Research On DC Side Short Circuit Fault Protection Strategy Of Multi-terminal DC Transmission System

Multi-terminal DC transmission(MTDC)technology can effectively solve the problems of large randomness of new energy and long distance from load center,and its development potential is tremendous.However,there is no natural zero crossing of DC side short-circuit fault current,which has the characteristics of fast rise rate,high peak value and rapid development.Therefore,the problem of DC side short-circuit fault isolation and protection limits the further development of MTDC transmission.In order to solve the above problems,firstly,the basic principle and control strategy of MTDC transmission are studied,and the system scenario is built;Then the topology of new current limiter and Hybrid DC circuit breaker are designed to realize short-circuit current suppression and fault isolation.The specific works of this paper are as follow:(1)An improved coordinated control strategy based on adaptive droop coefficient is proposed.The problems of traditional droop control are studied,such as unable to realize slope adaptive adjustment,possible voltage power out of limit and slow response speed.According to the changes of voltage and power,different droop coefficients are selected in real time to realize fast dynamic response and more reasonable power distribution;The power and voltage deviation control is added to avoid the output power and voltage exceeding the range that the converter can withstand;A lag link is added in the current loop to compensate the delay caused by the current loop and improve the response speed of the system.(2)A mutual inductance fault current limiter using coupling reactor and a few power electronic switches is proposed.The proposed current limiter mainly includes three parts: fault current limiting branch,energy absorption branch and freewheeling branch.Compared with the traditional series reactor,the proposed mutual inductance current limiter can automatically increase the equivalent inductance and enhance the ability of fault current suppression in the current rising stage;the equivalent inductance is automatically reduced to improve the attenuation speed of fault current in the current drop phase.In addition,it also has self-recovery function.(3)A new hybrid DC circuit breaker is proposed to realize the rapid suppression of bipolar short-circuit current.The proposed DC circuit breaker consists of three parts: nominal current branch,energy absorption branch and freewheeling branch.For transient faults,the energy absorption branch and freewheeling branch are mainly used to absorb and suppress the fault current,so as to avoid over-current damaging the line;For permanent faults,the energy absorption branch is disconnected after short-time operation to realize complete isolation between converter side and fault side.Taking the four terminal DC transmission system as an example to verify the feasibility of the above strategy and topology.The research results are helpful to solve the DC side fault protection problem of multi-terminal DC transmission system.