SUMMARYThe number of constraints in coordination of distance and directional overcurrent relays (D&DOCRs) in power systems increases significantly by considering changes in network topology, especially in large power systems. By increasing the number of coordination constraints, the complexity of the D&DOCRs coordination problem is increased. Therefore, in this paper, a new method is proposed to reduce the number of coordination constraints in the D&DOCRs coordination problem. In the proposed method, redundant constraints are identified before solving the coordination problem. The proposed method is independent from D&DOCRs settings and operating characteristics of directional overcurrent relays. In order to evaluate the proposed method, two different test systems, namely, an 8-bus and the IEEE 14-bus test systems, are used. Based on the presented results, it can be seen that the proposed method is efficient, and by utilizing it, many redundant coordination constraints are identified and removed, helping to obtain effective solutions for the optimal settings with less computational effort.
Recently, microgrid operation increased significantly with increasing distributed renewable energy resources in the power system. Microgrids can operate with and without utility. Fault currents are significantly different in island and utility connected operation modes. Therefore, microgrid protection is one of the important subjects in microgrid operation. In this paper, a hybrid particle swarm optimization (HPSO) approach has been developed for coordination of directional overcurrent relays (DOCRs) in a microgrid system. The coordination constraints include the utility connected and an autonomous condition of the microgrid operation. In the optimization procedure, the current setting (I set ) of relays is considered as discrete parameters and time multiplier settings (TMS) is assumed as continues parameter. The proposed algorithm has two parts, in the first part, PSO is used to calculate the I set and in the second part, linear programming is applied to calculate the TMS of each relay. In the case study, loads connected to the network are divided into critical and noncritical ones. In normal operation of the system, distributed generators (DGs) operate in parallel with the utility. When a fault occurs on the utility side, noncritical loads are disconnected from the network and DGs are operated in microgrid as islanded mode. Regarding to simulation results, DOCRs have a suitable and reliable operation in both conditions of microgrid operations. In addition, overall operating time of the primary relays is properly minimized.
Reliability of power system can be influenced by the maintenance strategy adopted for the system's equipments. This study focuses on power circuit breakers maintenance planning based on the concept of reliability-centered maintenance (RCM) and applies the proposed approach on the 400 kV transmission network in Khorasan province in Iran. The RCM strategy takes into account the technical condition and importance of each circuit breaker from the network's perspective. The technical condition index of a specific circuit breaker is a numerical representation of the physical health of the breaker and can be evaluated by some criteria. To determine this index, the performance of the circuit breakers during interval 2007-2012 is considered. The importance of each circuit breaker is related to the consequence of its failure on the network. As the main drawback of the available researches in this area is the lack of a generic tool to determine the importance of the circuit breakers, this study proposes a three-stage procedure to accurately evaluate the importance of the circuit breakers. The main idea in the proposed approach is to analyse all failure modes of the circuit breakers and simulate the effect of each failure mode on the overall expected energy not served. The obtained results show the efficiency of the proposed approach.
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