Fault Analysis And Transient Protection Of Flexible Direct Current System Connected With Doubly-Fed Wind Power

The flexible DC transmission system connected to doubly fed wind power has significant advantages in large-scale wind power long-distance transmission.However,the investment of a large number of power electronic equipment will lead to essential changes in the fault characteristics of the system,and the applicability of traditional protection will decrease,seriously affecting the safe and stable operation of the power grid.Therefore,this article focuses on the protection methods for the col lection lines inside the wind farm,the protection methods for the transmission lines between wind power and the flexible DC transmission system,and the protection methods for the DC lines of the flexible DC transmission system connected to the doubly-fed wind power.The feasibility and effectiveness of the protection methods have been verified through simulation.The achievements of this article are summarized as follows:A protection method based on high-frequency impedance model is proposed to address the problem of difficulty in quickly and reliably identifying faults in the internal collection lines of wind farms.Firstly,based on the fault transient process of a doubly-fed induction generator,considering the influence of physical constraints and control strategies,the short-circuit current of a single doubly fed fan is analyzed and calculated,and a high-frequency fault model of the doubly fed fan is constructed.On this basis,analyze the topology structure of wind farms in different locations of faults,establish an equivalent model of the wind farm,and derive high-frequency measurement impedance for the installation of protection at the end of the collection line and the transformer ports of each wind turbine.Finally,based on the measured high-frequency equivalent impedance and the matching degree of different fault scenarios,a protection criterion is constructed.The simulation verification based on RT-LAB shows that the proposed protection method is not affected by the fault location and fan control strategy,and can accurately identify internal and external faults.A pilot protection method based on high-frequency inductance difference coefficient is proposed to address the issue of reduced performance and poor applicability of the protection of the transmission line in wind power flexible DC systems.Firstly,based on the high-frequency model of doubly fed wind turbine faults and combined with the topology of the wind farm,an equivalent model of the wind farm during transmission line faults was established and simplified.Analyze the transient characteristics of the AC side of the converter,consider the switching of sub-modules,and establish an equivalent model of the converter.On this basis,analyze the difference between the measured inductance of the bus and the calculated inductance when faults occur inside and outside the transmission line area,extract high-frequency components,and construct protection criteria.Finally,simulation verification based on RT-LAB shows that the proposed protection method has fast action speed and good resistance to transition resistance.A pilot protection method for flexible DC transmission systems based on high-frequency energy polarity is proposed to address the issue of boundary components for protection,which makes it difficult to meet the requirements of quick action.Firstly,considering the fault transient process of the converter,the sub modules are combined and switched,and the equivalent high-frequency model of the converter is derived.Analyze the equivalent impedance of the flexible direct current converter,select high-frequency components,and construct a high-frequency fault additional network based on inductance resistance composition.Furthermore,establish a high-frequency energy expression and construct protection criteria by comparing the differences in the direction of high-frequency energy flow when faults occur at different positions inside and outside the zone.Finally,simulation verification based on RT-LAB shows that the proposed protection method has fast action speed and can accurately distinguish internal and external faults.