Chapter 3 MODELING OF SHUNT FACTS DEVICES. The Shunt FACTS Devices are used for voltage control and

Transcription

1 44 Chapter 3 MODELING OF SHUNT FACTS DEVICES 3.0 Introduction The Shunt FACTS Devices are used for voltage control and power flow control, but these are good at for voltage control. These are not in a position to effectively damp the power system oscillations. In some critical cases the voltage control can even amplify oscillations [2]. To achieve this objective it is necessary to improve the Shunt FACTS devices control concept by introducing auxiliary signals, which reflect power system oscillations. In literature survey most of damping controllers are developed by using global like generator speed deviation, generator power change, load angle deviation etc. [2][13][14]. In [4] the voltage across the shunt compensator considered as input signal to the damping controller. In this work voltage signal of a constant resistive load at Shunt FACTS device is installed, which reflects the oscillations of power at any point in the system. 3.1 Flexible AC Transmission System (FACTS) Power flow is a function of line impedance, the magnitude of the sending and receiving end voltages, and phase angle between the voltages. By controlling one or a combination of the power flow arguments, it is possible to control the active, as well as the reactive power flow in the transmission line. In the past, power systems were simple and designed to be self-sufficient. Active power exchange of near was rare as transmission systems cannot be controlled fast enough to handle dynamic changes in the system and therefore,

2 45 dynamic problems were usually solved by having generous stability margins so that the system could never from anticipated operating contingencies. Today, it is possible to increase the system laudability and security by using different approaches. It is a usual practice in power systems to install shunt capacitors to support system voltage at satisfactory level. Series capacitors are used to reduce transmission line reactance and thereby increase power transfer capability of lines. Phase shifting transformers are applied to control power flows in transmission lines by introducing additional phase shift between sending and receiving end. In the past, all these devices were controlled mechanically and were, therefore, relatively slow. They are very useful in steady state operation of power systems but from dynamical point of view, their time response is too slow to effectively damp transient oscillations. If mechanically controlled systems were made to respond faster, power system security would be significantly improved, allowing the full utilization of system capability while maintaining adequate levels of stability. This concept and advances in the field of power electronics led to a new approach introduced by the Electric Power Research Institute (EPRI) in the late 1980 and is referred as Flexible AC Transmission System or simply FACTS. Flexible AC Transmission System is defined as Alternating Current transmission system incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability FACTS Controllers Facts controllers are classified based on connections as shunt, series and combination of series and parallel Shunt FACTS controllers: Shunt FACTS controllers are variable shunt impedance or variable reactive current devices. All Shunt FACTS controllers inject

3 46 current into the power system at the point the device is connected [33] to maintain good voltage profile. i) Thyristor based shunt FACTS controllers Static VAR compensators (SVC): A shunt connected static VAR compensator is a device to exchange capacitive or inductive current with power system to maintain or control the bus voltage. These are generally a) TCR: Thyristor controlled Reactor b) TSR: Thyristor switched Reactor c) TSC: Thyristor switched Capacitor d) FC-TCR: Fixed Capacitor Thyristor controlled Reactor e) TSC-TCR: Thyristor switched Capacitor Thyristor controlled Reactor are as shown in figure Fig Basic type of thyristor based static VAR compensators

4 47 ii) Converter based shunt FACTS controller Static Synchronous Compensator (STATCOM) It is a converter based shunt FACTS controller whose output current inductive or capacitive can be controlled independent of the ac system voltage. The converter based shunt compensator as shown in figure Fig converter based shunt compensator Series FACTS controllers: Series Facts controllers are used to control power flow in the transmission lines by varying the line reactance or by injecting voltage in series with transmission line. i) Thyristor based series FACTS controllers It is a controller to control the power flow by varying the line reactance. These are generally

5 48 a) TCSC: Thyristor controlled series capacitor b) GCSC: GTO controlled series capacitor c) TSSC: Thyristor switched series capacitors are shown in figure Fig Basic type of thyristor based series compensators ii) Converter based series FACTS controller Static synchronous series compensator (SSSC) It is a converter-based series FACTS controller can generate a controllable compensating capacitive or inductive voltage, which implies that the amount of transmittable power can be increased as well as decreased from the natural power flow. The SSSC output

6 49 voltage is independent of the line current. The converter based series compensator as shown in figure Fig converter based series compensator Combination of shunt and series converter: Unified power flow controller (UPFC) It one of the familiar converter based shunt and series combination compensator. It has the ability to simultaneously and selectively control all of the parameters affecting flowing of power in transmission line, namely; voltage, impedance and phase angle. The UPFC can fulf aa these functions by adding the series voltage in series with the transmission line. The UPFC is as shown in figure

7 50 Fig Unified power flow controller (UPFC) Estimation of Power change in the constant Resistive Load connected at Shunt FACTS device using Locally Measured Variables and employing control concept If the Shunt FACTS device is considered at any node k in a power system, the voltage across the constant resistive load is Vk. Power across the resistive load: P Vk (3.1) Change in power across the load: P Vref 2 - Vk (3.2) The effectiveness of Shunt FACTS device for damping oscillations is limited by the maximum rating of the device. Maximum damping is thus achieved employing bang-bang control with correct phase angle of the signal thus utilizing the maximum device rating [2]. The equation (3.2) depicts a Shunt FACTS device control employing a damping signal. Additional filters are required in order to filter out interference signals from the relevant frequency range of oscillation from 0.3 to approximately 2 Hz. The transfer function [2] to filter out

8 51 harmonic content in the estimated change in power signal from (3.2) was employed for bang-bang controller. G( s) a b s s a 2 s 1 T T W W s s (3.3) 3.2 Fuzzy Logic Controller A general fuzzy controller consists of four modules: a fuzzy rule base, a fuzzy inference engine, a fuzzification / defuzzification modules. The interconnections among these modules and the controlled process are shown in Fig A fuzzy controller operates by repeating a cycle of the following four steps. First, measurements are taken of all variables that represent relevant condition of the controlled process. Next, these measurements are converted into appropriate fuzzy sets to express measurement uncertainties. This step is called fuzzification. The inference engine to evaluate the control rules stored in the fuzzy rule base then uses the fuzzified measurements. The result of this evaluation is a fuzzy set (or several fuzzy sets) defined on the universe of discourse of possible actions. This fuzzy set is converted, in the final step of the cycle, into a single (crisp) value (or a vector of values) that, in some sense, is the best representative of the fuzzy set (or fuzzy sets). This conversion is called a defuzzification. The defuzzified values represent action taken by the fuzzy controller in individual control cycles.

9 52 Fig Fuzzy logic controller SIMULINK Model: The Fig shows the MAMDANI Fuzzy Inference System (FIS) Editor. At the top is a diagram of the system with each input and output clearly labeled as DP and B for FLC POD of SVC. Double click on the input (DP) or output (B) boxes for editing of the member ship functions. The Fig shows the member ship function editor. The rules of FIS is edited by double-clicking on the fuzzy rule box. The Fig shows the Rule Editor.

10 53 Fig MAMDANI FIS Editor Fig Membership function editor

11 54 Fig Rule Editor 3.3 Shunt FACTS controllers modeling Static VAR Compensator (SVC) Configuration SVC is the primary invention FACTS device that is already in operation in different places in the world. The SVC is a shunt device of the FACTS group using power electronics to control power flow and improve transient stability on power grids [33]. The SVC regulates voltage at its terminals by controlling the amount of reactive power injected into or absorbed from the power system. When system voltage

12 55 is low, the SVC generates reactive power (SVC capacitive). When system voltage is high, it absorbs reactive power (SVC inductive). The configuration of the FC-TCR type SVC is shown in Fig.3.1 i. Fixed Capacitor (FC) that provides a permanently connected source of reactive power. ii. Thyristor Controlled Reactor (TCR) that consists of bi-directional thyristor valves in series with Shunt reactors, usually connected in delta configuration. The main task of an SVC is to keep up the voltage of particular bus by means of reactive power compensation (obtained by varying the firing angle of the thyristors). It can also afford the damping of power oscillations and enhance power flow over a line by using auxiliary stabilizing signals such as line active and reactive power, frequency, phase angle difference, speed etc. The essential Components of a static var compensator (FC-TCR) [1] [3]: Transformer between high voltage (HV) network bus and the medium voltage (MV) bus where power electronic equipment is connected. Generally a dedicated transformer is used, but sometimes the tertiary of an autotransformer is used. FC: Fixed Capacitor that provides a permanently connected source of reactive power.

13 56 TCR: Thyristor Controlled Reactor that consists of bi-directional thyristor valves in series with Shunt reactors, generally connected in delta configuration Harmonic filters connected to the medium voltage bus. At fundamental frequency, the filters are capacitive. Control system, generally with a primary function of regulating the transmission voltage. Fig.3.1 FC-TCR type SVC SVC V-I characteristic As long as the SVC susceptance B stays within the maximum and minimum susceptance values imposed by the total reactive power of capacitor banks and reactor banks, the voltage is regulated at the reference voltage Vref. The V-I characteristics of SVC are shown in Fig.3.2. The V-I characteristic is described by the following three equations:

14 57 V = Vref +Xs* I (3.4) SVC is in regulation range (-BCmax < B < BLmax) V = -I/BCmax SVC is fully capacitive (B = -BCmax) (3.5) V = -I/BLmax SVC is fully inductive (B = -BLmax) (3.6) Where V: Positive sequence voltage I: Reactive current (I > 0 indicate an inductive current and I < 0 Indicates an capacitive current) Xs: Slope or droop reactance Bcmax: Maximum capacitive susceptance Blmax: Maximum inductive susceptance Fig.3.2 V-I characteristic of SVC The SVC control characteristic is shown in Fig.3.2. This plots the ac bus voltage at the point where the SVC is connected against variation in SVC current (Isvc). It can be seen that ac system voltage

15 58 varies linearly with slope Xs from the reference setting Vref. This characteristic is also called the droop characteristic as it indicates the drop in voltage due to current drawn. Composite SVC and power system characteristics are in Fig.3.3. Three power system characteristics are considered. The middle characteristic represents nominal system condition, and is assumed to intersect the SVC characteristic at point A where V = Vref and Isvc is zero. If the bus voltage falls below Vref due to increase in system load level, the SVC holds the voltage at V3 that would otherwise drop to V2 without SVC. In this case the SVC must become more capacitive to raise the voltage. If, on the other hand, the voltage rises above Vref possibly due to a decrease in system loading, the system voltage will increase to V1 without SVC. With SVC operating point moves to point B. The SVC regulates the system voltage to V4 by absorbing inductive current I1. In this case the SVC becomes inductive to lower the voltage to the desired value. Outside the control range the SVC acts like a fixed capacitor or fixed inductor. Fig.3.3 Graphical operation of SVC for given system conditions

16 Basic Non-linear Model for Dynamic Performance of SVC The Fig.3.4 shows the Basic model for SVC suggested by IEEE Special Stability Controls Working Group [1]; in this Basic model, the voltage regulator is of integral plus proportional type as shown in Fig.3.5. In addition to the main job of the SVC controller, which is to control the SVC bus voltage, the reactance of the SVC controller maybe used to damp system oscillations, as denoted in Fig Fig.3.4 Basic Model 2 of SVC Fig 3.5 Voltage regulator model for Basic Model 2 of SVC

17 SIMULINK Modeling of SVC The SIMULINK SVC block developed as a phasor model, to perform transient stability studies in 3-Ph power systems. The SVC delivers or absorbs the reactive current as per the output susceptance of ac voltage regulator Conventional PI Voltage Regulator of SVC The SVC control block diagram is shown in Fig. 3.6 [1] [2]. The voltage controller is of integral plus proportional type and the slope Xs, realized through current feedback. It gives the appropriate susceptance (B) as per the voltage error (Ve=Vref-Vm). Fig.3.6 Conventional PI Voltage Regulator block diagram of SVC FLC POD controller along with PI Voltage Regulator of SVC The block diagram of FLC POD controller along with PI voltage regulator of SVC is shown in Fig PI voltage regulator is fed by one input that is voltage error (Ve) and FLC POD controller is fed by one input that is change in power (DP). FLC POD controller gives the

18 61 appropriate susceptance (B) during dynamic condition, which depends on DP; under steady state condition it gives zero susceptance. The PI voltage regulator gives appropriate susceptance (B) as per the Voltage error (Ve=Vref-Vm). The block diagram of FLCPOD for SVC is as shown in figure 3.7(a). Fig. 3.7(a) Block diagram of FLCPOD for SVC The rules for the proposed FLC POD controller are: i) If DP is DPN Then B is BN ii) iii) If DP is DPZ Then B is BZ If DP is DPP Then B is BP

19 62 These rules are in matrix form as given below Error (DP) DPN DPZ DPP Out put (B) BN BZ BP The membership functions for input and output to FLC POD controller, Change in power (DP) and Susceptance (B) are given in Fig.3.7 (b). Fig.3.7 (b) (a) Input membership function (DP) and (b) Output Membership Function (B) of FLC POD controller Fig.3.8 FLC POD controller along with PI Voltage Regulator block diagram of SVC

20 Static Synchronous Compensator (STATCOM) Configuration and operation STATCOM is one of the important shunt connected Flexible AC Transmission Systems (FACTS) controllers can be used for voltage regulation in power system, to control power flow and make better transient stability [2][33]. The basic structure of STATCOM [7] [8] [11] [16] [33] in schematic diagram is as shown in Fig.3.9. It regulates voltage at its terminal by changing the amount of reactive power in or out from the power system. When system voltage is low, the STATCOM inject reactive power. When system voltage is high, it absorbs reactive power. The change of reactive power is accomplished by means of a Voltage-Sourced Converter (VSC) connected on the secondary side of a coupling transformer. The power electronics based VSC generates 3 phase voltage with power system frequency V2 from a DC voltage source furnished by the charged capacitor. The basic Components of a static synchronous compensator (STATCOM) [12]: Transformer between high voltage (HV) network bus and the medium voltage (MV) bus where power electronic equipment is connected. Usually a dedicated transformer is used, but sometimes the tertiary of an autotransformer is used. Voltage-Source Converter (VSC) or Voltage Source Inverter (VSI). A DC capacitor

21 64 Harmonic filters connected to the medium voltage bus. At fundamental frequency, the filters are capacitive. Control system, usually with a primary function of regulating the transmission voltage. Operating Principle of the STATCOM The Fig. 3.9 shows the basic schematic diagram of the STATCOM. The energy storage device is a relatively small dc capacitor and hence the STATCOM is capable of only reactive power exchange with the power system. If a dc storage battery or other dc voltage source were used to replace the dc capacitor, the STATCOM can exchange real and reactive power with power system, extending its operating region from two to four quadrants as shown in Fig The real power is absorbed /supplied by the STATCOM to compensate converter losses due to switching and charging/discharging of the DC capacitor to maintaining satisfactory DC voltage level. This depends on the size of DC capacitor and losses due to switching. The real power exchange between power system and STATCOM is relatively small because the DC capacitor and the switching losses are small. This implies that the output current I of STATCOM has to be approximately ±90 0 with respect to V1. The Fig.3.11 shows the STATCOM output current and voltage diagram where IP and IQ are the quadrature and in phase components of STATCOM current I with respect to power system voltage V1. If the losses in the STATCOM are neglected and it is assumed that real power exchange with the power system is zero, then the active

22 65 component IP of STATCOM current I is equal to zero; the STATCOM current I is equal to reactive component IQ.During steady state working condition, the voltage V2 produced by the VSC is in phase with V1 (i.e δ=0), so that only reactive power is flowing (Active power P=0). Fig. 3.9 Schematic representation of STATCOM If the magnitude of voltage V2 produced by VSC is less than the magnitude of power system voltage V1, reactive power Q flows from power system to VSC (STATCOM absorbs reactive power) as shown in Fig.3.11 (a). If V2 is greater than V1, Q flows from VSC to power system (STATCOM produces reactive power) as shown in Fig.3.11 (b). If V2 is equal to V1 the reactive power exchange is zero. The amount of reactive power is given by Q = V1 2 V1 V X (3.5)

23 66 Fig.3.10 Four-quadrant operation of STATCOM Fig STATCOM operation STATCOM V-I characteristic Modes of the STATCOM operation: 1) Voltage regulation mode 2) VAR control mode When the STATCOM worked in voltage regulation mode, it implements the V-I characteristic [15] [33] as shown in Fig The V-I characteristic is depicted by the following equation: V = Vref + XS.I (3.6)

24 67 Where V = Positive sequence voltage (pu) I = Reactive current (pu/pnom) (I > 0 indicates an inductive current and I< 0 indicates capacitive current) XS=Slope (pu/pnom: usually between 1% and 5%) Pnom=Converter rating in MVA Fig.3.12 V-I characteristics of STATCOM The STATCOM control characteristic [15] is shown in Fig This plots the ac bus voltage at the point where the STATCOM is connected against variation in STATCOM current (Istatcom). It can be seen that ac system voltage varies linearly with slope Xs from the reference setting Vref. This characteristic is also called the droop characteristic as it indicates the drop in voltage due to current drawn.

25 68 Composite STATCOM and power system characteristics [15] are in Fig Three power system characteristics are considered. The middle characteristic represents nominal system condition, and is assumed to intersect the STATCOM characteristic at point A where V = Vref and Istatcom is zero. If the bus voltage falls below Vref due to increase in system load level, the STATCOM holds the voltage at V3 that would otherwise drop to V2 without STATCOM. In this case the STATCOM must become more capacitive to raise the voltage. If, on the other hand, the voltage rises above Vref possibly due to a decrease in system loading, the system voltage will increase to V1 without STATCOM. With STATCOM operating point moves to point B. The STATCOM regulates the system voltage to V4 by absorbing inductive current I1. In this case the STATCOM becomes inductive to lower the voltage to the desired value. Outside the control range the STATCOM acts like a fixed current source (capacitive or inductive). Fig.3.13 Graphical operation of STATCOM for given system Conditions

26 STATCOM control system The d q decoupled current control strategy [6][9][12] is implemented as shown in Fig The control system consists of: A phase-locked loop (PLL): it is used to synchronize the STATCOM current with bus voltage at which point of STATCOM connection. An AC voltage regulator: it gives the reference reactive current Iqref required by the system to maintain bus voltage at constant value or in specified range. A DC voltage regulator: it gives the reference active current Idref to maintain the capacitor voltage at a constant value or in specified range. The inner current regulator: it controls the magnitude and phase of the voltage generated by the PWM converter of STATCOM to deliver or absorb required reactive current by the STATCOM as per reference valve given by the AC and DC voltage regulators. Fig.3.14 control structure of STATCOM

27 SIMULINK Modeling of STATCOM The SIMULINK STATCOM block developed as a phasor model, to perform transient stability studies in 3-Ph power systems. The STATCOM delivers or absorbs the reactive current as per the output of ac voltage regulator Conventional PI Voltage Regulator of STATCOM The conventional PI voltage regulator block diagram of STATCOM is shown in Fig The voltage controller is of integral plus proportional type and the slope Xs, realized through current feedback. It gives the appropriate reactive current (IQ) as per the voltage error (Ve = Vref-Vm). Fig.3.15 PI voltage regulator block diagram of STATCOM FLC POD controller along with PI voltage regulator of STATCOM The FLC POD controller along with conventional PI voltage regulator block diagram of STATCOM is shown in Fig PI voltage

28 71 regulator is fed by one input that is voltage error (Ve) and FLC POD controller is fed by one input that is change in power (DP). FLC POD gives the appropriate reactive current (IQ), which is required by the system in dynamic condition which depends on DP; under steady state condition it gives zero reactive current. The PI voltage regulator gives appropriate reactive current (IQ) as per the Voltage error (Ve=Vref-Vm). The block diagram of FLCPOD for STATCOM is as shown in figure 3.16(a) 3.16(a) Block diagram of FLCPOD for STATCOM

29 72 The rules for the proposed FLC POD controller are: i) If DP is DPN Then IQ is IQN ii) iii) If DP is DPZ Then IQ is IQZ If DP is DPP Then IQ is IQP These rules are in matrix form as given below error (DP) DPN DPZ DPP Out put (IQ) IQN IQZ IQP The membership functions for input and output to FLC POD controller, Change in power (DP) and Reactive current (IQ) are given in Fig.3.16 (b). Fig.3.16 (b) (a) Input membership function (DP) and (b) Output Membership Function (IQ) for FLC POD controller

30 73 Fig.3.17 FLC POD controller along with conventional PI voltage regulator block diagram of STATCOM 3.4 Summary In this chapter, all FACTS devices are discussed briefly. Details of SVC and STATCOM have been discussed. SIMULINK implementation of the SVC and STATCOM has been discussed. The SVC and STATCOM with PI and FLPOD controllers allow the controls shunt suceptance and shunt injected reactive current respectively. Fuzzy logic controller and SIMULINK modeling of Fuzzy logic controller are discussed.