Research On Mitigation Of Sub-synchronous Oscillation In Wind Power System By MMC Type Static Synchronous Series Compensator

In recent years,the installed capacity of wind power continues to increase,and the large-scale wind farms dominated by doubly fed induction generator(DFIG)gradually increase.The problem of sub synchronous oscillation(SSO)of series-compensated grid-connected system is increasingly serious.The tandem type FACTS device is suitable for the suppression of SSO because of its better flexibility and small influence on the suppression effect of the operation mode.Modular Multilevel Converter(MMC)type Static Synchronous Series Compensator(SSSC)(MMC-SSSC)as a series FACTS device,can be used for power flow control of high-voltage large-capacity transmission lines,but it has not been used for the suppression of wind power SSO.In this paper,the following research is carried out on the problem of the suppression of wind power sub-synchronous oscillation based on MMC-SSSC:The topology of MMC-SSSC is given,and the modulation method of MMC and the working principle of sub-module are analyzed.Combined with the operation principle of MMC device and the modulation method of sub-module,the operation principle of MMC-SSSC is analyzed.A mathematical model of MMC-SSSC is established by combining the two-terminal transmission system.The control strategy of MMC-SSSC is explored,and the MMC DC bus voltage stabilization strategy suitable for MMC-SSSC is proposed.Based on the MMC-type SSSC topology structure,and taking into account both power flow control and sub-synchronous oscillation suppression functions,the guidelines for parameter design of the MMC-SSSC device are given.Combined with the transfer function of control system,the PI parameters of double loop control are calculated.Based on the PSCAD/EMTDC simulation platform,the effectiveness of the MMC-SSSC control strategy is verified.Aiming at the problem of sub-synchronous oscillation in doubly-fed wind farms,a strategy was proposed to use the MMC-SSSC device to inject sub-synchronous voltage at the same frequency and phase as the system sub-synchronous current to increase the equivalent positive resistance at the sub-synchronous frequency of the system,thereby suppressing sub-synchronous oscillation.Aiming at the characteristics of wideband and time-varying sub-synchronous component,a wide-band filter for sub-synchronous signal extraction is designed;the designed filter can accurately extract the sub-synchronous current component of the line.Combined with a practical doubly-fed wind power sub-synchronous oscillation simulation model of a series-compensated grid-connected system,different operating conditions were set up,and the effectiveness of the proposed suppression strategy was verified.The time-domain simulation results show that the proposed oscillation suppression strategy can effectively suppress the sub-synchronous oscillation in the system under different wind speeds,different string compensation levels,and different MMC sub-module voltage values;the suppression effect when the sub-module voltage value is small is better than the effect when the sub-module voltage value is large.The research conclusions of this paper have guiding significance for designing and using MMC-SSSC device to suppress sub-synchronous oscillation of wind power.