Research On Fast Fault Clearing Scheme For DC Overhead Lines Of MMC-HVDC Transmission System

In recent years,with large-scale renewable clean energy connected to the power grid,the previous pattern of local consumption of electric energy has been broken,and a new pattern of long-distance and cross regional large-scale electric energy interconnection has been formed.In the scenario of long-distance high-capacity transmission,the new generation transmission technology represented by flexible DC shows great development potential.Among them,the flexible DC power grid based on modular multilevel converters(MMC)has the advantages of multi-source power supply,multi drop point power collection,power flow reversal and no need to change the voltage polarity.It has become the preferred scheme to absorb large-scale renewable energy.However,there are still many technical problems in the development and application of flexible DC transmission system,one of which is the DC line fault current removal technology which can take into account rapidity,effectiveness and economy.Here,this paper mainly studies the fault current clearing technology of DC overhead line in flexible DC power grid.Taking MMC DC power grid as the research object,the change law of relevant electrical quantities before and after DC overhead line fault is analyzed,and two fault removal schemes are proposed respectively.Firstly,the DC fault current clearing technology and its applicable scenarios are analyzed,the topology and working principle of MMC converter and the rising trend of DC line fault current in MMC DC transmission system are introduced,and the fault current clearing and fault ride through capabilities of source side scheme and grid side scheme in the fault clearing scheme are discussed respectively.Then,aiming at the source network cooperation scheme,an adaptive fault removal scheme based on source network cooperation is proposed.In case of DC line fault,the scheme uses MMC active voltage regulation control strategy to reduce the input number of sub-modules on the source side,reduce the output voltage of converter bridge arm unit,and adaptively cooperate with the precharge capacitor voltage of network side circuit breaker,make the output voltage of MMC bridge arm unit less than the precharge capacitor voltage,and use the voltage difference to quickly reduce the fault current to zero,So as to turn off the fault current and clear the fault.The topology,working principle and control strategy of the proposed scheme are introduced in detail,and the key parameters are designed.Finally,aiming at the multi-port DC circuit breaker scheme,a multi-port DC circuit breaker sharing the main disconnect switch is proposed.The circuit breaker uses the unidirectional conductivity of the diode to select the fault current path.Regardless of the direction of the fault current,it can interrupt the fault current of any port and the bus fault current through two unidirectional main disconnect switches,which means that the number of semiconductor switches can be greatly reduced.The topology and working principle of the proposed multi-port circuit breaker are introduced,the fault clearing process is theoretically calculated and analyzed,and the fault current breaking performance is compared with other multi-port DC circuit breaker schemes.The simulation results by PSCAD/EMTDC software show that the proposed schemes can ensure the rapidity and effectiveness of fault current breaking.