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Improvement of Performance of Transmission System Using Optimal Allocation of UPFC Device by GA and PSO Algorithms A. Haritha, Hameed.S.K Abstract: Power losses and voltage instability are major problems in present power systems. It has become more complex day by day due to less security and reliability. Flexible AC transmission systems (FACTS) controllers have been mainly used for solving various power system steady state control problems. Flexible AC transmission systems or FACTS are devices which allow the flexible and dynamic control of power systems and enhancement of system stability using FACTS controllers. UPFC is the one of the most important device in FACTS controllers. It is in multiple line compensation are integrated into a generalized power flow controller that is able to maintain prescribed, and independently controllable, real and reactive power flow in the line. Based on PSO algorithm is an effective method for finding the optimal choice and location of FACTS controllers. It also increases loadability of the line and minimizes losses. This paper presents comparative study of GA and PSO algorithms for one of the FACTS controller i.e., UPFC device. The suggested algorithm has been applied to 30 bus tested data. Keywords: — Voltage Stability, FACTS Devices, Optimal Allocation, Genetic Algorithm. I. INTRODUCTION Modern, power systems are prone to widespread failures. With increased loading of existing power-transmission systems, operation of power system becomes more complex and power system will become less secure. Operating environment, conventional planning and operating methods can leave systems exposed to instabilities. Voltage instability is one of the phenomena which have result in a major blackout. Besides, with the electricity market deregulation, number of unplanned power exchanges increases due to the competition among utilities and direct contracts concluded between generation companies and costumers. If these exchanges are not controlled, some lines may become overloaded. 1Haritha.A, M.Tech Student, Department of EEE, JNTUA, Anantapur/ Quba College of Engineering and Technology, Venkatachalam, SPS Nellore district, Andhra Pradesh, India, (email: [email protected]). 2Hameed.S.K, Associate Professor, HOD, Department of EEE, JNTUA/ Anantapur/ Golden Valley Integrated Campus (GVIC), Angallu, madanaplli, Chittor district, Andhra Pradesh, India, (email: [email protected]). Because many of the existing transmission lines could not cope with increasing power demand, the problem of voltage stability and voltage collapse has also become a major concern in planning and operation of deregulated power systems. So control of power flow in order to have more efficient, reliable, and secure system is in the interest of the transmission system operator (TSO). To overcome this problem, FACTS devices are introduced. FACTS devices can regulate the active and reactive-power control as well as adaptive to voltage magnitude control simultaneously by their fast control characteristics and their continuous compensating capability and so reduce flow of heavily loaded lines and maintain voltages in desired level. Besides, FACTS devices can improve both transient and small signal stability margins. Controlling the power flows in the network, under normal and abnormal conditions of the network, can help to reduce flows in heavily loaded lines, reduce system power loss, and so improve the stability and performance of the system without generation rescheduling or topological changes in the network [1]. Because of the considerable costs of the FACTS devices, it is so mementos to find out the optimal location for placement of these devices to improve voltage stability margins and enhance network security [2-7]. reliability and loadability has been studied according to proper control objectives [5-15]. Some of papers have been tried to find suitable location for FACTS devices to improve power system security and loadability [14-17]. Optimal allocation of these devices in deregulated power systems has been presented in [18-19]. Some of papers use heuristic approaches and intelligent algorithms to find suitable location of FACTS devices [16-19]. In [20], voltage stability index has been used to find the suitable location of UPFC to improve power system security/stability after evaluating the degree of severity of considered contingencies. This paper presents a novel heuristic method based on GA to find optimal location of multi-type FACTS devices to enhance voltage stability level considering investment cost these devices and power system losses. Genetic Algorithm is previously used for many optimization problems like optimal power flow, economic dispatch and controller optimization, congestion management and etc in power systems [21-23]. Proposed method is tested on IEEE 30 bus system and results are presented. 26]. Models integrated into transmission line for TCSC and UPFC and SVC is modeled is incorporated into the bus as shunt element of transmission line. Mathe-matical models for FACTS devices are implemented by MATLAB programming language. II. FACTS DEVICES MODEL TCSC. TCSC acts as the capacitive or inductive compensator by modifying reactance of transmission line. This changes line flow due to change in series reactance. A. FACTS Devices In this paper, three different FACTS devices have been selected to place in suitable location to improve voltage stability margins in power system. These are: TCSC (Thyristor Controlled Series Capacitor), SVC (Static VAR Compensator) and UPFC (Unified Power Flow Controller). These are shown in Fig. 1. In this paper TCSC is modeled by changing transmission line reactance as below: Xij = Xline + XTCSC ………………….. (2) XTCSC = rTCRC. Xline ………………… (3) where Xline is reactance of transmission line and is compensation factor of TCSC. Rating of TCSC is depended on transmission line where it is located. To prevent overcompensation, TCSC reactance is chosen between -0.7 Xline to 0.2 Xline. SVC: SVC can be used for both inductive and capacitive compensation. In this paper SVC is modeled as an ideal reactive power injection at bus: ∆Qi = ∆QSVC ……………….. (4) Figure: 1 Considered FACTS Devices (a) TCSC (b) SVC (c) UPFC. Power flow through the transmission line i-j namely is depended on line reactance, bus voltage magnitudes, and phase angle between sending and receiving buses. This is expressed by Eq. 1 ………………….. (1) TCSC can change line reactance and SVC can be used to control reactive power in network. UPFC is the most versatile member of FACTS devices family and can be applied in order to control all power flow parameters (i.e. line impedance, bus voltage, and phase angle). Power flow can be controlled and optimized by changing power system parameters using FACTS devices. So optimal choice and allocation of FACTS devices can result in suitable utilization in power system. UPFC: Two types of UPFC models are reported in papers [26-29]. One is coupled model [26] and other is decoupled model [27-29]. In the first type, UPFC is modeled with series combination of a voltage source and impedance in the transmission line. In decoupled model, UPFC is modeled with two separated buses. First model is more complex compared with the second one because modification of Jacobian matrix in coupled model is inevitable. While decoupled model can be easily implemented in conventional power flow algorithms without modification of Jacobian matrix elements, in this paper, decoupled model used for modeling UPFC in power flow study (Fig. 2). B. Mathematical Model of FACTS Devices In this paper steady state model of FACTS devices are developed for power flow studies. So TCSC is modeled simply to just modify the reactance of transmission line. SVC and UPFC are modeled using the power injection models [24- Figure:2 Decoupled model for UPFC Although UPFC can control the power flow, but cannot generate the real power. So: maximum voltage stability margin. For a given network, with the increase in load/generation, the voltage magnitude and angles change near maximum-power-transfer condition and the propensity of voltage-stability is to be close to unity, indicating that the system is close to voltage collapse. The L index gives a scalar number to each load bus. Among the various indices for voltage-stability and voltage-collapse prediction, the L index gives fairly compatible results. The L indices for given loads conditions are calculated for all the load buses and the maximum of the L indices gives the proximity of the system to voltage collapse. Pu1 + Pu2 = 0 ………………. (6) IV. PROPOSED GENETIC ALGORITHM UPFC controls power flow of the transmission line where is installed. To obtain UPFC model in load flow study, it is represented by four variables: Pu1, Qu1, Pu2, and Qu2. Assuming UPFC to be lossless, real power flow from bus i to bus j can be expressed as: Pij = Pu1 ……………. (5) Each reactive power output of UPFC, Qu1 Qu2can be set to an arbitrary value depend on rating of UPFC to maintain bus voltage. III. VOLTAGE STABILITY INDEX Consider a power network where n is the total number of buses with 1, 2, …, g generator buses, and g+1, …, n remaining (n – g) buses. For a given system operating condition, using the load-flow (state-estimation) results, the voltage-stability L index is obtained as [20, 30]: Genetic Algorithm (GA) is one of the most famous metaheuristic optimization algorithms witch is based on natural evolution and population. Genetics which is usually used to reach to a near global optimum solution. In each iteration of GA (referred as generation), a new set of string (i.e. chromosomes) with improved fitness is produced using genetic operators (i.e. selection, crossover and mutation). A. Chromosome’s structure Chromosome structure of GA is shown in Fig. 3. This involves rating and location of FACTS devices. …………………. (7) where j = g+1, …, n and all the terms inside the sigma on the right-hand side of (7) are complex quantities. The complex values of Fij are obtained from the Y-bus matrix of power system. For a given operating condition: Figure:3 A typical chromosome. B. Selection ……………………… (8) where IG, IL, and VG, VL, represent complex current and voltage vectors at the generator nodes and load nodes. [YGG], …[YGL], …, [YLL], and [YlG] are corresponding partitioned portions of the Y-bus matrix. Rearranging (8), ………………. (9) For stability, the index Lj must not be more than one for any of the nodes j. Hence, the global index L demon-strating the stability of the complete sub-system is given by L= maximum of Lj for all j (load buses). An L-index value far away from 1 and close to 0 indicates improved voltage stability. For an unloaded system with genera-tor/load buses voltages, the L for load buses are close to zero, indicating that the system has In proposed GA, method of tournament selection is used for selection [31-32]. This method chooses each parent by choosing (tournament size) players random-ly and choosing the best individual out of that set to be a parent. In this paper is chosen tntn.4=tn C. Cross Over Cross over allows the genes from different parents to be combined in children by exchanging materials between two parents. Cross over function randomly selects a gene at the same coordinate from one of two parents and assign it to the child. For each chromosome, a random number is selected. If this number is between 0.01 and 0.3, two parents are combined; else chromosome is transferred with no cross over. D. Mutation GA creates mutation children by randomly changing the genes of individual parents. In this paper, GA adds a random vector from a Gaussian distribution to the parents. For each chromosome, random number is selected. If this number is between 0.01 and 0.1, mutation process is applied; else chromosome is transferred with no mutation. E. FACTS devices Cost Function Using Siemens AG Database [33], cost function for SVC, TCSC and UPFC are developed as follows: And Particle Swarm Optimization (PSO) Optimization techniques are used to find the placement of Unified Power Flow Controller (UPFC) to reduce the power losses. Simulation studies were done for different scenarios in 30 bus tested data. Scenario 2: one TCSC is installed Scenario 3: one SVC is installed TCSC: ……….. (10) Scenario 4: one UPFC is installed Scenario 5: Multi-type (TCSC, SVC and UPFC) FACTS devices are installed. SVC: …………… (11) UPFC: ………. (12) Where s is the operating range of the FACTS devices in MVAR and CTCSC, CSVC and CUPFC are in (USD/kVAR). These cost functions are shown in Fig. 4. Power system considering cost function of FACTS devices. So these devices should be place to prevent congestion in transmission lines and transformers and maintain bus voltages close to their reference value. The first scenario is normal operation of network without installing any device. In second, third and forth scenario just installation of one device is considered. Each device is placed an optimal location obtained by GA introduced in Chapter IV. Multi-type FACTS devices installation is considered in 5th scenario. In this case three different kinds of FACTS devices (shunt, series and combinational compensation device) are used to place in optimal location to enhance voltage stability margin of power system. Power losses with and without UPSC shown in below figure. Power Losses M 33 30 27 24 21 18 15 129 6 3 0 Without UPFC UPFC with GA Voltage stability index introduced in Chapter III, were used in objective function considering cost function of FACTS devices and power system losses. Fitness function is expressed as below: ……… (13) Figure: 5 Real power losses with and without UPFC TABLE I REAL POWER LOSSES WITH AND WITHOUT UPFC Scenarios Total power losses (MW) Without UPFC 29.3900 UPFC GA UPFC PSO 23.0900 22.8591 The coefficient a1 to a3 are optimized by trial and error to 2.78, 0.1 and 2.05 respectively. V. SIMULATION RESULTS In this project, MATLAB code have been developed for Load flow analysis and by using Genetic Algorithm (GA) This results show the power losses at different Scenarios by using GA and PSO. [7] S. Sung-Hwan, L. Jung-Uk, M. Seung-Il, “FACTS Operation Scheme for Enhancement of Power System Security,“ in Proc of IEEE Power Tech Conference, Bologna, 2003, Vol. 3, pp. 36-41. [8] K. Sun-Ho, L. Jung-Uk, M. Seung-Il, “Enhancement of Power System Security Level Through the Power Flow Control of UPFC,” in Proc. of the IEEE Power Engineering Society Summer Meeting, 2000, Vol. 1, pp. 38 – 43. [9] A. Kazemi, H.A. Shayanfar, A. Rabiee, J. Aghaie, “Power System Security Improvement using the Unified Power Flow Controller,“ in Proc.of the IEEE Power India Conference, 2006, pp. 1-5. Figure 6: Voltage profile with UPFC and without UPFC using GA The above figure show the comparison the Voltage profile in between UPFC GA and without UPFC. Here we absorbed, after placement of UPFC with GA voltage profile improved . Figure 7: Voltage Profile without UPFC and UPFC using PSO. The above figure shows the comparison the voltage profile in between UPFC PSO and without UPFC. Here we absorbed, after placement of UPFC with PSO voltage profile improved. VI. CONCLUSION In this paper, performance of transmission system with UPFC device is studied using Particle Swarm Optimization (PSO) and Genetic Algorithm (GA). It is concluded that with PSO the performance studies in terms of voltage profile and power loss are more accurate compared with GA. Using PSO accurate results can be obtained. Case Study was conducted on 30 bus tested data. So it says that performance of transmission system with UPFC device should be increased in PSO algorithm. REFERENCES [1] F.D. Galiana, K. Almeida, M. Toussaint, J. Griffin, and D. Atanackovic: “Assessment and Control of the Impact of FACTS Devices on Power System Performance”, IEEE Trans. Power Systems, Vol. 11, No. 4, Nov 1996 [2] N.G. Hingurani, L. Gyugyi, Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems, IEEE Press, New York, 2000. [3] M. Noroozian, L. Angquist, M. Ghandhari, G. Anderson, "Improv-ing Power System Dynamics by Series-connected FACTS De-vices," IEEE Trans. on Power Delivery, Vol. 12, No.4, October 1997. [4] M. Noroozian, L. Angquist, M. Ghandhari, "Use of UPFC for Optimal Power Flow Control," IEEE Trans. on Power Delivery,Vo1.12, No.4, October 1997. [5] R. Billinton, M. Fotuhi-Firuzabad, O.S. Faried, S. Aboreshaid, "Impact of Unified Power Flow Controllers on Power System Reliability," IEEE Trans. on Power Systems, Vo1.15, No.1, February 2000. [6] J.A. Momoh, J. Zhu, G.D. Boswell, S. Hoffman, "Power System Security Enhancement by OPF with Phase Shifter," IEEE Trans. on Power Systems, Vol. 16, No.2, May 2001. About Authors A.Haritha received the B.Tech Degree in Electrical and Electronics Engineering from Siddharth Institute of Engineering and Technology, puttur, University of JNTUA in 2010.She is currently working towards the Master’s Degree in Electrical Power Systems, in Quba College of Engineering and Technology, University of JNTUA. Her interest lies in the areas FACTS, Control Systems. Hameed.S.K Received B.Tech and M.Tech Degrees in Electrical and Electronics Engineering from Kakatheya University and NIT durgapur in 2000 &2002 respectively. Currently He is a Associate Professor and HOD in the Department of Electrical and Electronics Engineering at Quba College of Engineering and Technology –SPS Nellore. His current interests include FACTS and Neural networks