Research On Suppression Strategy Of Continuous Commutation Failure In HVDC Transmission System

In order to solve the problem of unbalanced distribution of energy resources and power load in my country,the state vigorously implements the strategies of "West-to-East Power Transmission" and "North-South Mutual Supply".HVDC plays an important role in the "West-East Power Transmission" strategy by virtue of its technical advantages.The thyristor,the core commutation device of the HVDC system,does not have the capability of self-turning off,so commutation failure is very likely to occur when the AC system on the inverter side fails.The commutation failure will lead to a surge of DC current,interruption of transmission power,and even DC blocking,which seriously threatens the safe operation of the power grid.Therefore,it is of great significance to study the commutation failure suppression measures in depth.This paper uses the method of combining theoretical analysis and electromagnetic transient simulation to discuss the occurrence mechanism of the first commutation failure and the continuous commutation failure.On this basis,two methods to suppress the DC continuous commutation failure is proposed.The main contents of this article are as follows:（1）Describe the composition,operation principle and control method of the HVDC system,and analyze the coordinated action process of the inverter and the rectifier in detail.Based on the 6-pulse commutation circuit,the mechanism of the first commutation failure is analyzed,and the relationship between the influencing factors and the extinction angle is analyzed.The mechanism of continuous commutation failure is analyzed based on the CIGRE standard DC system.（2）A dynamic extinction angle control strategy to suppress DC continuous commutation failure is proposed,which consists of constant turn-off area control and extinction angle compensator.The strategy dynamically adjusts the extinction angle setting value during the fault according to the sag amplitude of the AC voltage and the rate of change of the DC current to increase the commutation margin.（3）In order to take into account the commutation failure suppression capability and the system reactive power consumption,a joint control strategy is proposed to suppress the DC continuous commutation failure.It is composed of improved VDCOL input signal,virtual inductance current limit control and constant turn-off area strategy.The VDCOL improvement signal can slow down the fluctuation of the DC current,the virtual inductance strategy reduces DC current,and the turn-off area is determined to increase the commutation margin.The three coordinate functions to achieve optimal control.The simulation research using PSCAD/EMTDC on the CIGRE standard DC test system shows that the joint strategy proposed in this paper can more effectively suppress the continuous DC commutation failure,improve the fault recovery performance of the AC and DC power grid,and improve the stability of the AC /DC system and have certain engineering practice value.