Topology Design And Research Of Modular Hybrid DC Circuit Breaker

The flexible DC transmission technology provides effective technical support for the construction of the future DC grid.However,how to quickly isolate the DC-side fault has always been a problem that restricts the development of the DC grid.As a key DC breaking device,the DC circuit breaker has fast response and short recovery time,which can significantly reduce the over-current risk of power electronic devices in the HVDC power transmission system.However,it also has some research difficulties,such as difficult to extinguish DC arc,need to rapidly break in a few ms,and high overvoltage.In this dissertation,the existing modular full-control hybrid DC circuit breaker is taken as the research object and several researches have been done.Firstly,the improved topology of DC circuit breaker.is researched.The topology of the existing lower-cost diode bridge sub-modules modular fully controlled hybrid DC circuit breaker was improved.This new topology can not only achieve the fast bi-directional breaking function of the DC circuit breaker,but also increase the reclosing speed.The current limiting circuit is used to reduce the peak value of the current passing through the breaker and its over-voltage.And the operation process and principle of its closing,opening and reclosing are analyzed in detail.Secondly,the DC circuit breaker control strategy and its application in the DC grid are researched.The control strategy of "fault self-processing and self-recovery" and"pre-commutation" are used in this improved topology,and based on the analysis of short circuit fault characteristics of MMC multi-terminal DC system,the application strategy of DC circuit breaker,combination of "breaker on-site protection" and the upper relay protection system,is put forward.This control strategy can prevent malfunction of breakers on non-faulty lines.Then,based on the improved DC circuit breaker topology and the three-terminal DC system example in this dissertation,the DC circuit breaker parameter design applied in the multi-terminal DC system is performed.Finally,the rationality of the improved topology functions,parameter design,control strategy and application strategy in the multi-terminal DC system are analyzed by PSCAD/EMTDC simulation.And a low-voltage experiment platform was built to verify breaking function of the topology.