Code

NLTC

No-Load Tap Changer

NTDC

National Transmission And Dispatch Center

PI

Proportional Integral

PLL

Phase Locked Loop

PSO

Particle Swarm Optimization

PSS

Power System Stabilizer

PWM

Pulse Width Modulation

RTDS

Real Time Digital Simulator xv

SC

Synchronous Condenser

SMES

Superconducting Magnetic Energy Storage

SSR

Sub Synchronous Resonance

SSSC

Static Synchronous Series Compensator

STATCOM

Static Synchronous Compensator

STATCON

Static Synchronous Condenser

SVC

Static VAR Compensator

TCPAR

Thyristor Controlled Phase Angle Regulator

TCPS

Thyristor Controlled Phase Controlled Transformer

TCSC

Thyristor Controlled Series Compensator

THD

Total Harmonic Distortion

UPFC

Unified Power Flow Controller

VAR

Volt Ampere Reactive

VSC

Voltage Source Convertor

WAPDA

Water And Power Development Authority

xvi

Chapter # 01 Introduction

1. Introduction 1.1 Introduction to thesis title Voltage control and reactive-power management are two aspects of a single activity that both supports reliability and facilitates commercial transactions across transmission networks. On an alternating-current (AC) power system, voltage is controlled by managing production and absorption of reactive power. Reactive power compensation is done for; 

Better efficiency of power generation, transmission and distribution



Improvement in voltage profile



Improves system power factor



Reduction of KVA demand



Higher load capability



Reduction in system losses



Increases system capacity and saves cost on new installations

The Static Synchronous Compensator (STATCOM) is a shunt connected reactive compensation equipment which is capable of generating and/or absorbing reactive power whose output can be varied so as to maintain control of specific parameters of the electric power system. The STATCOM provides operating characteristics similar to a rotating synchronous compensator without the mechanical inertia, due to the STATCOM employ solid state power switching devices it provides rapid controllability of the three phase voltages, both in magnitude and phase angle. STATCOM provide voltage support to buses by modulating bus voltages during dynamic disturbances in order to provide better transient characteristics, improve the transient stability margins and to damp out the system oscillations due to these disturbances. The principle of STATCOM operation is as follows. The VSC generates a controllable AC voltage source behind the leakage reactance. This voltage is compared with the AC bus voltage system; when the AC bus voltage magnitude is above that of the VSC voltage magnitude, the AC system sees the STATCOM

Reactive power compensation using STATCOM

2

as an inductance connected to its terminals. Otherwise, if the VSC voltage magnitude is above that of the AC bus voltage magnitude, the AC system sees the STATCOM as a capacitance connected to its terminals. If the voltage magnitudes are equal, the reactive power exchange is zero. If the STATCOM has a DC source or energy storage device on its DC side, it can supply real power to the power system. This can be achieved adjusting the phase angle of the STATCOM terminals and the phase angle of the AC power system. When the phase angle of the AC power system leads the VSI phase angle, the STATCOM absorbs real power from the AC system; if the phase angle of the AC power system lags the VSC phase angle, the STATCOM supplies real power to AC system. STATCOM could have many topologies, but in most practical applications it employs the DC to AC converter, which can also be called a Voltage Source convertor (VSC) in 3-phase configuration as the primary block. The basic theory of VSC is to produce a set of controllable 3-phase output voltages/ currents at the fundamental frequency of the AC bus voltage from a DC input voltage source such as a charged capacitor or a DC energy supply device. By varying the magnitude and phase angle of the output voltage and current, the system can exchange active/reactive power between the DC and AC buses, and regulate the AC bus voltage 1.2 History The history of FACTS controllers can be traced back to 1970s when Hingorani presented the idea of power electronic applications in power system compensation. From then on, various researches were conducted on the application of high power semiconductors in transmission systems. In 1988, Hingorani defined the FACTS concept and described the wide prospects of the applications. Nowadays, FACTS technology has shown strong potential. Many examples of FACTS devices and controllers are in operation. The STATCOM was originally called as advanced SVC and then labelled as STATCON (static condenser) and now days commonly known as static


3

synchronous compensator (STATCOM). In advanced static VAR compensator, a voltage source convertor (VSC) is used instead of the controllable reactors and switched capacitors. Although VSCs require self-commutated power semiconductor devices such as GTO, IGBT, IGCT, MCT, etc. (with higher costs and losses) unlike in the case of variable impedance type SVC which use thyristor devices. Since 1980 when the first STATCOM (rated at 20 Mvar) using forcecommutated thyristor inverters was put into operation in Japan, many examples have been installed and the ratings have been increased considerably. In 1991, KEPCO and Mitsubish Motors installed a ±80MVar STATCOM at Inuyama Switching Station. In 1996, TVA, EPRI and Westinghouse installed a ±100MVar STATCOM at Sullivan 500 kV Substation. In 2001, EPRI and Siemens developed a ±200MVar STATCOM at Marcy 345kV substation. It is expected that more STATCOMs will be installed due to the advances in technology and commercial success.

S.N

Year Installed

Country

Capacity MVAR

Voltage Level (KV) 154

1

1991

Japan

2

1992

Japan

±80 MVA 50 MVA

3

1995

USA

±100 MVA

161

To regulate voltage

4

2001

UK

0 to +225

400

Dynamic reactive compensation

5

2001

USA

-41 to +133

115

6

2003

USA

±100

138

Dynamic reactive compensation during critical contingencies Dynamic var control during peak load conditions

500

Purpose

Place

Power System and voltage Stabilization Reactive compensation bus

Inumaya substation Shin Shinano Substation Nagona Sullivan substation in TVA power system East Claydon 400 kV Substation VELCO Essex substation SDG&E Talega substation

Table 1.1: Partial list of utility scale of STATCOM


4

1.3 Scope of thesis Reactive power is a subject of great concern for the operation of alternating current (AC) power systems. It has always been a challenge to obtain the balance between a minimum amount of reactive power flow (to maximize capacity for active power flow) and a sufficient amount of reactive power flow to maintain a proper system voltage profile. Although it does not do any useful work but its compensation is necessary for many reasons viz; 

Reactive power (VARS) is required to maintain the voltage to deliver active power (watts) through transmission lines and to maintain a System Healthy.



It is uneconomical to increase voltage level and it may be more profitable to give consideration to line compensation by means of capacitors or other compensation devices to increase the economic limit of power transmission.



The quality of the electrical energy supply can be evaluated basing on a number of parameters. However, the most important will be always the presence of electrical energy and the number and duration of interrupts. A long term, wide-spread interrupt

a blackout leads usually to

catastrophic losses. It is difficult to imagine that in all the country there is no electrical supply. One of the reasons leading to a blackout is reactive power that went out of the control. 

Energy supplier charge a customer for reactive power which force the industry plants and individual customers to minimize energy consumption, including reactive power.

Hence the reactive power must be controlled and maintained at required level by compensating devices. Due to draw backs in conventional compensation devices STATCOM, the 3rd generation flexible AC transmission system device, that nowadays getting most of attention for reactive power compensation because;



STATCOM has faster response



It requires less space as bulky passive components (such as reactors) are


5

eliminated 

It can be interfaced with real power sources such as battery, fuel cell or SMES



A STATCOM has superior performance during low voltage condition as the reactive current can be maintained constant.



It is even possible to increase the reactive current in a STATCOM under transient conditions if the devices are rated for the transient overload.



It does not contribute to short circuit current.



It has a symmetric lead-lag capability.



It has no moving parts and hence the maintenance is easier.



It has no problems of loss of synchronism under a major disturbance.

1.4 Outline of thesis First chapter gives introduction about thesis title, history, scope of thesis and also the outline of thesis. In second chapter preliminary theory about transmission line and reactive power, and conventional methods of reactive power and voltage control is discussed. Chapter three gives the idea of flexible AC transmission system (FACTS) and describes in detail about static synchronous compensator (STATCOM). In fourth chapter MATLAB software is used to get result after simulation of 48pulse GTO-based STATCOM. In last conclusions and future recommendations are given.


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Chapter # 02 Preliminary theory of Reactive power

2. Preliminary theory of Reactive power compensation 2.1 Transmission lines Electric power is produced at generating station and transmitted to a consumer through a complex network of individual components, including transmission lines, transformers and switching devices. It is common practice to classify the transmission network into following subsystems: 

Transmission system



Sub transmission system



Distribution system

The transmission interconnects all major generating stations and main load centers in the system. It forms the backbone of the integrated power system and operates at the highest voltage levels (typically, 500kV and 220kV). The generator voltages are usually line the range of 11kV to 15kV. These are steeped up to the transmission voltage level, and power is transmitted to transmission substations where the voltages are steeped to the sub transmission level (typically 66kV to 132kV). The generation and transmission subsystems are often referred to as the bulk power system. The sub transmission system transmits power in smaller quantities from the transmission substation to the distribution substations. Large industrial consumers are commonly supplied directly from the sub transmission system. The distribution represents the final stage in the transfer of power to the individual consumers. The primary distribution voltage is typically between 11kV and 33kV. Small industrial customers are supplied by primary feeders at this voltage level. The secondary distribution feeders supply residential and commercial customers at 230/440V. The overall system thus consists of multiple generating sources and several layers of transmission networks. This provides a high degree of structural


8

redundancy that enables the system to withstand unusual contingencies without services disruption to the consumers.[1]

Figure 2.1: 500 kV power system Network of WAPDA

2.1.1 Purpose of Transmission Transmission lines are essential for three purposes. 

To transmit power from a mater-power site to a market. These may be very long and justified because of the subsidy aspect connected with the



project.

For bulk supply of power to load centers from outlying steam stations. These are likely to be relatively short.



For interconnection purposes, that is, for transfer of energy from one system to another in case of emergency or in response to diversity in


9

system peaks. Frequent attempts have been made to set up definitions of ‘Transmission lines,” “distribution circuits” and “substations.” None has proved entirely satisfactory or universally applicable, but for the purposes of accounting the Federal Power Commission and various state commissions have set up definitions that in essence read: “A transmission system includes all land, conversion structures and equipment at a primary source of supply lines, switching and conversion stations between a generating or receiving point and the entrance to a distribution center or wholesale point, all lines and equipment whose primary purpose is to augment, integrate or tie together sources of power supply”.[3] 2.1.2 Choice of voltage The cost of transformers, switches, and circuit breakers increases rapidly with increasing voltage in the upper ranges of transmission voltages. In any investigation involving voltages above 230kV, therefore, the unit cost of power transmitted is subject to the law of diminishing returns. Furthermore, the increase of the reactance of the terminal transformers also tends to counteract the gain obtained in the transmission line from the higher voltage. There is, therefore, some value of voltage in the range being investigated beyond which, under existing circumstances, it is uneconomical to go and it may be more profitable to give consideration to line compensation by means of capacitors to increase the economic limit of power transmission than increase the voltage much above present practice.


10

Figure 2.2: Trends in transmission voltages in 60 years

2.1.3 Choice of Conductors Size The preliminary choice of the conductor size can also be limited to two or three, although the method of selecting will differ with the length of transmission and the choice of voltage. In the lower voltages up to, say, 30 kV, for a given percentage energy loss in transmission, the cross section and consequently the weight of the conductors required to transmit a given block of power varies inversely as the square of the voltage. Thus, if the voltage is doubled, the weight of the conductors will be reduced to one-fourth with approximately a corresponding reduction in their cost. This saving in conducting material for a given energy loss in transmission becomes less as the higher voltages are reached, becoming increasingly less as voltages go higher. [3] 2.1.4 Loadability Characteristics This concept was first introduced by H.P. St. Clair in 1953.The concept of “line Loadability” is useful in developing a fuller understanding of power transfer capability as influenced by voltage level and line length. Line Loadability is defined as the degree of line loading (expressed in percent of SIL) permissible given the thermal, voltage drop, and stability limits.


11

The limits to the line loading are governed by the following considerations: 

Thermal limits for the lines up to 80kms



Voltage drop limits for lines between 80kms and 320kms long



Stability limits for lines longer than 320kms

2.1.5 Effect of using bundled conductors At voltages above 230 kV (extra high voltage) and with circuits with only one conductor per phase, the corona effect becomes more excessive. Associated with this phenomenon is a power loss as well as interference with communication links. Corona is the direct result of high-voltage gradient at the conductor surface. The gradient can be reduced considerably by using more than one conductor per phase. The conductors are in close proximity compared with the spacing between phases. A line such as this is called a bundle-conductor line. The bundle consists of two or more conductors (sub conductors) arranged on the perimeter of a circle called the bundle circle. Another important advantage of bundling is the attendant reduction in line reactances, both series and shunt. The analysis of bundle-conductor lines is a specific case of the general multiconductor configuration problem. [2]

Figure 2.3: single Phase symmetrical bundle conductor circuit 2.2 Factors that effects the performance of transmission system Along with the economic development and the social improvement, the electric utilities must run more rapidly to meet the heavily increasing demands of electric power. However, in the procedure of expanding and interconnecting of the power System, accordingly various problems arise:


12



On account of the irregular distribution of the energy sources, and the vast amount of transfer electric power over long distance between the generation center and the load center, huge power losses in the lines occur.



In the interconnected power system, the power flow from the generator to the consumers is dependent on the location of the generation node, of the consumer nodes and on the transmission paths available, i.e. on the power system topology and the electrical characteristics of the lines involved, the result is transmission bottlenecks and unwanted parallel path or loop flows.



To meet the load and electric market demands, new lines should be added to the system, but because of a variety of environmental land use and regulatory pressures, the growth of electric power transmission lines in many parts of the world is restricted.



In the large-scale power system, the stability becomes more critical, several large-area power failures due to damaging of the power system stability resulted in enormous economic losses in the world.

On these backgrounds, there is an urgent demand to realize the rational transfer power allocation, to reduce the power losses and generation costs, and to improve the stability and the reliability of the power system greatly. The factors that limit the transmission system to do so are as listed below: a. System stability b. Loop Flow c. Voltage limits d. Thermal limits of line e. High short circuit limits f. Franettie Effect


13

a. System Stability: “Power system stability denotes the ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that system integrity is preserved.” Types of Power System Stability Controls and Possibilities for Advanced Control Stability controls are of many types including 

Generator excitation controls



Prime mover controls including fast valving



Generator tripping



Fast fault clearing



High-speed reclosing and single-pole switching



Dynamic braking



Load tripping and modulation



Reactive power compensation switching or modulation (series and shunt) Current injection by voltage source inverter devices (STATCOM, UPFC, SMES, battery storage)



Fast phase angle control



HVDC link supplementary controls



Adjustable speed (doubly fed) synchronous machines



Controlled separation and under frequency load shedding

b. Loop Flow: “The tendency of electricity to flow along the path of least resistance, which may not necessarily be the same as that intended in the contract between the two transmitting entities.” Loop flow can also be defined as;


14

“The movement of electric power from generator to load by dividing along multiple parallel paths; it especially refers to power flow along an unintended path that loops away from the most direct geographic path or contract path.”

c. Voltage Limits: In every power transmission system the power flow is always accompanied by voltage drops, these voltage drops are limited between 3%-6% of the standard voltage. In Pakistan according to National Transmission and Dispatch Company (NTDC), WAPDA the voltage at the end of transmission line must not be greater than 5% of sending end voltage. According to Wiring codes or regulations set an upper limit to the allowable voltage drop in a branch circuit. In the United States, the 2005 National Electrical Code (NEC) recommends no more than a 5% voltage drop at the outlet. The Canadian electrical code requires no more than 5% drop between service entrance and point of use. UK regulations limit voltage drop to 4% of supply voltage.

d. Thermal Limits: Thermal limit is due to the heat generated when current flows in the conductor. Heat generated by the line losses (I2R) causes a temperature rise. Since line temperature of overhead lines must be kept within a safe limit to prevent excessive line sag between transmission towers and to prevent irreversible stretching, the ground clearance must be maintained in the case of overhead transmission lines. This imposes condition on the maximum safe current in a line. Several factors other than the current flowing in conductor are responsible for increase in temperature such as design conditions (conductor size and geometry, spacing between towers, etc.) and operating conditions (ambient temperature, wind velocity, etc.). Cables are even more prone to thermal limit because of limited possibilities for heat transfer. However, there is no problem of sag in cables. But if the cable gets too hot, the insulation will begin to deteriorate and may fail in future. [5]


15

e. High Short Circuit Limits: A high short circuit limit of transmission system defines that how much value of short current can system withstands. As we know if the system is larger than during any short circuit fault, short circuit current will be greater; so in order to limit this current we have to install such protection scheme to handle this large current. Doing this will protect our system from some serious damages.

f. Ferranti Effect: The Ferranti Effect is a rise in voltage occurring at the receiving end of a long transmission line, relative to the voltage at the sending end, which occurs when the line is charged but there is a very light load or the load is disconnected. This effect is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance are responsible for producing this phenomenon. The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied. The relative voltage rise is proportional to the square of the line length. This unwanted voltage raise may lead to insulations breakdowns and other power system and utility damages. 2.3 Blackout and its impacts The quality of the electrical energy supply can be evaluated basing on a number of parameters. However, the most important will be always the presence of electrical energy and the number and duration of interrupts. If there is no voltage in the socket nobody will care about harmonics, sags or surges. A long term, wide-spread interrupt – a blackout leads usually to catastrophic losses. It is difficult to imagine that in all the country there is no electrical supply. In reality such things have already happened a number of times. One of the reasons leading to a blackout is reactive power that went out of the control. When consumption of electrical energy is high, the demand on inductive reactive power increases usually at the same proportion. In this moment, the transmission lines (that are well loaded) introduce an extra inductive reactive power. The local sources of capacitive reactive power become insufficient. It is necessary to


16

deliver more of the reactive power from generators in power plants. It might happen that they are already fully loaded and the reactive power will have to be delivered from more distant places or from abroad. Transmission of reactive power will load more the lines, which in turn will introduce more reactive power. The voltage on customer side will decrease further. Local control of voltage by means of autotransformers will lead to increase of current (to get the same power) and this in turn will increase voltage drops in lines. In one moment this process can go like avalanche reducing voltage to zero. In mean time most of the generators in power plants will switch off due to unacceptably low voltage what of course will deteriorate the situation.

In continental Europe most of the power plant is based on heat and steam turbines. If a generation unit in such power plant is stopped and cool down it requires time and electrical energy to start operation again. If the other power plants are also off -the blackout is permanent. Insufficient reactive power leading to voltage collapse has been a causal factor in major blackouts in the worldwide. Voltage collapse occurred in United States in the blackout of July 2, 1996, and August10, 1996 on the West Coast While August 14, 2003, blackout in the United States and Canada was not due to a voltage collapse as that term has traditionally used by power system engineers, the task force final report said that” Insufficient reactive power was an issue in the blackout” and the report also “overestimation of dynamics reactive output of system generation ” as common factor among major outages in the United States.

Demand for reactive power was unusually high because of a large volume of long-distance transmissions streaming through Ohio to areas, including Canada, than needed to import power to meet local demand. But the supply of reactive power was low because some plants were out of service and, possibly, because other plants were not producing enough of it” [5].


17

2.4 Reactive power and voltage control Voltage control and reactive-power management are two aspects of a single activity that both supports reliability and facilitates commercial transactions across transmission networks. On an alternating-current (AC) power system, voltage is controlled by managing production and absorption of reactive power.

Reactive Power is helpful to maintain a System Healthy. We always in practice to reduce reactive power to improve system efficiency .This are acceptable at some level, if system is purely resistively or capacitance it make cause some problem in Electrical system. AC systems supply or consume two kind of power: real power and reactive power.

While active power is the energy supplied to run a motor, heat a home, or illuminate an electric light bulb, reactive power provides the important function of regulating voltage. If voltage on the system is not high enough, active power cannot be supplied. Reactive power is used to provide the voltage levels necessary for active power to do useful work. Reactive power is essential to move active power through the transmission and distribution system to the customer [5].

For efficient and reliable operation of power systems, the control of voltage and reactive power should satisfy the following objectives:



Voltages at the terminals of all equipment in the system are within acceptable limits. Both utility equipment and customer equipment are designed to operate at a certain voltage rating. Prolonged operation of the equipment at voltages outside the allowable range should adversely affect their performance and possibly cause them damage.



System stability is enhanced to maximize utilization of the transmission system. Voltage and reactive power control have a significant impact on system stability.



The reactive power flow is minimized so as to reduce RI2 and XI2


18

practical minimum. This ensures that the transmission system operates efficiently, i.e., mainly for active power transfer. The problem of maintaining voltages within the required limits is complicated by the fact that the power system supplies power to a vast number of loads and is fed from many generating units. As loads vary, the reactive power requirements of the transmission system vary. Since reactive power cannot be transmitted over long distances, voltage control has to be effected by using special devices dispersed throughout the system. This is in contrast to the control of frequency which depends upon the overall system active power balance. The proper selection and coordination of equipment for controlling reactive power and voltages are among the major challenges of power system engineering. Voltage control is complicated by two additional factors. First, the transmission system itself is a nonlinear consumer of reactive power, depending on system loading. At very light loading the system generates reactive power that must be absorbed, while at heavy loading the system consumes a large amount of reactive power that must be replaced. The system’s reactive-power requirements also depend on the generation and transmission configuration. Consequently, system reactive requirements vary in time as load levels and load and generation patterns change. The bulk-power system is composed of many pieces of equipment, any one of which can fail at any time. Therefore, the system is designed to withstand the loss of any single piece of equipment and to continue operating without impacting any customers. That is, the system is designed to withstand a single contingency. Taken together, these two factors result in a dynamic reactive-power requirement. The loss of a generator or a major transmission line can have the compounding effect of reducing the reactive supply and, at the same time, reconfiguring flows such that the system is consuming additional reactive power. At least a portion of the reactive supply must be capable of responding quickly to changing reactive-power demands and to maintain acceptable voltages throughout the system. Thus just as an electrical system requires real-power


19

reserves to respond to contingencies, so too it must maintain reactive-power reserves. Loads can also be both real and reactive. The reactive portion of the load could be served from the transmission system. Reactive loads incur more voltage drop and reactive losses in the transmission system than do similar-size (MVA) real loads. Vertically integrated utilities often include charges for provision of reactive power to loads in their rates. With restructuring, the trend is to restrict loads to operation at near zero reactive power demand (a 1.0 power factor). The system operator proposal limits loads to power factors between 0.97 lagging (absorbing reactive power) and 0.99 leading. This would help to maintain reliability of the system and avoid the problems of market power in which a company could use its transmission lines to limit competition for generation and increase its prices. 2.5 Production and Absorption of Reactive Power The study of generation and absorption of reactive power in the power system is essential since the reactive power is very precious in keeping the voltage of the power system stable. Whereas frequency is the indicator of active power balance, voltage is the sole indicator of reactive power balance. The components responsible for the generation and absorption of reactive power in the power system are: • Synchronous Generator • Transmission Line • Transformers • Loads 2.5.1 Synchronous Generators Synchronous generators can generate or absorb reactive power depending on the excitation. When overexcited they supply reactive power, and when under excited they absorb reactive power. The capability continuously supply are absorb reactive power is, however, limited by field current, armature current, and end-region heating limits synchronous generators are normally equipped


20

with automatic voltage regulators which continually adjust excitation so as to control the armature voltage. 2.5.2 Transmission line Transmission line is divided into two parts: Overhead and Underground lines. Overhead lines, depending on load current, either absorb or supply reactive power. At loads below the natural (surge impedance) load, the lines produce reactive power; at loads above the natural load the lines absorb reactive power. Underground cables, owing to their high capacitance, have high natural loads. They are always loaded below their natural loads, and hence generate reactive power under all operating conditions. 2.5.3 Transformers Transformers always absorb reactive power regardless of their loading; at no load, the shunt magnetizing reactance affects predominate; and at full load, the series leakage inductance effects predominate. 2.5.4 Loads Loads normally absorb reactive power. A typical load bus supplied by power system is composed of large number of devices. The composition changes depending on the day, season and weather conditions. The composed characteristics are normally such that a load bus absorbs reactive power. Both active power and reactive power of the composite loads vary as a function of voltage magnitude. Loads at low-lagging power factors cause excessive voltage drops in the transmission network and are uneconomical to supply. Industrial consumers are normally charged for reactive power as well as active power; this gives them an incentive to improve the load power factor y using shunt capacitors. Compensating devices are usually added to supply or absorb reactive power and thereby control the reactive power balance in a desired manner. In what follows, we will discuss the characteristics of these devices and the principles of


21

application. 2.6 Reactive Power Compensation VAR compensation is defined as the management of reactive power to improve the performance of ac power systems. The concept of VAR compensation embraces a wide and diverse field of both system and customer problems, especially related with power quality issues, since most of power quality problems can be attenuated or solved with an adequate control of reactive power [4]. 2.6.1 Need of Reactive Power Compensation The need for adjustable reactive power compensation can be divided into three basic classes: i) The need to maintain the stability of synchronous machines. We shall see that voltage control by reactive power compensation can have a positive stabilizing influence on the system during disturbances cause the rotor angles of synchronous machines to change rapidly. Both the transient stability and the dynamic stability of a system can enhance. It is even possible with controlled compensators to drive voltages deliberately out of their normal steady-state bounds for several seconds following a fault or other major disturbance to enhance the stabilizing influence still further

ii) The need to control voltage within acceptable bounds about the desired steady-state value to provide quality service to consumer loads. Following certain abrupt changes in the load, or in the network configuration as a result of switching actions, it may be necessary to make a voltage correction in as short a time as a few cycles of the power frequency. For other voltage disturbances, a correction within a few seconds will suffice. Uncorrected voltage deviations, even if temporary, may lead to an outage or damage to utility or consumer-owned equipment. Even small variations, particularly those that cause flicker, are often objectionable. Reactive power compensation using STATCOM

22

iii) The need to regulate voltage profiles in the network to prevent unnecessary flows of reactive power on transmission lines. To this end, reactive power compensation can be used to maintain transmission losses to a practical minimum. While the reactive compensation must be adjusted or changed periodically to maintain minimum losses, the adjustments can be made quite infrequently with several minutes to effect the desired change. 2.6.2 Compensator requirements The functional requirement of reactive shunt compensators used for increased power transmission, improved voltage and transient stability and power oscillation damping can be summarized as follows: The compensator must stay in synchronous operation with the AC system at the compensated bus under all operating conditions including major disturbances. Should the bus voltage be lost temporarily due to nearby faults, the compensator must be able to recapture synchronism immediately at fault clearing. The compensator must be able to regulate the bus voltage for voltage support and improved transient stability, or control it for power oscillation damping and transient stability enhancement, on a priority basis as system conditions may require. For a transmission line connecting two systems, the best location for VAR compensation is in middle, whereas for a radial feed to a load the best location is at the load end. [6] 2.7 Compensation Techniques The principles of both shunt and series reactive power compensation techniques are described below: 2.7.1 Shunt compensation Shunt compensation, especially shunt reactive compensation has been widely used in transmission system to regulate the voltage magnitude, improve the Reactive power compensation using STATCOM

23

voltage quality, and enhance the system stability. Shunt-connected reactors are used to reduce the line over-voltages by consuming the reactive power, while shunt-connected capacitors are used to maintain the voltage levels by compensating the reactive power to transmission line. [8]

The Figure 2.4 comprises of a source V1, a power line and an inductive load. The figures show the system without any type of compensation. The phasor diagram of these is also shown above. The active current Ip is in phase with the load voltage V2.

Figure 2.4: Power System without compensation

Here, the load is inductive and hence it requires reactive power for its proper operation and this has to be supplied by the source, thus increasing the current from the generator and through the power lines. Instead of the lines carrying this, if the reactive power can be supplied near the load, the line current can be minimized, reducing the power losses and improving the voltage regulation at the load terminals. This can be done in three ways: i) A voltage source. ii) A current source. iii) A capacitor.


24

Figure 2.5: Power System with shunt compensation As shown in Figure 2.5, a current source device is used to compensate Iq, which is the reactive component of the load current. In turn the voltage regulation of the system is improved and the reactive current component from the source is reduced or almost eliminated. This is in case of lagging compensation. For leading compensation, we require an inductor. Therefore we can see that, a current source or a voltage source can be used for both leading and lagging shunt compensation, the main advantages being the reactive power generated is independent of the voltage at the point of connection. 2.7.2 Series Compensation Series compensation aims to directly control the overall series line impedance of the transmission line. The AC power transmission is primarily limited by the series reactive impedance of the transmission line. A series-connected can add a voltage in opposition to the transmission line voltage drop, therefore reducing the series line impedance. [12] Series compensation can be implemented like shunt compensation, i.e. with a current or a voltage source as shown in Figure 2.6. We can see the results which are obtained by series compensation through a voltage source and it is adjusted to have unity power factor at V2. However series compensation techniques are different from shunt compensation techniques, as capacitors are used mostly for series compensation techniques. In this case, the voltage Vcomp has been added Reactive power compensation using STATCOM

25

between the line and the load to change the angle V2’. Now, this is the voltage at the load side. With proper adjustment of the magnitude of Vcomp, unity power

factor

can

be

reached

at

V2.

Figure 2.6: Power System with series compensation

2.8 Method of Voltage Control The control of voltage levels is accomplished by controlling the production, absorption, and flow of reactive power at all levels in the system. The generating units provide the basic means of voltage control; the automatic voltage regulators control field excitation to maintain a scheduled voltage level at the terminals of the generators. Additional means are usually required to control voltage throughout the system. The devices used for this purpose may be classified as follows: i) sources or sinks of reactive power, such as shunt capacitor, shunt reactor, synchronous condensers and static VAR compensators(SVCs) ii) Line reactance compensators, such as series capacitors iii) Regulating transformers, such as tap changing transformers and boosters Shunt capacitors and reactors, and series capacitors provide passive compensation. They are either permanently connected to the transmission and distribution system, or switched. They contribute to voltage control by modifying the network characteristics.


26

Synchronous condensers and SVCs provide active compensation; the reactive power absorbed/supplied by them is automatically adjusted so as to maintain voltage of the buses to which they are connected together with the generating units, they establish voltages at specific points in the system. Voltage at other location in the system are determined by active and reactive power flows through various circuit elements, including the passive compensating devices. The following is the description of the basic characteristics and forms of application of devices commonly used voltage and reactive power control. 2.8.1 Shunt Reactors Shunt reactors are used to compensate for the effect of line capacitance, particularly to limit voltage rise on open circuit or light loads. They are usually required for EHV overhead lines longer than 200km. A shorter overhead line may also require shunt reactors if the line is supplied from a weak system (low short-circuit capacity) as shown in Figure 2.7 when the far end of line is opened, the capacitive line-charging current flows through the large source inductive reactance (Xs) will cause a rise in voltage Es at the sending end of the line[1].

Figure 2.7: EHV line connected to a weak system

2.8.2 Shunt capacitors Shunt capacitors supply reactive power and boost local voltages are used thought the system and are applied in a wide range of sizes. Shunt capacitors


27

i.e. capacitors connected in parallel with lines, are extensively used in distribution systems. Shunt capacitors supply the type of reactive power or current to counteract the out of phase component of current required by an individual load. In a sense, shunt capacitors modify the characteristic of an inductive load by drawing a leading current which counteracts some or the entire lagging component of inductive load current at the point of installation. Therefore a shunt capacitors has the same effect as the as an overexcited synchronous condenser generator or motor [11]. As shown in Figure 2.8 by the application of shunt capacitor to a feeder, the magnitude of the source current can be reduced, the power factor can be improved, and consequently the voltage drop between the sending end and the load is reduced. However, shunt capacitors do not affect current or power factor beyond their point of application. Figure 2.8 a and c shows the single line diagram of the line and its voltage phasor diagram before the addition of the shunt capacitors and Figure 2.8 b and d show then after addition.

Figure 2. 8: Voltage Phasor Diagram for a feeder circuit of lagging power factor: (a) and (c) without and (d) with shunt capacitor


28

2.8.3 Series Capacitors Series

capacitors

conductors

are

connected

in

series

with

the

line

to compensate for the inductive reactance of the line. Series

capacitors i.e. capacitors connected in series with line have been used to a very limited extent on distribution circuits due to being a more specialized type of apparatus with a limited range of application. Also because of the special problem associated with each application, there is requirement of large amount of complex engineering investigation, therefore in general, and utilities are reluctant to install series capacitors, especially of small sizes. As shown in Figure 2.9 a series capacitor compensates for individual reactance. In other words a series capacitor is a negative (capacitive) reactance in series with the circuit positive (inductive) reactance with the effect of compensating for part or all of it. Therefore, the primary effect of the series capacitors is to minimize, or even suppress, the voltage drop caused by

the

inductive

reactance

in

the

circuit

[11].

Figure 2.9: Voltage Phasor Diagram for a feeder circuit of lagging power factor: (a) and (c) without and (d) with series capacitor


29

2.8.4 Synchronous Compensators Synchronous compensator (or synchronous condenser) shown if Figure 2.10 is a synchronous machine running without a prime mover or a mechanical load. By controlling the field excitation, it can be made to either generate or absorb reactive power. With a voltage regulator, it can automatically adjust the reactive power output to maintain constant terminal voltage. It draws a small amount of active power from the power system to supply losses.

Figure 2.10: Adjustable Synchronous Condenser 2.8.5 Regulating Transformer The following are the methods of voltage control in a transmission system: 1) By transformer tap changing 2) Booster (or regulating) transformer

1) Tap Changing Transformer Transformer tap changing is the basic and easiest way of voltage control of transmission, sub-transmission and distribution system. The transformer does not generate any reactive power (rather it consumes) and only transfers the reactive power from one side to another side by changing the in-phase component of the system voltage. In this method, a number of tapings are provided on the secondary of the transformer. The voltage drop in the line is supplied by changing the secondary e.m.f. of the transformer through the adjustment of its number of turns.


30

There are two types of tap changing: off load and on load tap changing which are described below:



No-Load Tap Changer (NLTC)

Figure 2. 1 1 shows the arrangement where a number of tapings have been provided on the secondary. As the position of the tap is varied, the effective number of secondary turns is varied and hence the output voltage of the secondary can be changed. movable arm

makes

Thus referring to Figure 2.11 when the

contact

with

stud

1,

the secondary voltage is

minimum and when with stud 5, it is maximum. During the period of light load, the voltage across the primary is not much below the alternator voltage and the movable arm is placed on stud 1.When the load increases, the voltage across the primary drops, but the secondary voltage can be kept at the

previous value by placing the movable arm on to a higher stud.

Whenever a tapping is to be changed in this type of transformer, the load is kept off and hence the name off load tap-changing transformer. The principal disadvantage of the circuit arrangement is that it cannot be used for tap changing on load. Suppose for a moment that tapping is changed from position 1 to position 2 when the transformer is supplying load. If contact with stud 1 is broken before contact with stud 2 is made, there is break in the circuit and arcing results.

Figure 2.11: Off-load tap changer


31



On Load tap Changing Transformer

In supply system, tap-changing has normally to be performed on load so that there is no interruption to supply. Figure 2.12 shows diagrammatically type

of

on-load

one

tap- changing transformer. The secondary consists of two

equal parallel windings which have similar tapings 1a to 5a and 1b to 5b. In the normal working conditions, switches a, b and

tapings with the

same number remain closed and each secondary winding carries one-half of the total current.

Referring to Figure 2.12 the secondary voltage will be

maximum when switches a, b and 5a, 5b are closed. However, the secondary voltage will be minimum when switches a, b and 1a, 1b are closed. Suppose that the transformer is working

with

tapping

position at 4a, 4b

and it is desired to alter its position to 5a, 5b. For this purpose, one of the switches a and b, say a, is opened. This takes the secondary winding controlled by switch an out of the circuit. Now, the secondary winding controlled by switch b carries the total current which is twice its rated capacity. Then the tapping on the changed to 5a and switch a is closed. position is changed without interrupting the

In this way, tapping

supply. This method has the

following disadvantages. • During switching, the impedance of transformer is increased and there will be a voltage surge. • There are twice as many tapings as the voltage steps.

Figure 2.12: On-load tap changer


32

2) Booster A single-phase booster consists of two parts; an exciting transformer connected across the supply along with a series transformer with the supply as shown in Figure 2.13. The output voltage of the series transformer can be added to the input voltage in phase or in reverse phase by changing the position of a switch. The output voltage of the regulating transformer can be varied by changing the taps of the exciting transformer. These transformers are often used when it is inconvenient to have tapings in the main transformers. A booster transformer is costlier not versatile in use (though it can be used in some distribution feeders).

Figure 2.13: Boosters or Regulating Transformer

2.9 Reactive Power Limitations Reactive power does not travel very far. Usually necessary to produce it close to the location where it is needed. A supplier/source close to the location of the need is in a much better position to provide reactive power versus one that is located far from the location of the need. Reactive power supplies are closely tied to the ability to deliver real or active power.


33

Though reactive power is needed to run many electrical devices, it can cause harmful effects on your appliances and other motorized loads, as well as your electrical infrastructure. Since the current flowing through your electrical system is higher than that necessary to do the required work, excess power dissipates in the form of heat as the reactive current flows through resistive components like wires, switches and transformers. Keep in mind that whenever energy is expended, you pay. It makes no difference whether the energy is expended in the form of heat or useful work. We can determine how much reactive power your electrical devices use by measuring their power factor, the ratio between real power and true power. A power factor of 1 (i.e. 100%) ideally means that all electrical power is applied towards real work. Homes typically have overall power factors in the range of 70% to 85%, depending upon which appliances may be running. Newer homes with the latest in energy efficient appliances can have an overall power factor in the nineties. The typical residential power meter only reads real power, i.e. what you would have with a power factor of 100%. While most electric companies do not charge residences directly for reactive power, it’s a common misconception to say that reactive power correction has no economic benefit. To begin with, electric companies correct for power factor around industrial complexes, or they will request the offending customer to do so at his expense, or they will charge more for reactive power. Clearly electric companies benefit from power factor correction, since transmission lines carrying the additional (reactive) current too heavily industrialized areas costs them money. Many people overlook the benefits that power factor correction can offer the typical home in comparison to the savings and other benefits that businesses with large inductive loads can expect. Most importantly, you pay for reactive power in the form of energy losses created by the reactive current flowing in your home. These losses are in the form of heat and cannot be returned to the grid. Hence you pay. The fewer kilowatts expended in the home, whether from heat dissipation or not, the lower


34

the electric bill. Since power factor correction reduces the energy losses, you save. As stated earlier, electric companies correct for power factor around industrial complexes, or they will request the offending customer to do so, or they will charge for reactive power. They’re not worried about residential service because the impact on their distribution grid is not as severe as in heavily industrialized areas. However, it is true that power factor correction assists the electric company by reducing demand for electricity, thereby allowing them to satisfy service needs elsewhere. But who cares? Power factor correction lowers your electric bill by reducing the number of kilowatts expended, and without it your electric bill will be higher, guaranteed. We’ve encountered this with other electric companies and have been successful in getting each of them to issue a retraction. Electric companies do vary greatly and many show no interest in deviating from their standard marketing strategy by acknowledging proven energy saving products. Keep in mind that promoting real energy savings to all their customers would devastate their bottom line. Power factor correction will not raise your electric bill or do harm to your electrical devices. The technology has been successfully applied throughout industry for years. When sized properly, power factor correction will enhance the electrical efficiency and longevity of inductive loads. Power factor correction can have adverse side effects (e.g. harmonics) on sensitive industrialized equipment if not handled by knowledgeable, experienced professionals. Power factor correction on residential dwellings is limited to the capacity of the electrical panel (200 amp max) and does not over compensate household inductive loads. By increasing the efficiency of electrical systems, energy demand and its environmental impact is lessened profound effects of Reactive

Power

in

Various

elements

of

Power


System

[5]

35

Chapter # 03 Static synchronous compensator (STATCOM)

3. Static synchronous compensator (STATCOM) 3.1 Introduction to FACTS The AC transmission system has various limits classified as static limits and dynamic limits. These inherent power system limits restrict the power transaction, which lead to the underutilization of the existing transmission resources. Traditionally, fixed or mechanically switched shunt and series capacitors, reactors and synchronous generators were being used to solve much of the problem. However, there are restrictions as to the use of these conventional devices. Desired performance was not being able to achieve effectively. Wear and tear in the mechanical components and slow response were the heart of the problems. There was greater need for the alternative technology made of solid state devices with fast response characteristics. The need was further fuelled by worldwide restructuring of electric utilities, increasing environmental and efficiency regulations and difficulty in getting permit and right of way for the construction of overhead transmission lines. This, together with the invention of Thyristor switch (semiconductor device), opened the door for the development of power electronics devices known as Flexible AC Transmission Systems (FACTS) controllers. The path from historical Thyristor based FACTS controllers to modern state-of-the-art voltage source converters based FACTS controllers, was made possible due to rapid advances in high power semiconductors devices. FACTS controllers have been in use in utilities around the world since 1970s, when the first utility demonstration of first family of FACTS named as Static VAR Compensator (SVC) was accomplished. Since then the large effort was put in research and development of FACTS controllers. FACTS technology provides the opportunity to [13] 

Increase loading capacity of transmission lines.



Prevent blackouts.



Improve generation productivity.



Reduce circulating reactive power.


37



Improves system stability limit.



Reduce voltage flicker.



Reduce system damping and oscillations.



Control power flow so that it flows through the designated routes.

3.1.1 Basic definitions Flexibility of electric power transmission: “The ability to accommodate changes in the electric transmission system or operating conditions while maintaining sufficient steady state and transient margin”. Flexible AC transmission system (FACTS): “Alternating current transmission system incorporating power electronics based and other static controllers to enhance controllability and increase power transfer capability”. FACTS controller: “A power electronic based system and other static equipment that provide control of one or more AC transmission system parameters”. [14] 3.1.2 Objectives of FACTS The concept of FACTS was established in order to solve the problem which was emerging in power systems in the late 1980s as there are restrictions on the construction of transmission line and to promote power growth of import and export. The main objectives behind FACTS based controllers are; 

Power transfer capability of transmission systems is to be increased



The power flow is to be kept at the designated route

The first objective indicates the power flow in a given transmission line can be increased up to its thermal limits. The second objective indicates that the flow of power in the line can be Reactive power compensation using STATCOM

38

restricted to select proper transmission corridors by controlling current in the line. If these two objectives are fulfilled there will be significant increase in the utilization of new and existing transmission lines. It will promote the deregulation of power system and there will be minimum requirement for new transmission lines. In order to implement these objectives, high power compensators and controllers are required. [7]

Table 3.1: Comparison of Basic Types of Compensators

3.1.3 Benefits and cost Primarily, the FACTS controllers provide voltage support at critical buses in the system (with shunt connected controllers) and regulate power flow in critical lines (with series connected controllers). Both voltage and power flow are


39

controlled by the combined series and shunt controller (UPFC). The power electronic control is quite fast and this enables regulation both under steady state and dynamic conditions (when the system is subjected to disturbances). The benefits due to FACTS controllers are listed below. 

They contribute to optimal system operation by reducing power losses and improving voltage profile.



The power flow in critical lines can be enhanced as the operating margins can be reduced due to fast controllability. In general, the power carrying capacity of lines can be increased to values up to the thermal limits (imposed by current carrying capacity of the conductors).



The transient stability limit is increased thereby improving dynamic security of the system and reducing the incidence of blackouts caused by cascading outages.



The steady state or small signal stability region can be increased by providing auxiliary stabilizing controllers to damp low frequency oscillations.



FACTS controllers such as TCSC can counter the problem of sub synchronous resonance (SSR) experienced with fixed series capacitors connected in lines evacuating power from thermal power stations (with turbo generators).



The problem of voltage fluctuations and in particular, dynamic over voltages can be overcome by FACTS controllers.

The capital investment and the operating costs (essentially the cost of power losses and maintenance) are offset against the benefits provided by the FACTS controllers and the `payback period' is generally used as an index in the planning. The major issues in the deployment of FACTS controllers are (a) the location (b) ratings (continuous and short term) and (c) control strategies required for the optimal utilization. Here, both steady-state and dynamic operating conditions have to be considered. Several systems studies involving power flow, stability, short circuit analysis are required to prepare the specifications. The design and testing of the control and protection equipment is based on Real Time Digital Simulator (RTDS) or physical simulators. It is to be


40

noted that a series connected FACTS controller (such as TCSC) can control power flow not only in the line in which it is connected, but also in the parallel paths (depending on the control strategies). [8] FACTS CONTROLLERS

COST (US $)

Shunt capacitor

8/KVAR

Series capacitor

20/KVAR

SVC

40/KVAR Controlled portions

TCSC

40/KVAR Controlled portions

STATCOM

50/KVAR

UPFC series portions

50/KVAR through power

UPFC shunt portions

50/KVAR controlled

Table 3.2: Cost comparison of different FACTS controllers

3.1.4 Generation of reactive power compensation [15] 1. First Generation; Mechanically switched devices are: 

Fixed shunt reactor (FR)



Fixed shunt capacitor (FC)



Mechanical switched shunt reactor (MSR)



Mechanical switched shunt capacitor (MSC)

2. Second Generation; Thyristor-based devices are: 

Thyristor controlled Reactor (TCR)



Thyristor switched capacitor (TSC)



Static VAR compensator (SVC)



Thyristor

switched

series

compensator

(Capacitor

or

reactors)

(TSSC/TSSR) 

Thyristor

controlled

series

compensator

capacitors

or

reactors

(TCSC/TCSR). 

Thyristor controlled braking resistors (TCBR)



Thyristor controlled phase shifting transformers (TCPST)


41



Line commutated converter compensator (LCC)

3. Third Generation; Converter-based devices: 

Static synchronous compensator (SATCOM)



Static Synchronous Series compensator (SSSC)



Unified power flow controller (UPFC)



Interline power flow controller (IPFC)



Self-commutated compensator (SCC)

Figure 3.1: Overview of conventional and FACTS devices 3.1.5 Classification of FACTS devices The classification of the FACTS Controllers done on the bases of their types of arrangement in the Power syst


42

Figure 3.2: Classification of FACTS devices

1. Series FACTS controllers: These FACTS controller could be variable impedance such as capacitor, reactor or a power electronics based variable source which in principle injects a voltage in series with the line as illustrated in fig. As long as the voltage is in phase quadrature with the line current, the series controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well. Applications: The main application of series compensators are i) Reduction of voltage fluctuations within defined limits during changing power transmissions ii) Improvement of oscillation damping of the system iii) Limitations of short circuit currents in networks or substations 2. Shunt FACTS controllers: These FACTS controller could be variable impedance such as capacitor, reactor or a power electronics based variable source which is shunt connected to the line in order to inject variable current , as shown in figure. As long as the injected current is in phase quadrature with the line voltage, the shunt controller only


43

supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well. Applications: The primary function of shunt connected compensator is to provide reactive power compensation. The main application of these type of controllers in transmission, distribution and networks are i) Reduction in unwanted reactive power flows and reduction in losses ii) Compensation of consumers and power quality improvement in those applications where huge demand fluctuations occur such as industrial machines, metal melting plants, railway or underground train system iii) Improvement of transient stability

3. Combine series-series FACTS controllers: These controllers are the combination of separate series FACTS controllers, which are controlled in a coordinated manner in a multi-line transmission system, as illustrated in figure. This configuration provides independent series reactive power compensation for each line but also transfers real power among the lines via power link. The presence of power link between series controllers names this configuration as “Unified Series-Series controller” 4. Combined series –shunt FACTS controllers: These controllers are the combination of separate shunt and series FACTS controllers, which are controlled in a coordinated manner or a unified power flow controller, with series and shunt elements. When shunt and series FACTS controllers are unified there can be real power exchange between series and shunt

controllers

via

power


link.

44

Figure 3.3 Use of Power Electronics in High-Voltage Systems - “Ranking” of the Controllers 3.1.6 FACTS Application FACTS controllers can be used for various applications to enhance power system performance. Once of the greatest advantages of using FACTS controllers is that it can be used in all the three states of power system, namely Steady state, Transient and Post transient steady state. However, the conventional devices find little application during system transient or contingency condition. a. steady state application: Various steady state applications of FACTS controllers includes voltage control (low and high), increase of thermal loading, post-contingency voltage control, loop flows control, reduction in short circuit level and power flow control. SVC and STATCOM can be used for voltage control while TCSC is more suitable for


45

loop flow control and for power flow control. i) Congestion management Congestion management id a serious concern for independent system operator (ISO) in present deregulated electricity markets as it can arbitrarily increase the prices and hinder the free electricity trade. FACTS devices like TCSC, TCPAR (Thyristor controlled Phase Angle Regulator) and UPFC can help to reduce congestion smooth locational marginal price (LMP) and to increase the social welfare by redirecting power from congested interface to underutilize line.

ii) ATC improvement In many deregulated market, the power transaction between buyer and seller is allowed based on calculation of ATC. Low ATC signifies that the network is unable to accommodate further transaction and hence does not promote free competition. FACTS controllers like TCSC, TCPAR and UPFC can help to improve ATC by allowing more power transactions.

iii) Reactive power and voltage control The use of shut FACTS controllers like SVC and STATCOM for reactive power and voltage control is well known.

iv) Loading margin improvement Several blackouts in many part of the world occur mainly due to voltage collapse at the maximum load ability point. Series and shut compensations are generally used to increase the maximum transfer capabilities of power networks. The recent advancement in FACTS controllers have allowed them to be used more efficiently for increasing the loading margin in the system.

v) Power flow balancing and control


46

FACTS controllers especially TCSC, SSSC and UPFC, enable the load flow on parallel circuits and different voltage levels to be optimized and controlled with a minimum of power wheeling , the best possible utilization of the lines , and a minimizing of overall system losses at the same time.

b. Dynamic applications Dynamic Application of FACTS controllers include transient stability improvement, oscillation damping (dynamic stability) and voltage stability enhancement. One of the most important capabilities expected of FACTS applications is to be able to reduce the impact of the primary disturbance.

i) Transient stability enhancement Transient instability is caused by large disturbances such as tripping of a major transmission line or a greater and problem can be seen from the first swing of the angle. FACS devices can resolve the problem by providing fast and rapid response during the first swing to control voltage and power flow in the system.

ii) Oscillation damping Electromechanical oscillations have been observed in many power systems worldwide and may lead to partial power interruption if not controlled. Initially, power system stabilizer (PSS) is used for oscillation damping in power system. Now this function can be more effectively handled by proper placement and setting of SVC, STATCOM and TCSC.

iii) Dynamic voltage control Shunt FACTS controllers like SVC and STATCOM as well as UPFC can be utilized for dynamic control of voltage during system contingency


47

and save the system from collapse and black out.

iv) SSR elimination Sub synchronous resonance (SSR) is a phenomenon which can be associated with series compensation under certain adverse conditions. TCSC have dynamic characteristics that differ drastically at frequencies outside the operating frequency range and hence is used in Stode, Sweden for the elimination of SSR in the power system.

v) Power system interconnection Interconnection of power system is becoming increasingly widespread as part of power exchange between countries as well as regions within countries in many parts of the world .There are numerous examples of interconnection of remotely separated region within one country. In case of long distance AC transmission , as in interconnected power systems care has to be taken for safeguarding of synchronism as well as stable system voltages, particularly in conjunction with system fault .With series compensation, bulk AC power transmission over distances of more than 1,000 km are a reality today. With the advent of TCSC, further potential as well as flexibility is added to AC power transmission. [16]


48

Figure 3.4: HVDC and FACTS worldwide by Siemens


49

3.2 Introduction to static synchronous compensators (STATCOM) Over the last couple of decades, researchers and engineers have made pathbreaking research on FACTS devices and by virtue of which, many STATCOM controllers based on self-commutated solid state voltage source converter (VSC) have been developed and commercially put in operation to control system dynamics under stressed conditions. STATCOM is qualitatively superior then line commutating static VAR compensator (SVC) and so this controller has given many names as Static compensator advanced static VAR compensator, advanced static VAR generator or static VAR generator, static condenser, synchronous solid state VAR compensator, VSC-based SVC or self-commutated SVC or static synchronous compensator, static condenser (STATCON). With the advent of voltage-source converter (VSC) technology built upon selfcommutating controllable solid state switches has ushered a new family of FACTS controllers such as static synchronous compensators (STATCOM) and unified power flow controller (UPFC) have been developed. The selfcommutating VSC, called as DC-to-AC converter, is the backbone of these controllers being employed to regulate reactive current by generation and absorption of controllable reactive power with various solid-state switching techniques. [17] The Static Synchronous Compensator (STATCOM) is a shunt connected reactive compensation equipment which is capable of generating and/or absorbing reactive power whose output can be varied so as to maintain control of specific parameters of the electric power system. The STATCOM provides operating characteristics similar to a rotating synchronous compensator without the mechanical inertia, due to the STATCOM employ solid state power switching devices it provides rapid controllability of the three phase voltages, both in magnitude and phase angle. STATCOM provide voltage support to buses by modulating bus voltages during dynamic disturbances in order to provide better transient characteristics, improve the transient stability margins and to damp out the system oscillations due to these disturbances.


50

3.2.1 Definition STATCOM is defined by IEEE as “a self-commutated switching power converter supplied from an appropriate electric energy source and operated to produce a set of adjustable multiphase voltage, which may be coupled to an AC power system for the purpose of exchanging independently controllable real and reactive power.” When two AC sources of same frequency are connected through a series inductance, active power flows from leading source to lagging source and reactive power flows from higher voltage magnitude AC source to lower voltage magnitude AC source. Active power flow is determined by the phase angle difference between the sources and the reactive power flow is determined by the voltage magnitude difference between the sources. Hence, STATCOM can control reactive power flow by changing the fundamental component of the converter voltage with respect to the AC bus bar voltage both phase wise and magnitude wise. 3.2.2 Basic Circuit Configuration of STATCOM The STATCOM has been defined as per CIGRE/IEEE with following three operating structural components. First component is Static: based on solid state switching devices with no rotating components; second component is Synchronous: analogous to an ideal synchronous machine with 3 sinusoidal phase voltages at fundamental frequency; third component is Compensator: provided with reactive compensation [15] The typical connection of STATCOM to AC bus is shown in Figure 3.5. That consists of the coupling transformer, input filter, Voltage Source Converter and a controller.


51

Figure 3.5: Connection of STATCOM with AC bus The STATCOM is a static compensator is composed of inverters with a capacitor in its dc side, coupling transformers, and a control system. The inverters are, in conventional STATCOMs, switched with a single pulse per period and the transformers are connected in order to provide harmonic minimization. The equipment action is made through the continuous and quick control of capacitive or inductive reactive power. Its output voltage is a waveform composed of pulses that approaches a sinusoidal wave. To obtain voltage harmonic content, that clearly agrees with strict standards, without the necessity of filters, it is necessary at least a set of eight inverters and transformers to produce a 48-pulse voltage waveform. Figure 3.21 shows one example of such a STATCOM and Figure 3.23 shows its voltage. 3.2.3 The major attributes of STATCOM The major attributes of STATCOM over SVC are; 

Faster response



Requires less space as bulky passive components (such as reactors) are eliminated



Inherently modular and relocatable



It can be interfaced with real power sources such as battery, fuel cell or SMES (superconducting magnetic energy storage)



A STATCOM has superior performance during low voltage condition as the reactive current can be maintained constant (In a SVC, the capacitive reactive current drops linearly with the voltage at the limit (of capacitive susceptance). It is even possible to increase the reactive current in a STATCOM under transient conditions if the devices are rated for the


52

transient overload. In a SVC, the maximum reactive current is determined by the rating of the passive components reactors and capacitors. [13] 3.2.4 STATCOM voltage sources In addition to voltage source using batteries and capacitors, STATCOMs can be operated with an inductor, which provides a source of direct current rather than voltage, Figure 3.6. A three-phase, current-source converter then generates a set of three-phase output currents which, by appropriate switching action, lag or lead the system voltages. The basic output current is a square or block wave and harmonic reduction requires PWM, or multi-level or multi-phase techniques, and/or harmonic filters. The energy in the current source can be sustained by drawing energy from the supply system or by using an external energy source. However the losses of a current-sourced converter tend to be higher than those of voltage-sourced converter.

Figure 3.6: Current sourced convertor CSC A further interesting concept is to design the converter as an AC to AC frequency charge. The “source “can be a three-phase high frequency generator, a resonant circuit (a parallel capacitor and inductor in each phase), or even a transformer which itself connected to the supply system Figure 3.7.


53

Figure 3.7: power doubling converter arrangement In this power-doubling arrangement, there are no energy storage components as such; the output connection to the output transformer can be considered to behave as a current source for the converter. However, the input terminals then need to behave as a voltage source and therefore an input filter needs to be connected to the input terminals of the converter. This type of converter requires special device which have both bi-directional current carrying and forward and reverse voltage blocking capabilities. Suitable devices to implement the powerdoubling arrangement are not yet commercially available, so this scheme is only of theoretical interest at present. [9]


54

Figure 3.8: Capacitors in a STATCOM (photo courtesy of ABB)


55

Difference between CSC and VSC 

CSC is the lowest cost convertor.



CSC does not have high short circuit current as does VSC.



For CSC the rate of rise of fault current during external or internal faults is limited by the reactor. Whereas in the VSC the capacitor discharges current would rise very rapidly and can damage the valves.



In CSC the valves are not subjected to high dv/dt due to the presence of the AC capacitors.



Interface of CSC with AC system is more complex.



Continuous losses in DC reactor of a CSC are much higher than the losses in the DC capacitor in VSC.



With the presence of capacitors in VSC, which are subjected to commutation charging and discharging, this convertor will produce harmonic voltages at a frequency of resonance between the capacitor and AC system inductances. These harmonics as well as DC reactor can result in over voltages on the valves and transformers.



Wide spread adoption of asymmetrical devices, IGBTs and GTOs, has made VSC a favorable choice. [6]

Figure 3.9: (a) voltage source converter (b) current source converter


56

Figure 3.10: STATCOM substation and close-up of the converter valves (photo courtesy of ABB)


57

3.2.5 Power industries Manufacturer of different FACTS devices such as GE, Siemens, ABB, Alsthom, Mitsubishi, Toshiba and so on, with their in-house R&D facilities have given birth to many versatile STATCOM projects presently in operation in highvoltage transmission system to control system dynamics under stressed conditions. [17] 3.3 Working principle of STATCOM A STATCOM is comparable to a Synchronous Condenser (or Compensator) which can supply variable reactive power and regulate the voltage of the bus where it is connected. (Synchronous condenser is a salient pole synchronous generator without prime mover).

Figure 3.11: Synchronous condensor The equivalent circuit of a Synchronous Condenser (SC) is shown in Figure 3.12, which shows a variable AC voltage source (E) whose magnitude is controlled by adjusting the field current. Neglecting losses, the phase angle ( ) difference between the generated voltage (E) and the bus voltage (V) can be assumed to be zero. By varying the magnitude of E, the reactive current supplied by SC can be varied. When E = V, the reactive current output is zero. When E > V, the SC acts as a capacitor whereas when E < V, the SC acts as an inductor.


58

When

= 0, the reactive current drawn (Ir) is given by

Figure 3.12: A synchronous condenser A STATCOM (previously called as static condenser (STATCON)) has a similar equivalent circuit as that of a SC. The AC voltage is directly proportional to the DC voltage (Vdc) across the capacitor (see Figure 3.13 which shows the circuit for a single phase STATCOM). If an energy source (a battery or a rectifier) is present on the DC side, the voltage Vdc can be held constant. The self-commutated switches T1 and T2 (based on say GTOs) are switched on and off once in a cycle. The conduction period of each switch is 1800 and care has to be taken to see that T1 is off when T2 is on and vice versa. The diodes D1 and D2 enable the conduction of the current in the reverse direction. The charge on the capacitors ensures that the diodes are reverse biased. The voltage waveform across PN is shown in Figure 4.14. The voltage

=

when T1 is conducting (T2 is off) and

=−

when T2 is conducting (and T1 is off). The switches are synchronized with the supply voltage (V) which is assumed to be sinusoidal of frequency

. The fundamental component, rms value (E1) is

obtained as

=

√2 2

sin

=

√2


59

When E1 > V, the STATCOM draws a capacitive reactive current, whereas it is inductive if E1 < V. Note that, to be compatible with the convention used for SVC, the inductive current drawn is assumed to be positive. At the instant when T1 is switched on and Ir is inductive, the current (Ir) flowing through the circuit is negative (as it is a lagging current) and flows through T1 (as iT1 is negative of Ir). After 900, the current through T1 becomes zero and as Ir rises above zero and becomes positive, the diode D1 takes over conduction. Similar events occur when T2 turns on and off. Thus, both T1 and T2 cease conduction before they are turned off. On the other hand, when Ir is capacitive, the current Ir is positive at the instant of turning on T1 and flows through the diode D1. After 900, the current reverses its sign and flows through T1. At the time of switching off T1, the current through it is at its peak value. Thus, we need self-commutated devices such as GTOs when the STATCOM draws capacitive reactive current. In contrast, T1 and T2 carry peak current at turn on when Ir is inductive. Note that diode D1 or D2 turns off automatically when the parallel device (T1 or T2) turns off. Also, the capacitors can be charged from the source through the diodes.

Figure 3.13: A single phase STATCOM


60

Figure 3.14: The waveform of VPN In comparing SC and STATCOM, we note that while rotation of the DC field winding on the rotor results in the generation of AC voltages in the stator windings through magnetic induction, the synchronous operation of the switches in a STATCOM results in the AC voltage at the output. Unlike in a SC, this output voltage also contains many harmonics and some solution has to be found to eliminate them. Unlike in the case of a SC, the capacitors can be charged from the AC side and there is no need of an energy source on the DC side if only reactive current is to be provided in steady state. The losses in the STATCOM can be met from the AC source. The advantages of a STATCOM over a SC are: 

The response is much faster to changing system conditions.



It does not contribute to short circuit current.



It has a symmetric lead-lag capability.



It has no moving parts and hence the maintenance is easier.



It has no problems of loss of synchronism under a major disturbance. [13]


61

Figure 3.15: Reactive power compensation SC & controlled voltage source switching convertor 3.4 STATCOM characteristics 3.4.1 STATCOM operating characteristics The steady state control characteristics of a STATCOM are shown in Figure 3.16. The losses in the STATCOM are neglected and ISTATCOM is assumed to be purely reactive. As in the case of a SVC, the negative current indicates capacitive operation while positive current indicates inductive operation. The limits on the capacitive and inductive currents are symmetric (±Imax). The positive slope BC is provided for the V-I characteristic to 

prevent the STATCOM hitting the limits often and



to allow parallel operation of two or more units.

The reference voltage (Vref ) corresponds to zero current output and generally, the STATCOM is operated close to zero output during normal operating conditions, such that full dynamic range is available during contingencies. This is arranged by controlling the mechanically switched capacitors/reactors connected in parallel with a STATCOM. [13]


62

Figure 3.16: Control characteristics of a STATCOM

3.4.2 Transient response Because the operation of a STATCOM is based on the generation of the sinusoidal voltage, its response to transient disturbances inherently good and extremely rapid. The steady state operating condition of a STATCOM is dependent on the system voltage (and impedance) and the STATCOM source voltage and its coupling impedance. Thus in Figure 3.17, with an open circuit system voltage

slightly larger than the target voltage of STATCOM steady

state characteristics, the STATCOM draws a small capacitive current . In order to generate this current the STATCOM source voltage

must be slightly

higher than the target voltage. If now the system voltage is depressed, due to a fault, to a value

, the point of

intersection of the system characteristics and the STATCOM controlled characteristic demands a current . Initially, before there has been any change of STATCOM source voltage, the STATCOM current increases substantially from to

(given by the intersection of the system characteristics and the natural

STATCOM characteristics; this is increased by the control action to the required value

by an increase of source voltage to

, normally within one half cycle.


63

Figure 3.17: Response of a STATCOM to a system voltage changes Figure 3.18 illustrates how a STATCOM responds to voltage disturbances. Prior to the voltage dip, the STATCOM is operating at about its rated lagging current. A dip of system voltage suddenly occurs, to about 50% of its steady state value. This STATCOM inherently responds to this disturbance by generating a capacitive current to support the system voltage but, even on the natural characteristics, there would be a capacitive overload current. To prevent this, the STATCOM control system defects the sudden change and reduce the target voltage to limit the STATCOM current to its rated capacitive value.

Figure 3.18: Response of a STATCOM to a depression system voltage When the fault is cleared and the system voltage is recovers to its pre-fault value, this will trend to cause an inductive overload current in the STATCOM. Again the STATCOM control system is able to detect change and adjust the


64

target voltage appropriately to reach rated lagging current. Although there is an unavoidable transient distortion of the STATCOM current at each step change, it can be seen from Figure 3.18 that the changes from inductive to capacitive and capacitive to inductive current to take place within a half cycles. [9] 3.4.3 Harmonics Both SVCs and STATCOMs generate harmonics. The TCR of an SVC is a harmonic current source. Network harmonic voltages distortion occurs as a result of the currents entering the power system. The STATCOM is a harmonic voltage source. Network voltage harmonic distortion occurs as a result of voltage division between the STATCOM phase impedance and the network impedance. The major harmonic generation in SVCs is at low frequencies; above the 15th harmonic the contribution is normally small. At lower frequencies the generation is large and filters are needed. SVCs normally have at least 5th and 7th harmonic filters. The filter rating is in the range of 25–50% of the TCR size. STATCOMs with PWM operation have their major harmonic generation at higher frequencies. The major contributions are at odd multiples of the PWM switch frequency; at even multiples the levels are lower. The harmonic generation decays with increasing frequency. STATCOMs might also generate harmonics in the same spectra as the conventional SVCs. The magnitudes depend on converter topology and the modulation and switching frequency used. In most cases STATCOMs as well as SVCs require harmonic filters. [19] 3.4.4 Source voltage ripple Ideally, the dc source should be so strong that its voltage remains effectively constant at the chosen level, under steady state conditions. In practice, especially for capacitor voltage sources, this would require extremely large, bulky and expensive dc capacitors. A compromise is necessary to allow the capacitor to charge and discharge to some extent between each switching operation, i.e. a constant average voltage can be maintained but with a super-imposed ripple


65

voltage of a few percent as in Figure 3.19. This ripple must be taken into account in selecting switching instants and in evaluating the overall harmonic behavior of the convertor system.

Figure 3.19: Converter output voltage and current waveforms and dc capacitor current and voltage during var generation and absorption

3.5 Snubber circuit GTO devices are available in a wide range of voltage and current ratings including, in particular, the current turn off capability. A widely used GTO has a peak voltage rating of 4.5kV and a peak turn-off current of 4kA. As with conventional thyristor, it is important to protect individual GTO devices against both forward and reverse overvoltage and against excessive rates of change of inrush current and of voltage at turn-off. Figure 3.20 illustrate a typical snubber circuit arrangement. In order for the GTO to turn off safely at 4kA, the snubber capacitor, CS must have a high value, about 6mF. The energy stored in this capacitor must be dissipated after every switching. If a smaller capacitor is used, say 3mF, the switching losses are substantially reduced but the safe turn-off current is reduced


66

to about 3kA. Energy stored in the di/dt limiting inductor of snubber circuit at turn-off is dissipated via the discharge resistor and diode. Some of the dv/dt and di/dt circuit energy can be recovered by additional circuits. The added complexity and cost of these energy recovery techniques must be weighed against the saving in losses and thepossibility that they might enable simple PWM techniques

to

be

applied

to

the

GTO

converters.

[9]

Figure 3.20: Typical snubber circuit arrangement for GTO in a STATCOM

3.6 STATCOM control Following are the STATCOM control strategies 

VSC

using

GTO-based

square-wave

inverters

and

special

interconnection transformers. Typically four three-level inverters are used to build a 48-step voltage waveform. Special interconnection transformers are used to neutralize harmonics contained in the square waves generated by individual inverters. In this type of VSC, the fundamental component of voltage is proportional to the voltage V . Therefore V dc

dc

has to be varied for

controlling the reactive power.


67



VSC using IGBT-based PWM inverters. This type of inverter uses Pulse-Width Modulation (PWM) technique to synthesize a sinusoidal waveform from a DC voltage source with a typical chopping frequency of a few kilohertz. Harmonic voltages are cancelled by connecting filters at the AC side of the VSC. This type of VSC uses a fixed DC voltage Vdc. The fundamental component of voltage is varied by changing the modulation index of the PWM modulator.

The controller of a STATCOM is used to operate the inverter in such a way that the phase angle between the inverter voltage and the line voltage is dynamically adjusted so that the STATCOM generates or absorbs desired VAR at the point of connection. [20]

Figure 3.21: 48-pulse STATCOM diagram


68

The control used for the model of STATCOM shown in Figure 3.21 is a very simple one. It uses measurements of voltages and currents at the point where the STATCOM is connected to the AC system bus. These measured signals are worked in two ways as shown in Figure 3.22. In one way, the voltages are fed to the PLL (phase locked loop) block in order to detect the frequency and phase angle and to generate the synchronizing signal to the switching logic. In the second way of the control, the voltage is fed together with the measured currents to the “Instantaneous Power Theory” block, in order to calculate the instantaneous imaginary power q. This imaginary power q is compared with a reference q* and the error observed is fed to proportional integral controller block. The proportional-integral controller outputs a signal that gives the leading or lagging phase angle necessary to adjust the voltage on the dc side capacitor, thus controlling the energy flow in or out of it. The leading or lagging signal is added to the PLL synchronism signal output and delivered to the switch logic control block. [21]

Figure 3.22: Control diagram


69

The interaction between the AC system voltage and the inverter-composed voltage provides the control of the STATCOM VAR output. When these two voltages are synchronized and have the same amplitude, the active and reactive power output is zero. Figure 3.23 (a) shows this situation. However, if the amplitude of the STATCOM voltage is smaller than that of the system voltage, it produces a current lagging the voltage by 90o (see Figure 3.23 (b)), and the compensator behaves as an inductive load, which reactive value depends on the voltage amplitude. Making the STATCOM voltage higher than the AC system voltage the current will lead the voltage by 90o, (see Figure 3.23(c)), and the compensator behaves as a variable capacitive load. As in the previous case, the reactive power depends on the voltage amplitude. This amplitude control is done through the control of the voltage on the dc capacitor. This voltage is related to the energy stored at the dc capacitor. By lagging or leading the STATCOM voltage, it is possible to charge or discharge the dc capacitor, as a consequence, change the value of the dc voltage and the STATCOM’s operational characteristics.

Figure 3.23: STATCOM 48-pulse voltage and compensating current


70

3.7 Solid-state switching devices The conventional thyristor, a line commutating switching device available commercially at very high power ratings, is a mature technology and forms basic switching element for SVC, a second generation FACTS controller being used as a dynamic reactive power compensator. This power semiconductor device has no turn-off capability and relatively high response time. The emerging technology is solid-state controllable turn-off switches. These switches viz. GTO, IGBT, IGCT are being used extensively in converter circuits for state-of-the-art FACTS controllers. These turn-off devices have different operating characteristics in respect to switching frequency/speed, device ratings, turn-off and turn-on timings, forward and reverse breakdown voltage, on-state voltage drop, switching losses and so on. Drive circuit requirements, switching frequency/speed switching losses and cost of each device are the trade-off to use these devices effectively. Among the turn-off power switches, GTO thyristor is a mature technology and commercially available at high power ratings. Its extensive applications in high power rating converter-cum-compensator circuits have ushered in a new era of FACTS controllers, for example, STATCOM, UPFC, convertible static compensator (CSC), static synchronous series compensator (SSSC) and so on. Solid-state IGBT switching device is a relatively new technology in power electronics is employed in medium-to-high power ratings PWM-based FACTS controllers due to its high switching frequency and speed. Among the turn-off switches, IGCT is the most promising and emerging solid-state technology and has the merits of low switching loss, higher switching frequency/speed, no snubber circuit requirements. [17] 3.8 STATCOM topologies and configurations Switching topologies such as PWM or power frequency switching depend upon the type of solid-state devices used in STATCOM. Primarily, fundamental frequency method of switching (pulsed one per line frequency cycle) and PWM techniques (pulsed multi times per half cycle) are widely accepted methods. In multi-pulse and multi-level converters, there is only one turn-on, turn-off per Reactive power compensation using STATCOM

71

device per cycle. But in pulse width modulation multiple pulses per half-cycle is achieved and also the width of the pulses can also be varied to change the amplitude of the AC voltage. 3.8.1 Pulse width modulation (PWM) convertor In PWM control, solid-state switches are operated many times at frequent intervals within the same cycle of output voltage, and an improved quality of output AC voltage waveforms (in terms of low amplitude of low-order harmonics/low total harmonic distortion (THD)) can be obtained. Based on the frequency and amplitude of triangular shape carrier signal and modulating control signal, PWM converters are designed, in general, to eliminate triplen and other low order harmonics (5th/7th), and by means of suitable filter design, predominantly higher-order harmonics are reduced in the AC voltage output.

Figure 3.24: VSC using IGBT-based PWM inverters

As the converter conduction and switching losses are a function of switching frequency, the PWM technique is not generally adopted in high rating STATCOMs on account of high switching losses, whereas low-to medium rating STATCOMs used in power distribution system are built upon PWM


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control and such STATCOMs are generally termed D-STATCOM. Switching frequency of solid-state devices is one of the key factors in designing PWMVSC and it can be typically 3 kHz for IGBT and 500 Hz for IGCT or GCT. The PWM technique in such converter circuit has been found to be unpopular due to its higher gating energy requirements and switching losses. As GTO is well-proven solid-state device and commercially available with power-handling levels as that of the conventional thyristor, GTO-VSC is the backbone of the high power rating STATCOMs that are used extensively in high-voltage transmission system. STATCOMs built upon GTO-VSCs are designed primarily to operate it in a square-wave mode of operation. [17]


73

Figure 3.25: Single converter valve unit in a STATCOM


74

3.8.2 Multi-pulse convertor In a multi-pulse converter configuration, the displacement angle between two consecutive six-pulse converter is 2π/(6N) and three-phase voltage contains odd harmonics component of the order of (6Nk+1), where k = 1, 2, 3, . . . . With the increase in pulse number, lower-order harmonics are neutralized and a very close to sinusoidal AC output voltage waveform can be realized. Basic six pulse convertor is shown in Figure 3.26.

Figure 3.26: Two level six pulses VSC Bridge and its AC (Phase & Line) voltage output wave form in square wave mode

Compared with basic six-pulse converter, the multi-pulse configuration of STATCOM increases the achievable VAR rating, improves the harmonic performance, decreases the DC side current harmonics and reduces significantly the overall filter requirements. The increase in pulse order increases the number of electronics devices, magnetics and associated components and thus added to the cost. However, the high pulse-order STATCOM enables to improve harmonics and operational performances. To reduce total harmonic distortion (THD), multi-pulse converter topology derived from the combination of multiple number (N-numbers) of elementary six-pulse converter units to be triggered at specific displacement angle(s), is


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widely adopted, and output AC voltage waveforms from each unit is electromagnetically added with an appropriate phase shift by inter-phase transformer(s) to produce a multi-pulse (6xN pulses) waveform close to sinusoidal wave.

Figure 3.27: Magnetics of 48-pulse, two-level ± 80 MVA STATCOM and its output AC waveform

3.8.3 Multi-level convertor Another variant of topology is a multi-level VSC structure to generate multistepped voltage waveform close to sinusoidal nature. In multi-level topology, a synthesized staircase voltage waveform is derived from several levels of DC voltage sources obtained normally by using capacitor voltage sources. Owing

to

the

complex

series-parallel

connection

of

transformers

windings/circuits in multi-pulse converters, multi-level configurations have been Reactive power compensation using STATCOM

76

receiving increasing attention for high voltage and high power rating applications. In multi-level structures, three-level converter topologies with square-wave mode of operation are most common. An N-level topology is achieved by splitting of DC capacitors into (N-1) sections produce N-level output phase voltage and a (2N-1) level output line voltage waveform. When number of levels is high enough, harmonic content in AC output voltage is reduced to low enough to avoid the need of filters. The main features of multi-level converter are the low harmonic content of the output voltage compared with a square-wave pulse converter, decreased device voltage stress (a fraction of the total DC bus voltage) and potentially higher converter voltage and thus power rating. Three basic types of multi-level VSCs are; i) Multi-point clamped converter ii) Chain converters based on standard H-bridge arrangements iii) Nested-cell converter or flying capacitor multi-level converter.

Figure 3.28: Single Phase of a three-level and four levels NPC converter


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Figure 3.29: A view on technology by Siemens: STATCOM


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3.9 Optimal Location of STATCOM Transmission lines are often driven close to or even beyond their thermal limits in order to satisfy the increased electric power consumption and trades due to increase of the unplanned power exchanges. Due to this the bus voltage of load buses falls. This also leads to an increase in Transmission losses of the system. Thus to improve the overall voltage profile we require shunt FACTS. STATCOM is one of the better shunt FACTS available. PSO, BFO and Plant Growth Optimization etc. are techniques for finding the optimal STATCOM location with objective function as transmission losses. Particle Swarm Optimization (PSO) is an evolutionary computation technique developed by Eberheart & Kennedy in 1995 and is based on bird flocking and fish schooling. Its simplicity and faster convergence make it an attractive algorithm to employ. The bacterial foraging optimization (BFO) algorithm is inspired from biomimicry of the e-coli bacteria and is a robust algorithm for non-gradient optimization solution, proposed in 2002 by Kevin M Passino. It consists of four steps: chemotaxis, swarming, reproduction & elimination-dispersal PLANT GROWTH ALGORITHM is a Bionic random algorithm. According to the plant growth characteristics, an artificial plant growth model is built including leaf growth, branching, phototropism and spatial occupancy. [22] 3.10 STATCOM losses The forward voltage drop of GTO thyristors is greater than that of conventional thyristors because of more complex system of semi conducting-junction and the energy requires for the turn-off duty. Figure 4.30 shows the approximate variation of STATCOM losses (% of rated current) through the operating range from rated leading to rated lagging current.


79

Figure 3.30: Typical loss curve for a STATCOM

Figure 3.31: Typical loss curves for STATCOM applications

In many cases, the STATCOM output will need to be biased, generally towards the capacitive side for SVC application. Figure 4.31 shows the loss patterns for the same output range (+1.0 to -0.5pu current). If the STATCOM is rated for an output of ±1.0pu current, for this range, the upper half of the inductive range is not used. The losses in the float condition (0 Mvar) and within the lagging range Reactive power compensation using STATCOM

80

are quit low, but become high in the upper part of the capacitive range. These capacitive losses can be reduced by halving the rating of the STATCOM and combining it with a TSC of about 0.6pu to reduce the losses (and probably the overall costs). An intermediate option is also illustrate with a STATCOM of ±0.75 rating, to cover the total dynamic range, biased by a fixed capacitor (or filter bank) of 0.25pu output. This may give an overall optimization of cost and losses especially if the predominant range of operation of compensator is from about 0.1 to 0.6pu capacitive current. [9] 3.11 Capability of STATCOM to exchange real power For applications requiring active (real) power compensation it is clear that the STATCOM in contrast to SVC can interface suitable energy storage with the AC system for real power exchange. That is, the STATCOM is capable of drawing controlled real power from an energy source (large capacitor, battery, fuel, super conducting magnetic storage, etc.) at its DC terminal and deliver it as AC power to the system. It can also control energy absorption from the AC system to keep the storage device charged. This potential capability provides a new tool for enhancing dynamic compensation, improving power system efficiency and, potentially, preventing power outages. The reactive and real power exchange between STATCOM and the AC system can be controlled independently of each other and any combination of real power generation and absorption is achievable. Thus, by equipping the STATCOM with an energy storage device of suitable capacity, extremely effective control strategies for the modulation of reactive and real output power can be executed for the improvement of transient stability and damping of power oscillation. It should be noted that for short term dynamic disturbances an energy consuming device (e.g. a switched resistor) may be effectively used in place of the more expensive energy storage to absorb power from the AC system via STATCOM. With this simple scheme, the STATCOM would transfer energy from the AC system to the DC terminal where it would be dissipated by


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the energy consuming device that would be switched on whenever surplus energy at those terminals is detected ( by e.g., the increase of DC voltage). [6] 3.12 Physical size and installation From the stand point of physical installation, because the STATCOM not only controls but also internally generates the reactive output power (both capacitive and inductive) , the large capacitor and reactor banks with their associated switchgear and protection, used in conventional thyristor controlled SVCs, are not needed. This results in a significant reduction in overall size (about 30 to 40%), as well as in installation, labor and cost. The small physical size of the STATCOM makes it eminently suitable for installations in areas where land cost is at a premium and for applications where anticipated system changes may require the relocation of the installation. [6]

Figure 3.32: Footprints of SVC and STATCOM


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3.13 STATCOM applications 3.13.1 Transmission applications 1. STATCOM installation of ±80 Mvar in Japan in 1991 One prototype STATCOM installation of ±80 Mvar was installed for service in Japan in 1991. The main converter circuit configuration is given in Figure 4.33.This STATCOM uses eight voltage-sourced converters, each of 10 MVA ratting, connected to a main STATCOM transformer via eighty converter transformer producing 7.5o phase angle displacement from each other, resulting in 48-pulse operation. The control system incorporates power system voltage control, power oscillation damping and constant reactive power output control. The control system varies the width of the rectangular output voltage of each converter to achieve voltage magnitude control and to ensure low losses. A gapped-core design is used for the eight phase-displacement transformer to reduce the effects of dc magnetization, decrease magnetic impedance, and improve the uniformity of voltage sharing between windings. This is especially important when the STATCOM is energized from the power system during startup sequences or following system faults. Initially the converter start-up system used a relatively large, separate, “start-up converter” to supply dc voltage to the STATCOM main converter. This method was found to be slow and a new system now allows the STATCOM converter to start immediately after the energisation of the STATCOM transformer and the eight converter transformers from the power system. [9]


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Figure 3.33: ±80 Mvar in Japan 2. STATCOM installation of ±100 Mvar in USA The Sullivan substation is supplied by a 500 kV bulk power network and by four 161 kV lines that are interconnected through a 1200 MVA transformer bank. Seven distributors and one large industrial customer are served from this substation. The STATCOM, shown in Figure 4.34 is implemented with a 48 pulse, two-level voltage source inverter that combines eight, six pulse threephase inverter bridges, each with a nominal rating of 12.5 MVA. The system also comprises a single step-down transformer having a wye and delta secondary to couple the inverter to the 161 kV transmission line, and a central control system with operator interface. The STATCOM system is housed in one building that is a standard commercial design with metal walls and roof and measured 27.4 x 15.2m. The STATCOM regulates the 161 kV bus voltage during daily load increases to minimize the activation of the tap changing mechanism on the transformer bank, which interconnects the two power systems. The use of this VAR compensator to regulate the bus voltage has resulted in the reduction of the use tap changer from about 250 times per month to 2 to 5 times per month. Tap changing


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mechanisms are prone to failure, and the estimated cost of each failure is about $ 1 million. Without the STATCOM, the transmission company would be compelled either to install a second transformer bank or to construct a fifth 161 kV

line

into

the

area;

both

are

costly

alternatives.

[23]

Figure 3.34: The ±100 MVAr STATCOM at Sullivan Substation 3. +225/-52 Mvar SVC including ±75 Mvar STATCOM in England in 1996 In 1996, the National Grid Company plc of England and Wales sought relocatable dynamic reactive compensation equipment for its 400 kV transmission network, capable of generating 0 to 225 Mvar at 0.95p.u. system voltage, with a particular reference to the inclusion of a STATCOM of 150 Mvar range. The design adopted includes a ±75 Mvar STATCOM in conjunction with a 127 Mvar TSC and 23 Mvar harmonic filter to provide a full controlled range of output +225 to -52 Mvar, Figure 4.35. This STATCOM design is required to meet stringent emission levels and immunity to existing and future prospective harmonic levels. It uses multi-level converters in a chain circuit configuration. The control system incorporates voltage control, reactive set point regulation, and a coordinating control for the STATCOM and the associated TSC. Provision is also made to include power


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oscillation damping control in the future. All the controls and power electronic equipment are housed in weatherproof, transportable GRP (glass reinforced plastic) cabins and the outdoor components are grouped together on frameworks to satisfy the requirement for easy relocation to another substation when this is required. [9]

Figure 3.35: +225/-52 Mvar SVC including ±75 Mvar STATCOM in England


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Figure 3.36: STATCOM substation in East Claydon, United Kingdom


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3.13.2 Other Applications of STATCOM STATCOM technology has multi-dimensional applications to control power system parameters in steady state and dynamic system conditions. STATCOM is used for power quality improvements, reactive power control, voltage regulation, power swings or oscillations damping, damping torsional oscillations or SSR damping, transmission line capacity enhancement, dynamic stability improvement including steady state, transient and voltage stability, and for application under power system faults. Energy storage applications; some manufacturing processes require absolute continuity of supply to maintain product quality and/or safety, for example, float glass, paper, semi-conductor devices, and some chemical and nuclear processes. The cost of disruption may be so great that auxiliary or emergency power sources are economically justified In distribution system, this controller is named as D-STATCOM being widely used for power quality improvement, custom power, voltage regulation, compensation and balancing of nonlinear loads and/or unbalanced loads, load power factor improvement, harmonic elimination and so on. Other applications of smaller STATCOMs, in service or under consideration, are for the reduction of lamp flicker due to arc furnaces, for voltage control for wind farms and for balancing of single-phase traction loads. These smaller units generally use PWM to obtain a satisfactory harmonic performance. STATCOM back-to-back inter-tie is a relatively new area of application to exchange power between two inter-ties and to improve voltage stability. It is analogous to HVDC back-to-back system named as HVDC light with inherent MVAR supporting feature.


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Figure 3.37: Various applications of STATCOM


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Chapter # 04 Simulation results and Discussion

4. Simulation results and discussion 4.1 Circuit Description

Figure 4.1: Detailed model of STATCOM

The STATCOM regulates voltage by generating or absorbing reactive power. The STATCOM model described here is a detailed model with full representation of power electronics. It uses a square-wave, 48-pulse VSC and interconnection transformers for harmonic neutralization. This type of model requires discrete simulation at fixed type steps (25 µs in this case) and it is used typically for studying the STATCOM performance on a much smaller time range (a few seconds. This model of the 100 Mvar STATCOM on a 500 kV Power System represents a three-bus 500 kV system with a 100 Mvar STATCOM regulating voltage at bus B1.The internal voltage of the equivalent system connected at bus B1 can be varied by means of a Three-Phase Reactive power compensation using STATCOM

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Programmable Voltage Source.

S. No.

Parameters of Transmission Line

Value of Parameters

1

Positive sequence Inductance

0.9337e-3 H/Km

2

Zero Sequence Inductance

4.1264e-3 H/Km

3

Positive sequence Resistance

4

Zero Sequence Resistance

0.02546 Ω/Km 0.3864 Ω/Km

5

Positive sequence Capacitance

12.74e-9F/Km

6

Zero Sequence Capacitance

7.751e-9 F/Km

7

Transmission line length L1

200 Km

8


75 Km


180 Km

9

Table 4. 1: Transmission line parameters

4.2 STATCOM Components The STATCOM consists of a three-level 48-pulse inverter and two seriesconnected 3000 µF capacitors which act as a variable DC voltage source. The variable amplitude 60 Hz voltage produced by the inverter is synthesized from the variable DC voltage which varies around 19.3 kV. 4.2.1 48-Pulse three-Level Inverter

Figure 4.2: Voltage Source Converter Reactive power compensation using STATCOM

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The STATCOM uses this circuit to generate the inverter voltage V2. It consists of four 3-phase 3-level inverters coupled with four phase shifting transformers introducing phase shift of +/-7.5 degrees. Except for the 23rd and 25th harmonics, this transformer arrangement neutralizes all odd harmonics up to the 45th harmonic. Y and D transformer secondaries cancel harmonics 5+12n (5, 17, 29, 41,...) and 7+12n (7, 19, 31, 43,...). In addition, the 15° phase shift between the two groups of transformers (Tr1Y and Tr1D leading by 7.5°, Tr2Y and Tr2D lagging by 7.5°) allows cancellation of harmonics 11+24n (11, 35,...) and 13+24n (13, 37,...). Considering that all 3n harmonics are not transmitted by the transformers (delta and ungrounded Y), the first harmonics that are not canceled by the transformers are therefore the 23rd, 25th , 47th and 49th harmonics. By choosing the appropriate conduction angle for the three-level inverter (θ = 172.5°), the 23rd and 25th harmonics can be minimized. The first significant harmonics generated by the inverter will then be 47th and 49th. Using a bipolar DC voltage, the STATCOM thus generates a 48-step voltage approximating a sine wave. The following figure reproduces the primary voltage generated by the STATCOM 48-pulse inverter as well as its harmonics contents. 4.2.2 STATCOM Control System The control system task is to increase or decrease the capacitor DC voltage, so that the generated AC voltage has the correct amplitude for the required reactive power. The control system must also keep the AC generated voltage in phase with the system voltage at the STATCOM connection bus to generate or absorb reactive power only (except for small active power required by transformer and inverter losses).A voltage droop is incorporated in the voltage regulation to obtain a V-I characteristics with a slope (0.03 pu/100 MVA in this case). Therefore, when the STATCOM operating point changes from fully capacitive (+100 Mvar) to fully inductive (-100 Mvar) the SVC voltage varies between 10.03=0.97 pu and 1+0.03=1.03 pu. To explain the regulation principle, let us suppose that the system voltage Vmean becomes lower than the reference voltage Vref. The voltage regulator will then ask for a higher reactive current output (positive Iq= capacitive current). To


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generate more capacitive reactive power, the current regulator will then increase phase lag of inverter voltage with respect to system voltage, so that an active power will temporarily flow from AC system to capacitors, thus increasing DC voltage and consequently generating a higher AC voltage.

Figure 4.3: STATCOM Controller As explained in the preceding section, the conduction angle θ of the 3-level inverters has been fixed to 172.5°. This conduction angle minimizes 23rd and 25th harmonics of voltage generated by the square-wave inverters. Also, to reduce non characteristic harmonics, the positive and negative voltages of the DC bus are forced to stay equal by the DC Balance Regulator module. This is performed by applying a slight offset on the conduction angles θ for the positive and negative half-cycles.


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4.3 Steady-State and Dynamic Performance of the STATCOM We will now observe steady-state waveforms and the STATCOM dynamic response when the system voltage is varied. Following are the different cases of system voltage w.r.t. reference set voltage. i) System voltage equals to reference voltage ii) System voltage less than the reference voltage iii) System voltage greater than the reference voltage

Figure 4.4: Waveforms Illustrating STATCOM Response to System Voltage


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4.3.1 System voltage equals to reference voltage Initially the programmable voltage source is set at 1.0491 pu, resulting in a 1.0 pu voltage at bus B1 when the STATCOM is out of service. As the reference voltage Vref is set to 1.0 pu, the STATCOM is initially floating (zero current). The DC voltage is 19.3 kV. As shown in Figure 4.5 that STATCOM current (Iaprim) is zero and system voltage and STATCOM voltage are in Phase.

Figure 4.5: STATCOM Current is zero, System voltage and STATCOM voltage are in Phase


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4.3.2 System voltage less than the reference voltage At t=0.1s, voltage is suddenly decreased by 4.5% (0.955 pu of nominal voltage). The STATCOM reacts by generating reactive power (Q=+70 Mvar) to keep voltage at 0.979 pu. The 95% settling time is approximately 47 ms. At this point the DC voltage has increased to 20.4 kV. Figure 4.6 shows that system voltage is less than the reference voltage. Figure 4.7 show that the STATCOM current is leading the voltage, establishing the capacitive mode, and so generating the reactive power as shown in Figure 4.8.

Figure 4.6: System voltage less than the reference voltage.

Figure 4.7: STATCOM Current leading the System voltage and system voltage and STATCOM voltages are in Phase


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Figure 4.8: STATCOM produces reactive power

Notice that when the STATCOM is operating in capacitive mode (Q=+70 Mvar), the 48-pulse secondary voltage (in pu) generated by inverters is higher than the primary voltage (in pu) and in phase with primary voltage. Current is leading voltage by 90°; the STATCOM is therefore generating reactive power.


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4.3.3 System voltage greater than the reference voltage Then, at t=0.2 sec. the source voltage is increased to1.045 pu of its nominal value. The STATCOM reacts by changing its operating point from capacitive to inductive to keep voltage at 1.021 pu. At this point the STATCOM absorbs 72 Mvar and the DC voltage has been lowered to 18.2 kV. Observe in Figure 4.10 the first trace showing the STATCOM primary voltage and current that the current is changing from capacitive to inductive in approximately one cycle. Figure 4.11 show that the STATCOM is now absorbing reactive power.

Figure 4.9: System voltage greater than reference voltage


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Figure 4.10: STATCOM Current lagging the System voltage and System voltage and STATCOM voltages are in Phase

Figure 4.11: STATCOM absorbing reactive power


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Chapter # 05 Conclusions and recommendations

5. Conclusions and recommendations 5.1 Conclusion In this thesis comprehensive study of reactive power compensation has been done by using shunt connected FACTS device i.e. STATCOM. We had studied the need of reactive power and its compensation by STATCOM. The overall conclusions are:



Reactive power compensation is been used nowadays to increase the transmittable power in AC power systems. Fixed or mechanically switched capacitors and reactors are being employed to increase the steady state power transmission by controlling the voltage along the lines. However these devices do not provide high speed control. Furthermore, control cannot be initiated frequently because mechanical devices wear out quickly compared to static devices.



STATCOM is a controlled reactive-power source. It provides the desired reactive-power generation and absorption entirely by means of electronic processing of the voltage and current waveforms in a voltage-source converter (VSC).



STATCOM has number of advantages over conventional methods of compensation viz; quick response time, less space requirement, optimum voltage platform, higher operational flexibility and excellent dynamic characteristics under various operating conditions.



STATCOM is better device then SVC. For country like Pakistan having large interconnected system the SVC is better option from economic point of view but due to other aspects like stability margin, voltage improvement and power system performance, STATCOM is preferred.


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5.2 Future recommendations 

STATCOM should be implemented in Industrial plants, using arc furnaces that operate with large random peaks of reactive power demand and causing undesirable effects in the plant itself and in the ac power network.



The supplier of electric power charges the consumer also for reactive power demand so STATCOMs should be implemented in distribution system applications to reduce the reactive power demand.



The future work should include introduction of the development of transient and steady state models to improve the, transient stability margin and steady state power transfer capacity respectively.



Current source convertor based STATCOM be further studied for improvement in the performance of STATCOM for various applications.



Work on HVDC implementing the STATCOM should be carried out.


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References [1] P. Kundur: ‘Power system stability and control’ [2] Mohamed E. El-Hawary: ‘Introduction to Electrical Power Systems’ [3] Book by Central Station Engineers of the Westinghouse Electric Corporation EAST PITTSBURGH, PENNSYLVANIA: ‘Electrical Transmission and Distribution Reference’ [4] T. J. Miller: ‘Reactive power Control in Electric Systems’ John Willey & Sons, 1982. [5] Jignesh.Parmar: ‘Importance of Reactive Power for System’ [6] Narain G. Hingorani & Laszlo Gyugyi: ‘Understanding FACTS: Concept and technology of flexible AC transmission systems’ [7] U.A Bakshi & MV Bakshi: ‘Transmission and distribution’ fourth revised edition [8] K. R. Padiyar: ‘FACTS Controllers in Power Transmission and Distribution’ [9] Yong-Hua Song and Allan Johns: ‘Flexible AC transmission systems (FACTS)’ [10] Courseware Sample 86371-F0 LabVolt: ‘Static Synchronous Compensator (STATCOM)’. [11] Juan Dixon , Luis Morán, José Rodríguez , Ricardo Domke: ‘Reactive Power Compensation Technologies, State- of-the-Art Review’ [12] Yongan Deng, MASc student at Concordia University: ‘Reactive Power Compensation of Transmission Lines’ [13] Dr. S. Titus, B.J.Vinothbabu and I. Maria Anton Nishanth: ‘Power System Stability Enhancement Under Three Phase Fault with FACTS Devices TCSC, STATCOM and UPFC’


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[14] ‘Proposed terms and definitions for Flexible AC transmission system (FACTS)’ Paper prepared by the FACTS terms & definitions task force of the FACTS working group of the DC and FACTS subcommittee [15] Tariq Masood, R.K. Aggarwal, S.A. Qureshi, R.A.J Khan: ‘STATCOM Model against SVC Control Model Performance Analyses Technique’ [16] Naresh Acharya, Arthit Sode-Yome and Nadarajah Mithulananthan: ‘Facts about Flexible AC transmission system (FACTS) controller: Practical installation and benefits’ [17] B. Singh, R. Saha, A. Chandra & K. Al-Haddad: ‘Static synchronous compensators (STATCOM)’: a review [18] Molinas, M. Jon Are Suul Undeland, T, Dept. of Electrical. Power Eng. Norwegian Univ. of Sci. & Technol., Trondheim, Power Electronics, IEEE Transactions. [19] M. Noroozian, SM IEEE and C.W. Taylor, Fellow IEEE: ‘Benefits of SVC and STATCOM for Electric Utility Application’ [20] Kalyan K Sen, Member, IEEE: ‘STATCOM - STATic synchronous compensator: Theory, Modeling, and Applications’ [21]Carlos A.C. Cavaliere , Edson H. Watanabe

and Maurício Aredes:

‘Analysis and Operation of STATCOM in Unbalanced Systems’ [22] Vikram Singh Chauhan, Jitendra Meel and T Jayabarathi: ‘Optimal Location of STATCOM on Transmission Network using Evolutionary Algorithms’ [23] Juan Dixon (SM), Luis Morán (F), José Rodríguez (SM) and Ricardo Domke: ‘Reactive Power Compensation Technologies, State of-the-Art Review’ (Invited Paper)


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Appendix Battery energy storage system (BESS) A chemical based energy storage system using shunt-connected switching converters to supply or absorb energy to or from an ac system which can be adjusted rapidly.

Current source In current source the current flowing through it can not undergo a discontinuity due to the external circuit variation. The most representative example is the inductance since an instantaneous change in current would correspond to an instantaneous change in its flux which would require an infinite voltage.

Flexibility of electric power transmission The ability to accommodate changes in the electric transmission system or operating conditions while maintaining sufficient steady-state and transient margins.

Flexible ac transmission system (FACTS) Alternating-current transmission systems incorporating power electronic–based and other static controllers to enhance controllability and increase power transfer capability.

FACTS controller A power electronic–based system and other static equipment that provide control of one or more ac transmission system parameters.

Inter-phase power controller (IPC) A series-connected controller of active and reactive power consisting, in each phase, of inductive and capacitive branches subjected to separately phase-shifted voltages. The active and reactive power can be set independently by adjusting the phase shifts and/ or the branch impedances using mechanical or electronic switches. In the particular case where the inductive and capacitive impedances form a conjugate pair, each terminal of the IPC is a passive current source dependent on the voltage at the other terminal.


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Power system stability Power system stability denotes the ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that system integrity is preserved.

PWM convertor In PWM control, solid-state switches are operated many times at frequent intervals within the same cycle of output voltage, and an improved quality of output AC voltage waveforms can be obtained.

Phase-shifting transformer A phase shifting transformer is a special type of system intertie transformers which control the power flow through specific lines in a complex power transmission network by providing the possibility to insert a voltage with an arbitrary phase angle in the power system.

Reactive power Reactive power is power that flows back and forth between the inductive windings of the generator and the inductive windings of motors, transformers, etc., which are part of the electrical load. Reactive power (VARS) is required to maintain the voltage to deliver active power (watts) through transmission lines and to maintain a system healthy.

Static condenser (STATCON) Synchronous condenser is a salient pole synchronous generator without prime mover which can supply variable reactive power and regulate the voltage of the bus where it is connected. This term is deprecated in favor of the static synchronous compensator (SSC or STATCOM).


107

Self-commutating converters Two types of self-commutating converters; The current sourced converters in which direct current has one polarity, and the power reversal takes place through reversal of dc voltage polarity. The voltage sourced converters in which the dc voltage always has one polarity, and the power reversal takes place through reversal of dc current polarity.

Static synchronous compensator (SSC or STATCOM) A self-commutated switching power converter supplied from an appropriate electric energy source and operated to produce a set of adjustable multiphase voltage, which may be coupled to an AC power system for the purpose of exchanging independently controllable real and reactive power.

Static synchronous generator (SSG) A static, self-commutated switching power converter supplied from an appropriate electric energy source and operated to produce a set of adjustable multiphase output voltages, which may be coupled to an ac power system for the purpose of exchanging independently controllable real and reactive power.

Static synchronous series compensator (SSSC or S3C) A static synchronous generator operated without an external electric energy source as a series compensator whose output voltage is in quadrature with, and controllable independently of, the line current for the purpose of increasing or decreasing the overall reactive voltage drop across the line and thereby controlling the transmitted electric power. The S3C may include transiently rated energy-storage or energy absorbing devices to enhance the dynamic behavior of the power system by additional temporary real power compensation, to increase or decrease momentarily, the overall real (resistive) voltage drop across the line.

Static var compensator (SVC) A shunt-connected static var generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system (typically bus voltage).


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Static var generator or absorber (SVG) A static electrical device, equipment, or system that is capable of drawing controlled capacitive and/ or inductive current from an electrical power system and thereby generating or absorbing reactive power. Generally considered to consist of shuntconnected, thyristor-controlled reactor(s) and/ or thyristor-switched capacitors.

Static var system (SVS) A combination of different static and mechanically switched var compensators whose outputs are coordinated.

Superconducting magnetic energy storage (SMES) A superconducting electromagnetic-based energy-storage system using shuntconnected switching converters to rapidly exchange energy with an ac system.

Thyristor-controlled braking resistor (TCBR) A shunt-connected, thyristor switched resistor, which is controlled to aid stabilization of a power system or to minimize power acceleration of a generating unit during a disturbance.

Transmission system A transmission system includes all land, conversion structures and equipment at a primary source of supply lines, switching and conversion stations between a generating or receiving point and the entrance to a distribution center or wholesale point, all lines and equipment whose primary purpose is to augment, integrate or tie together sources of power supply.

Turn-off devices The devices having the capability of both turn on and turn off (e.g. GTO, IGBT, etc.) are called turn off devices.


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Unified power-flow controller (UPFC) A combination of a static synchronous compensator (STATCOM) and a static synchronous series compensator (S3C) which is coupled via a common dc link, to allow bidirectional flow of real power between the series output terminals of the S3C and the shunt output terminals of the STATCOM, and are controlled to provide concurrent real and reactive series line compensation without an external electric energy source. The UPFC, by means of angularly unconstrained series voltage injection, is able to control, concurrently or selectively, the transmission line voltage, impedance, and angle or, alternatively, the real and reactive power flow in the line. The UPFC may also provide independently controllable shunt-reactive compensation.

Voltage source In voltage source, the voltage across its terminals can not undergo a discontinuity due to the external circuit variation. The most representative example is the capacitor since an instantaneous change of voltage across its terminals would mean an instantaneous change of its charge which would require an infinite current.

Voltage Source Converter (VSC) It has the capability to transfer power in either direction. With a voltage source converter, the magnitude, the phase angle and the frequency of the output voltage can be controlled. In these converters the dc side voltage always has one polarity, and the power reversal takes place through reversal of dc current polarity .On dc side the voltage is supported by a capacitor. This capacitor is large enough to at least handle a sustained charge/discharge current that accompanies the switching sequence of the converter valves and shifts in phase angle of the switching valves without significant change in the dc voltage.


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