Research On The Mechanism And Countermeasures Of Inrush Current Of Charging MMC-HVDC

As the connecting link between Modular Multilevel Converter(MMC)and communication system,connecting transformer is the core component of Modular Multilevel Converter Based High Voltage Direct Current System(MMC-HVDC).During the sequential control operation of charging MMC-HVDC,applied voltage of connecting transformer will change abruptly,and according to Constant-linkage Theorem,one side of that change will create a large number of inrush current under some circumstances.The production rule and magnitude of inrush current caused by charging MMC-HVDC is obviously different from those caused by power transformer in general,Inrush Current from station direct current will bring some specific problems.Therefore,through intensively studying the inrush current caused by connecting transformer during the process of charging MMC-HVDC,this paper analyzes the mechanism and properity of its production,and provides solutions for inrush current.First of all,the MMC in the initial stage of charging is simplified equivalently.The electrical components used to construct MMC are diverse and numerous.The same power electronic device will have different states in the circuit.Based on the particularity of MMC in the initial charging stage,the whole MMC is simplified equivalently and the simplified equivalent circuit of MMC-HVDC is drawn in stages without changing the characteristics of the converter port.Then,the dynamic process of winding end voltage of the connected transformer during the sequential control operation of MMC-HVDC charging is studied theoretically by using a simplified equivalent circuit.Based on the results of the research and the law of magnetic chain conservation,the expressions of magnetic flux transient components of different transformer cores connected by MMC-HVDC two-end converter stations are deduced respectively,taking into account factors such as the status of the primary equipment,the closing sequence of three-phase breakers on the AC side of the connected transformer,the charging sequence of the two-end converter stations,and the closing time difference of the first closing breaker on the AC side of the two-end connected transformer.This transient component is the main cause of excitation inrush.Next,the charging process of MMC-HVDC at both ends is simulated and analyzed in the PSCAD/EMTDC electromagnetic transient program.In the simulation,using the single-phase classical model of transformer and connecting them to YNyn0 connection mode,the saturation characteristics of transformer are achieved through three parameters: transformer knee voltage,hollow reactance and rated excitation current.The simulation results verify the correctness of the theoretical derivation in this paper.Finally,based on the phase selection closing technology,the closing strategy of connecting the AC network side circuit breaker of the transformer under various working conditions is proposed;this paper discusses the natural suppression effect of the charging current limiting resistor set on the AC network side of the connecting transformer on the inrush current;The corresponding solutions are formulated around the unique problems caused by MMC-HVDC charging inrush current in DC transmission.