Coordination Strategy Of HVDC Grid Fault Protection Including The Action Of Fault Current Limiting Devices

With the continuous development of renewable energy sources such as wind power and PV worldwide,the modular multilevel converter(MMC)based flexible HVDC grids have become one of the most important research hotspots in the field of HVDC transmission as an effective means for renewable energy integration.In the flexible HVDC grid,the capacitor of the converter will discharge rapidly when a serious DC short-circuit fault occurs,resulting in a rapid rise of the fault current.However,the HVDC grid,which is mainly composed of power electronic components,are weak in overcurrent capability.If the fault current reach the upper limit of tolerance within a few milliseconds after the fault,the fault current will be difficult to break,resulting in the outage of the whole HVDC grid and further seriously endangering the safe and stable operation of AC and DC systems.The fault current limiting of the HVDC grid can ensure reliable breaking of the DC fault and the safety margin of the device before the fault is broken,thereby avoiding the blocking of the converter and the damage of the power electronics.At present,the DC fault current limiting is still in its infancy at home and abroad.However,with the continuous development of the HVDC grid,the configuration of the DC fault current limiting devices will affect the transient characteristics of the HVDC grid.Therefore,it is necessary to study the fault evolution mechanism with DC fault current limiter action and the protection coordination strategy between different types of fault current limiting devices.Regarding the issue above,firstly,this paper proposes DC fault current calculation method considering the prompt actions of fault current limiter(FCL)and DC circuit breaker(DCCB)along with the energy consumption characteristic of the metal-oxide arrester(MOA),which includes six substages.The applicability of phase-domain frequency dependent line model and cable line model to this calculation method is discussed.In addition,this paper optimizes the configuration of the fault current limiting reactors in whole DC grid with the following two objective functions:the minimum sum of the cut-off currents of the DCCB at each DC bus,the minimum sum of the energy consumption of the shunt MOAs of FCLs and the MOAs of DCCBs.Secondly,this paper proposes a novel DC/DC converter topology with DC fault isolation capability applicable for HVDC grid,which is developed from the traditional half-bridge MMC based DC/DC converter.It has additional fault current transfer branch to make the fault current interruption easier.The new approach is more efficient in terms of both economy and technology by minimizing the requirements for DCCB.Finally,the fault protection coordination of muti-type fault current limiting devices such as FCL,DCCB,half-bridge based DC/DC converters,and hybrid MMC compound of half-bridge and full-bridge sub-modules are studied.Different current limiting devices are configured in different areas of the compound multi-terminal HVDC grid system including the meshed network and radial structure.According to various fault areas,the protection coordination between fault current limiting devices is proposed.The backup protection coordination of some current limiting devices,such as the rejecting operation of DCCBs,are provided.