Research on Markov Model Based Risk Assessment of Protective Relaying for High Voltage Transmission Line

It is of significance for the assessment on protective relayings to evaluate possible consequences caused by being operated protective relayings. Combining with the state of transmission lines and the failure mechanism of protective devices, a risk assessment method of protective relayings for high-voltage transmission lines is proposed. Firstly, ten states of overall system composed of transmission lines and protective devices are given and further according to the property of self-checking of microcomputer-based protective devices, four kinds of risks, namely the detected unready operation failure, the undetected unready operation failure, the detected unnecessary operation failure and the undetected unnecessary operation failure, are concluded. Secondly, according to the division of the states of transmission lines and protective devices, a 10-state Markov model is established, and using this Markov model a computing method for above-mentioned four kinds of risk indices is given. Finally, based on the measured failure data of protective devices in a certain power system, the differences in evaluating reliability by the proposed risk indices and traditional reliability indices are compared, and the impacts of self-checking rate on risks are analyzed. Analysis results show that there are obvious differences of both risk indices and unavailability that change with the cycle of periodical survey; the risk indices and unavailability of the system can be effectively reduced bydecreasing the failure rate or increasing the self-checking rate.