Optimal TCSC and SVC Placement for Voltage Profile Enhancement and Loss Minimization Using Bee Colony Optimization

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1 Optimal TCSC and SVC Placement for Voltage Profile Enhancement and Loss Minimization Using Bee Colony Optimization Sudipta Das, Biswa Ranjan Kuanr,Niladri Chakraborty Department of Power Engineering Jadavpur University Kolkata, India Abstract This paper elaborates a research work that proposes a methodology to reduce transmission losses in a power system and improve voltage stability by incorporating two commonly used Flexible AC Transmission devices (FACTS) known as Thyristor Controlled Series Compensators (TCSC) and Static Var Compensator (SVC). In order to determine the optimum size and location of FACTS devices a Bee Colony Optimization (BCO) based technique is proposed. In order to validate the proposed methodology, it is implemented on standard IEEE-14 bus system. A thorough analysis of obtained result established the potential of FACTS devices in reducing transmission losses and improving voltage profile. The result also demonstrates worth of BCO as a global optimizer while dealing with complex optimization problem. Keywords loss minimization; voltage profile enhancement; bee colony optimization; thyristor controlled series compensator, staitc var compensator. I. INTRODUCTION An ever increasing power demand and restructured power systems has made minimization of loss and power quality two important aspects that needs to be taken care of. It is more pragmatic to meet a fraction of increased load by loss minimization than satisfying the increased load fully from generation increment. In a restructured power scenario consumers not only want cheap power but also good power quality as they have multiple options to buy power from vendors. A uniform voltage profile is a must for good power quality. Both of the above objectives can be attained to a greater extent by integrating FACTS devices into power distribution systems. The concept of FACTS, which is a widely used terminology for achieving higher controllability in power system by implementing power electronics devices, was introduced byhingorani[1].these devices have got capabilities to manage power flow, reduce transmission loss, satisfy voltage security constraints and increase system efficiency and security.they are also very useful for enhancing stability of the network and improving dynamic behavior of the system.all these advantages of FACTS devices are primarily due to their fast response to disturbances in urgent circumstances and flexibility in normal operating conditions. Thyristor Controlled Series Compensators (TCSC) is one of the most popular FACTS devices that has got wide application in power system engineering. In several research works TCSC has been successfully implemented to mitigate power system oscillations and enhance its stability [2-4].Gitizadeh et al. successfully utilized TCSC to minimize voltage deviation as well as system cost [5]. Sizing and optimal placement of TCSC in this case is achieved by applying Multi-objective artificial bee colony optimization technique. They also studied the effect of switching loss on system performance. In another research work TCSC is also used for improving static voltage stability [6]. In this work line stability index is used as an indicator of system stability. Optimal placement of TCSC has been accomplished by optimizing a parameter known as line stability index using particle swarm optimization with time varying acceleration co-efficient (PSO - TVAC). TCSC is also used for transmission loss minimization [7] and congestion management elsewhere [8]. Another FACTS device that has got several applications in power system engineering is Static Var Compensator (SVC). Dixit et al. successfully integrated SVC to IEEE-30 bus power system to minimize real power loss and voltage deviations at load bus [9]. They used continuous genetic algorithm (CGA) as an optimization tool for optimal placement of SVC.In another work optimal SVC placement is carried out in IEEE-14 bus and IEEE-57 bus systems by using ant colony optimization to achieve four objectives: minimization of real power loss and FACTS device cost, improvement in loadability and voltage profile of the system [10]. Several other applications of SVC includes congestion management [8, 11], enhancing security margin and voltage profileenhancement while reducing congestion [12].Again reducing small signal oscillations in multi-machine power network [13], enhancing voltage stability under critical line outage contingency [14]etc was addressed by other research groups. Though FACTS devices have many advantages as discussed above, its high cost due to incorporation of sophisticated power electronics devices is a matter of concern. In order to maximize the economical benefit optimal placement and sizing of FACTS devices is a must. In last two decades techniques based on swarm intelligence optimization such as particle swarm optimization (PSO), ant colony /14/$ IEEE

2 optimization (ACO), bat algorithm (BA) and bee colony optimization (BCO) have been successfully applied to solve many complex engineering optimization problem. Bee colony optimization is a population based meta-heuristics that imitates the mutual cooperation shown by bees in food foraging process. This is a very powerful and efficient optimization technique which has already been implemented to several complex optimization problems such as load dispatch problems [15], congestion management [16], PID controller parameter tuning [17] and many others [18-20].As long as optimal placement of FACTS devices are concerned several meta-heuristics such as bacteria foraging optimization [3], firefly algorithm [7], genetic algorithm [9] andant colony optimization [10] has already been successfully implemented. However, implementation of bee colony optimization for optimal placement of FACTS devices is very rare. Hence, in this work bee colony optimization is preferred as the optimization tool. In this work two widely used FACTS device TCSC and SVC are incorporated in IEEE-14 bus test system, to reduce the transmission loss as well as voltage deviation in buses. As there are two objectives of the FACTS placement, acombined objective function is formulated by weight factor approach. Then bee colony based optimization technique is implemented to solve the optimization problem. The result thus obtained is presented and analyzed in a lucid manner. From the result it is observed that integration of FACTS device reduces the system loss and improves the voltage profile to a great extent. Results also established the superior computational efficiency of bee colony algorithm. II. FACTS DEVICE MODELING In this work line power flow, losses in line, bus voltages are computed by Newton-Rapson load flow technique. In order to introduce facts devices in the system their effects on various parameters such as line reactance, reactive power injection at buses, change in phase angle should be mathematically modeled. A. Modeling of TCSC TCSC is a series compensation component that controls power flow in transmission line by changing effective line impedance. A TCSC consists of a series of capacitor banks connected in parallel to a thyristor controlled reactor. As a TCSC is capable to provide both capacitive and inductive compensationit can both decrease and increase effective line impedances. The compensation provided by TCSC varies between 20% inductive to 80% capacitive[21]. Equivalent impedance of the line connecting bus i to bus j in which TCSC is placed can be expressed as: = + (1) Where denotes the impedance of the uncompensated line and denotes the amount of compensation provided by TCSC. B. Modeling of SVC Basically a static Var compensator consists of a thyristor controlled reactor parallel with a capacitor bank. By changing firing angles of thyristor compensation provided by SVC can be varied. Like TCSC, SVC also provides both inductive and capacitive compensation. It either injects or withdraws reactive power from the bus to which it is connected and there by maintain the voltage magnitude at the point of connection.the expression for current drawn or injected by SVC is given as: = (2) Where is the current drawn by SVC, is the susceptance of SVC and is the bus voltage at the point of connection. Reactive power injected or absorbed by SVC can be expressed as: = (3) In this study SVC is configured as a variable susceptance which can both inject and absorb maximum upto 100 MVar [22]. III. PROBLEM FORMULATION In the present work objective is to minimize transmission loss in the network as well as voltage deviation at buses. Hence, both should be represented in the objective function which has to be optimized. A. Minimization of voltage deviation In order to obtain a uniform voltage profile summation of voltage deviation is to be minimized. In this work voltage stability index is used as a criterion to minimize the voltage deviation. Voltage stability index (P) of a network is defined as the net voltage deviation of each bus in the net work from unity. In a n-bus system Voltage stability index (,) is mathematically expressed as: = 1 (4) Where is voltage magnitude of i th bus. B. Minimization of system loss Redistribution of reactive power in network occurs primarily because of transmission loss. So transmission loss minimization brings in change in real power generated by slack bus. The mathematical expression for calculating losses in a network is given as: = + 2 (5) Where is real power loss, represents conductance of l th transmission line, nl represents the number of transmission line, and denotes voltages at bus iand j and denotes power angle. C. Formulation of combined objective function Basically a multi-objective optimization problem takes multiple objectives into consideration that needs to be optimized simultaneously. In the present work, multiple objectives of minimization of loss and voltage deviations are linearly combined to formulate an objective function which is given below.

3 = + (6) In the above expression is the combined objective function and, denotes weight factor. Some of weight factor should be unity. For minimization of voltage deviation should be set at 1 and should be set to 0 while for minimization of transmission loss should be set at 0 and should be set to 1. For simultaneous optimization of both voltage deviation and power loss both weight factors should be set at zero. D. Constraints of the problem Above optimization problem is subjected to various constraints such as constraints of load flow, voltage constraints, reactive power generation limit and FACTS device constraints. All these constraints can be further divided into equality constraints and inequality constraints. =1,. =1,. Where NB represents number of buses, and represents active and reactive power demand at bus i, and denotes active and reactive power generation at bus iand, stand for transfer conductance and susceptance between bus i and bus j. Active power generation and reactive power generation constraints and voltage limit constrains are inequality constraints which are mathematically expressed below. (9) (10) (11) Where,, represents minimum permissible active power generation, reactive power generation and bus voltage at bus i,,, denotes maximum permissible active power generation, reactive power generation and bus voltage at bus i (12) (13) Where denotes the impedance of the uncompensated line, denotes the amount of compensation provided by TCSC and denotes the reactive power injected or withdrawn by SVC at the point of connection. IV. IMPLEMENTATION OF BEE COLONY ALGORITHM FOR OPTIMIZATION Bees behavior in nature is basic building block of bee colony optimization (BCO)technique [17-20]. Naturally, bees perform dances that inform other bees about the availability of food and distance of the patch from nest. The primary motivation behind BCO is to create a multi-agent system that will successfully and efficiently solve various complex combinatorial optimization problems. In BCO a population comprising of artificial bees search for The equality constraints are basic load flow equations which are the given optimal below. solution in search space. Every artificial bee represents the quality of the solution via objective function value.one new solution to the optimization problem is cos sin = 0 (7) generated from each bee.this algorithm operates in two phase known as forward pass and backward pass. During the forward sin cos =0 pass each bee explores search space. Every forward pass consists of a few constructive moves that improve the quality (8) of solution. After obtaining new partial solutions bees go back to the nest and backward pass starts in which bee shared information regarding their new solution. During backward pass every bee has to take a decision within a certain probability whether to become an uncommitted follower by rejecting the created partial solution or become a recruiter before moving back to the created partial solutions. In the next step by Roulette-wheel every follower chooses a recruiter and next forward pass starts. Another inequality constraint arises due to the existence of upper and lower limit of compensation provided by FACTS devices. As in this work two FACTS devices TCSC and SVC are used there will be two constraints due to their compensation limits which is expressed below. Fig. 1. Flow chart for application of BCO for optimal placement and sizing of FACTS devices

4 Flow chart for implementation of BCO algorithm to solve the present optimization problem is given in Fig. 1. For every FACTS device there will be two decision variable: location and size. Hence, number of decision variable in a bee should be twice the number of FACTS devices incorporated in the system. Number of artificial bees in bee population should be 5 to 10 times of number of decision variable. So if a single SVC or TCSC to be incorporated number of decision variable is 2 and number of bees in the population should be in between 10 to 20. Incase, when both SVC and TCSC are to be incorporated number of decision variable increases to four and number of bees in population should be increased accordingly. V. SIMULATION AND RESULT In order validate the proposed technique it is applied on standard IEEE-14 bus system [7]. Programs were developed with help of MATLAB 8.1 computational environment and executed in a PC with 4GB RAM and Intel corei5 2.3GHz processor. In order to judge the effects of FACTS devices on system loss and voltage deviation, corresponding values of these parameters need to be determined. By carrying Newton- Raphson load flow it is found that for uncompensated IEEE- 14 bus system transmission loss and voltage deviation stands at pu and pu respectively. A. Optimal Placement of TCSC Optimal location and magnitude for a single TCSC to be installed in IEEE-14 bus system is obtained for three different conditions: minimization of loss, minimization of voltage deviation and minimization of both. Corresponding values are presented in table I. Individual line losses and bus voltage for simultaneous minimization of loss and voltage deviation is shown in Fig. 1 and Fig. 2 respectively. From these figures it should be observed that line losses in line 1-5 is much more in comparison to other line while in several other lines such as line 7-9 loss is almost zero. This is because resistance of line 1-5 is comparatively higher than other lines and these lines carry much more load than others. Lines whose resistances are very low have power loss almost equal to zero. As long as bus voltage deviations are concerned bus 6 and bus 8 has got maximum voltage deviation. Fig. 2. Individual line losses for optimal placement of TCSC for voltage TABLE I. OPTIMUM MAGNITUDE AND LOCATION OF TCSC FOR DIFEERENT OPERATION REQUIREMENT Parameter to be minimized Location Magnitude Loss Voltage dev. Loss Line Voltage dev. Line Loss & Voltage deviation Line From the table it is observed that a minimum loss of pu will occur for a TCSC with capacitive compensation of magnitude pu in line 9 which connects bus 4 to bus 9. Similarly for minimum voltage deviation a TCSC providing a capacitive compensation of magnitude pu needs to be placed in series with line 1. For this case voltage deviation comes out to be pu with a corresponding loss of pu. In third case, where both loss and voltage deviation is to be minimized, optimum location is found out to be 16 while optimum size stands at pu. It should be observed that in case of minimum loss corresponding voltage deviation is higher than uncompensated system while for minimum voltage deviation scenario corresponding loss is higher than that of uncompensated system. However, while both are simultaneously optimized both values are found to be better than uncompensated system. Hence, it is prudent to simultaneously optimize both. Fig. 3. Individual bus voltage for optimal placement of TCSC for voltage B. Optimal Placement of SVC Optimal location and magnitude of SVC for different operating conditions are presented in Table II. TABLE II. OPTIMUM MAGNITUDE AND LOCATION OF SVC FOR DIFEERENT OPERATION REQUIREMENT Parameter to be minimized Location Magnitude (Mvar) Loss Voltage dev. Loss Bus Voltage dev. Bus Loss & Voltage deviation Bus

5 From the table it is observed that forminimization of loss SVC drawing a reactive power of Mvar should be connected to bus 9. This corresponds to a loss of pu which is pu less than uncompensated system. For minimization of voltage deviation optimum location of SVC is found out to be bus 8 while optimum magnitude stands at Mvar. This reduces voltage deviation from pu in uncompensated system to pu. In order to minimize loss and voltage deviation both, an SVC that injects a reactive power of pu should be connected to bus 6. This leads to a transmission loss of pu and voltage deviation of pu. Fig. 4 and Fig. 5 represent individual line losses and bus voltage for combined optimization of loss and voltage deviation. location and magnitudes of TCSC and SVC for different conditions are presented in Table III. For minimum loss scenario optimum loss comes out to be pu which is less than that of when either only TCSCor SVC is used to minimize the loss. For this case a TCSC of magnitude pu is to be placed in line 9 and an SVC injecting Mvar should be placed at bus 9. Minimum voltage deviation for simultaneous placement of TCSC and SVC comes out to be pu for a TCSC of magnitude pu in series with line 1 and an SVC of magnitude pu at bus 8. In this scenario also combined performance of TCSC and SVC is found to be better than either TCSC or SVC alone. For simultaneous optimization of loss and voltage deviation TCSC of magnitude pu has to be placed in line 16 and SVC providing a reactive power of Mvar should be placed at bus 6. This results in a optimized system loss of pu and voltage deviation of pu. Individual line loss and bus voltage for this scenario is presented in Fig. 6 and Fig. 7. Like previous cases in this case also losses in line 1-6 constitutes a major part of total system loss due to their high line impedance. However, voltage profile in this case is found to be uniform as out of 14 buses magnitude of voltage at 12 buses is found to be almost 1 pu. TABLE III. OPTIMUM MAGNITUDE AND LOCATION OF TCSC & SVC FOR DIFEERENT OPERATION REQUIREMENT Fig. 4. Individual line losses for optimal placement of SVC for voltage Parameter to be minimized Device Location Magnitude Loss Voltage dev. Loss TCSC Line pu SVC Bus Mvar Voltage TCSC Line pu dev. SVC Bus Mvar Loss & Voltage deviation TCSC Line pu SVC Bus Mvar Fig. 5. Individual bus voltages for optimal placement of SVC for voltage From the graph it is observed like TCSC here also a large portion of total system loss takes place in line 1-5 due to their high line resistances loss while loss in several other lines remains almost zero. It should be noted that in this case total system loss is higher than of corresponding value in case of TCSC while voltage deviation is lower than that of the corresponding value in TCSC. C. Simultaneous Placement of TCSC and SVC After individually placing TCSC and SVC they are placed simultaneously to improve the system performance. Optimized Fig. 6. Individual line losses for optimal placement of TCSC-SVC for voltage

6 Fig. 7. Individual line losses for optimal placement of TCSC-SVC for voltage VI. CONCLUSION In the present work a solution technique based on bee colony optimization is proposed to determine the optimal location and magnitude of FACTS devices such as TCSC and SVC to minimize transmission loss as well as voltage deviation.before applying the optimization technique, the biobjective optimization problem is first converted to a single objective optimization problem by weight factor approach. Result obtained by proposed approach for IEEE-14 bus system is presented and analyzed. From the result it is observed that installing TCSC and SVC of proper magnitude at locations identified by the proposed technique will reduce the transmission loss considerably while improving the voltage profile. ACKNOWLEDGMENT We would like to acknowledge and thank Jadavpur University, Kolkata, India for providing all necessary help to carry out this work. REFERENCES [1] N.G. Hingorani, I. 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