Research On Suppression Strategies Of DC Circuit Breaker Operation Overvoltage In Multi-Terminal Flexible DC Grids

In order to cope with the global energy crisis and environmental changes,as well as to promote sustainable development and carbon neutrality goals,there is an urgent need for a clean and low-carbon transition of energy,the best way to use and transport renewable energy conveniently in large quantities is to convert it into electric energy,realizing grid connection and consumption in time.Voltage Source Converter(VSC)based on High Voltage Direct Current technology emerged as the times require.Converter stations can be connected through DC lines to form a multi-terminal flexible DC power grid.When a fault occurs on the DC side of the multi-terminal flexible DC grid,it is necessary to clear fault from DC side quickly by breaking the DC circuit breaker,but the operation of DC circuit breakers may cause operating overvoltage in some areas of the multi-terminal flexible DC grid.Therefore,researches on impact analysis and suppression strategy of DC circuit breaker operating overvoltage have important theoretical significance for ensuring safety,stability and transmission reliability of multi-terminal flexible DC grids,as well as practical applications in optimizing equipment insulation coordination and improving economic benefits.At present,the most recognized DC circuit breaker in the field of multi-terminal flexible DC grid research is the hybrid DC circuit breaker.ABB’s fully controlled DC circuit breaker(FCCB)and the Global Energy Internet Research Institute’s cascaded full bridge DC circuit breaker(CFCB)have been used most widely.Taking FCCB and CFCB as the research objects,basing on the simulation,strategies to suppress DC circuit breaker operating overvoltage in multi-terminal flexible DC grid operation by optimizing their topology are proposed.Based on a brief analysis of faults may occurs on multi-terminal flexible DC grid,the breaking principle of hybrid DC circuit breaker and the generation mechanism of DC circuit breaker operating overvoltage in multi-terminal flexible DC grid during the fault clearing process are described,the factors that may affect DC circuit breaker operating overvoltage are analyzed,making theoretical preparations for the research on the suppression of DC circuit breaker operating overvoltage in multi-terminal flexible DC grid.Based on the topological structure and related parameters of the Zhangbei four-terminal flexible DC grid project,a multi-terminal flexible DC grid electromagnetic transient simulation model is built in PSCAD/EMTDC,and its voltage stability and anti-interference ability are tested,preparing for the simulation verification of the research on the suppression of DC circuit breaker operating overvoltage in multi-terminal flexible DC grid.Based on the analysis of the topology and breaking process of FCCB and CFCB,topological structure optimization scheme in order to suppress DC circuit breaker operating overvoltage in multi-terminal flexible DC grid are proposed separately and creatively.And then set up simulation models of FCCB,optimized FCCB,CFCB and optimized CFCB in PSCAD/EMTDC,determine the optimal parameters of the optimization strategy.Finally,verify the effectiveness of DC circuit breaker operation overvoltage suppression schemes by taking test in the multi-terminal flexible DC grid simulation model,which reduces the difficulty of system overvoltage insulation coordination and improves economic benefits in engineering practice for reference.