Subsynchronous resonance (SSR) may occur in DFIG based wind farms connected to series compensated transmission systems. Different from the traditional SSR accident, the converter control parameters have an important influence on the SSR phenomenon because of the access of power electronic equipment. In order to facilitate research on the new SSR problems caused by the access of power electronics devices, this paper establishes an equivalent model based on the actual parameters of some wind farms, where this kind of SSR accident has taken place. The equivalent model can be used as a standard model for studying the mechanism, characteristics and mitigation of this kind of SSR problem. Then, the validity of the proposed model is verified. Further, a time-domain simulation method is used to analyze this study model, so as to obtain the influence of the changes of various factors on DFIG stability. Some measures are put forward to mitigate such SSR accidents based on the results of the analysis. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900104), Science and Technology Project of State Grid Corporation of China (No. SGXJ0000KXJS1700841), and Integration Project of Smart Grid Joint Fund of National Natural Science Foundation of China (No. U1866601).