Study On Access Mode And Energy Storage Of Medium DC Distribution

Due to the continuous development of urbanization in China,more and more DC loads appear in urban distribution,such as electric vehicles,LED lighting,etc.With the addition of the vigorous promotion of renewable energy,DC distribution has developed rapidly,making it a hot research topic at home and abroad in recent years.Compared with AC distribution,DC distribution network has high reliability because it does not contain reactive power components,and can directly supply power for DC load in the distribution,reducing the substation link.In addition,renewable energy has been vigorously promoted in recent years.Compared with AC distribution network,DC distribution is more convenient for the access of distributed generation.Therefore,it is significant to make a study of DC distribution.Aiming at the access mode between DC distribution and AC network,the paper puts forward a new connection mode between DC distribution and AC network,which does not use direct connection transformer.Without transformer installed,the paper analyses the impact of AC system faults on DC system and the impact of DC system faults on AC system.On the basis of not using connected transformer,the paper analyses the strategy of optimizing the control mode of converter in order to restrain the impact of zero sequence current flowing to DC power grid when asymmetric short circuit fault occurs on AC side.The converter adopts modular multi-level(MMC)structure because of its flexibility.By improving the control mode,the zero sequence current can be effectively prevented from circulating between AC system and DC system,so the reliability of DC distribution network can be improved.In addition,in view of the DC fault such as short circuit,the paper improving the topology structure of the MMC converter.By improving the topology structure of sub-module,the short fault can be isolated between AC side and DC side.A simulation model is built in PSCAD,the results validate the effectiveness of the proposed converter improvement strategy and the feasibility of the proposed transformerless connection of DC distribution.In addition,with regard to DC distribution,in view of the problem that distributed power may cause the fluctuation of DC bus voltage,the paper proposes hybrid energy storage system(HESS)combining supercapacitors and batteries,which can effectively suppress the fluctuation components of DC bus voltage according to different demands.A simulation model is built in PSCAD,the result verifies the effectiveness of the proposed hybrid energy storage,the proposed hybrid energy storage system can effectively suppress the DC voltage fluctuation.