A Flexible Alternating Current Transmission System (FACTS) is a system composed of static equipment used for the AC transmission of electrical energy. It is.
CONTENTS CHAPTER (1) INTRODUCTION
CHAPTER (2) SHUNT COMPENSATION
CHAPTER (3) SERIES COMPENSATION
CHAPTER (4) CASE STUDY
REFERENCES
1
CHAPTER (1) INTRODUCTION 1.1 Introduction: A Flexible Alternating Current Transmission System (FACTS) is a system composed of static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability of the network. It is generally a power electronics-based system. In recent years, the need for fast reactive compensation in power transmission systems has become increasingly evident. The utility industry is facing unprecedented problems related to energy cost, environmental, social, and regulatory issues, as well as to the profound changes in the U.S. industrial structure and the geographic shifts of highly populated areas. The present situation may be briefly summarized as follows. The power demand has shown a steady but geographically uneven growth. The available power generation is often not close to the growing load centers. The locations of new power generation are largely determined by regulatory policies, environmental acceptability, and the cost of available energy. In order to meet the power demand under these often contradictory requirements, the utilities increasingly rely on the utilization of existing generation facilities via power import/export arrangements. Power exportation and importation requires the interconnection of (previously independent) power systems into an ever growing grid, in which individual transmission systems may play no other part but to ―wheel‖ the power from the exporting system to the importing one[1]. However, the existing traditional transmission facilities were not designed to handle the control requirements of an interconnected power system. The power flow in the individual lines of the transmission grid is determined by their impedance and it often cannot be restricted to the desired power corridors. As a consequence, power flow loops develop and certain lines become overloaded, with the overall effect of deteriorating voltage profiles and decreased system stability.
2
Furthermore, while the power transmission requirements have been rapidly growing, the difficulties and escalating cost of right-of-ways have stymied the construction of new lines. This overall situation demands the review of traditional power transmission theory and practice, and the creation of new concepts that allow the full utilization of existing power generation and transmission facilities without decreasing system availability and security. The Electric Power Research Institute (EPRI) has initiated the development of (FACTS) in which power flow is dynamically controlled by various power electronic devices. The two main objectives of FACTS are to increase the transmission capacity of lines and control power flow over designated transmission routes [2]. Technological advancements in the semiconductor industry led to the production of a power grade gate turn–off thyristor (GTO). The commercial availability of GTOs in the mid–1980s made it possible to construct large voltage– sourced converters (VSCs) [3]. In principle, VSCs are capable of generating multiphase alternating voltage of controlled magnitude and phase. The application of voltage source converters VSCs in the transmission industry became the subject of considerable research effort in the late 1980s and through the 1990s.The ―flexible AC transmission system‖ (FACTS) refers to a concept of power flow control through AC transmission lines using static converters [4]. Examples of converter based FACTS controllers include the advanced static compensator (STATCOM), the series static synchronous compensator (SSSC) and the unified power flow controller (UPFC). Voltage source inverter (VSI) based static VAr compensators with only small capacitors on the dc side have been considered for reactive power control. These are known as Advanced Static VAr Compensators (ASVC) or Static Compensator (STATCOM). In contrast to the TCR/FC or TCR/TSC schemes, bulky and expensive passive elements are not required [5, 6], FACTS-based devices have been used for power flow control and for damping power system oscillations. They can also be used to increase transmission line capacity, steady state voltage regulation; provide transient voltage support to prevent system collapse; and damp power oscillations. FACTS devices can be used in wind power systems to improve the transient and 3
dynamic stability of the overall power system. The STATCOM is from the family of FACTS devices that can be used effectively in wind farms to provide transient voltage support to prevent system collapse. Series and shunt Compensation connected to power network as shown in fig1.1, where in high voltage transmission systems, a thyristor controlled series capacitor
Fig 1.1 Power network
TCSC may be used to reduce the electrical length of long transmission lines, increasing power transfer and stability margins. An HVDC link may be used for the purpose of long distance ,bulk power transmission .An SVC or a STATCOM may be 4
used to provide reactive power support at a network
location far away from
synchronous generators .at the distribution level ,33KV and 11Kv , a D-STATCOM may be used to provide voltage magnitude support ,power factor improvement and harmonic cancellation .The interfacing of embedded DC generators ,such as fuel cells , with the AC distribution system would require a thyristor based converter[7] .
Classification of FACTS as listed in table 1.1:
Table 1.1 Application for compensation Compensator
Function
Static Shunt Compensator Voltage support and stability VAR compensation STATCOM (PF correction) Current harmonic Load current balancing Flicker effect compensation
5
Static Synchronous Series Compensator Protection against voltage sags and swells Voltage balancing
SSSC
Voltage regulation Flicker attenuation
Unified Power Flow Control Static Series/Shunt Compensator
UPFC
Protection against voltage sags Protection against voltage swells Voltage balancing ,Voltage regulation Flicker attenuation ,Interface with DG Active and reactive power control VAR compensation Harmonic compensation 6
Static Var Compensator
SVC
Voltage support and stability VAR compensation
Thyristor Controlled Series Capacitor
TCSC
Varies the electrical length of the compensated transmission line Fast active power flow regulation Increase stability margin Static Phase Shifter
SPS
Control of phase angle of the end line voltages with little delay
7
The examples of FACTS studied in this report, where this report divided into three chapters as follow Chapter 2:
This chapter
introduces the theory of shunt compensators,
mathematical model for STATCOM and SVC (TCR/FC) and its applications. Chapter 3:
Presents series compensators where we study the theory, the
modeling of (TCR/SC) and SSSC and its applications. Chapter
4:
shows
the
simulation
MATLAB/SIMULINK.
8
results
of
STATCOM
using
CHAPTER (2) Shunt Compensator
2.1 Introduction:
The SVC and STATCOM have a strong influence on voltage quality improvement and show medium performance with respect to overall system stability. The unified power flow controllers (UPFC) have shown efficient performance in terms of load flow support, stability and voltage quality. The main objective in this chapter is to present the theory and applications of The STATCOM and SVC. A STATCOM is a shunt-connected reactive power compensation device that is capable of generating and/or absorbing reactive power and in which the output can be varied to control the specific parameters of an electric power system. The STATCOM is a static compensator and is used to regulate voltage and to improve dynamic stability [8]. SVCs are extensively treated in the literature [9]. A typical shunt-connected static var compensator, composed of fixed capacitors FC and thyristor controlled reactors TCRs, With proper coordination of the capacitor switching and reactor control, the var output can be varied continuously between the capacitive and inductive ratings of the equipment. The compensator is normally
operated to regulate the voltage of the transmission system at a
selected terminal, often with an appropriate modulation option to provide damping if power oscillation is detected. A STATCOM can supply the required reactive power under various operating conditions, to control the network voltage actively and thus, improve the steady state stability of the network. The STATCOM can be operated over its full output current range even at very low voltage levels and the maximum VAr generation or absorption changes linearly with the utility or ac system voltage. The maximum compensating current of the SVC decreases linearly with the ac system voltage and the maximum VAr output decreases with the square of the voltage. This implies that for the same dynamic performance, a higher rating SVC is
9
required when compared to that of a STATCOM. For an SVC, the maximum transient capacitive current is determined by the size of the capacitor and the magnitude of the ac system voltage. In the case of a STATCOM, the maximum transient capacitive over current capability is determined by the maximum turn-off capability of the power semiconductors employed [8]. Fig. 2.1 shows the schematic of SVC and its VI characteristics. Fig. 2.2 shows the schematic of the STATCOM and its VI characteristics. The main function of a STATCOM is to provide reactive power support and thus improve voltage stability. The main objective of using a UPFC in a system is to be able to control both active and reactive power in the associated line in which it is placed. The STATCOM has better reactive power control than an SVC as seen in Figs. 2.1 and 2.2. Mechanically switched capacitors do not have a better performance at lower voltages and hence a higher rating device is needed for the same performance. Also, the reactive power support provided by the SVC is dependent on the ac system voltage and hence its capability is de-rated at lower voltages. The UPFC is not very economical and requires more complicated control techniques for exploiting its complete capabilities.
Fig. 2.1 V-I characteristic of SVC
Fig. 2.2 V-I characteristic of STATCOM
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Part1: SVC 2-1 SVC OPERATION Thyristor Controlled Reactor (TCR) with thyristor switched capacitor shown in Fig. 2.4 is used as static var compensator in the power system [10]. Static means it has no moving part. Static compensators have many practical applications in electric power system, maintaining constant voltage, improving system stability, improving power factor and correcting phase unbalances.
(a)
(b)
C
Fig.2.4 typical structure of SVC's (a) TCR with TSC (b) TCR with FC
(C) variable susceptance
TCR with shunt capacitor compensator is used as the variable reactive power generator. The compensator is controlled to meet the exact reactive power demand of the connected bus. Two oppositely poled thyristors in Fig. 2.4.a conduct on alternate half-cycles. If the thyristors are gated into conduction at the peaks of the voltage, full conduction results in the
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reactor. If the gating is delayed in the thyristors partial conduction is obtained in the reactor. The waveforms in Fig. 2.5 show the current in the inductor for different gating angles
Fig.2.5 voltage and line current waveforms of a basic single phase the TCR for firing angles. (a) (b)=160
(c)
(d)=60
The instantaneous current I is given by
I={
√
(
)
}
(2.1)
Where V is the rms voltage; XL = L is the fundamental-frequency reactance of the reactor (in Ohms); =2f; and is the gating angle. The time origin is chosen to coincide
12
with a positive going zero crossing of the voltage. The fundamental component is found by Fourier analysis and is given by.
I=
(2.2)
Where is the conduction angle, related by by the equation (2.3) then the equation (2.2) can be written as I1=BL()V
(2.4)
Where BL () is an adjustable fundamental frequency susceptance controlled by the conduction angle by the equation,
BL ( ) =
(2.5)
With the required capacitance command the conduction angle for the inductor is calculated. The relationship between them and is given in Fig.2.5, which is the control law of the TCR. According to Fig.2.5 a look-up table is constructed between the BL () and firing angle . The required capacitor value is extracted from the total capacitor value. The remaining is the excess capacitor that should be compensated by the TCR. Therefore the adjustable susceptance of the TCR BL () is equated to the susceptance of the excess capacitor value.
Fig.2.5 Control law of the TCR
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2.2 CONTROLLER DESGIN OF SVC A SVC has two arrangements, first one is TSR with fixed capacitance and second one has thyristor switched capacitance. In the case of TSR/TSC , the response time is usually 3060µs depending upon the SVC configuration and system strength .when SVC operated in a voltage regulation , it is fastest thyristor controlled in FACTS controller , with settling time almost one period in case of TCR/FC. Fig.2.6 shows the simplified block diagram of a voltage control for a typical SVC application. This controller has essential units as follow as [11]: Measuring circuits unit: the main function of these circuits is to measure the voltages and currents at different points of power network, which provide information for SVC control and protection purposes.
Fig. 2.6 simplified voltage control block diagram of a typical SVC application.
Voltage regulator: the voltage regulator performs the closed loop voltage control .the difference between the voltage reference and voltage measured at the point of connection of the SVC is fed as the control error signal to PI regulator that provides the total SVC susceptance reference required to minimize the error.
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Power oscillation damping: when a power modulation control circuit is included in the SVC controller, this high –level control function utilizes the power system response as input and acts on the voltage regulation to provide damping for slow electro-mechanical swings in power system. Allocator: this block has the function of converting the susceptance reference from the voltage regulator into specific information which is then processed in order to determine the number of reactive banks that must be switched on and the required firing angle. Linearizer: the linearizer converts the susceptance from the allocator to a firing angle α, to maintain the same control response over the entire SVC operating range, the angle α is determined as non- linear function of susceptance reference order. This function is normally given as a table that is derived from the following formula 1-XL B (α) = α+
(
)
(2.6)
Where B (α) is susceptance of the TCR fired at the angle α.
Hunting detection and gain
adjustment :the stability
controller supervises the
operation of the voltage controller .unstable operation (hunting) , which may take place during weak system operating conditions, will be detected and the gain of the PI controller would be reduced by half to try to achieve stable operation .
2.3 APPLICATION OF SVC Stabilization of the voltage of weak system Reduction of the transmission losses Increment of transmission capacity makes the existing power grid play its maximum efficiency Increment of transient stability limit Enhancement of voltage control and stability Elimination of power fluctuation
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Part 2: STATCOM 2-1 STATCOM DESIGN The STATCOM ratings are based on many parameters which are mostly governed by the amount of reactive power the system needs to recover and ride through typical faults on the power system and to reduce the interaction of other system equipment that can become out of synchronism with the grid. Although the final rating of the STATCOM is determined based on system economics, the capacity chosen will be at least adequate for the system to stabilize after temporary system disturbances. The type of faults that the system is expected to recover from also determines the size of the STATCOM. For example, a three phase impedance fault of low impedance requires a very high rating STATCOM while a high impedance short circuit fault needs a lower rating device to support the system during the fault and help recover after the fault. The converter current ratings and the size of the capacitor also decide the capability of the STATCOM. The STATCOM can be connected to the system at any voltage level by using a coupling transformer as shown in Fig. 2.7. The devices in a voltage source converter are clamped against over-voltages across the DC link capacitor bank to minimize losses and not have to withstand large spikes in reverse over-voltage [12].
Fig. 2.7 STATCOM connected to network.
2.2
LOCATION OF STATCOM STATCOM provides effective voltage support at the bus to which it is connected to.
The STATCOM is placed as close as possible to the load bus for various reasons. The first reason is that the location of the reactive power support should be as close as possible to the
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point at which the support is needed. Secondly, in the studied test system the location of the STATCOM at the load bus is more appropriate because the effect of voltage change is the highest at this point. The location of the STATCOM is based on quantitative benefits evaluation. The main benefits of using a STATCOM in the system are reduced losses and increased maximum transfer capability. The location of STATCOM is generally chosen to be the location in the system which needs reactive power. To place a STATCOM at any load bus reduces the reactive power flow through the lines, thus, reducing line current and also the I2 R losses. Shipping of reactive power at low voltages in a system running close to its stability limit is not very efficient. Also, the total amount of reactive power transfer available will be influenced by the transmission line power factor limiting factors. Hence, sources and compensation devices are always kept as close as possible to the load as the ratio ΔV/ Vnom will be higher for the load bus under fault conditions [13].
2.3 STATCOM OPERATION The operating principles of the STATCOM can be explained by using its equivalent single-phase circuit given by Fig. 2.8, which is inherently a combination of two converters as shown in Fig. 2.9 after rearrangement of the circuit of Fig. 2.8 If the fundamental component of the output voltage of the inverter Voa is in phase with the supply voltage Vsa, the current flowing out the STATCOM or toward the STATCOM is always at 90 to the network voltage because of reactive coupling. According to the circuit shown in Fig. 2.9, converter 1 operates as an Uncontrollable rectifier through which a small quantity of real power flows from the ac to dc side, and converter 2 operates as an inverter when the real power flows in the reverse direction. Initially, when the converter 2 is not conducting, the capacitor C is charged up to the peak value of supply voltage through converter 1, and remains at this voltage for as long as there is no real power transfer between the circuit and the supply. If the switches of converter 2 are operated to obtain the fundamental of the STATCOM output voltage slightly leading the ac main voltage, converter2 conducts
17
Fig .2.8
Equivalent circuit of single phase of STATCOM
Converter 1
Fig. 2.9
converter 2
STATCOM single phase diagram as a combination of two converters
For a superior period to the Fig.2.9 main circuit of STATCOM. One of the converters 1, causing a transfer of real power from the dc side to the ac side. In this way the dc capacitor voltage is decreased, and a reactive power is absorbed by the STATCOM system (lagging mode). The inverse is true when the fundamental of the STATCOM output voltage slightly lags the supply voltage: The dc capacitor voltage is increased, and a reactive power is generated by the STATCOM (leading mode). We can conclude that the reactive power either generated or absorbed by the STATCOM can be controlled only by one parameter: the phase angle between the STATCOM output voltage and the supply voltage. Thus, as indicated by the pharos diagram shown in Fig. 2.10, when the amplitude of the inverter output voltage Voa is smaller than the supply voltage Vsa, the reactive power is absorbed by the STATCOM. Otherwise, the STATCOM generates the reactive power when the amplitude of the supply voltage is larger than the output voltage of the inverter [14].
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Fig. 2.10 Phasor diagram for leading and lagging mode
2.4
MATHEMATICAL MODEL OF STATCOM The three- phase, three-level inverter can be constructed as shown in fig 2.11 using
twelve switching devices and eighteen diodes. In this configuration, the number of levels refers to the positive voltage levels including zero which appear in STATCOM output line voltage (V, etc.) when very large capacitors are used on the dc side. The inverter is connected to the supply system via a single transformer and the leakage reactance of that transformer is represented by X.
Fig. 2.11 Power circuit of the three- level inverter Fig.2.12 shows equivalent circuit of the three- level inverter, including a DC-side capacitor, an inverter, and series inductance X in the three lines connecting to the transmission line. This inductance accounts for the leakage of the actual power transformers, and resistance R in series with the AC lines to represent the inverter and transformer
19
conduction losses. The inverter block in the circuit is treated as an ideal, lossless power transformer [15].
Fig. 2.12
Equivalent circuit of three level inverter
Refer to equivalent circuit of STATCOM; mathematical model is represented as follow.
Assuming balanced three phases of supply can be represented in abc frame as (
[
]=√
[
)
( (
)] )
(2.7)
The ac side of the inverter system is modeled by differential equation for each phase such that
Voa(t)-Vsa(t)=Ls
+Rsia(t).
(2.8)
Vob(t)-Vsb(t)=Ls +Rsib(t).
(2.9)
Voc(t)-Vsc(t)=Ls +Rsic(t).
(2.10)
Then, three phase currents are described as,
20
() ] [ ( )] ()
= [
[ ]
() () ()
[
() ( )] ()
(2.11)
Under the assumption that harmonic components generated by the switching pattern are negligible, the switching function S can be defined as follows: (
Sa,bc=√
[
)
( (
)], )
(2.12)
Where: m: is modulation index m=
.
: is the phase angle which relates the phase difference between source voltage and output voltage of inverter. Now, the output voltage of the inverter and the capacitor dc current can be expressed as follows: The voltage inverter is described as fellow VO, a b c=S V dc I dc =ST I abc
[
() ( )] ()
[
]{
[
]
[
]
}
(2.13) With SkI function of connection, defining the state of the switch, SkI= 1 if the switch is closed and 0 in the contrary case. K: number of the arms. (k=1, 2, 3). I: number of the switch of the arm. (I= 1, 2, 3, 4). The switching state and the operating device can be summarized in the following table.
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1 Sa o 1
[
if S11 and S12 are ON} if S12 and D(0 - a) or S13 { if S13 and S14 are ON}
and D(0 - a' ) are ON}
The DC side of dc side is modeled by ]= [
]
(2.14)
=S11.S12ia+S21.S22ib+S31.S32ic
(2.15)
=S13.S14ia+S23.S24ib+S33.S34ic
(2.16)
From these equations in abc frame we can represent in d-q frame to make easier to make analysis and decouple control.
2.4.1 MODEL IN D-Q FRAME The park’s transformation is used to develop the time invariant frame from equation (2.7) in the rotating reference frames (d-q), the park’s transformation [13, 14] expressions used are given by the following relationship: In d-q reference frame using Xdq=k Xabc.
K=
[
( ) ( ) ⁄√
( (
) ) ⁄√
( (
) ) ] ⁄√
(2.17) Where W is rotation speed of rotation
and W=0 for stationary reference frame, for
rotor reference frame W=Wr and W=Ws synchronous reference frame. Fig. 2.13 shows the transformation in d-q frame. Then, applying park’s transformation
for above equation in
ABC frame to develop model in d-q frame as below for equation (2.7) to (2.17). Fig .2.14 represents the STATCOM in d-q frame.
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Fig.2.13 definition of rotating reference frame
Fig .2.14 STATCOM in d-q
frame
For three -balanced phase generator voltages
Vs d-q=K Vs, a-b-c
[
]= K [
(2.18)
]
(2.19)
Idq=K iabc
(2.20)
Three phases Current in d-q frame
[ ]= [
][ ]
[
]
(2.21)
For Dc side circuit, we include the inverter and DC-side circuit equation in the model. The instantaneous power at the AC- and DC-terminals of the inverter is equal, giving the following power balance equation: Assume idc1=idc2 = idc, then 2Vdc idc= (vodid+voqiq)
(2.22)
23
∫(
Vdc=
)
(2.23)
By taking differential for two sides we get =
(
)
(2.24)
Full model in state variables
[
=
[
] [
]
(2.25)
][ ]
Then, active and reactive power of ASVC at the inverter bus can be calculated as
2.5
P= (vodid+voqiq).
(2.26)
Q= (-vodiq+voqid).
(2.27)
CONTROLLER DESGIN OF STATCOM There are two types of inverter control based for two equation of inverter voltage (Assume harmonic eliminated)
(2.28) (2.29)
Where m is a factor for the inverter which relates the DC-side voltage to the amplitude (peak) of the phase-to neutral voltage at the inverter AC-side terminals, and is
the angle by
which the inverter voltage vector leads the line voltage vector. It is important to distinguish between two basic types of voltage-sourced inverter that can be used in ASVC systems. Inverter Type I: allows the instantaneous values of both control purposes. Provided that
and m to be varied for
is kept sufficiently high, VOd and VOq, can be
independently controlled. This capability can be achieved by various pulse-width-modulation (PWM) techniques that invariably have a negative impact on the efficiency, harmonic content, or utilization of the inverter. Type I inverters are presently considered uneconomical for transmission line applications.
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Inverter Type II: is of primary interest for transmission line ASVCs. In this case, m is a constant factor, and the only available control input is the angle of the inverter
voltage
vector. This control type will be used in proposed system for voltage regulation at the point of common collector bus of variable speed wind farm connected to grid [13]. Details of output control feedback are shown in Fig. 2.15.
Droop
Vref
Iqref
PI
PI
α
Θ+ α
SPWM
Θ Vm
Iqm
Vector magnitude
Vsd
Rotating axis coordinator transformation
Vsq
Isa Isb Isc
Fig. 2.15 Vector current control
2.6 APPLICATION OF STATCOM Stabilization of weak system voltage Reduced transmission losses Enhance transmission capacity Power oscillation damping Improve power factor Reduce harmonics Flicker mitigation Assist voltage after grid faults 25
Phase Locked Loop (PLL) Vsd
Vsq
ABC/DQ
Vsa Vsb Vsc
CHAPTER (3) SERIES COMPENSATOR
3.1 Introduction: Series capacitive compensation is a very economical way for increased transmission capacity and improved transient stability of the transmission grid. FACTS controllers have the flexibility of controlling real and reactive power which, in addition to this control, could provide an excellent tool for improving power system dynamics. Several studies have investigated the potential of using this capability in mitigating SSR of series capacitivecompensated transmission grids [14]–[15]. The use of the thyristor-controlled series capacitor (TCSC), static synchronous compensator (STATCOM), and static synchronous series compensator (SSSC) in their balanced mode of operations has been implemented and/or studied as a means for damping SSR. In series compensation, the FACTS are connected in series
with
the
power
system.
It
works
as
a
controllable
voltage
source.
Series inductance exists in all AC transmission lines. On long lines, when a large current flows, this causes a large voltage drop. To compensate, series capacitors are connected, decreasing the effect of the inductance.
In the case of a no-loss line as shown in fig.3.1, voltage magnitude at the receiving end is the same as voltage magnitude at the sending end: Vs = Vr=V. Transmission results in a phase lag
that depends on line reactance X .
Fig.3.1 Transmission on a no-loss line
Where VS, Vr and I can be calculated from these equations
26
VS=V cos ( ) +j Vsin ( )
(3.1)
)
(3.2)
Vr=V cos ( ) - j Vsin (
I=
=
( )
(3.3)
As it is a no-loss line, active power P is the same at any point of the line: ( )
Ps=Pr=P=V cos ( ).
( )
=
(3.4)
Reactive power at sending end is the opposite of reactive power at receiving end: ( )
Qs= -Qr=Q=V sin ( ). As
=
(
( ))
is very small, active power mainly depends on
(3.5) whereas reactive power mainly
depends on voltage magnitude [16].
3-2 SERIES COMPENSATION OPERATION FACTS for series compensation modify line impedance: X is decreased so as to increase the transmittable active power. However, more reactive power must be provided. According to fig.3.2 the active and reactive power can be calculated as below.
27
Fig .3.2
P=
Q=
Series compensation
( )
(
(3.6)
( ))
(3.7)
3-3 FACTS TYPES FOR SERIES COMPENSATION Thyristor- controlled series compensators have not yet been used in practical applications. The development of such compensators using thyristor-switched capacitors, or a fixed
capacitor in parallel with a thyristor controlled reactor, have been recently reported
[16]. These schemes are shown in Figs.3.2.a and 3.2.b. In the thyristor-switched capacitor scheme f Fig. 3.3. A, the degree of series compensation is controlled by increasing or decreasing the number of capacitor banks in series. To accomplish this, each capacitor bank is controlled by a thyristor bypass switch, or valve. The operation of the thyristor switches is coordinated with voltage and current zero crossings; the thyristor switch can be turned on to bypass the capacitor bank when the applied ac voltage crosses zero, and its turn off has to be initiated prior to a current zero at which it can recover its voltage blocking capability to activate the capacitor bank. In the fixed capacitor, thyristor controlled reactor scheme of Fig. 3.3.b, the degree of series compensation in the Capacitive operating region (the admittance of the TCR is kept below that of the parallel connected capacitor) is increased (or decreased) by increasing the current in the TCR. Minimum series compensation is reached when the TCR is off The TCR
28
may be designed for a substantially higher maximum admittance at full thyristor conduction then that of the fixed shunt connected capacitor. In this case, the TCR, with an appropriate surge current rating, can be used essentially as a bypass switch to limit the voltage across the capacitor during faults and other system contingencies of similar effect.
. Fig. 3.2 Controllable series compensator using a) thyristor-switched capacitors, And (b) a thyristor-controlled-reactor with a fixed capacitor
An alternative approach is to use feedback control over Voltage-Source Converters or Current-Source Converters [based on Gate-Turn-Off- Thyristors (GTOs)] so that they function as electronic capacitors [17]. Since converters are not the capacitors, it is not expected that they will resonate with the line inductance and the danger of SSR instability can be avoided. This line of thinking has given rise to the Static Synchronous Series Compensator (SSSC) as shown in fig.3.4 [17]. Although no stand-alone SSSC has been in service, it is the series converter of the Unified Power Flow Controller (UPFC) in the American Electric Power (AEP) system [18]. Because the capacitors are cheaper than GTOs, the TCSC is price-wise more competitive than the SSSC. In order to lower the overall cost, this paper proposes a hybrid system in which the SSSC is only a fraction of the compensation which is provided mainly by fixed capacitors. Fractional Mvar of the SSSC also implies corresponding reduction of
29
conduction and switching losses in the GTOs. It is possible that the same approach can be applied to compensation using the TCSC. Further research is required to confirm this.
Fig.3.3 schematic diagram of SSSC
Part 1: TCSC 3-1 TCSC OPERATION
The TCSC varies the electrical length of the compensated transmission line with little delay. Owing to this characteristic, it may be used to provide fast active power flow regulation. It also increases the stability margin of the system and has proved very effective in damping SSR and power oscillations (Larsen et al., 1992). Fig. 3.4 shows the main circuit of a TCSC in the steady state: The capacitor voltage varies by controlling the current pulses through the thyristor branch. Steady state relation of the apparent reactance of TCSC can be deduced by determining the fundamental frequency component of voltage bus and dividing by line current. Fig.3.5 shows the characteristic of the TCSC apparent reactance.
30
Fig 3.5 TCSC (a) structure formed by fixed capacitor and TCR (b) variable reactive representation
Fig .3.5 TCSC steady state reactance characteristic
3-2 Controller Design for TCSC
The TCSC power flow model is based on the concept of a non-linear series reactance, which is adjusted using Newton's algorithm to satisfy a specified active power flow across the variable reactance representing the TCSC (Fuerte-Esquivel and Acha, 1997).
31
For power swing damping studies, a TCSC can be modeled as a variable reactance. The active power transfer Plm across an impedance connected between nodes l and m is determined by the voltage magnitude | Vl| and | Vm|, the difference in voltage phase angles θl and θm and the transmission line resistance Rlm and reactance Xlm. In high voltage transmission lines, Xlm>>>>> Rlm and the following approximate equation may be used to calculate the active power transfer Plm Plm=
sin (θl - θm) for TCSC Xlm will be XTCSC
Fig. 3.6 shows the single-phase TCSC is modeled in the EMTP-RV as a single module using ideal thyristor pair and an RC snubber circuit .A phase locked loop (PLL) is used to extract phase information of fundamental frequency line current, which will be used to synchronize TCSC operation. The thyristor gating control is based on the synchronous voltage reversal (SVR) technique [19], [20]. The TCSC impedance is measured in terms of a boost factor kB, which is the ratio of the apparent reactance of the TCSC seen from the line to the physical reactance of the TCSC capacitor bank. Positive value of kB is considered for capacitive operation. A low-pass filter based estimation algorithm is used to estimate the voltage and the current phasors [20], [21]. A boost measurement block performs complex impedance calculations for the boost factor of the TCSC as (Vic/Ic)/XcTCSC, where VC and IC are the estimated phase voltage and current and XcTCSC is the capacitive reactance of the TCSC capacitor branch at the fundamental frequency. A proportional-integral (PI) control based boost level controller is implemented to control the TCSC boost level to the desired value by adjusting the instant of the expected capacitor voltage zero crossing. The integral part of the controller helps in removing the steady state errors [22]. The controller parameters were determined by Ziegler – Nicholas method.
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Fig.3.6 Block diagram of TCSC controller
Part 2: SSSC
3-1 SSSC OPERATION The single-phase SSSC is modeled using a three-level insulated- gate bipolar transistor (IGBT)-based pulse width-modulation (PWM) converter. The schematic diagram of an SSSC connected to the transmission system is shown in Fig.3.7. The line voltage and current as well as other controllable variables are measured and passed to the controller. A coupling series transformer is used to inject series voltage into the transmission line. An LC filter is used to filter out the high-frequency switching noises. The dc capacitor size is selected so that the dc transient voltage overshoot is limited to 20% [23]. A detail of a three-level converter has been discussed in part 2 of chapter (2), where its operation and mathematical model of it has been presented.
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Fig.3.7 schematic diagram of the single phase of SSSC implementation
3-2 CONTROLLER DESIGN FOR SSSC
This section describes the implementation and control of the adopted SSSC. Fig.3.8 shows the block diagram of SSSC controller. The difference between the SSSC impedance and the reference value is processed in a proportional-integral (PI) controller. The output of the PI controller acts as a modulation index for the PWM converter. A positive value of is taken for capacitive compensation and a negative value for inductive compensation. A phaselocked loop (PLL) is used to estimate the phase angle of the fundamental frequency of the line current. The estimated phase angle is used to synchronize the SSSC operation. The PLL consists of a phase detection block, a loop filter, and a voltage-controlled oscillator (VCO).
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Fig 3.8 Schematic diagram of SSSC controller
The rate of convergence of the PLL increases with the increase of the gain values and the speed of the response increases with an increase in; however, it creates oscillations in the response. The proper choice of the gain values is a compromise between speed and accuracy. For the FACTS applications, a fast startup period and zero steady-state error are desired [24]. The dc voltage is controlled at a reference value by implementing a separate PI controller. The difference between the actual dc voltage and the reference dc voltage is processed in this controller to generate a small angle for Δθdc charging/ discharging the dc capacitor. The angle is added to the PLL output angle. The value ±π/2 is added to the angle so that the injected voltage is in quadrature with the transmission- line current, quadrature leading for inductive compensation, and quadrature lagging for capacitive compensation, respectively. The cosine reference wave and a triangular wave are compared according to the gate signal generation logic.
3-3 APPLICATION OF SIERES COMPENSATION Increased transmission capacity Damping Sub synchronous Resonance (SSR) Improved transient stability of the transmission grid
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CHAPTER (4) CASE STUDY In this part, dynamic behavior of STATCOM is presented. Inductive and capacitive mode when voltages at the point of connection changes between over and less than 10% from its value. Test STATCOM behavior when it connected to threephase supply .Details of block diagrams of STATCOM using MATLAB/SIMULINK are described as follow, Fig.4.1 shows the block diagram of generation the SPWM at frequency ratio 21 and modulation index 0.9 .
Fig .4.1 Simulink model of SPWM
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Fig4.2 presents the voltage of inverter as function of switching instants and DC voltage.Fi.g.4.3 illustrates the simulink model for differential equations of STATCOM where DC side and AC side of STATCOM.
Fig .4.2 Simulink model of three- level inverter
Fig .4.3 Simulink model of STATCOM equations
Fig 4.4 shows the block diagram of PLL and Fig.4.5 presents the proposed control diagram using MATLAB/SIMULIMK.
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Fig.4.4 Simulink model of PLL
Fig.4.5 Simulink model of proposed control of STATCOM
Fig.4.6 shows the three-phase voltage that is decreased to16KV at t=0.4 sec and increased to 19.7KV at 0.6 sec for 20msec then remain constant at previous value 18KV.
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Fig.4.6 Three-phase supply voltage
According to proposed control of STATCOM .PLL Block control used to detect the phase angle of the three -phase generating voltage that introduced using the d-q synchronous reference frame , Fig.4.7 represents the PLL block diagram where threephase voltage is converted to d-q frame ,to get angular speed of the supply source the q-component of voltage compares with zeros reference and the PI controller decides the exact angular speed.PLL is important to make the alpha control synchronized with the supply voltage and make sense for any change in voltage and fast response to meet change.
Fig.4.7 PLL block diagram
The simulation results of the PLL block are shown in Fig.4.8.where Vsd and Vsq component of three-phase supply are obtained from park transformation as shown in Fig.4.4,Fig.4.8.a and Fig.4.8.b show Vsd and Vsq of three-phase supply.
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Fig.4.8.a Direct supply voltage
Fig.4.8.b Quadratic supply voltage
Fig .4.8.c shows the synchronous speed of the voltage supply, Fig.4.8.d presents supply frequency; Fig .4.8.e illustrates the phase angle of the three phase voltage of supply.
Fig.4.8.c Angular speed
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Fig.4.8.d Supply frequency
Fig.4.8.e Phase angle of supply voltage
Fig.4.9 represents STATCOM variables where reference and actual current Iq of STATCOM . At 0.4 sec Iq has positive value 400 A when voltage of three-phase supply drops to 16KV, in this case STATCOM acts in leading mode operation and generates rated current .so, voltage at DC link drops to 1.55KV where capacitor discharges current as shown in Fig.4.20.When voltage of three-phase supply increases to 19.7KV at 0.6sec ,Iq has negative value 200A where STATCOM acts in this case Lagging mode operation (rectifier) where capacitor charges in this case to 16.8 KV at 0.6.
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Iqm Iqref
Fig .4.9 Iqref, Iq
Fig .4.10 Voltage at the DC bus
Fig.4.12 illustrates the change of alpha control during voltage change, where at leading mode alpha has positive value to get STATCOM in leading mode and negative value to get it in lagging mode.
Fig .4.12 Variation of alpha
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Fig.4.13 represents the three-phase current of STATCOM, the phase voltage of the three-phase supply and current are shown in Fig.4.13 at 0.4 sec current leads voltage but at 0.6sec current lags voltage.
Fig .4.12 Three-phase current
Fig .4.13
Vsa (V), Isa (A)
STATCOM reactive power and active power are presented in Fig.4.14 the amount of active power is very small but reactive power have value between 6-10 MVAr (lagging-leading mode).
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P
Q
Fig .4.14
P, Q STATCOM
Fig.4.15 shows the SPWM generation, the switching signal voltage and the output voltage of inverter.
(a)
(b)
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(c) Fig .4.15 (a) SPWM generation
(b) The switching signal of phase
(c) The phase voltage of the inverter
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