Research On Topology Of A Cascaded Hybrid High Voltage Dc Circuit Breaker

In recent years,with the rapid development of DC power grid technology,it has become possible to connect large-scale renewable energy sources to the power grid.However,with the requirements of high voltage and high current power transmission,how to effectively and quickly isolate DC side short-circuit faults has become a difficulty that restricts its vigorous development.DC circuit breaker has become a research hotspot due to its selective,fast and reliable fault isolation,reducing unnecessary fault diffusion and its influence on DC system.At present,the research situation of high-voltage DC circuit breakers at home and abroad is mainly due to the problems of high actual development and production cost,poor reliability of fault detection and identification,and weak stability of circuit breaker control strategy,which leads to the fact that high-voltage DC circuit breakers cannot be widely applied in actual DC network projects.In order to improve the performance of DC circuit breaker,reduce its production cost,and make DC circuit breaker more practical in engineering,this paper compares and analyzes the typical DC circuit breaker topology and working principle,and based on this,proposes a new type of current-limiting modular hybrid DC circuit breaker topology structure,which is mainly designed to improve the structure of the transfer branch circuit.Diode group and IGBT module are used to replace the traditional IGBT series branch circuit.While ensuring the circuit breaker to realize bidirectional flow,it can also reduce a large number of IGBT and reduce the production cost.The IGBT module designed can also solve the problem that a large number of IGBT series voltage sharing is difficult.At the same time,in order to prolong the service life of the circuitbreaker,the auxiliary current limiting module is embedded in the transfer branch,which can protect the sub-devices of the circuit breaker from the impact of large current,avoid the influence of the reactor on the power flow control of the system and improve the safety and stability of the system.On the other hand,the fault detection and identification methods of DC power lines are analyzed.Based on this method,fault lines can be quickly identified and located,providing margin for the breaking time of circuit breakers.In view of the problems that the traditional "pre-transfer" control strategy of DC circuit breaker may cause the circuit breaker to be in a pre-transfer state all the time,resulting in over-temperature of sub-modules,as well as additional energy loss,causing misoperation of the circuit breaker,etc.,an improved and optimized pre-transfer control strategy of the circuit breaker is proposed,and the time and threshold value of its pre-transfer process are adjusted through formula derivation,so as to reduce the breaking time of the circuit breaker and improve the control stability of the breaking process of the circuit breaker.Finally,by studying the fault types of DC power grid,the coordinated control strategy between circuit breaker and converter station is proposed.Based on PSCAD/EMTDC software,the single-end and three-end system simulation research of DC circuit breaker proposed in this paper is carried out,which verifies that the proposed circuit breaker topology combined with the improved control strategy can effectively realize fault breaking.