CUSTOM POWER DEVICES AND APPLICATIONS IN ...

NATIONAL CONFERENCE ON ELECTRICAL, ELECTRONICS AND INDUSTRIAL AUTOMATION (NCEEIA–2016)

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CUSTOM POWER DEVICES AND APPLICATIONS IN POWER NETWORKS: STATCOM and SVC Reshu Singh, Deepti Singh and Brijesh Singh, Member, IEEE

 Abstract-- The objective of the present paper work is to represents a bibliographical survey and role of flexible AC transmission systems (FACTS) in power networks. The main aim of using FACTs system is to obtain constraints free smooth operations of power systems during abnormal conditions. Various types of FACTS controllers with their construction and operational methodologies have been discussed in the present paper. Index Terms-- AC power flow; power system operation and control; transmission system; security constraint power system operation; flexible AC transmission systems.

I. INTRODUCTION emergence of custom power devices has led to THE development of new and fast compensators for effective control of real and reactive power flows in distribution systems [5]. The custom power devices include compensators like Distribution Static Compensator (DSTATCOM), Dynamic Voltage Restorer (DVR), Unified Power Quality Conditioner (UPQC), Battery Energy Storage System (BESS), and many more such controllers. These devices may be quite helpful in solving power quality problems. However, due to high cost, and for effective control, these are to be optimally placed in the system. Very limited attempt seems to be made in optimal placement of custom power devices in interconnected power systems [1]-[12]. Placement of Static VAR Compensator (SVC), Static Compensator (STATCOM) and DVR for voltage sag mitigation in a predominantly meshed sub-transmission network and a predominantly radial distribution network has been considered in [13]. However, placement of Flexible AC Transmission System (FACTS) controllers have been considered at an arbitrarily selected bus and no specific criterion has been suggested to determine optimal location of

Reshu Singh, M. Tech. Student is with Department of Electrical Engineering, Faculty of Engineering and Technology, Mangalayatan University, Aligarh (UP). Deepti Singh, Assistant Professor is with the Department of Electrical Engineering, Faculty of Engineering and Technology, Mangalayatan University, Aligarh (UP). Brijesh Singh, Associate Professor is with Department of Computer Science and Engineering, Faculty of Engineering and Technology, Mangalayatan University, Aligarh (UP). (corresponding author’s mail: [email protected])

such controllers. In the present work, the operations and applications of STATCOM and SVC have been reviewed [14]-[17]. II. FLEXIBLE AC TRANSMISSION SYSTEMS CONTROLLERS Flexible AC Transmission Systems (FACTS) is defined as “alternating current transmission systems incorporating power electronics based and static controllers to enhance controllability and increase capability.” The FACTS controller is defined as a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters. The FACTS controller can be classified as 1. Shunt connected controllers 2. Series connected controllers 3. Combined series –shunt controllers 4. Combined shunt-series controllers Depending upon the power electronic devices used in the control, the FACTS controllers can be classified into two types; Variable impedance type and Voltage source converter (VSC) based The variable impedance type controllers include: i. Static Var Compensator (SVC); shunt connected ii. Thyristor Controlled Series Capacitor or Compensator (TCSC); series connected iii. Thyristor Controlled Phase Shifting Transformer (TCPST) or static PST; combined shunt and series. The VSC based FACTS controllers are: i. Static Synchronous Compensator (STATCOM); shunt connected ii. Static Synchronous Series Compensator (SSSC); series connected iii. Interline Power Flow Controller (IPFC); combined series-series iv. Unified Power Flow Controller (UPFC); combined shunt –series Some of the special purpose FACTS controllers are i. Thyristor controller braking resistor ( TCBR) ii. Thyristor controlled voltage limiter (TCVL) iii. Thyristorcontrolled voltage regulator (TCVR)

Department of Electrical Engineering & Department of Electronics and Communication Engineering, Faculty of Engineering and Technology, Mangalayatan University Aligarh- INDIA

NATIONAL CONFERENCE ON ELECTRICAL, ELECTRONICS AND INDUSTRIAL AUTOMATION (NCEEIA–2016)

iv. Interphase power controller (IPC) v. NGH-SSR damping VSC based FACTS controller have several advantages over the variable impedance type. For example, a STATCOM is much more compact than a SVC for similar rating and is technically superior. It can supply required reactive current even at low values of the bus voltage and can be designed to have inbuilt short term overload capability. Also a STATCOM can supply active power if it has an energy source or large energy storage at its DC terminals. The only drawback with VSC based controllers is that requirement of using selfcommutating power semiconductor devices such as GATE Turn off (GTO) thyristors. Insulated Gate Bipolar Transistors (IGBT), Integrated Gate Commutated Thyristors (IGCT).

complex turns ratio with magnitude of unity. The power flow in a lossless transmission line with an ideal PST is given by where Φ = Φ1 – Φ2

P=

PST is not faster enough in dynamic conditions. Therefore thyristor switches can ensure fast control on Ø. Now it is to be noted that V varies with the load and can be expressed as V2 = V1 Cos (Φ1 – Φ2) It is possible to regulate the bus voltage magnitude. The reactive power (Qc) that has to be injected is given by Qc = V22 – V1V2Cos

)/X

III. ROLE OF FACTS CONTROLLERS

IV. BENEFITS WITH THE APPLICATION OF FACTS

Mainly grid of transmission lines operating at high or extra high voltages is required to transmit power from the generating stations to the load centers. Basically of Power transmission networks are operated at different voltages (10 kV- 800 kV). Normally power flow (P) is given by

CONTROLLERS

P=

)

A. Control of Power Flow in AC Transmission Line Power flow in an AC transmission line is used to enhance power transfer capacity; to change power flow under dynamic conditions such as sudden increase in load, line trap or generator outage and to ensure system stability and security. Note: The stability can be affected by growing low frequency, power oscillations (due to generator rotor swings) loss of synchronism and voltage collapses caused by major disturbances. Now, Maximum power (Pmax) transmitted Pmax =

max

where

max = 30°-40°

The max is selected depending on the stability margins and the stiffness of the terminal buses to which the line is connected. The line length increases in a linear fashion and P max reduces. B. Power transfer capacity as a function of line length The series compensation using series connected capacitors increases Pmaxas the compensated value of the series reactance ( Xc) is given by Xc = X (1-ksc) where ksc is the degree of series compensation. The value of ksc depends upon the several factors including the resistance of the conductors. Typically ksc does not exceed 0.7.Fixed capacitors have been used since a long time for increasing the power transfer in long lines. Control of series capacitors is uneconomical. Now Thyristor switches have been used to control the series capacitor for series compensation (TCSC).In the lines of short lengths, the power flow can be controlled by introducing phase shifting transformers (PST) which has a

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These controllers contribute an important role in optimal system operation by redirecting power losses and improving voltage profile. The power flow in critical lines can be enhanced and the operating margins can be reduced due to fast controllability. The transient stability limit is increased thereby improving dynamic security of the system and reducing the incidence of blackout caused by outages. The steady state or small signal stability region can be increased by providing auxiliary stabilizing controllers to draw 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. The problem of voltage fluctuations and in particular, dynamic over voltages can be overcome by FACTS controllers The major issues in development of FACTS controllers are  Location  Ratings(continuous and short term)  Control strategy required for optimal utilization V. STATIC VAR COMPENSATOR (SVC) It is a first generation FACTS controller. It is a variable impedance device where the current through a reactor is controlled using back to back connected thyristor values. The main application of SVC was initially for load compensation of fast changing loads such as steel mills and arc furnaces. Now in transmission line it is used for compensation. The objectives for use of SVC are as follows:  Increases power transfer in long lines  Improve stability with fast acting voltage regulations  Damp low frequency oscillations due to swing (rotor) modes.  Damp subsynchronous frequency oscillations due to torsional modes  Control dynamic overvoltages. As a part of analysis the SVC Location is an important parameter for effectiveness of the SVCs in transmission lines.

Department of Electrical Engineering & Department of Electronics and Communication Engineering, Faculty of Engineering and Technology, Mangalayatan University Aligarh- INDIA

NATIONAL CONFERENCE ON ELECTRICAL, ELECTRONICS AND INDUSTRIAL AUTOMATION (NCEEIA–2016)

Let us take an example: Consider a symmetric lossless transmission line with SVC at midpoint

     

Fig. 1 A transmission line with SVC connected at midpoint Then, without SVC, the voltage at the midpoint is given as Vmo = When Φ=βl is the electrical length of the line, l is the length in Km and β is β=ω = 2πf where l and c are positive sequence inductance and capacitance of the line for per unit length and f is frequency. Now it is shown that the voltage variation of midpoint is maximum (due to variation in δ). SVC helps to limit the variations of voltage by suitable control. The steady state characteristics of SVC are shown as follows:



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Faster response Requires less space as bulky passive components ( such as reactors) are eliminated Inherently modular and re-locatable. It can be interfaced with real power sources such as better fuel cell or SMES ( superconducting magnetic energy storage) A STATCOM has superior performance during low voltage conditions as the reactive current can be maintained constant. Note : 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 transient overload.

The STATCOM was originally called as an advanced SVC and then labeled as STATCOM (static condenser). It is basically used for supplying variable reactive power and regulates the voltage of the bus where it is connected. Equivalent circuit of a synchronous condenser:

Fig. 3 A synchronous condenser

Fig. 2 Control characteristic of SVC The SVC current is considered as a positive when SVC susceptance is inductive. Thus ISVC = -BSVCVSVC The slope of OA is Bc (susceptance of the capacitor) and slope of OBC is BL (susceptance of the reactor). A positive slope (in the range of 1-5%) is given in the control range to enable parallel operation of more than one SVC connected at the same or neighboring buses and also to prevent SVC hitting the limits frequently. The steady state value of SVC bus voltage is determined from the intersection of the system characteristics and the control characteristics.

In this; a variable AC voltage source (E) whose magnitude is controlled by adjusting the field current. Neglecting losses, the phase angle (δ) difference between the generated voltage (E) and the bus voltage V, the reactive current supplied by synchronous condenser can be varied. When E=V, the reactive power output is zero. When E>V, the synchronous condenser acts as a capacitor whereas when E