Improvement Strategies For Commutation Failure And Trigger Phase Control Of HVDC

With the gradual deepening of China's west-east electricity transmission project,more and more high voltage direct current(HVDC)transmission projects have been included in the planning and construction operations.HVDC is already an indispensable component of China's power grid structure,and is a key factor for the safe and stable operation of China's large power grids.It will play an increasingly important role in the future operation and management of power grids.Compared with AC transmission,a significant advantage of HVDC is that it can quickly and extensively adjust system operating parameters through the control system,helping to improve the performance of AC and DC systems.As the basic control of inverter side,extinction angle control plays an important role in resisting commutation failure.In view of the shortcoming of large error in extinction angle calculation of existing extinction angle control during fault,an improved predictive extinction angle control method is proposed.This method takes into account change of DC current and commutation voltage during commutation process.Using moving average filter and the second-order generalized integrator,the DC current and commutation voltage are respectively processed for solving firing angle of each converter valve,and the minimum firing angle is taken as the output of extinction angle control.Through PSCAD/EMTDC simulation,the commutation failure immunity index is tested under different strengths of AC system.Simulation results proved that the improved predictive extinction angle control can better resist commutation failure.Trigger phase control is the basis for the HVDC control system,and the reference phase provided by phase-locked loop has an important influence on the accuracy of trigger phase control.Aiming at the shortcomings of the slow dynamic response speed of synchronous reference frame phase-locked loop,which is commonly used in equidistant firing control,an improved phase-locked loop is proposed to improve the accuracy of trigger phase control.The improved phase-locked loop has a relatively fast dynamic response speed.At the same time,the cascaded delayed signal cancellation method and the mathematical operation filter are used in series to form a pre-filtering link to ensure a strong anti-interference capability.In the CIGRE benchmark model of HVDC,the original phase-locked loop and the improved phase-locked loop are used for triggering respectively.The simulation test of the commutation failure under AC fault is carried out.The results show that the improved phase-locked loop can effectively reduce the probability of commutation failure during the duration of the fault.The effect of trigger phase control on the performance of HVDC is very obvious under asymmetric fault conditions.Aiming at the poor anti-interference of zero-crossing detection of traditional individual phase control and the low degree of control freedom for equidistant firing control,an individual phase control scheme based on single-phase locked loop is proposed.The phase-locked process of the proposed method has strong ability of suppressing harmonics and negative sequence voltage interference,and can obtain more detailed voltage phase information under asymmetrical conditions which reduces the difference of actual trigger angle of each valve.The switching function modeling method is adjusted to make it suitable for the quantitative analysis of harmonic performance of HVDC under the new individual phase control.Based on the CIGRE benchmark model of HVDC,the steady state and transient performance of HVDC under the condition of proposed method are tested and analyzed with PSCAD/EMTDC.The simulation results show that the new individual phase control has no adverse effect on the stability of HVDC,and effectively reduces the probability of commutation failure during AC fault recovery,which verifies the superiority of the proposed method.Finally,the above research contents are summarized,and the future research about the interaction and cooperation between trigger phase control and pole control system is prospected.