| With the increasing demand of electrical energy,more and more MMC-HVDC project start investing in construction.However,the DC fault seriously restricts the application of MMC-HVDC,so it is necessary to study the DC fault characteristics and protection method of MMC-HVDC.According to the operation principle of MMC-HVDC and the modulation mode,a 11-level model of MMC-HVDC is built in PSCAD/EMTDC software.The control strategy of sub-module capacitor voltage balance and the control strategy of sub-module capacitor voltage balance are realized.On the basis,the bipolar and unipolar fault characteristics and short circuit current flow paths of the DC side of the MMC-HVDC system are analyzed..And the influence of the MMC control system on the ground fault is considered.The formula of DC side short-circuit current in the DC side bipolar circuit and the unbipolar fault is derived and the simulation results are verified.On the basis,this paper analyzes the principle of the double-thyristor method for DC fault clearing of MMC-HVDC.To solve the shortcoming of double-thyristor method,such as the misleading of thyristor under high positive voltage change rate,the damage of the freewheeling diode under over current and the high time constant,the improvement scheme of double-thyristor method is proposed,and finally the improve method is verified by simulation.The protection methods for DC side short circuit fault of ring topology and star topology in MMC-MTDC are proposed separately: In ring topology,combining the improved method of double-thyristor with the principle of handshake,short-circuit current can be eliminated quickly by locking converter and inputting additional resistance and double-thyristor;In star topology,by using fault location method based on the phase characteristics of hyperbolic sine function,the problem that can not decide to switch on or not by judging the charged information in star topology.Finally,simulation results verify the effectiveness of the above methods. |
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