Research On Sub-Synchronous Oscillation Suppression Method In Power System With DFIG-based Wind Farms Using SSSC

In the process of constructing large-scale wind power bases in China,the mainstream doubly-fed wind farms face the risk of inducing sub-synchronous oscillation(SSO)when transmitting power through a fixed series compensation channel.The existing wind power SSO solutions mainly include upgrading and modifying the control strategy of doubly-fed fan(DFIG)and designing the parallel FACTS suppression device.Few literatures have reported series-type FACTS suppression devices.Aiming at the SSO problem in the grid-connected system of doubly-fed wind farm,this paper studies the SSO suppression method of wind power based on h-bridge static synchronous series compensator(H-SSSC).The main research work is as follows:A doubly-fed wind farm-fixed series compensation radiating grid-connected system model is established,which mainly includes the mathematical model of DFIG and RLC transmission line with fixed series compensation.The time-domain simulation model of doubly-fed wind farm grid-connected system was built,and the SSO phenomenon occurred in the field was reproduced.The sub synchronous current signal which can reflect the system oscillation information quickly and accurately is determined by analyzing the characteristics of the sub synchronous electric quantity on the grid side.Aiming at the scenario where H-SSSC is connected to a double-fed wind farm grid-connected system,the influence of sub-synchronous current and modulation mode of H-SSSC on DC capacitor voltage fluctuation of H-bridge module is analyzed.An H-SSSC voltage equalization strategy with active voltage vector superposition is established,and a time-domain simulation model is established based on the H-SSSC parameter design method to verify the effectiveness of the voltage sharing strategy.At the same time,the DC capacitor voltage simulation analysis verifies the correctness of the theoretical analysis.The induced mechanism of SSO in the grid-connected system of doubly-fed wind farm is analyzed,and the H-SSSC additional active damping control(SADC)strategy is proposed.By using the sub-synchronous voltage of the output of H-SSSC and the sub-synchronous current in the line in the same phase,H-SSSC is equivalent to a positive resistance,so as to ensure that the total resistance of the system is positive without the conditions for SSO.Based on the time-domain simulation model,both frequency-domain and time-domain analysis verify the effectiveness of the strategy.Moreover,SADC strategy parameters show better robustness under various operating conditions.The structure of hybrid series compensation(HSC)device is constructed by replacing part fixed series complement(FSC)with H-SSSC.With the help of H-SSSC,the series compensation of transmission lines can be flexibly adjusted,and SSO can be suppressed by SADC strategy.The HSC device can provide a solution to the SSO suppression problem of the grid connected system of doubly-fed wind farms.Based on simulation examples,the effectiveness of the HSC device under various system operating conditions is verified.