Research On Mitigating Sub-synchronous Oscillation Of DFIG Connected To Series-Compensated Transmission Line By Energy Storage

During recent years,China's wind power industry has developed rapidly.Affected by geographical conditions,wind power is mainly installed in Northwest,North and Northeast China,which is far from load center in Eastern and Central China.Series capacitor compensation technology is an important means to realize wind power long-distance transmission.The existing research shows that when the doubly-fed induction generator(DFIG)is connected to series-compensated transmission line,there is a risk of sub-synchronous oscillation(SSO).The study on mitigating SSO by parallel FACTS device has strong theoretical and practical value for improving the safety and stable operation capability of wind farms.In this paper,taking DFIG based wind farm as research object,the parallel energy storage device is analyzed to mitigate SSO of DFIG based wind farm which is connected to series-compensated transmission line.The mathematical models of components in the system are established.Superconductor magnetic energy storage(SMES)is chosen as research subject,which has specialties in fast response and high power density.An supplementary sub-synchronous damping controller(SSDC)based on proportional resonance is proposed.The electric damping relationship between the energy storage device and DFIG at sub-synchronous frequency is studied based on the complex torque coefficient method,which shows that when active and reactive SSDC are activated simultaneously,a larger electrical damping is provided.The time domain simulation method is used to verify the mechanism analysis results,which shows that the active or reactive SSDC has obvious effect on mitigating SSO,the mitigation effect is better when both are added.The only use of active SSDC has smaller impact on the dynamic characteristics of DFIG voltage.Based on the actual structure and parameters of a multi-DFIG wind farm in northern North China,the centralized access and distributed access methods for energy storage are studied.The time domain simulation results of the typical access method show that under the same energy storage access capacity,when SMES are installed on the 220 kV bus respectively in distributed access,the mitigation effect is better than installed on the 220 kV bus in centralized access.Even if the energy storage capacity of distributed access is slightly higher than the energy storage capacity of centralized access,the mitigation effect is inferior to centralized access.In distributed access,more distributed device accesses help to increase the mitigation effect.