Mechanism Analysis And Suppression Strategy Of Subsequent Commutation Failure In HVDC Transmission

Line commutated converter high voltage direct current(LCC-HVDC)plays a very important role in China’s "West East Power Transmission" and "National Networking" strategies.LCCHVDC relies on the AC grid to provide commutation voltage,a fault in the inverter side of the AC system may trigger a commutation failure,if not handled properly will lead to subsequent commutation failure,which may cause a sharp reduction in DC power delivery and weak grid voltage collapse and other serious consequences.Compared with the first commutation failure,the mechanism of subsequent commutation failure is more complex and its development process lasts a longer time scale,so it is realistic and feasible to reduce its occurrence probability by optimizing the control strategy.This paper takes the subsequent commutation failure as the research object,analyzes its causes and main influencing factors,and proposes corresponding suppression strategies,which is of great significance for the safe and stable operation of LCC-HVDC.Through in-depth research on the mechanism and influencing factors of subsequent commutation failure,theoretical analysis and simulation experiments,it is pointed out that the poor value of the voltage dependent current order limiter(VDCOL)control parameter on the inverter side and improper switching between multiple control modes are the main causes of subsequent commutation failure.Based on this,targeted improvement plans will be proposed for the two causes that induce subsequent commutation failure in the following text.VDCOL control in HVDC transmission systems can limit the DC current after a fault and can reduce the probability of subsequent commutation failures in the system,but the traditional VDCOL control parameters are fixed and difficult to meet the system recovery requirements under different operating conditions.In this paper,a VDCOL adaptive improvement control strategy is proposed,taking into account the variability of system recovery requirements at different fault severity levels.Adaptive adjustment of VDCOL control parameters based on the degree of AC bus voltage drop after a fault to meet the recovery needs of the system under different operating conditions,simulation results show that the VDCOL adaptive improvement control strategy can reduce the initial current command value of the system recovery under severe operating conditions and avoid the subsequent commutation failure of the system;Under non-serious faults,the VDCOL adaptive improvement control strategy can increase the current command value without guaranteeing the subsequent commutation failure of the system,help the system recover to steady state as soon as possible and reduce the adverse impact of commutation failure on system operation.Improper switching of multiple control modes on the LCC-HVDC inverter side may lead to subsequent commutation failure.For this reason,this paper analyzes the mechanism of the interactive impact of the converter control at each time after the failure and designs a strategy to reduce the risk of subsequent commutation failure.Based on the action logic of the inverter-side controller,the system is divided into four phases from the occurrence of fault to the restoration of steady state,based on the dynamic trajectory analysis of each phase,the influence law of the inverter-side controller on the recovery process of commutation failure is obtained,and the results show that the secondary interaction process between the constant-extinction-angle-control and the constant current control of the inverter-side,i.e.,the current deviation control phase,is very likely to trigger the subsequent commutation failure.An improved current deviation control strategy is proposed to improve the accuracy of extinction angle state identification by compensating for the current deviation during the controller secondary interaction,thereby suppressing the subsequent commutation failure caused by improper controller interaction.Finally,the proposed improved control strategy is tested in the PSCAD /EMTDC simulation platform with the CIGRE standard model under different operating conditions,and the results show that the analyzed post-fault dynamic process is accurate and the invented subsequent commutation failure suppression strategy is effective.