Reactive On DC Side Fault Analysis And Protection Scheme Based On MMC-HVDC

The high voltage direct current(MMC-HVDC)transmission based on modular multilevel converter is an inevitable trend in the development of future DC power grid.It has obvious advantages such as independent control of active power and reactive power,easy expansion of voltage levels and low harmonic content etc.However,the fault current on the DC side developed rapidly due to the "low inertia and weak damping" characteristics of the DC power grid,which poses a serious threat to the stable operation of the DC system.Therefore,introducing the research aim to the current limiting technology can effectively limit the fault current level,then realize the safe and reliable breaking of the DC circuit breaker.The main work of this thesis is as follows:First of all,this thesis makes an in-depth study of MMC’s topology,working principle and modulation mode,which provides important theoretical support for DC side fault characteristic analysis.Aiming at three typical DC side faults of MMC-HVDC system,this thesis mainly derived the expression of fault current after bipolar short-circuit.The analysis results of DC side faults are simulated and verified by PSCAD/EMTDC simulation platform.It is concluded that if current limiting measures can be put in time before converter station lockout,the rapid increase of fault current caused by rapid discharge of submodule capacitance can be effectively suppressed.At the same time,the lockout of converter station can be reduced or avoided.It paves the way for the introduction of current limiting protection scheme later.Secondly,a new type of DC fault current limiter(FCL)topology is proposed as an effective auxiliary method to suppress overcurrent in order to solve the practical problem that the traditional DC circuit breaker only has fault isolation function and does not have current limiting function in this thesis;Furthermore,a new protection scheme of DC current limiter and DC Current Breaker(DCCB)is proposed considering the reliability of subsequent DC circuit breakers.The scheme can limit the fault current by putting the current limiting branch of the new current limiter in time during the fault process.And at the same time,the fault isolation speed is accelerated with the DC circuit breaker.It is verified that the proposed new DC current limiter can limit the fault current level effectively,reduce the energy dissipation pressure of the lightning arrester,and improve the ability of the DC circuit breaker to remove the DC side short-circuit fault based on PSCAD/EMTDC simulation platform.Finally,a protection scheme of hybrid DC circuit breaker combined with current limiting capability in order to further study the problems of high fault current level and difficult breaking on DC side in this thesis.By introducing inductance current-limiting loop,capacitance discharge loop and resistorcapacitor buffer branch,the peak value and rising rate of fault current can be reduced,and the short-circuit fault can be quickly cleared.Detail analysis and theoretical derivation are carried out to analyze the steady state operation,current limiting and disconnection process respectively.It is verified that the proposed protection scheme can not only reduce the electrical stress in the process of fault breaking significantly,but also reduce the breaking difficulty of DC circuit breaker based on PSCAD/EMTDC simulation platform.