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Reliability assessment of protective relaying based on operation risk theory

WANG Qing-liang,HOU Yuan-bin,FU Zhou-xing,LI Zhong(Xi'an University of Science and Technology,Xi'an 710054,China)  
Based on the risk theory,a new reliability index is proposed considering correct inaction,failure events and the failure probability of protected equipment in protective relays.The economical losses resulted from failure events are taken into consideration in the method.A general 8-state Markov probability model of line protection is set up in which resist-operation failures are divided into extended state and no-extended state.Using the state space method,the reliability indices have been calculated.Studies have been conducted to ascertain the relation between the protection reliability and failure probability of equipment involving the economical loss of incorrect operation.Then,the optimal maintenance interval is derived by taking the minimum of risk assessments as target function for protective relays.The obtained interval is significant to predict the protection maintenance.The general and detailed reliability model with its algorithm was validated with the protective relays test.It proves a new idea and method to improve protection system and they can enhance the security of distribution power supply.
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