Enhanced Genetic Algorithm Approach for Security Constrained Optimal Power Flow Including FACTS Devices

Abstract
his paper presents a genetic algorithm basedapproach for solving security constrained optimal power flowproblem (SCOPF) including FACTS devices. The optimal location ofFACTS devices are identified using an index called overload indexand the optimal values are obtained using an enhanced geneticalgorithm. The optimal allocation by the proposed method optimizesthe investment, taking into account its effects on security in terms ofthe alleviation of line overloads. The proposed approach has beentested on IEEE-30 bus system to show the effectiveness of theproposed algorithm for solving the SCOPF problem.
Keywords—Optimal Power Flow, Genetic Algorithm, FlexibleAC transmission system (FACTS) devices, Severity Index (SI),Security Enhancement, Thyristor controlled series capacitor (TCSC).
I. INTRODUCTION
N any power system, unexpected outages of lines ortransformers occur due to faults or other disturbances.These events, referred to as contingencies, may causesignificant overloading of transmission lines or transformers,which in turn may lead to a viability crisis of the power system.The principle role of power system control is to maintain asecure system state, i.e., to prevent the power system, movingfrom secure state into emergency state over the widest range ofoperating conditions. Optimal Power Flow (OPF) is major toolused to improve the security of the system. The security of thesystem can be improved either through preventive control orpost contingency corrective action. Alsac and Stott [2] extendedthe penalty function method to security constrained optimalpower flow problem in which all the contingency caseconstraints are augmented to the optimal power flow problem.In this method the functional inequality constraints are handledas soft constraints using penalty function technique. Thedrawback of this approach is the difficulty involved in choosingproper penalty weights for different systems and differentoperating conditions which if not properly selected may lead toexcessive oscillatory convergence. This combined withprohibitively large computing time makes this methodunsuitable for online implementation. Apart from usingR.Narmatha Banu (narmi800[at]yahoo.co.in) is with Department ofElectrical & Electronics Engineering , Kalsalingam University, KrishnanKoil-626 190, India.D.Devaraj is with Department of Electrical & Electronics Engineering ,Kalsalingam University, Krishnan Koil-626 190, India.preventive approach for security enhancement, the postcontingency state corrective action can also be used for securityenhancement. The resulting stage has the same security level asthe usual security – constrained optimal power flow case withlower operating cost. The power electronics-based FACTSdevices can also be employed for corrective action due to itshigh speed of response. Thyristor controlled series capacitor(TCSC) is one such device which offers smooth and flexiblecontrol of the line impedance, with much faster responsecompared to the traditional control devices. TCSC can be usedeffectively in maintaining system security in case of acontingency; by eliminating or alleviating overloads along theselected network branches. It is important to ascertain thelocation for the placement of these devices due to theirconsiderable costs. In this paper, the location of the TCSC isidentified based on the overload index.The optimal base case control variables and the postcontingency TCSC settings are obtained as the solutions toSCOPF problem of minimizing over loaded lines for single lineoutages. The various formulation aim at either minimizing thetotal fuel cost or minimizing some defined objective function ieminimizing/alleviating the line overloads with system securityconstraints [1,2]. A number of mathematical programmingbased techniques have been proposed to solve the optimalpower flow problem. These include the gradient method [2-4],Newton method [5] and linear programming [6].The gradientand Newton methods suffer from the difficulty in handlinginequality constraints. To apply linear programming, the inputoutputfunction is to be expressed as a set of linear functionswhich may lead to loss of accuracy. Recently globaloptimization techniques such as the genetic algorithm havebeen proposed to solve the optimal power flow problem [8, 9].A genetic algorithm [10] is a stochastic search technique basedon the mechanics of natural genetics and natural selection. Inthis paper, Genetic Algorithm is used to solve the securityconstrained optimal power flow problem.The proposed algorithm solves the SCOPF problem subjectto the power balance equality constraints, limits on controlvariables namely active power generation, controllable voltagemagnitude pertaining to the base case, thyristor control seriescapacitor (TCSC) for contingency case studies. Theeffectiveness of the proposed approach is demonstrated throughpreventive and corrective control action for a few harmfulcontingencies in the IEEE -30 bus system

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