Research On Improved Method Of VDCOL Controller To Suppress Continuous Commutation Failure Of HVDC Transmission

At present,with the application of DC engineering,China’s power grid has formed a large-scale AC / DC hybrid system.AC side fault leads to continuous commutation failure of HVDC transmission system.If reasonable suppression methods are not taken,it will lead to converter valve locking,power imbalance and other problems,and even power grid collapse when it is more serious.Therefore,effective suppression of continuous commutation failure is of far-reaching significance for HVDC transmission.In order to effectively suppress the continuous commutation failure of HVDC transmission system under different fault degrees,the suppression of continuous commutation failure is studied for the control of the voltage dependent current order limiter(VDCOL).Firstly,this paper studies the main circuit and control system of HVDC transmission system,and introduces the basic principle of commutation failure.Then the mechanism of continuous commutation failure is analyzed by commutation area.It can be seen that DC current,commutation voltage,lead trigger angle and other variables directly affect the change of turn-off angle.In order to further determine the influencing factors of continuous commutation failure,according to the response changes of each control link of HVDC transmission system after the failure,the fault recovery process is analyzed,and it is deduced that the unreasonable control of DC current command by conventional VDCOL is an important reason for the continuous commutation failure of the system.Then,the conventional VDCOL control under different fault degrees of HVDC transmission system is analyzed.Aiming at the problem that the conventional VDCOL in HVDC transmission system can not effectively suppress the continuous commutation failure under severe fault degrees due to the unreasonable regulation of DC current command,a dynamic nonlinear VDCOL control method is designed,and a new mathematical model is proposed to design the linear relationship between voltage and current of conventional VDCOL into nonlinearity,It overcomes the disadvantage that the command current can not be adjusted reasonably.At the same time,considering the variable voltage before and after commutation failure,the second-order generalized integral is introduced to determine the fault coefficient and modify the voltage.Then PSCAD / EMTDC simulation shows that the control method can effectively suppress the continuous commutation failure under the degree of severe fault,and provide a certain guarantee for the stability of the system.However,it can not effectively inhibit mild and moderate faults.Finally,aiming at the deficiency of dynamic nonlinear VDCOL control mode,a dynamic nonlinear VDCOL control method with additional virtual capacitor is proposed.The input voltage of dynamic nonlinear VDCOL is compensated by adding virtual capacitor,which can effectively suppress the large and rapid decline of input voltage and improve the stability of the system.Through PSCAD / EMTDC simulation,the deficiency of dynamic nonlinear VDCOL control is effectively overcome,and the continuous commutation failure under different fault conditions can be suppressed.