Analysis Of Commutation Process And Countermeasures For Single Phase Tripping Of AC Side Of DC System

High Voltage Direct Current(HVDC)is an important way for long-distance,largecapacity transmission and large-area networking in China.Under the objective conditions of "strong and weak contact",the failure of the AC system is likely to cause the commutation failure of the DC system.The continuous failure of the commutation will cause DC blocking.In severe cases,the AC/DC hybrid system will be unstable.Therefore,in-depth analysis of the impact of the chain reaction caused by the failure of the AC system on the system and targeted development of effective measures is of great significance for ensuring the safety of HVDC transmission systems.This paper first analyzes the working principle of HVDC converter.According to the basic working principle of the rectifier and the inverter,the commutation process of the LCC-type inverter is analyzed in detail,and the variation of the DC system voltage during commutation is formulated.Finally,starting from the angle of commutation voltage time area,the relationship between commutation angle,turn-off angle and other electrical quantities is revealed,and the influence of commutation voltage on the turnoff angle is analyzed.The phase characteristics of the voltage at the converter bus after the single-phase tripping of the inverter-side AC system are studied.The symmetric component method is used to analyze the phases of the voltages on the converter bus after a single-phase trip.The results show that when there are multiple AC lines sent from the inverter station,the three-phase voltage phases on the converter bus are asymmetric after the fault phase is removed;on this basis,the inverter commutation process after singlephase tripping of the AC system is analyzed,and it is found that in the existing equal interval triggering method,the three-phase asymmetry will cause the DC system to face commutation The risk of failure,and further proposed a critical criterion for commutation failure caused by three-phase asymmetry.For DC systems where trigger angle fluctuations do not meet this criterion,single-phase tripping will cause commutation failure;the use of commutation voltage is proposed.The countermeasures for adjusting the pulse interval of the converter in the steady state phase difference have played a very good role in preventing and suppressing commutation failures that occur during non-all-phase operation.Aiming at the problem of commutation failure caused by single-phase reclosing in fault,this paper starts with the characteristics of fault arc in single-phase ground fault,and derives the characteristic expression of arc current by double-ended electric quantity,and calculates the expression by the expression.The result sequence is analyzed by marginal spectral energy,and the adaptive identification method of AC line adaptive reclosing is proposed,which effectively avoids the commutation failure caused by coincidence.