Research Of Restrain Strategyof Commutation Failure Based On Static Voltage Stability Curve

Line Commutated Converter High Voltage Direct Current(LCC-HVDC)transmission system has received significant attention over the last several decades worldwide.HVDC transmission system enjoys great popularity in the field of long distance bulk power transmission,etc.As one of the most important equipment of the LCC-HVDC system,the thyristor has a high voltage withstand capability and large power capacity.The voltage level and power capacity of the LCC-HVDC system have been significantly increased due to the use of thyristors.However,the thyristors are not fully controlled power electronics devices,i.e.the devices cannot be switched off without ac voltage offered by ending ac system.As a result,the LCC system is prone to the commutation failure.The commutation failure will impact the ac system more seriously in a multi-infeed HVDC system,which exerts a significantly negative impact on the stability of the ac system.This thesis first analyses the principle and influence factor of the commutation failure of the HVDC.This thesis points out that decreasing the power reference of HVDC during the ac system fault is helpful to achieve a successful commutation in the LCC converter.Therefore the key issue is to confirm the power reference during any fault.Through the analysis of the voltage stability of the ac system,describing the relationship between the active power and voltage—PV curve and the relationship between reactive power and voltage—QV curve,this thesis points out that QV curve cannot be used in the control system because the traditional HVDC cannot control the reactive power directly.In the end of the second part,the commutation restrain method based on the PV curve is presented.Next,since the PV curve is changing during the system's operation,it is not applicable to control the direct current system by a fixed curve.In the third chapter,through the ac equivalent system,a new curve calculation method is derived.The proposed algorithm is verified by comparing this method with the PSCAD simulation results.In the end of the third chapter,the influence of ac system parameters has been analyzed,a conclusion is drawn that the control result is better in the ac system which has a low short-circuit current ratio.The commutation failure has more serious impact on multi-infeed system.In the fourth chapter,the PV curve used for multi-infeed system has been analyzed.Verified by the PSCAD simulation results,the proposed algorithm can be used in the control system of HVDC.In this chapter,the influence of the ac system parameters and the reactance of the transmission line between different HVDC converter stations have been analyzed.Based on the study presented in this thesis,it is concluded that no matter what ac system parameters are,the PV curve control method can decrease the critical commutation voltage effectively.It is helpful to restrain the commutation failure.