Research On Hybrid Unified Power Flow Controller Based On Phase Shifting Transformer To Suppress Sub-synchronous Oscillation

Double-fed induction generator(DFIG)-based wind farms connected to the grid via series compensation may cause sub-synchronous oscillation(SSO).Unified power flow controller(UPFC)can simultaneously or selectively control voltage phase angle,amplitude and various line parameters to suppress SSO in DFIG-based wind farms connected to series compensated transmission system.But the UPFC is composed of large-capacity power electronic devices,which is expensive and difficult to popularize and apply on a large scale.New approaches are urgently needed to address this bottleneck.Aiming at the SSO problem of DFIG-based wind farms connected to series compensated transmission system,this paper studies the SSO suppression method based on hybrid unified power flow controller(HUPFC).The main researchs are as follows:Mathematical model of doubly-fed wind turbine and AC system with series-compensated transmission line are established.The mathemat ical model of the double-fed wind turbine includes the dynamic model of the wind turbine,the model of shafting,the model of the DFIG,the model of phase-locked loop(PLL),and the model of converter.Based on the equivalent circuit of DFIG-based wind farms connected to series compensated transmission system,the mechanism of SSO is analyzed.The working principle of UPFC is analyzed,and a time-domain simulation model is established.The supplementary active resistance control(SARC)strategy of UPFC was proposed.By tracking sub-synchronous current in the transmission line,the SARC strategy makes UPFC inject a sub-synchronous voltage to the grid.The sub-synchronous voltage has the same phase and frequency as the sub-synchronous current and the amplitude is variable.Thereby,the system equivalent resistance is increased to suppress SSO.Based on an actual wind farm in North China,the effectiveness of the SARC strategy under various system operating conditions is verified.Simulation results show,the higher the wind speed,the lower the risk of SSO;the higher the degree of series compensation,the slower the speed of system instability.In order to solve the problems of high cost and difficult to popularize and apply on a large scale of UPFC,a method of using HUPFC to suppress SSO in DFIG-based wind farms connected to series compensated transmission system is proposed.Various topological structures of HUPFC are compared and analyzed,the analysis shows that the HUPFC composed of a large-capacity phase shifter(ST)and a small-capacity UPFC in series can achieve the same effect of continuously regulating the system power flow as a large-capacity UPFC.Applying the SARC strategy to HUPFC to realize the function of suppressing SSO in DFIG-based wind farms connected to series compensated transmission system.Finally,based on simulation examples,The results of the frequency scanning and the time domain simulation both show that the HUPFC can effectively suppress SSO under various system operating conditions such as load degree changes,series compensation changes,the capacity ratio of ST and UPFC changes and fault types changes in DFIG-based wind farms connected to series compensated transmission system.