Research On The Control Strategy To Suppress The Subsequent Commutation Failures Of High-voltage DC Transmission System

With the rapid development of high-voltage direct current transmission engineering,the coupling between AC and DC is becoming increasingly close.Subsequent commutation failures in the LCC-HVDC(line commuted converter high voltage direct current)system will bring huge risks to the safety and stability of the AC and DC systems.At present,control strategies for suppressing subsequent commutation failures mainly focus on fault characteristics,while the impact mechanism of DC system control characteristics on subsequent commutation failures has not been thoroughly studied.Therefore,it is necessary to study control strategies for suppressing subsequent commutation failures in HVDC transmission systems.This article analyzes the mechanism of subsequent commutation failures,and proposes a commutation capability index to improve the description of DC fault severity.Furthermore,a suppression measure for subsequent commutation failures is proposed which takes into account the CEC(current error controller)characteristics.The main work is as follows:Firstly,the impact mechanism of subsequent commutation failures in HVDC transmission system is discussed.The critical commutation area theory in DC commutation is mainly analyzed and the conditions are clarified that need to be met for the DC system to restore normal commutation.Besides,this paper analyzes the influence of the DC control system behavior on the subsequent commutation failure,then expounding that the slope of the CEC ramp function will affect the switching of the DC control mode.That is to say,increasing its slope can enhance the ability of the system to resist the subsequent commutation failure within a certain range.Secondly,a CEC slope dynamic calculation suppression method based on the commutation capability index(SCCl)was proposed.In response to the above mechanism analysis conclusion,a detailed discussion was conducted on the "over compensation" and "under compensation" problems existing in current control.Based on the critical commutation area theory considering rate of change increment,the commutation capability of the HVDC system was analyzed.In addition.the commutation capability index SCCl was proposed to quantitatively evaluate the impact of AC faults at the inverter side on DC system.A suppression method for CEC slope calculation based on SCCl was designed,which dynamically adjusts the CEC slope to meet the system commutation requirements and achieve the effect of suppressing subsequent commutation failures.The effectiveness of the proposed method was verified by comparing the suppression effects of the CIGRE HVDC benchmark model and the proposed suppression strategy through simulation on the PSCAD/EMTDC platform.Finally,a strategy for CEC slope dynamic adjustment with additional compensation increment was proposed to solve the problem of CEC slope insufficient adjustment under severe single line-to-ground fault.By analyzing the influence of factors such as amplitude drop and phase shift of AC phase voltage caused by the single line-to-ground fault on the inverter side,an expression for the compensation of CEC output was derived based on the critical commutation area theory,further overcoming the problem of "under compensation" under severe single line-to-ground fault.A complete suppression strategy has been formed and validated by combining reasonable criteria such as startup,input and shutdown.