Study On Transient Protection Strategy Of Direct Drive Wind Power Connected To MMC-HVDC System

The wind farm of permanent magnet direct drive wind turbine cluster has become one of the mainstream of wind power generation due to its high efficiency and low energy consumption.The MMC-HVDC transmission systems has become an effective way to solve the problem of unbalanced distribution of wind energy and load in China,because of its low cost,long distance and no traditional HVDC commutation failure.The input of power electronic equipments lead to the more complex transient process of electric power system fault,the difficulty of accurate extraction of power frequency,and the risk of failure of linear circuit superposition.The protection based on power frequency is no longer applicable to the line protection of large-scale wind farms connected to MMC-HVDC systems.Therefore,this paper takes the direct driven wind power connected to MMC-HVDC systems as the research object,and study on the fault transient model of the direct driven wind turbine and MMC-HVDC system,the protection method of the direct driven wind farm collection line,and the protection method of the direct driven wind farm connected to MMC-HVDC system outgoing line.The main contents and achievements are as follows:Aiming at the resear-ch on the fault transient model of direct driven wind turbine and MMC-HVDC system.considering the complexity of converter control system,the dynamic change characteristics of control strategy before and after fault,and the nonlinearity of power electronic equipment during fault,combined with the voltage equation of the electrical main circuit of the converter and the control equation of the control system of the direct drive wind turbine and MMC-HVDC system respectively.The fault short circuit current of the direct driven wind turbine and MMC-HVDC system in the transient frequency domain is derived to realize the coordinated control of the converter equipment during the fault.Analyze the fault characteristics in detail.and establish the frequency domain impedance model of the direct drive wind turbine and MMC-HVDC system suitable for the transient analysis.Aiming at the problem of difficulty to identify the fault of the direct driven wind farm collection line,a protection method of the direct driven wind farm collection line based on the sudden variable high frequency energy is proposed.First,based on the frequency domain impedance model of the wind turbine and the change in the frequency band of the system transient characteristics,a sudden variable high frequency model of the wind farm collection line composed of the resistive and inductive impedance is established.Further,establish the sudden variable high frequency network under internal and external faults,derive the sudden variable high frequency energy equations under different fault scenarios,and construct the protection criteria according to the difference of the flow direction of sudden variable high frequency energy.Finally,the RT-LAB simulation platform verifies that the proposed method can identify all kinds of faults quickly and accurately,and eliminates the potential possibility that the direct driven wind power fault current leads to misoperation or non-operation of the collection line protection.Aiming at the problem of misoperation caused by the fault of the outgoing line protection of the traditional wind farm connected to MMC-HVDC system,a protection method of the direct drive wind farm connected to MMC-HVDC system outgoing line based on the high frequency action impedance difference is proposed.First,based on the frequency domain impedance models of the wind farm side and MMC-HVDC side,establish the fault transient network of the wind farm connected to MMC-HVDC system.On this basis,define a action impedance,according to the difference between the measured action impedance and the actual action impedance at different fault locations inside and outside the outgoing line,extract the high-frequency frequency point at the maximum value of the difference,and construct the protection criterion based on the high-frequency action impedance difference.Finally,the simulation verifies the accuracy of this method in identifying faults inside and outside the zone under various fault conditions.