Study On The Improvement Method Of Commutation Failure Based On Minimum Turn-off Angle Of Thyristor

In order to effectively solve the reverse distribution of load centers and energy resources in China,HVDC has been widely used in the world for its large capacity and long transmission distance.At the same time,because the thyristor valve used in the HVDC converter is a semi-controlled element,the AC system fault is easy to cause the HVDC transmission system to have a commutation failure,which will lead to the DC blocking and transmission power interruption,which brings a severe challenge to the safe and stable operation of the AC and DC system.Commuting failure has become a short board to further improve the reliability of HVDC transmission system,so it is necessary to conduct in-depth analysis of commutation failure and study its suppression measures.This article first analyzes the turn-off characteristics of the thyristor,Through the analysis of the current and voltage waveforms during the turn-off process of the thyristor,it is pointed out that the minimum turn-off time of the thyristor is a variation associated with the actual operation situation,through the thyristor turn-off characteristic testing platform,completed the minimum turn-off Angle of the thyristor mathematical model research,suitable for engineering application was optimized based on the fitting parameters of the thyristor minimum turn-off Angle formula.Since the minimum turn-off Angle of thyristor valve is dynamically related to operating conditions and operating environment,the variation of the turn-off angle of thyristor valve is introduced into the control system,and a control strategy based on the measured minimum turn-off angle of thyristor valve is proposed,The proposed control method was improved on the CIGRE standard test model and realized by PSCAD/EMTDC simulation.The simulation results show that the change of the minimum turn-off angle of the thyristor has a certain influence on the control of the DC transmission system.The control strategy proposed in this paper can mitigate the commutation failure caused by single-phase and three-phase short-circuit faults better.It can be beneficial for the control and protection system of DC transmission to adjust the control margin of turn-off angle of DC system,improve failure recovery characteristics,which has practical engineering significance.Finally,a intelligent monitoring and control system equipped with the commutation failure detection and suppression function has been designed,the system includes trigger intelligent monitoring board and the commutation failure monitoring chassis,which can measure thyristor trigger intelligent monitoring interface card real turn-off Angle,commutation failure monitoring chassis can calculate minimum thyristor turn-off Angle,Based on TTM system,the proposed minimum turn-off Angle commutation failure predictive control strategy is verified,and the simulation results show that the commutation failure monitoring technology of commutation valve can effectively improve the system’s ability to resist commutation failure.