Integration of DFIG-Based Wind Farms Into Series-Compensated Transmission Systems

Recent events have shown that doubly-fed induction generator (DFIG) wind turbines close to series capacitors are prone to subsynchronous oscillations (SSO). This paper analyzes these subsynchronous interactions (SSI) and a countermeasure to enable a reliable and secure integration of DFIG-based wind farms into series-compensated transmission systems. A case study based on a real power system is considered in this work. It has several transmission lines (many of them series compensated), various conventional and wind power plants, and a distributed load. This system allows to study the SSI among various wind farms and multiple subsynchronous modes, unlike previous studies which considered a single DFIG and one series-compensated line (i.e., one subsynchronous mode). An analytical investigation and development of fundamental concepts of the SSO are also discussed. A supplementary damping control (SDC) is designed to enhance the operating range and to damp the SSO of these series-compensated wind farms. A detailed analysis is carried out using eigenvalues and nonlinear time-domain simulations to evaluate the performance and robustness of the SDC. Besides the parameters usually studied, such as wind speed, series compensation level, control gains, the effects of conventional synchronous generation, type of load model, and system demand level on the SSI are also analyzed.