Research On The Grounding Mode And Fauit Transient Analysis Of Flexible Medium Voltage DC Distribution Network

In recent years,with the gradual transformation of China's industrial structure,the diversification of load types and the vigorous development of distributed power supply,the traditional ac distribution network is under impact.At the same time,with the rapid development of high-power power electronics technology,the flexible dc power transmission technology based on voltage source converters has been extended to the distribution network,making the flexible dc distribution network become a hot topic of domestic and foreign scho ars.Firstly,the advantages and disadvantages of the existing flexible dc distribution system structure are compared,and a flexible medium-voltage dc distribution system based on double-end power supply is established.The medium-voltage dc line voltage grade of the system is 10kV.A three-phase two-level voltage source converter is adopted and its mathematical model is established.DC solid state transformer(DCSST)based on Dual Active Bridge(DAB)converter with high frequency isolation is selected and single phase shift control is adopted.The mathematical models of the photovoltaic system and energy storage system are established,and the maximum power point tracking controller,constant power controller and fixed voltage controller of the photovoltaic system are respectively designed.Finally,the simulation analysis is carried out when the system is running steady-state.Secondly,the influence of different grounding modes of converter transformer on the transient characteristics of system fault is studied.The fault models of single-phase grounding fault of ac side line and single-pole grounding fault of dc side line are established.Through theoretical analysis and simulation verification,the existing problems of different system grounding methods were explained in detail,and the wiring of the converter transformer Yn/delta and the dc side capacitance of the converter were finally determined to adopt the split-midpoint high-resistance grounding method.Thirdly,based on the confirmed system grounding method,the fault characteristics of grounding fault in each fault area are studied,the corresponding fault transient model is established,and the influence of grounding fault on other fault areas is analyzed.Finally,the electromagnetic transient model of the system in this paper is built in PSCAD/EMTDC software,and the simulation verifies that the grounding mode of the system can effectively limit the transient overcurrent level of grounding fault current,maintain the stability of dc voltage,improve the reliability of power supply of the system,and have strong fault crossing ability and fault recovery ability.