Continuous Commutation Failure Supression And Coordinated Control For Ultra High Voltage AC/DC Hybird System

With the accelerated construction of UHV DC transmission channels,the circumstance of‘strong DC systems connect to weak AC systems’ has been gruadully formed in the AC/DC grid of our country.Commutation failure is an inherent characteristic of a half-controlled thyristor valve composed of HVDC.Once commutation failure is not effectively controlled,it will cause continuous commutation failure.In severe cases,the DC system will trigger blocking,power interruption,which pose a great threat to the safe and stable operation of the AC/DC hybird system.Therefore,it is significant to carry out continuous commutation failure suppression and coordinated control in UHV AC/DC hybrid grids.In view of this,this paper first proposes two sets of suppression measures for commutation failures at the level of the DC control system,and secondly proposes a set of coordinated control measures at the level of the AC system for the problems of safe and stable operation of the AC system caused by commutation failures.The main work and progress achieved in this article are as follows:(1)To effectively suppress the continuous commutation failure in UHVDC transmission systems,based on the analysis of the basic principles of VDCOL in DC system control and the effects of different input methods on the control performance of the valve system of the DC system,an improved VDCOL module that automatically switches the input signal with the turnoff angle as the criterion is proposed.This module effectively suppresses the continuous commutation failure of the multi-feed DC system and improves its coordinated recovery speed by reasonably designing the input signal conversion control strategy.(2)To suppress the temporary overvoltage at the sending end system and low voltage at the receiving end system caused by the commutation failure in the UHV DC transmission system,based on analyzing the feasibility of the bypass control to suppress the commutation failure,a bypass pair control strategy to suppress the multi-feed UHV DC commutation failure is proposed,the example analysis verifies the proposed control strategy to UHV DC The improvement effect of coordinated recovery after commutation failure.(3)To avoid the instability of the AC system caused by the commutation failure,a set of serious fault analysis methods for the receiving grid is proposed.The method includes power grid structure analysis,safety and stability check and countermeasure design.Taking the actual UHV DC transmission project as an example,it analyzes the operational risks that the receiving power grid may face after UHV DC input,and proposes counter measures for serious faults to ensure its safe and stable operation.