International Journal of Smart Grid and Clean Energy Coordination of directional overcurrent relays for distribution system using particle swarm optimization Deepak Vyas , Praghnesh Bhatt ,Vipin Shukla Pandit Deendayal Petroleum University, Raisan, Gandhinagar, Gujarat, India Abstract The protection of electrical distribution network with multiple loops and bidirectional power flow needs optimal coordination of directional overcurrent relays (DOCRs). The DOCRs coordination is highly non-linear and largely constrained optimization problem where plug settings (PS) and time dial settings (TDS) of DOCRs are set as control variables. The objective function in this paper is based on minimizing the operating time of all primary DOCRs considering far-end and near-end fault approach. The optimization is performed with particle swarm optimization (PSO) for standard test systems of 3-bus, 4-bus and 6-bus and the results are compared with the existing methods of relay coordination reported in literature. The optimized PS and TDS for DOCRs has resulted in the least value of objective function along with proper coordination time interval for all test systems. Keywords: Directional overcurrent relay coordination, coordination time interval, optimization, particle swarm optimization 1. Introduction Directional overcurrent relays (DOCRs) are required for the protection of feeders at sub-transmission and distribution level which carry bidirectional power flow. The relaying system must be selective, sensitive, reliable and fast in its operation to isolate the faulty section of electrical network Selectivity of relaying systems identifies the primary and backup relays. Backup relays will operate only in case of failure of primary relay with proper coordination time interval. The problem of relay coordination requires the knowledge of maximum possible load and fault current for each possible fault location to cover whole network with primary and backup overcurrent protection. The optimal relay coordination guarantees the correct relay operating sequence with the least fault clearing time for each fault location. DOCRs coordination has been obtained by various optimization techniques in literature. Linear programming (LP) is one of the popular techniques to solve this problem. LP formulates the problem in linear form and solved with the help of simplex methods [1-3]. In LP technique, PS requires to be assumed while allowing TDS as a linear function which will determine time of operation (TOP) for each relay. In [4], Random Search Technique (RST) is applied to achieve optimal relay settings. To optimize both TDS and PS, Sequential Quadratic Programming was proposed in [5-6]. Relay coordination is formulated by Mixed integer nonlinear programming problem and optimized by different variants of PSO in [7-11]. Various evolutionary methods such as Differential Evolutionary (DE) algorithm and its modified variants [12-15], Seeker algorithm [16], Teaching Learning Based Optimization (TLBO) [17- 18], Ant colony algorithm [19], Group search optimization method [20], Chaotic firefly algorithm [21] and real coded genetic algorithm (RGA) with bounded exponential cross over and power mutation [22] have been reported to solve this highly constrained optimization problem of DOCRs coordination. Hybrid methods which are the combination of two or more methods such as GA-LP [23], GA-NLP [24], Gravitational Search Algorithm (GSA)-SQP [25], PSO-GSA [26], Biogeography-Based Optimization * Manuscript received May 6, 2019; revised January 17, 2020. doi: 10.12720/sgce.9.2.290-297