Study Of Improved Scheme And Backup Strategy Of Arrester Based Last Circuit Breaker Protection

Ultra high voltage direct current(UHVDC) transmission system has been widely applied, with the characteristics of low power loss, variety of operation mode, flexible adjustment and so on. The capacity of reactive power compensation equipment on UHVDC inverter station bus is larger than which on HVDC. Once the inverter side suddenly lose all AC load and the conventional last circuit breaker protection miss trip, there will be very serious overvoltage. To this end, arrester based last circuit breaker protection is often configured in the UHVDC transmission system to deal with overvoltage cause by conventional last circuit breaker protection's miss tripping. While original arrester based last circuit breaker may act in a relatively long time and it may invalid in certain operating conditions. To solve these problems and ensure UHVDC converter station operate safely and stably, improved scheme and backup strategy of arrester based last circuit breaker protection are studied in this paper.A UHVDC transmission system model is built with EMTDC / PSCAD according to the actual parameters of Xiangjiaba- Shanghai UHVDC project, and arrester model is deployed in the simulation system to meet the requirement of the research. On this basis, this paper proposes an improved scheme about arrester based last circuit breaker protection and the corresponding strategy of setting trip value. Real-time voltage is adopt in the improved scheme instead of constant voltage to integrate the arrester power. In addition, this paper proposes a dynamic tuning strategy about arrester based last circuit breaker protection action threshold based on the inverse relationship between voltage and arrester energy. The dynamic tuning strategy proposed can shorten time of system equipment withstanding high time overvoltage under the premise of guaranteed operating reliability, so that it can protect the main equipment better.This paper studies last circuit breaker protection strategy when the UHVDC transmission system is in 1+1/2 operation mode and analyzes its flaws. On this basis, two last circuit breaker protection backup strategies are proposed to deal with the scene of high-end converter transformer quitting running and simulation results show the effectiveness of the strategy. And D-S evidence theory is adopt in this paper to form a suitable UHVDC last circuit breaker protection by integrating three criteria information of the main last circuit breaker protection and backup protection.