Optimization And Prototype Research Of The Current Limiting DC Hybrid Circuit Breaker

The access of large scale renewable clean energy needs to develop the flexible DC (transmission and distribution) system. However, due to its low impedance, the short circuit current in the flexible DC system raises fast once the short circuit fault occurs in DC side. It costs 30-100 milliseconds to remove the fault current from AC switchyard using mechanical AC circuit breakers, which is too long for VSC based converters to sustain. Also all of the AC circuit breakers may trip to isolate faulted region on DC cables or to extinguish arc on overhead lines. Therefore, HVDC circuit breakers and corresponding fast DC current breaking technologies become one of the technical bottlenecks in developing the DC system.This paper is based on previous research results. Firstly, the basic feature and topology structure of DC circuit breaker is summarized and discussed, which contains mechanical, all-state, and hybrid DC breakers both domestic and abroad. And the overview points out that the hybrid circuit breaker, which relies on mechanical switch conducting working current and solid-state switch turning off fault current, combines the advantages of them such as high controllability, low on-state losses, no arc breaking and high reliability. Secondly, the structure of a current limiting DC hybrid circuit breaker based on natural commutation type is studied and optimized by connected in series with small inductor loop in the mechanical switch branch, of which the operating characteristics, breaking principle and energy transferring process is analyzed to be foundation of the prototype. Further, the key technologies which are involved is discussed in the paper:the requiring short-circuit parameters based on flexible HVDC engineering and an equalizing circuit is proposed. Meanwhile, the auxiliary circuit of the breaker is designed to support the prototype. Moreover, the new topology of the DC hybrid circuit breaker and equalizing circuit is simulated through the power system simulation platform PSCAD, in order to verify the feasibility and effectiveness of the novel structure. Finally, a laboratory prototype is built, software and hardware architecture of the control system is designed as well, which uses NI as the controller. The experiment is done to prove above theory. The result of simulation and experiment shows that:The improved current limiting DC hybrid circuit breaker can instantaneous trigger solid-state switch with promoting the commutation speed during the short circuit fault. The novel topology reduces the requirement for mechanical switch and does not produce extra loss in normal operation, which leaves out cooling system to reduce the volume. The current limiting circuit can not only limit the fault current rising rate but also improve working conditions of switch branches. Furthermore, the auxiliary circuit can significantly protect the device to get rid of the insulation damage of operating overvoltage.