VIEWS: 35 PAGES: 12 CATEGORY: Business POSTED ON: 9/7/2013
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH ISSN International Journal of Advanced Research in Engineering and Technology (IJARET),IN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME ENGINEERING AND TECHNOLOGY (IJARET) ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) IJARET Volume 4, Issue 5, July – August 2013, pp. 164-175 © IAEME: www.iaeme.com/ijaret.asp ©IAEME Journal Impact Factor (2013): 5.8376 (Calculated by GISI) www.jifactor.com ENHANCEMENT OF VOLTAGE STABILITY USING STATIC SYNCHRONOUS SERIES COMPENSATOR (SSSC) WITH PI CONTROLLER-LLLG FAULT B. Suresh Kumar Assistant Professor, Department of Electrical Electronics Engineering, CBIT ABSTRACT This project presents the enhancement of voltage stability using Static Synchronous Series Compensator (SSSC). The continuous demand in electric power system network has caused the system to be heavily loaded leading to voltage instability. Under heavy loaded conditions there may be insufficient reactive power causing the voltages to drop. This drop may lead to drops in voltage at various buses. The result would be the occurrence of voltage collapse which leads to total blackout of the whole system. Flexible AC transmission systems (FACTS) controllers have been mainly used for solving various power system stability control problems. In this project, a static synchronous series compensator (SSSC) is used to investigate the effect of this device in controlling active and reactive powers as well as damping power system oscillations in transient mode. The transient mode is created by LLLG fault. The PI controller is used to tune the circuit and to provide the zero signal error. Simulations have been done in MATLAB/Simulink environment. Keywords: Static Synchronous Series Compensator (SSSC), Proportional-Integral, Real and Reactive Power Flow, Voltage Stability. I. INTRODUCTION In recent years, greater demands have been placed on transmission network and the increase in demands will rise because of the increasing number of non utility generators and heightened competition among utilities themselves. Increasing demands, lack of long term planning, and the need to provide open access electricity market for generating companies and utility customers, all of them have created tendencies toward less security and reduced quality of supply. The power systems of today, by and large, are mechanically controlled. There is widespread use of microelectronics, computers and high speed communications for control and protection of present transmission systems; however, when operating signals are sent to the power circuits, where the final power control action is taken, the switching devices are mechanical and there is little high speed control. Another problem with mechanical devices is that 164 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME control cannot be initiated frequently, because these mechanical devices tend to wear out very quickly compared to static devices. New installations of power stations and other facilities are primarily determined based on environmental and economic reasons. In addition, new transmission lines are expensive and take considerable amount of time to construct. Given these conditions, in order to meet ever-increasing load demands, electric utilities have to rely on power export/import arrangements through the existing transmission system, deteriorating voltage profiles and system stability in some cases. This situation has resulted in an increased possibility of transient, oscillatory and voltage instability, which are now brought into concerns of many utilities especially in planning and operation [1, 2]. Moreover, the trend of the deregulated power system has led to some unexpected problems, such as voltage instability, etc. In effect, from the point of view of both dynamic and steady state operation, the system is really uncontrolled. Power system planers, operators, and engineers have learned to live with this limitation by using a variety of ingenious techniques to make the system work effectively, but at a price of providing greater operating margins and redundancies. These represent an asset that can be effectively utilized with prudent use of FACTS technology on a selective, a needed basis. i.e. Proportional Integral Controller, Real and Reactive Power Flow, Voltage Stability. The FACTS devices (Flexible AC Transmission Systems) could be a means to carry out this function without the drawbacks of the electromechanical devices such as slowness and wear. FACTS can improve the stability of network, such as the transient and the small signal stability, and can reduce the flow of heavily loaded lines and support voltages by controlling their parameters including series impedance, shunt impedance, current, and voltage and phase angle. Controlling the power flows in the network leads to reduce the flow of heavily loaded lines, increased system load ability, less system loss and improved security of the system [3]. The static synchronous series compensator (SSSC) FACTS controller is used to prove its performance in terms of stability improvement. A Static Synchronous Series Compensator (SSSC) is a member of FACTS family which is connected in series with a power system. It consists of a solid state voltage source converter (VSC) which generates a controllable alternating current voltage at fundamental frequency. When the injected voltage is kept in quadrature with the line current, it can emulate as inductive or capacitive reactance so as to influence the power flow through the transmission line. While the primary purpose of a SSSC is to control power flow in steady state, it can also improve transient stability of a power system. Here PI controller is used to control the parameters of the power system [4]. II. BASIC OPERATIONAL PRINCIPLE OF SSSC Fig.1 Functional Model of SSSC 165 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME Fig.1 shows a functional model of the SSSC where the dc capacitor has been replaced by an energy storage device such as a high energy battery installation to allow active as well as reactive power exchanges with the ac system. The SSSC's output voltage magnitude and phase angle can be varied in a controlled manner to influence power flows in a transmission line. The phase displacement of the inserted voltage , with respect to the transmission line current , determines the exchange of real and reactive power with the ac system [5]. In Fig.2, it is assumed that the SSSC losses are zero and, therefore, the series injected voltage is in perfect quadrature with the line current, leading or lagging. The operating conditions that limit the SSSC operation from the power system point of view are also depicted inFig.2. The SSSC can increase as well as decrease the power flow in the transmission line by simply reversing the operation from capacitive to inductive mode. In the inductive mode, the series injected voltage is in phase with the voltage drop developed across the line reactance, thus, the series compensation has the same effect as increasing the line reactance. If the series inserted voltage magnitude is larger than voltage drop across the uncompensated line, i.e., , the power flow will reverse. This fact can limit the SSSC operation to values of , as in practice, it would be unlikely to use the SSSC for power reversal. Also, if the rating of the SSSC controller is high, it is possible to increase or decrease the receiving end voltage above or below the typical operating voltage range of 0.95p.u.– 1.05p.u, but with possible negative consequences for other system devices. The SSSC output current corresponds to the transmission line current, which is affected by power system impedance, loading and voltage profile, as well as by the actions of the SSSC. Thus, the relationship between the SSSC and the line current is complex. The fundamental component of the SSSC output voltage magnitude is, on the other hand, directly related to the dc voltage that is either constant or kept within certain limits, depending on the chosen design and control of the SSSC. The SSSC output voltage phase angle is correlated to the line current phase angle by plus or minus few degrees for example, to account for changes in the dc voltage. It has to be noted that the injected SSSC voltage is different from the SSSC output voltage ,due to the voltage drop or rise across the series transformer reactance, i.e., (1) Where, the negative sign corresponds to capacitive operation, while the positive sign corresponds to inductive operation of the SSSC and stands for the series transformer reactance. This voltage difference between the injected and output SSSC voltage can be small in the case of small transmission line currents, but it can be significant in high loading conditions [6]. Fig.2 Series Compensation by a SSSC 166 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME The active and reactive power exchanged between the SSSC and the transmission line can be calculated as follows (2) (3) Where, represents the angle between the injected SSSC voltage and transmission line current. Inspection of the equations (2) and (3), considering that the angle between the SSSC output voltage and line current is approximately 90o, shows that the SSSC real power should be small compared to the reactive power. This is expected, since the real power going into the SSSC is used only to cover for the losses and charging of the dc capacitor, i.e. (4) The losses in the SSSC circuit are due to the transformer windings and especially due to the switching of the GTO valves. III. CONTROL SYSTEM OF SSSC SSSC is similar to the variable reactance because the injected voltage and current to the circuit by this device are changing depend upon to the system conditions and the loads entering/getting out. For responding to the dynamic and transient changes created in system, SSSC utilizes the series converter. One side of the converter is connected to the AC system and the other side is connected to a capacitor and battery which in the system we assume DC source as battery. If a dynamic change in system will be occurred, SSSC circuit works such that according to the control circuit, the energy of battery will be converted to the ac form by converter and then injecting this voltage to the circuit the changes will be damped appropriately. For controlling the powers, first, sampling from the voltage and current is reactive powers of bus 2 are calculated using their voltage and current in dq0 references and compared with the determined reference and the produced error signal is given to the PI controllers. Adjusting parameters of the PI controllers, we are trying to achieve the zero signal error, such that powers can follow the reference powers precisely. Then, the output of the controllers are transformed to the abc reference and given to the PWM. Fig.3 Scheme of the SSSC Fig.3 shows a single line diagram of a simple transmission line with an inductive reactance, , connecting a sending end voltage source, Vs, and a receiving end voltage source, respectively [7]. 167 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME The real ad reactive power (P and Q) how at the receiving end voltage source given by the expressions (5) Q= (6) Where and are the magnitudes and and are the phase angles of the voltage sources and respectively. For simplicity, the voltage magnitudes are chosen such those = = V and the difference between the phase angles is (7) A SSSC limited by its voltage and current ratings, is capable of emulating a compensating reactance (both inductive and capacitive) in series with the transmission line inductive reactance .Therefore, the expressions for power flow given as (8) (9) Where is the effective reactance of the transmission line between its two ends, including the emulated variable reactance inserted by the injected voltage source of the Static Synchronous Series Compensator (SSSC). The compensating reactance is defined to be negative when the SSSC is operated in an inductive mode and positive when the SSSC is operated in a capacitive mode. PI controller will eliminate forced oscillations and steady state error resulting in operation of on-off controller and P controller respectively. However, introducing integral mode has a negative effect on speed of the response and overall stability of the system. IV. TWO MACHINE POWER SYSTEM MODEL The dynamic performance of SSSC is presented by real time voltage and current waveforms. Using MATLAB software the system shown in Fig.2, has been obtained [3]. In the simulation one SSSC has been utilized to control the power flow in the 500 kV transmission systems. Fig.4 Two machine Power System Model This system which has been made in ring mode consisting of 4 buses (B1 to B4) connected to each other through three phase transmission lines L1, L2-1, L2-2 and L3 with the length of 280, 150, 150 and 50 km respectively. System has been supplied by two power plants with the phase-to-phase voltage equal to 13.8 kV. Active and reactive powers injected by power plants 1 and 2 to the power system are presented in per unit by using base parameters =100 MVA and =500 kV, which active and reactive powers of power plants 1 and 2 are (24-j3.8) and (15.6-j0.5) in per unit, respectively. 168 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME Fig.5 Two Machine System with SSSC The SSSC is connected at bus 2. And the effect of SSSC in this transmission line was observed. The bus 2 is at the middle of the transmission line so that it was selected as candidate bus to connect the SSSC. The SSSC is connected to control the active and reactive powers. And also could fairly improve the transient oscillations of the system. The Static Synchronous Series Compensator (SSSC), one of the key FACTS devices, consists of a voltage-sourced converter and a transformer connected in series with a transmission line. The SSSC injects a voltage of variable magnitude in quadrature with the line current, thereby emulating an inductive or capacitive reactance. This emulated variable reactance in series with the line can then influence the transmitted electric power. In this the SSSC is used to damp power oscillation on a power grid following a three-phase fault. The power grid consists of two power generation substations and one major load centre at bus B3. The first power generation substation (M1) has a rating of 2100 MVA; representing 6 machines of 350 MVA and the other one (M2) has a rating of 1400 MVA, representing 4 machines of 350 MVA. The load centre of approximately 2200 MW is modeled using a dynamic load model where the active & reactive power absorbed by the load is a function of the system voltage. The generation substation M1 is connected to this load by two transmission lines L1 and L2. L1 is 280-km long and L2 is split in two segments of 150 km in order to simulate a three-phase fault (using a fault breaker) at the midpoint of the line. The generation substation M2 is also connected to the load by a 50-km line (L3). When the SSSC is bypass, the power flow towards this major load is as follows: 664 MW flow on L1 (measured at bus B2), 563 MW flow on L2 (measured at B4) and 990 MW flow on L3 (measured at B3). The SSSC, located at bus B2, is in series with line L1. It has a rating of 100MVA and is capable of injecting up to 10% of the nominal system voltage. This SSSC is a phasor model of a typical three- level PWM SSSC. On the AC side, its total equivalent impedance is 0.16p.u on 100 MVA. This impedance represents the transformer leakage reactance and the phase reactor of the IGBT bridge of an actual PWM SSSC. V. SIMULATION RESULTS WITHOUT SSSC First, power system with two machine and four buses has been simulated in MATLAB environment, and then powers and voltages in all buses have been obtained. The results have been given in Table.1. Using obtained results bus 2 has been selected as candidate bus to which the SSSC be installed. Therefore,the simulation results has been focused on bus 2. 169 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME Table.1 Voltage, Current, Active and Reactive Powers at all buses without SSSC Active power (p.u) Time (sec) Fig.6 Active Power of bus 2 wihout SSSC Reactive power (p.u) Time (sec) Fig.7 Reactive Power of bus 2 without SSSC Voltage (p.u) Time (sec) Fig.8 Voltage of bus 2 without SSSC 170 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME Current (p.u) Time (sec) Fig.9 Current of bus 2 without SSSC VI. SIMULATION RESULTS WITH SSSC Table.2 Voltage,Curent,Active and Reactive Powers at all buses with SSSC,when SSSC Connected at bus 2 Active power (p.u) Time (sec) Fig.10 Active Power of bus 2 with SSSC 171 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME Reactive power (p.u) Time (sec) Fig.11 Reactive Power of bus 2 with SSSC Voltage (p.u) Time (sec) Fig.12 Voltage of bus 2 with SSSC Current (p.u) Time (sec) Fig.13 Current of bus 2 with SSSC Table.3 Comparision between the Valuses of the System with and without SSSC,when SSSC Connected to bus 2 172 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME SSSC is controlling the active and reactive powers, beside these could fairly improve the transient oscillations of system. Obtained results of bus 2 had proven that the stability of power system parameters has been increased. When SSSC is connected at bus 4. The results are as follows. Time (sec) Fig.14 Active power of bus 4 with SSSC Reactive power (p.u) Time (sec) Fig.15 Reactive power of bus 4 with SSSC Voltage (p.u) Time (sec) Fig.16 Voltage of bus 4 with SSSC 173 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME Current (p.u) Time (sec) Fig.17 Current of bus 4 with SSSC Table.5.4 Comparison between the Values of the System with and without SSSC, when SSSC Connected at bus 4 CONCLUSION Accoding to the obtained results of bus 2 had proven that the stability of power system parameters has been increased and also observed that the transient stability of the system also increased. It has been found that the SSSC is capable of controlling the flow of power at a desired point on the transmission line. It is also observed that the SSSC injects a fast changing voltage in series with the line irrespective of the magnitude and phase of the line current. After installation of SSSC, besides controlling the power flow in bus 2, the transient oscillations of system also improved. When SSSC is connected at bus 4, it is observed that the power system parameters have been increased at bus 4 also. According to the obtained simulation results it is observed that SSSC is capable of controlling the flow of power at a desired point on the transmission line. It is also observed that by installing the SSSC, active power, reactive powers will be damper faster compare to the mode without SSSC. REFERENCES [1] B. H. Lee and K. Y. Lee, "A Study on Voltage Collapse Mechanism in Electric Power Systems," IEEE Transactions on Power Systems, Vol. 6, pp. 966-974, August 1991. [2] B. H. Lee and K. Y. Lee, "Dynamic and Static Voltage Stability Enhancement of Power Systems," IEEE Transactions on Power Systems, Vol. 8, No. 1, pp. 231-238, 1993. [3] N. G. Hingorani, L. Gyugyi, “Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems”, New York: IEEE Press, 2000. 174 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME [4] M. Faridi, H. Maeiiat, M. Karimi, P. Farhadi and H. Moslesh (2011) “Power System Stability Enhancement Using Static Synchronous Series Compensator (SSSC)” IEEE Transactions on Power System, pp. 387-391. [5] Kalyan K. Sen, (1998) “SSSC- Static Synchronous Series Compensator (SSSC): Theory, Modeling and Applications”, IEEE Transactions on Power Delivery, Vol. 13, pp. 241-246. [6] H. Taheri, S. Shahabi, Sh. Taheri and A.Gholami (2009) “Application of Static Synchronous Series Compensator (SSSC) on Enhancement of Voltage Stability and Power oscillation Damping”, IEEE Transaction on Power System, pp. 533-539. [7] B.N. Singh, A. Chandra, K.AI-Haddad and B. Singh (1999) "Performance of Sliding- mode and Fuzzy Controllers for a Static Synchronous Series Compensator", IEEE proceedings, Volume 146, No.2, pp. 200-206. [8] D. Bala Gangi Reddy and M. Suryakalavathi, “Availability Transfer Capability Enhancement using Static Synchronous Series Compensator in Deregulated Power System”, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 2, 2012, pp. 12 - 28, ISSN Print : 0976-6545, ISSN Online: 0976-6553. [9] Suresh J. Thanekar, Waman Z. Gandhare and Anil P. Vaidya, “Voltage Stability Assessment of a Transmission System -A Review”, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 2, 2012, pp. 182 - 191, ISSN Print: 0976-6545, ISSN Online: 0976-6553. [10] Champa Nandi, Sumita Deb and Minakshi DebBarma,, “Voltage Stability Improvement using Static Synchronous Compensator in Power System with Variable Load Impedance”, International Journal of Electrical Engineering & Technology (IJEET), Volume 1, Issue 1, 2010, pp. 108 - 117, ISSN Print : 0976-6545, ISSN Online: 0976-6553. 175