Study On The Mechanism And Mitigation Of Sub-synchronous Interactions In Dfig-based Wind Farms

With the high growth of the cumulative installed capacity of the wind farms in China,wind power plays an important role in renewable energy supplies.The central location of energy supplies and loads are mismatch in China.Therefore,the ability of high-rating and long-distance transmission of large-scale wind power will affect the utilization of renewable energy.It has long been understood that series compensation can effectively increase the capacity of transmission lines and is being widely used in long distance transmission of large scale wind power.However,the use of series compensated transmission lines will introduce the risk of sub-synchronic interaction(SSI)between wind turbine generators(WTGs)and the transmission system.Through the simulation technology of isolating characteristics,the influence of key factors in the DFIG-based wind farms on SSI is analyzed in this thesis.With the investigation of the characteristics of subsynchronous interaction loop,a new perspective of the mechanism of SSI is presented.Moreover,the mechanism of SSI caused by the inter-harmonics in DFIG is investigated.Based on the Inter-modulation principle,the mechanism of DFIG’s inter-harmonics is studied and the expression of their frequency components is derived.Meanwhile,the connection between interharmonics with sub-synchronous frequency and rotor speed is studied and the cause is discussed.After that,the mechanism that inter-harmonics participate in the cause of SSI phenomenon is investigated.Finally,a practical SSI mitigation strategy on the basis of filters is presented.For the purpose of mitigating SSI and the limiting negative influence on the controllers’ performance,a practical mitigation strategy that adding low-pass filters into the control of Rotor Side Converter(RSC)is presented and the filters’ design method is proposed.Finally,the effectiveness and generality of this strategy are valid through simulation.The research results of this thesis have been used to analyze the cause of the unplanned unit outage in one DFIG-based wind farm in the north of China,and the results have been accepted by the wind turbine manufacturer.Moreover,the SSI mitigation strategy based on filters has been applied to practical wind farms.