Research On Dc Fault Protection Method Of Mmc Based DC Grid With DC Circuit Breaker

Modular multilevel converter(MMC)based DC grid has many adavantages,such as the rapidity of power flow reversal,the ability to supply passive network,the independent control of active and reactive power,no commutation failure,and so on.It has broad application prospects in renewable energy integration,urban power supply and asynchronous grid interconnection,etc.However,due to the low damping of MMC based DC grid,the problem of DC fault handing becomes one of major bottlenecks which hamper the applocation of DC grids.With the rapid development of DC circuit breaker(DC CB)technology,utilizing DC CBs to isolate DC fault has proved to be a practical and feasible technical route recognized by industry and academia.Thus,this thesis studies the DC fault protection methods and strategies of MMC based DC grid embedding DC CBs.Firstly,a DC fault protection method based on the change rate of DC voltage is proposed for half bridge MMC based DC grids with hybrid DC CBs.Influence factors including current limiting inductance,fault location,fault type,fault resistance and measured time constant are researched to provide a basis for the selection of setting values.The simulations on PSCAD/EMTDC verify that according to the proposed method,hybrid DC CBs can be correctly triggered to isolate DC faults rapidly.The DC voltage change rate method above has some shortcomings.For example,it is susceptible to noise interference,and its performance deteriorates with the increase of fault resistance.To overcome these shortcomings,this thesis further analyzes the difference between the transient energy of internal and external fault and presents a DC fault protection method based on high-frequency transient voltage attenuation characteristics.For half bridge MMC based DC grid with hybrid DC CBs,a DC fault protection procedure is designed.In this procedure,the change rate of DC voltage acts as the startup criterion.The high-frequency transient voltage energy is used as the fault identification criterion.The difference between the integral of the positive and the negative voltage is taken as the fault pole selection criterion.The autorecloser criterion is determined based on the voltage of the fault line.The accurate and rapid identification of fault line using the presented method is verified through simulations on PSCAD/EMTDC.In order to prevent the risk of blocking after the occurrence of DC faults in half bridge MMC based DC grids,this thesis finally studies the fault protection method for DC grid based on uninterruptable hybrid MMCs with DC fault ride-through capability.The operation principle and DC voltage operating region of the hybrid MMCs composed of half bridge sub-modules and full bridge sub-modules are analyzed.A DC fault ride-through control strategy of hybrid MMC is also presented.DC fault protection strategies for hybrid MMC based DC grids with DC disconnectors or mechanical DC CBs are proposed respectively.The simulations on PSCAD/EMTDC verify the feasibility and validity of these strategies.