Research On DC-side Protection Of Flexible HVDC Transmission System Based On MMC

Compared with conventional thyristor HVDC transmission technology,flexible HVDC transmission system based on modular multi-level converter(MMC)has no reactive power compensation and commutation failure.It can supply power for passive system,and can independently regulate the active and reactive power of the system.It has broad application prospects in the fields of new energy grid connection,asynchronous grid interconnection,island and weak grid power supply.However,the problem of HVDC line protection in flexible HVDC transmission system has become a major obstacle to its development.The protection of flexible line mainly exists in the following two aspects: on the one hand,flexible line has fewer boundary characteristics and requires high speed of fault identification,so the traditional protection scheme of HVDC transmission line is no longer applicable.On the other hand,the half-bridge MMC sub-modules commonly used in flexible system are blocked completely after DC fault occurs,forming uncontrolled rectifier bridge,which leads to excessive fault current and the existing DC circuit breakers can not achieve fault isolation.Aiming at the above problems,this paper studies the protection of flexible HVDC transmission lines from two aspects: fault identification and fault isolation.Firstly,in order to facilitate the verification of DC line protection scheme later,a three-terminal MMC-HVDC model is built on PSCAD platform,and its topology and control mode are introduced in detail.On the basis of this model,the expression of DC line current before and after the fault occurs is deduced by taking the block of sub-module as the boundary.Then,the three-terminal model is simulated and validated under steady-state operation mode and DC side short-circuit operation mode.The results show that the model built in this chapter runs stably and has obvious fault characteristics.It is suitable for DC line protection research.Then the directional characteristics of voltage and current sudden changes in fault lines are analyzed in detail with the additional network diagrams of internal and external faults of DC lines,and the concept of angle cosine is introduced.A backup protection scheme for DC line faults based on angle cosine of voltage and current sudden changes is proposed.This scheme utilizes both ends of DC line under fault and non-fault conditions when faults occur.The different cosine values of voltage and current sudden changes are used as fault identification criteria inside and outside the region,and the fault pole criterion is constructed by using the numerical difference of the voltage between the two poles of the DC line when the fault occurs.The three-terminal model is built to verify the feasibility of the scheme under different fault types.The results show that the proposed fault identification scheme can accurately identify the faults in and out of the region,andis less affected by distributed capacitance.It can operate reliably in the transient process without waiting for the end of the transient process.It meets the requirements of DC line's quickness and can effectively determine the fault type.Aiming at the isolation of DC fault lines,a new fault current limiting module is proposed in this paper.The current limiting module has simple topology and can effectively suppress DC side fault overcurrent.By cooperating with DC circuit breaker,fast isolation of fault lines can be achieved.From the aspects of topology structure,current limiting principle,parameter setting of each component and line loss,the current limiting module is analyzed in detail.Based on this module,a complete fault clearance strategy is designed.The fault isolation scheme is simulated by using the three-terminal flexible DC system model.The results show that the proposed fault clearance strategy can effectively limit the fault current and achieve fast isolation of fault lines without affecting the normal operation of non-fault lines.