Research On Failure Detection And Maintenance Strategies Of Relay Protection

With the grid scale expansion and the network structure becoming more complex, the grid security is more dependent on the reliability of relay protection. Since the late 1990’s, microcomputer-based protection is widely applicated in power systems, which has significantly improved the correct action rate of relay protection. However, incorrect tripping events happen more frequently than desired due to the software defects, hardware failure, secondary circuit malfunction, and human error, and some of which even lead to power system outages and safety problems.Therefore, it is necessary to analyse the reason of incorrect tripping and put forward corresponding preventive measures to minimize the risk of incorrect tripping. Based on the analysis of the failure mechanism, the failure detection and fault tolerant technology is studied, and the main breakthrough and conclusion are as follows.(1) Relay failure mechanism including failure effect and failure cause is analized. A three-dimensional integrated maintenance strategy based on failure modes, effects and risk analysis (FMECA) is proposed. A health index evaluation model based on time and space information is developed to guide the maintenance decision. The model is consisted detection health factor, random failure heath factor, application environment heath factor, reliability health factor, improvement health factor. The present health status of individual equipment is assessed by using deterministic information and the future trend of health status is predicted by using probability and statistics information. The model overcoms the drawback that the individual equipment maintenance schedule cannot be determined by the status evaluation result obtained by other methods. A Markov state space model is established taking into consideration of manual inspection costs and the relationship between manual inspection cycle and maintenance economy costs is revealed. The problem has been solved that there is no sufficient theoretical basis for the determination of manual inspection cycle of relay devices under unattended mode.(2) The reason for the existence of failure detection blind spots of digital relay is revealed. A new fault tolerant technology is proposed by combining action characteristic matching and redundancy design. An action characteristics plane is established which includes the protective action zones, measurement anomaly and detection blocking zone. The Venn diagram is applied to analyse the match degree of failure detection and action discriminating characteristic so that a general design principle is developed for detection blocking area of relay protection. An adaptive impedance offset characteristic of distance III is proposed to consummate PT breakage discriminant function after start up. It prevents incorrect tripping of distance III during an external fault accompanied by PT breakage. The reliability analysis model is developed for double redundancy design.The method of active redundancy design for relay protection is put forward to reduce the incorrect tripping during the shutdown process prior to scheduled maintenance.(3) Online monitoring method based on multi-source information mining technology is proposed to enhance failure detection ability of relay protection at both the substation level and system level. Relay self-check information model and general evaluation model of relay protection are developed. The models reveal the corresponding relationship of self-check information and device condition status, and achieve real-time dynamic evaluation of detection health factor. An associated monitoring method based on homology information comparison is proposed to improve hidden failure detection capability of secondary circuits. A fault recording data model based on XML Schema and a internal intermediate node information model of protection device are established. A multi-sampling frequency rate transformation algorithm for fault recording data is proposed to solve the problems of intelligent channel grouping, sampling frequency normalization, and action logic visualization under complex fault conditions and to impove the protection action analysis and diagnosis ability based on wide area information.(4) State detection method for SV, GOOOSE and protection function based on IEC61850 standard is proposed to achieve maintenance-free relay protection in intelligent substation. It reveals the cause of poor reliability of anolog merging unit. Sampling data validity determination method is proposed based on equal differential of sampling value and harmonic components, and improves the falure detection and fault tolerance ability of analog merging unit data quality. Relay protection automatic testing method based on model matching is proposed in intelligent substation, and it realizes host based automatic test, solves the problem of excessive human intervention and non-intuitive test results and improves maintenance efficiency and safety. A closed loop monitoring method of GOOSE circuit based on extension CID model command monitoring data points is proposed and it can achieve self-automated testing, improve the status detection capability of relay protection in intelligent substation.