Research On Transient Characteristics And Overvoltage Suppression Strategy Of Wind Turbines Considering HVDC System Commutation Failure

HVDC transmission is an important approach to delivery large-scale wind power for a long distanc e.When the commutation failure occurs,the voltage of sending end will decrease first and then rise continuously due to the reactive power fluctuation caused by the change of DC current,which threatens the stable operation of wind farm.Meanwhile,the transient voltage characteristics during the commutation failure is different from the short circuit fault in AC system,so the transient voltage characteristics and the interaction mechanism between it and wind turbine under the commutation failure should be researched,and the transient overvoltage suppression method which is suitable for this scenario should be proposed.The main work is as follows:(1)The mathematical model and simulation model of the doubly fed wind power HVDC transmission system are established.Firstly,the mathematical model of DFIG and the converter control strategy are deduced in detail,and the steady-state model of the HVDC system and the converter station control strategy are established.Secondly,a simulation model of the doubly fed wind power HVDC transmission system is established in PSCAD based on the mathematical model.(2)The form characteristics and formation mechanism of the sending end transient voltage due to commutation failure are analyzed.Firstly,the characteristics of the sending end transient voltage are extracted,and the differences between the transient voltage under the commutation failure and the AC grid short-circuit fault are summarized,including first low voltage and then high voltage,continuous voltage amplitude variation,short transient process,three phase symmetry and only own power frequency component.Secondly,the reactive power exchange between AC and DC system is analyzed according to the fault sequence,which reveals the formation mechanism of the sending end transient voltage.(3)The interaction mechanism between the sending end transient voltage and wind turbines are studied.On the one hand,the influence of transient voltage on DFIG characteristics is studied.The stator flux of DFIG under commutation failure and AC short circuit fault is analyzed by piecewise linear description method.On the other hand,the influence of DFIG control strategy on transient voltage is studied,including the influence of different steady-state control modes and low voltage ride through control modes on transient voltage,which pointed out that the provisions of low voltage ride through in China’s existing wind power grid integration criteria are not applicable under the commutation failure.(4)An overvoltage suppression control strategy based on low-high voltage cascading ride through of DFIG is proposed.According to the transient characteristics and reactive power output equation of DFIG,an overvoltage suppression control strategy based on low-high voltage cascading ride through of DFIG is proposed.The simulation results show that the proposed strategy can not only suppress the power fluctuation,but also provide reactive power support according to voltage change,which is effective on overvoltage suppression,and improves the stability and flexibility of the power system.