Research On Control Strategy And Topological Structure To Improve The Current Limiting Capability Of Hybrid DC Circuit Breakers

The technology of grid-connected and outsourced distributed renewable energy through Voltage Source Converter based High Voltage Direct Current Transmission system(VSC-HVDC)can effectively alleviate the energy crisis and environmental pollution problems.This technology has received extensive attention and research from countries all over the world.After VSC-HVDC fault occurs on the DC side,its fault range will rapidly expand and the current will rise rapidly.The DC Circuit Breaker(DCCB)with the function of suppressing and quickly breaking faults can effectively solve this critical problem that seriously threatens the safe operation of the system.This paper mainly analyzes the hybrid DCCB,which is currently a research hotspot in the field of high-voltage DCCB,and conducts research from the aspects of its control method and topology.The main research results and innovations are as follows:The classic topology of hybrid DCCB and its working principle are studied,and its internal core components and control logic,protection level,internal parameters and auxiliary systems are analyzed in detail.Build a hybrid DCCB test system through simulation software to provide a theoretical basis and a simulation basis for subsequent optimization and improvement of its control method and topology.A breaking strategy with the gradual withdrawal of the current transfer branch is proposed,which is beneficial to reduce the peak value of the fault current breaking of the hybrid DCCB.This strategy calculates the action time of each Insulated Gate Bipolar Transistor(IGBT)solid-state switch sub-module in the current transfer branch through theoretical analysis of the breaking process and turns off step by step.It solves the problem of a high rate of rise of fault current on the DC side and rapid expansion of the fault range.It is verified by simulation software that this control strategy can obviously limit the rate of rise of fault current,so it can reduce the peak value of fault current,shorten the fault breaking time and reduce the total energy absorption of the arrester.An H-type bidirectional hybrid DCCB topology with current limiting function is proposed.The topology is improved based on the typical hybrid DCCB topology and controllable current-limiting branches are added.It decides whether to use a current-limiting inductor by judging the fault,which can effectively suppress the fast-rising DC fault current while maintaining the dynamic performance of the system during normal operation.It solves the problems that the typical hybrid DCCB topology cannot suppress the fault current rise rate,the fault breaking time is long,the energy dissipating branch arrester has a large capacity,and the current transfer branch cannot be turned on at zero voltage during reclosing.The simulation software verifies that the topology has a good current-limiting capability during the fault breaking process.At the same time,it has the advantages of fast fault clearing speed,buffering energy consumption,early separation of fault points and small arrester capacity.In addition,it can realize the zero voltage condition of the current transfer branch to be turned on during the reclosing period.By studying the hybrid DCCB control strategy and topology,this paper realizes the suppression and rapid breaking of the fault current on the DC side of the VSC-HVDC,reduces the harm of the DC side fault to the system,and effectively improves the safety of the system operation.