| High voltage direct current(HVDC)transmission has the advantages of far transmission distance,large transmission capacity,low costs,good control performance and so on.HVDC has been widely developed and applied in recent years.Based on the significant advantages in long distance and large capacity transmission and power grid interconnection,HVDC plays an important role in west-east power transmission and national networking project.Commutation failure is a common failure of HVDC transmission system,if is not properly controlled,it will cause serious consequences.With the completion and operation of ?800 kV and ?1100 kV UHVDC,the capacity of HVDC transmission is becoming larger,and the influence of commutation failure on DC system is bigger.The resilience and recovery characteristics of DC system on commutation failure are closely related to HVDC inverter control strategy.At present,there are two main control strategies at home and abroad: constant extinction angle control strategy and constant voltage control strategy.The inverter side is in the constant extinction angle control state when the system is in the steady-state operation under constant extinction angle control strategy,while the inverter side is in the constant voltage control state when the system is in the steady-state operation under constant voltage control strategy.Commutation failure has occurred in systems under both the two control strategies.Less of the existing literature has studied on the influence of inverter control strategy on commutation failure.In this paper,the control methods of controllers under the constant extinction angle and the constant voltage control strategy are analyzed.Based on the actual engineering parameters and control logic,the PSCAD/EMTDC models of the HVDC transmission system under constant extinction angle and constant voltage control strategy are established.Model control systems are set up in accordance with the actual engineering control systems.The influence of operating power,AC system strength,steady extinction angle on commutation failure is analyzed based on the established model.Comparative study of the resilience of constant extinction angle control strategy and constant voltage control strategy on commutation failure is made,and control mode of converter after commutation failure under the two kinds of control strategy is analyzed.The duration of the commutation failure and the power recovery time are taken as two indexes to compare the recovery characteristics of the two control strategies after commutation failure.The research shows that,there are no significant differences between the two control strategies on commutation failure resilience.Under the single phase fault,the constant extinction angle control strategy is better than the constant voltage control strategy.Under the same fault condition,the fault duration of the constant extinction angle control strategy is less than the constant voltage control strategy,and the constant voltage control strategy is better than the constant extinction angle control strategy in the commutation failure recovery time.To improve the commutation failure resilience of DC system and the recovery characteristics after commutation failure,a current fuzzy controller is designed by MATLAB and PSCAD/EMTDC simulation based on the principle of fuzzy control.The current fuzzy controller can reduce the DC current to prevent commutation failure in the early stage of AC fault according to the variation of AC voltage and magnify the DC current command value to speed up the recovery of DC power system after the elimination of the AC fault,based on the recovery of DC voltage.The research has certain guiding significance for the selection and optimization of the control strategy in the future. |
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