PROTECTION COORDINATION IN TRANSMISSION AND DISTRIBUTION SYSTEMS

Abstract

Proper protection coordination is one of the inherent requirements to run a distribution system with highest reliability. It is known that most of the faults of power system occur in distribution system. Therefore, if the protection schemes are not coordinated properly then many consumers have to suffer unnecessary outages. For reliable operation of the distribution system, its primary protection scheme must react to a fault as quickly as possible to isolate the faulty parts from the healthy parts, but if primary protection fails only then its backup protection scheme should operate. This condition is the most desired feature of any protection system as primary protection removes only faulted part whereas the operation of the backup protection isolates a larger portion of the system. Commonly, directional overcurrent relays (DOCRs) are used for issuing the trip signal for circuit breakers (CBs) to isolate the faulted section in distribution or sub-transmission networks. In ring or multiple source power distribution networks, DOCRs are necessary. The operation of DOCRs depends on its two parameters, namely time multiplier setting (TMS) and plug setting (PS). The time gap between operations of primary protection and its corresponding backup protection, known as coordination time interval (CTI), can be achieved by optimum settings of TMS and PS of all the DOCRs used and thus a proper protection coordination can be obtained. In general, optimum settings of the relays are obtained by solving protection coordination problem as an optimization problem where the objective is to minimize the sum of the operating times of all the relays in their primary mode of operation while maintaining coordination constraints among all relays. In radial distribution systems, the feeders are protected by reclosers and fuses. Fuses are placed at feeders which are at more remote position from substation. Whenever a transient fault occurs in any feeder, the corresponding fuse does not melt because the recloser’s fast operation allows the transient fault to self-clear. But, whenever a permanent fault occurs, the concerned fuse must melt just before the last delay trip of the recloser in order to prevent the loads between the recloser and the fuse from being interrupted. Therefore, there must be a proper coordination between the operation of recloser and fuses. Moreover, the increasing penetration of distributed generation (DG) in distribution systems has added more complexity to the protection coordination problems i (as presence of DG changes the magnitudes and directions of the short circuit currents in the distribution system). Thus, an appropriately coordinated protection scheme (in absence of DG) may not be able to perform its coordination function correctly, in the presence of DGs. The protection coordination problem of DOCRs is formulated as either linear programming (LP), nonlinear programming (NLP) or mixed-integer nonlinear programming (MINLP) problems and various optimization techniques are applied to solve these problems. In LP formulation, the PS of the relays are assumed to be known and the sum of operating times of the relays are expressed as a linear function of the TMS of the relays. When both PS and TMS are to be determined simultaneously, the coordination problem becomes an NLP problem. Further, when it is necessary to find discrete optimum values of PSs then this problem is termed as an MINLP one. Normally, electromechanical/static relays need discrete values whereas numerical/digital relays need continuous values of variables. Both analytical as well as meta-heuristic optimization algorithms have been used for solving this problem. As this problem is very complex, meta-heuristic algorithms have been preferred as they are independent of the nature of problem formulation and the types of variables. Now, because of the availability of several meta-heuristic optimization methods for coordination of DOCRs, it is a natural curiosity to find the most effective meta-heuristic optimization method for practical implementation. Further, in the medium and large interconnected distribution systems, it is necessary to minimize the operating times of primary and the corresponding backup relays without any mis-coordination in reasonable time. To achieve these requirements, it is necessary to develop a new formulation which also minimizes the operating times of backup relays along with the operating times of primary relays without any constraint violation. Also, protection coordination of DOCRs needs to be obtained which should be robust enough to protect the systems under various system operating conditions such as (N-1) contingency. Initially, a comparative study of different meta-heuristic optimization approaches, which had been proposed in the literature for DOCRs, is presented. Towards this goal, five most effective meta-heuristic optimization approaches such as genetic algorithm (GA), particle swarm optimization (PSO), differential evolution (DE), harmony search (HS) and seeker optimization algorithm (SOA) have been considered. The performances of these optimization methods have been investigated on several power system networks of different sizes. The comparative performances of these methods have been studied by executing each method 100 times with the same initial conii ditions and based on the obtained results, the best meta-heuristic optimization method for solving the coordination problem of DOCRs is identified. Subsequently, for minimizing the operating times of primary and backup relays simultaneously, a new objective function (NOF) has been developed. In the proposed formulation, different types of relays (electromechanical, static and numerical) with different characteristic curves (IDMT, VI and EI) have been considered. As a result this problem becomes a MINLP one because of discrete nature of PS values of electromechanical and static relays. Further, to solve this MINLP problem, two interior point method (IPM) based algorithms have been developed. Both these algorithms are two phase optimization techniques and are named as IPM-BBM and IPM-IPM, respectively. In the first phase of both the methodologies, IPM is used to obtain continuous values of TMSs and PSs of DOCRs. In the second phase of IPM-BBM technique, branch and bound method (BBM) and in the second phase of IPM-IPM technique, IPM is used to obtain final settings (continuous TMS values and discrete PS values) of DOCRs. The effectiveness of the proposed solution methods and the developed objective function has been investigated on three power system networks of different sizes. The suitability of the proposed method for coordination of DOCRs in meshed networks has been established by comparing its performance with that obtained by GA, DE and two hybrid algorithms (IPM with GA and DE) for the developed objective function. Also, the superiority of the developed objective function has been established by comparing the protection coordination results obtained by using NOF with those obtained by the other objective functions reported in the literature. Further, a contingency constrained robust protection coordination scheme of DOCRs is discussed. The robust protection coordination scheme provides single settings of DOCRs which will be valid for the credible (N-1) topologies created after outage of any element. For selecting the feasible contingencies, a composite security index (CSI) has been used. The robust protection coordination scheme has been posed as an optimization problem and solved using an IPM based algorithm. The feasibility of the proposed formulation and solution algorithm has been demonstrated on three power system networks of different sizes. For the protection of radial distribution networks, an optimum recloser-fuse coordination scheme is proposed in the presence of DGs. The proposed approach formulates optimum recloser-fuse coordination problem as an optimization problem and applies IPM based algorithm to solve this iii optimization problem for obtaining the optimum recloser and fuse settings. The proposed scheme gives a single set of settings of the reclosers and fuses which is robust enough to coordinate the operations of the reclosers and fuses properly with and without the presence of single/multiple DGs in the system. The proposed approach has been tested on two radial distribution systems for three different scenarios: i) no DG in the system, ii) a single DG in the system and iii) multiple DGs in the system. The test results prove the robustness and effectiveness of the presented scheme. Finally, an optimum recloser-fuse coordination in reconfigurable radial distribution systems in the presence of DGs is proposed. In the proposed scheme, the problem of recloser-fuse coordination in reconfigurable radial distribution networks has been formulated as an optimization problem. The formulated recloser-fuse coordination problem has been solved using IPM based algorithm. In order to obtain all possible reconfigurable radial networks, a new graph theory based approach has been developed. The proposed approach has been applied to obtain optimum recloser-fuse settings for two radial distribution systems in the presence of DG. Further, to test the effectiveness of the proposed approach, cases of mis-coordination have been analyzed in all feasible configurations of the radial systems in the presence of DGs at single and multiple locations. The test results prove the effectiveness of the presented scheme.