Research On Fault Analysis And Isolation Technology Of DC Distribution System

With the rapid development of distributed generation,energy storage devices and DC loads,DC distribution system has been widely focused by domestic and foreign scholars because it can reduce the loss of conversion and improve the reliability of power supply.Voltage Source Converter(VSC)is well adapted to the DC distribution system due to the advantage of fast decoupling active and reactive power control,power supply to passive networks and having no commutation failure.And Modular Multilevel Converter(MMC)which is one of the VSC topologies has broad application prospects in the construction of DC distribution system with the advantages of flexible modulations,high output waveform quality and low switching loss.While in the MMC-based flexible DC distribution system,the traditional MMC based on half-bridge sub-module cannot isolate the fault current.What's more,because the impedance of DC system is small,the short-circuit current rises rapidly and has no zero-crossing,which makes the fault isolation much more difficult.Therefore,the research on the fault isolation technology of DC distribution system is of great significance to ensure the system safe and reliable operation.In this paper,the fault characteristics of the most severe bipolar short-circuit fault on the DC side are analyzed based on the operation principle of MMC.The analytical expression of fault current is derived and the accuracy of the obtained result is verified by simulation.And then on this basis,two fault isolation techniques including the fault self-clearing sub-module and the DC circuit breaker(DCCB)are investigated in the study.The details are as follows:(1)To solve the problem that MMC using half-bridge sub-module being incapable of isolating fault current,a new sub-module topology with DC-fault isolation capability is proposed in this paper,which applies the thyristor to the topology and is more economical than the others.Firstly,the fault control strategy of the new sub-module is studied and the mechanism of fault isolation is analyzed.It is pointed out that the fault can be isolated reliably by controlling the IGBT to force the thyristor to turn off and then blocking the converter.Then,the economic comparison of various fault self-cleaning sub-modules is analyzed considering the investment cost and operating loss.Finally,a simulation model is built in PSCAD,which verifies that the fault can isolated effectively by the proposed sub-module topology.(2)To solve the problem that the current injection DCCB needs to be equipped with capacitor pre-charge power supply and the reclosing function is difficult to realize,this paper proposes an improved topology of the circuit breaker.By adding auxiliary branches to the improved topology,the charging voltage of the capacitor can be obtained from the DC system directly,and no additional power supply is needed.In addition,the capacitor can return to the pre-charging state quickly after the fault is turned off,and the reclosure can be operated in a short time.The fault breaking process and reclosing scheme of the improved topology are studied,and the design method of each device parameter is proposed.Then,the feasibility of the proposed topology is verified according to the results of the simulation model.