Performance of Indirectly Controlled STATCOM with IEEE 30-bus System

Transcription

1 Performance of Indirectly Controlled STATCOM with IEEE 30- System Jagdish Kumar Department of Electrical Engineering, PEC University of Technology, Chandigarh, India jk_bishnoi@yahoo.com Abstract - STATCOM is one of the Flexible AC Transmission System (FACTS) devices primarily used for reactive power compensation in power system and also for improvement of voltage profile in the system. In this article a CMLI based indirectly controlled STATCOM has been placed in IEEE30- system for its performance evaluation in regulating the voltages of different es and also its capability to operate under fault condition of power system. The whole systems i.e. STATCOM and IEEE 30- system are simulated on MATLAB Simulink environment using SimPower Systems Block Sets. Simulation studies have been carried out under load variations at different es and its operation under fault conditions. It has been found from the simulation studies that it works well in maintaining voltages and it also supports system voltages under fault conditions. Keywords - Cascade multilevel inverter, direct control, indirect control. I. INTRODUCTION For the application of fast voltage regulation of power systems, static synchronous compensator (STATCOM) is superior over existing var compensating devices due to its many advantages as described in [1-2]. Basically three different types of STATCOM topologies have been reported in the literature depending on the type of voltage source inverter (VSI) used. These topologies can be classified as: i) PWM, ii) multi-pulse and iii) multilevel type [3-6]. Because of various disadvantages of PWM inverters such as high dv/dt per switching, low efficiency, high electromagnetic interference etc. [5], for high power applications, either multi-pulse or multi-level inverters are used. Presently multi-pulse inverters are not considered suitable for STATCOM applications because of requirement of zigzag transformer for interconnection of basic six-pulse inverter units because this makes the system complex [6-7]. In a multi-level inverter, the desired output voltage is synthesized from several levels of input dc voltages. By connecting sufficient number of dc levels, a nearly sinusoidal fundamental frequency voltage of high magnitude can be produced at the output of a multi-level inverter. The multi-level topology is further classified as: diode clamped type, flying capacitors type and cascade or isolated series H-bridges type. Due to modular configuration and least number of component requirements among all multilevel topologies, cascade multi-level inverter (CMLI) is considered to be the most suitable topology for power system applications [7-9]. Consequently, applications of CMLI based STATCOM for reactive power compensation in power systems has been studied in [10-11] in order to improve the voltage profile. This compensation is achieved by STATCOM by generating or absorbing reactive power at the point (PCC) where voltage needs to be controlled. II. STATIC SYNCHRONOUS COMPENSATOR A. Basic Operating Principle A STATCOM is basically a shunt connected voltage source inverter (VSI) which is connected to the power system through a coupling transformer or inductor (L C ). The voltage difference between the STATCOM output voltage (v c ) and the power system voltage (v l ) decides reactive power injection or absorption to the system [1]. This voltage difference can be achieved by two different ways: either by changing the modulation index (m) at constant dc link voltage (v dc ) (direct control) or by varying v dc at fixed m (indirect control) [2]. In indirect control, variation of v dc is achieved by phase shifting v c with respect to v l. The single-line diagram of basic configuration of CMLI based STATCOM is shown in Fig. 1. The various parameters as shown in Fig. 1 are given in Appendix-I. 51

2 B. Cascade Multilevel Inverter The CMLI consists of a number of H-bridge inverter units with separate dc source for each unit, and is connected in cascade or series. Each H-bridge can produce three different voltage levels: +V dc, 0, and V dc by connecting the dc source to ac output side by different combinations of the four switches S 1, S 2, S 3, and S 4. The ac output of each H-bridge is connected in series such that the synthesized output voltage waveform is the sum of all of the individual H-bridge outputs [4, 7-9]. The five-level CMLI configuration and its output voltage waveform are shown in Fig. 2. By connecting sufficient number of H-bridges in cascade and using proper modulation scheme, a nearly sinusoidal output voltage waveform can be synthesized. The number of levels in the output phase voltage is 2s+1, where s is the number of H-bridges used per phase. In this work, an 11-level CMLI has been considered. In 11-level CMLI there are five H-bridges and corresponding firing angles are α 1, α 2, α 3, and α 4 and α 5. As the harmonic content of the output voltage depends very much on the angles α 1 α 5, these angles should be chosen properly so that the output voltage waveform is nearly sinusoidal. In this work, the switching angles have been calculated by minimizing the total harmonic distortion due to all non-triplen odd harmonic components up to 49 th order [11] which is represented by the index THD 49 as shown in eqn. (1) below. 2 2 V V... V THD 100 (1) 49 V 1 In eqn. (1), V 1, V 5, V 7, and V 49 are magnitudes of the fundamental, fifth, seventh and forty ninth harmonic components respectively [11]. 2 C. Control Strategies In literature two control schemes, namely, direct and indirect control exists for STATCOM [12-13]. In case of direct control scheme, the output voltage is varied by varying the modulation index (m) i.e. by varying the switching angles of individual H-bridges of CMLI while keeping the dc capacitors voltages constant. In case of indirect control scheme, the output voltage is controlled by varying the dc capacitor voltages at constant modulation index (i.e. at fixed switching angles). The indirect control scheme employed for the STATCOM in this work is shown schematically in Fig. 3 for a CMLI having s number of H-bridges. 2V dc V dc -2V dc V dc + _ V dc + _ (a) V dc H 2 H 1 v ac 2 t So urce v 0 s i s L S R S Line param eters v l i l (b) Fig. 2. (a) Configuration of single-phase five-level CMLI, and (b) its waveform. Co upling Inducto r P aram eters R C L C i c v c R L L L Lo ad V L_ref m + - PI controller Switching angles look-up table 1 2 s Switching angles modulation and swapping STATCOM and Power system V L Co ntro l i dc R P i p C eq _ + v dc CM LI STATCOM Fig. 1: Single-line diagram of CMLI based STATCOM. Fig. 3: Schematic diagram of indirect control scheme for CMLI STATCOM. In Fig.3, the reference ac system voltage (V L_ref ) is compared with the actual voltage V L of the ac system and corresponding error (i.e. the difference between these two) is processed through a PI controller which produces a desired value of load angle (φ) by which the STATCOM voltage needs to be phase shifted 52

3 (lagging/leading) with respect to the ac system voltage in order to charge/discharge the dc capacitors. The charging or discharging of the dc capacitors increase or decrease the output voltage of the STATCOM which eventually controls the ac system voltage by proper exchange of the reactive power. The performance indirectly controlled STATCOM with IEEE 30- system has been carried out through simulation study in the MATLAB/SIMULINK [14] environment. III. PERFORMANCE OF STATCOM WITH IEEE 30-BUS SYSTEM The performance of the STATCOM has been investigated on a smaller system whose configuration is shown in Fig. 1 and the results are shown in Fig. 4 [13]. In Fig. 4 (a) (b) load voltage variations and dc capacitor voltage variations are shown for following sequence of load change: (a) Initial load on is P = 0.2 pu and Q = 0.4 pu, (b) the STATCOM is connected to load at =0.2 sec., (c) an inductive load of P (active power) = 0.4 pu and Q (reactive power) = 0.6 pu is connected at t = 1 sec. The STATCOM is maintaining load voltage at 1 pu level by supplying reactive power to the load (when load voltage goes down) and (d) at t = 2 sec. an a capacitive load of P =0.2 pu and Q = 0.6 pu is applied, the STATCOM is maintaining load voltage at 1 pu by absorbing reactive power from the power system (load voltage goes up). The exchange of reactive power is carried out by changing dc capacitor voltage (Fig. 4 (b) ) which further changes the STATCOM output voltage. The STATCOM and loads are connected to the load with the help of switches (circuit breakers). It has been found that STATCOM is working well in maintaining the load voltage. To investigate its performance on a relatively large distribution system, in this work, the IEEE-30 system as shown in Fig. 5 has been considered. The data of this system are given in the Appendix-II. The STATCOM is assumed to be connected at 8 through a 2 mh coupling inductor. It is further assumed that the STATCOM is required to maintain the voltage of 8 at 0.93 pu (specified value). To investigate the performance of the STATCOM under heavy load variation, it is also assumed that there is an isolator present in the section 9-10, which is OFF initially. With this initial configuration, the system identification technique as described in [IITM] has been followed and subsequently the PI controller has been designed using this identified transfer function. To test the effectiveness of the designed controller following sequences of events has been studied. The corresponding simulation results are shown in Fig. 6. a) Initially it is assumed that STATCOM is not connected at 8. As observed from Fig. 6 (a) the voltage of 8 without STATCOM is 0.92 pu, which is less than the STATCOM reference voltage. b) At t = 0.5 sec. STATCOM is connected to 8 and as a result, voltage of 8 is now maintained at 0.93 p.u. as shown in Fig. 6 (a) due to reactive power injected by the STATCOM. Corresponding variation in load angle and dc total capacitor voltage is shown in Fig. 6 (b) - (c) respectively. Reference voltage (0.93 pu) of 8 has been achieved. c) At t = 1.5 sec., loads of have been added by connecting the isolator existing between Due to the addition of loads connected to 10-14, the voltage of 8 falls momentarily but STATCOM restores it to 0.93 pu in a very short duration. The corresponding results are shown in Fig.6 (a) (c). Fig. 4: (a) Load voltage regulation and (b) total dc capacitors voltage variation. 53

4 d) A three phase short circuit fault of three cycles (60 ms) is created at 14 at t = 2.5 sec., which causes the 8 voltage to fall. To arrest this drop in the 8, the controller increases the DC capacitor voltage by increasing the load angle (as shown in Fig. 6 (b) and (c) ) and consequently the voltage is restored at its desired value after the fault is cleared. e) At t = 4 sec., loads connected to have been removed. After removing these loads, steadystate conditions are reached with minor variations in load voltage, load angle and total DC capacitor voltage as shown in Fig. 6. IV. CONCLUSION In this paper, the performance of CMLI based indirectly controlled STATCOM has been evaluated for voltage regulation of any of IEEE 30- system. The performance of the STATCOM found to be quite satisfactory although it was designed to operate for single system. M ain Substatio n Fig. 5: Configuration of IEEE 30 distribution system Fig. 6: (a) Load voltage regulation, (b) variation of load angle and (c) total capacitors voltage change when STATCOM is connected at 8 of the IEEE 30- distribution system (L C = 2 mh). APPENDIX-I Parameters of the ±5MVAr, 13.8kV STATCOM and power system are given below: Base voltage = 13.8 kv, Base power = 5MVA, v s = 1.0, Short Circuit Capacity = 10 (pu), ω = 314 rad./sec., X/R Ratio = 4, R S (series resistance) = 0.45Ω, L S (series inductor) = 4.8 mh, R C (resistance of coupling inductor) = 0.01 Ω, L C (coupling inductor) = 2 mh, C = 4800 µf, v dcref = V; load voltage controller s, K P = 3.5, K I = 330, modulation index = APPENDIX-II TABLE I Data for IEEE 30 distribution system Base kv = From To Resistance Reactance Line charging 0 (ss)

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