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ISSN (Print): 2278 - 8948, Volume - 1, Issue - 2, 2012 51 Performance of Indirectly Controlled STATCOM with IEEE 30 - bus System Jagdish Kumar Department of Electrical Engineering, PEC University of Technology, Chandigarh, India E - mail : 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 - bus system for its performance evaluation in regulating the voltages of different buses and also its capability to operate under fault cond ition of power system. The whole systems i.e. STATCOM and IEEE 30 - bus system are simulated on MATLAB Simulink environment using SimPower Systems Block Sets. Simulation studies have been carried out under load variations at different buses and its operation under fault conditions. It has been found from the simulation studies that it works well in maintaining bus voltages and it also supports system bus voltages under fault conditions. Keywords - Cascade multilevel inverter, direct control, indirect control. I . I NTRODUCTION 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 topolo gies 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 be cause 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 syste m 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. S TATIC S YNCHRONOUS C OMPENSATOR A. Basic Operating Principle A STATCOM is basically a shunt connected voltage source inverter (VSI) which is connected to the power system bus through a coupling transformer or inductor (L C ). The voltage difference between the STATCOM output voltage (v c ) and the power system bus 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 t o 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. International Journal of Advanced Electrical and Electronics Engineering (IJAEEE) ISSN (Print): 2278 - 8948, Volume - 1, Issue - 2, 2012 52 B. Cascade Multilevel Inverter The CMLI consists of a number of H - bridge inverter units with s eparate 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 , a nd 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 2 s +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 n corresoning firing ngles re α 1 , α 2 , α 3 , n α 4 and α 5 . As the harmonic content of the output voltage eens very much on the ngles α 1 … α 5 , t hese 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 or der [11] which is represented by the index THD 49 as shown in eqn. (1) below. (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]. Fig. 1 : Single - line diagram of CMLI based STATCOM. 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 volt ages 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. Fig. 2. (a) Configurati on of single - phase five - level CMLI, and (b) its waveform. 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 rouces  esire vlue of lo ngle (φ) y hich the STATCOM voltage needs to be phase shifted International Journal of Advanced Electrical and Electronics Engineering (IJAEEE) ISSN (Print): 2278 - 8948, Volume - 1, Issue - 2, 2012 53 (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 reactiv e power. The performance indirectly controlled STATCOM with IEEE 30 - bus system has been carried out through simulation study in the MATLAB/SIMULINK [14] environment. III. P ERFORMANCE OF S TATCOM WITH IEEE 30 - BUS S YSTEM 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 “()” – “()” lo voltge vritions n c capacitor voltage variations are shown for following sequence of load change: (a) Initial load on bus is P = 0.2 pu and Q = 0.4 pu, (b) the STATCOM is connected to load bus 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 bus volta ge at 1 pu level by supplying reactive power to the load bus (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 t he power system (load bus voltage goes up). The exchange of reactive power is carried out by chnging c ccitor voltge (Fig. 4 “()”) hich further changes the STATCOM output voltage. The STATCOM and loads are connected to the load bus with the help o f switches (circuit breakers). It has been found that STATCOM is working well in maintaining the load bus voltage. To investigate its performance on a relatively large distribution system, in this work, the IEEE - 30 bus 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 bus 8 through a 2 mH coupling inductor. It is further assumed that the STATCOM is required to maintain the voltage of bus 8 at 0.93 pu (specified va lue). 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, hich is „OFF‟ initilly. With this initial configuration, the system identification technique as desc ribed 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 simulat ion results are shown in Fig. 6. a) Initially it is assumed that STATCOM is not connecte t us 8. As oserve from Fig. 6 “()” the voltge of us 8 ithout STATCOM is „0.92 pu, which is less than the STATCOM reference voltage. b) At t = 0.5 sec. STATCOM is connected to bus 8 and as a result, voltage of bus 8 is now maintained t 0.9 .u. s shon in Fig. 6 “()” ue to rective power injected by the STATCOM. Corresponding variation in load angle and dc total capacitor voltge is shon in Fig. 6 “()” - “(c)” respectively. Reference voltage (0.93 pu) of bus 8 has been achieved. c) At t = 1.5 sec., loads of bus 10 - 14 have been added by connecting the isolator existing between bus 9 - 10. Due to the addition of loads connected to bus 10 - 14, the voltage of bus 8 falls momentarily but STATCOM restores it to 0.93 pu in a very short duration. The corresponding results are shown in Fig.6 “()” – “(c)”. Fig. 4 : (a) Load voltage regulation and (b) total dc capacitors voltage variation. International Journal of Advanced Electrical and Electronics Engineering (IJAEEE) ISSN (Print): 2278 - 8948, Volume - 1, Issue - 2, 2012 54 d) A three phase short circuit fault of three cycles (60 ms) is created at bus 14 at t = 2.5 sec., which causes the bus 8 voltage to fall. To arrest this drop in the bus 8, the controller increases the DC capacitor voltage by increasing the load angle (as shown in Fig. 6 “()” n “(c)”) n co nsequently the bus voltage is restored at its desired value after the fault is cleared. e) At t = 4 sec., loads connected to bus 10 - 14 have been removed. After removing these loads, steady - state conditions are reached with minor variations in load voltage, load angle and total DC capacitor voltage as shown in Fig. 6. IV. C ONCLUSION In this paper, the performance of CMLI based indirectly controlled STATCOM has been evaluated for voltage regulation of any bus of IEEE 30 - bus system. The performance of the ST ATCOM found to be quite satisfactory although it was designed to operate for single bus system. Fig. 5 : Configuration of IEEE 30 bus distribution system. Fig. 6 : (a) Load voltage regulation, (b) variation of load angle and (c) total capacitors vo ltage change when STATCOM is connected at bus 8 of the IEEE 30 - bus distribution system (L C = 2 mH). A PPENDIX - 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 Ccity  10 (u), ω  14 rad./sec., X/R Ratio = 4, R S (series resistnce)  0.45Ω, L S (series inductor) = 4.8 mH, R C (resistance of coupling inuctor)  0.01 Ω, L C (coupling inductor) = 2 mH, C = 4800 µF, v dcref  12500 V; lo voltge controller‟s, K P = 3.5, K I = 330, modulation index = 0.9240. A PPENDIX - II TABLE I Data for IEEE 30 bus distribution system Base kV = 23.00 From bus To bus Resistance (ohm) Reactance (ohm) Line charging 0 (ss) 1 0.896 0.155 0.000 1 2 0.279 0.015 0.000 2 3 0.444 0.439 0.000 3 4 0.864 0.751 0.000 4 5 0.864 0.751 0.000 5 6 1.374 0.774 0.000 6 7 1.374 0.774 0.000 7 8 1.374 0.774 0.000 8 9 1.374 0.774 0.000 9 10 1.374 0.774 0.000 10 11 1.374 0.774 0.000 11 12 1.374 0.774 0.000 12 13 1.374 0.774 0.000 13 14 1.374 0.774 0.000 International Journal of Advanced Electrical and Electronics Engineering (IJAEEE) ISSN (Print): 2278 - 8948, Volume - 1, Issue - 2, 2012 55 From bus To bus Resistance (ohm) Reactance (ohm) Line charging 8 15 0.864 0.751 0.000 15 16 1.374 0.774 0.000 16 17 1.374 0.774 0.000 6 18 0.864 0.751 0.000 18 19 0.864 0.751 0.000 19 20 1.374 0.774 0.000 6 21 0.864 0.751 0.000 3 22 0.444 0.439 0.000 22 23 0.444 0.439 0.000 23 24 0.864 0.751 0.000 24 25 0.864 0.751 0.000 25 26 0.864 0.751 0.000 26 27 1.374 0.774 0.000 1 28 0.279 0.015 0.000 28 29 1.374 0.774 0.000 29 30 1.374 0.774 0.000 Table II Active power, reactive power and bus voltages Bus No. Active Power (kW) Reactive Power (k (kVAR) Bus voltages (pu) 0 ----- ----- 1.0000 1 ----- ----- 0.9702 2 572.0 174.0 0.9628 3 ----- ----- 0.9478 4 936.0 312.0 0.9307 5 ----- ----- 0.9157 6 ----- ----- 0.8939 7 ----- ----- 0.8783 8 ----- ----- 0.8628 9 189.0 63.0 0.8527 10 ----- ----- 0.8433 11 336.0 112.0 0.8339 12 657.0 219.0 0.8258 13 ----- ----- 0.8201 Bus No. Active Power (kW) Reactive Power (k (kVAR) Bus voltages (pu) 14 729 243.0 0.8173 15 477.0 159.0 0.8591 16 549.0 183.0 0.8554 17 477.0 159.0 0.8537 18 432.0 144.0 0.8901 19 672.0 224.0 0.8873 20 495.0 165.0 0.8856 21 207.0 69.0 0.8934 22 522.0 174.0 0.9419 23 1917.0 639.0 0.9366 24 ----- ----- 0.9310 25 1116.0 372.0 0.9254 26 549.0 183.0 0.9223 27 792.0 264.0 0.9197 28 882.0 294.0 0.9688 29 882.0 294.0 0.9631 30 882.0 294.0 0.9603 R EFERENCES [1] L. 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