Key Technologies Of The Protection For The Flexible DC Grid

The flexible dc grid is drawing growing attention in the power system field,which is a dc power system based on the voltage source converter(VSC)where the dc lines connect with each other directly.Compared with the ac system,it is featured with reduction of the power loss and transmission corridors,as well as smooth integration of the renewable energy generation.In addition,the flexible dc grid has the advantages like the better power quality,no commutation failure and the more flexible power control compared with the conventional dc system.Therefore,the flexible dc grid becomes the developing trend of the future power system along with the fast development of the power electronic technology.However,several technique difficulties are still confronting us when it comes to this new power system form.This paper mainly researched the protection of the flexible dc grid.Firstly,it analyzed the dc fault transient characteristics of the typical flexible dc system.By the detailed analysis of the equivalent circuit,the mathematical analysis method of the dc fault transient characteristics is proposed.And the corresponding simulation cases were carried out to verify the correctness of the theoretical analysis.Secondly,the applicability of the traditional protection principles was discussed.Then it analyzed the transient characteristic differences between the internal and external dc faults of the dc grid.Afterward,the single-ended dc line protection principle based on the current transient energy and the directional pilot protection scheme based on the transient voltage ares proposed in this paper.The proposed protection principles satisfy the speed and selectivity requirements of the flexible dc grid,which could be used as the main protection and backup protection of the dc line.Finally,this paper discussed the principles of the typical dc isolation techniques and their applicability in the flexible dc grid.In additional,for the point-to-point flexible dc system,a novel fault reclosing strategy is proposed based on the uncontrolled rectifier operation of the converter,which avoids the overcurrent and overvoltage damages completely compared with the existing fault reclosing method.