Mechanism Of Fault-induced Inrush Current Of MMC-HVDC And Its Influences

The modular multilevel converter(MMC)high-voltage direct-current(HVDC)system has advantages such as flexible control,no commutation failure risk,and compact modular design compared to the line commutated converter based high-voltage DC transmission,and has been developed rapidly in recent years.The converter transformer is located between the AC busbar and the converter,working in a more complex electromagnetic environment,with a greater risk of failure or accident.Following a valve-side single-phase-to-ground(SPG)fault of the converter transformer,a large amount of DC components in the valve-side currents will continuously penetrate the converter transformer,leading to the saturation of the converter transformer and the generation of a special surge current phenomenon.This special surge current is named"fault-induced inrush current"(FIIC)by our research group.This article focuses on valveside SPG fault of the converter transformer,the characteristics of the converter transformer in different stages after the valve-side SPG fault,the mechanism of FIIC generation,the transmission characteristics of FIIC in AC/DC systems and its impact on relevant protection and suppression measures for FIIC.The main content is as follows:Firstly.taking the valve-side SPG fault of the converter transformer in the MMCHVDC system as an example,the charg,ing and discharging circuits formed by the preand post-blocking current paths of upper-and lower-arms of the converter transformer are analyzed in different stages,and the distribution of the fault currents in each phase winding after the fault is theoretically calculated,in consideration of the fault current paths.Finally,the mechanism and variation characteristics of the FIIC generated by the DC component on the valve side continuously flowing into the converter transformer after the SPG fault is clarified.Secondly,in consideration of the configuration of relay protections in actual engineerings,the impact of FIIC caused by the SPG fault on the opening time of the AC circuit breakers on the grid side and relevant protections is analyzed.After considering the FIIC,the grid-side current will be shifted,speeding up the appearance of the zerocrossing of the grid-side current,but the AC circuit breakers on the grid side will still be delayed.For the differential protection of the converter transformer,when the fault occurs in the case of internal fault,the FIIC may cause the differential protection to reject or delay action;when the fault occurs in the case of external fault,the FIIC may cause the differential protection to mal-operation.Due to the presence of high amplitude harmonics in the FIIC,it may result in harmonic overvoltage at the end of the AC transmission lines where it propagates through the lines.Finally.the impact of existing valve-side fault clearance strategies on FIIC is analyzed.After the valve-side SPG fault of the converter transformer occurs,the gridside circuit breakers cannot be opened normally.To solve this problem,a scheme of configuring circuit breakers on the valve side of the converter transformer and allowing the valve-side circuit breakers to open firstly has been adopted.This phase-selection tripping scheme may cause the generation of FIIC and recovery surge current successively.This paper proposes three schemes by constructing three-phase short circuit of the bridge arms,adding resistance in grounding pole and adding thyristor pair modules and damping modules at the bridge arms.The advantages and disadvantages of the above technical schemes are analyzed,which provides a reference for selecting different fault inrush current suppression schemes in practical engineerings.