Abstract-This article aims to investigate the sub-synchronous resonance issue that may be caused by the wind farms connected to an electrical network through a series-compensated transmission line. Two wind turbine types based on a doubly fed induction generator and a permanent magnet synchronous generator are modeled for sub-synchronous resonance analysis in detail, and the proportional and integral parameters of the control loops are optimized based on a computational intelligence method. The studies are carried out on the IEEE first benchmark model. Small-signal analysis is conducted to reveal the impacts of series compensation level, parameters of proportional and integral controllers, and wind speed on the existing oscillatory modes.