Subsynchronous resonance mitigation using variablespeed wind energy conversion systems

This work proposes a strategy to mitigate subsynchronous resonance (SSR) in synchronous generators using variable-speed wind energy conversion systems (WECSs) based on full converter wind turbines. A supplementary active and reactive power to be delivered by WECSs located near synchronous generators is designed to reduce the SSR phenomenon. These supplementary signals are calculated using an observer-based controller tuned through an optimal quadratic technique which allows to minimise the WECS control effort, whereas a good torsional oscillation damping is accomplished. The power ratio between the WECS and the synchronous generator, impact on the power quality and communication time-delay requirements are also discussed. The authors compare two control approaches: the first one uses mechanical measurements of the synchronous generator (measures which have high observability of torsional modes), whereas the second one uses voltage measurements at the wind farm connection point. In this way, two control schemes using remote and local measurements are proposed, and advantages and disadvantages of both schemes are presented. The proposed scheme can provide satisfactory torsional damping under a wide range of operating points, avoiding to include dedicated SSR damping equipment, and using the new wind farms installed in the network. Eigenvalue analysis and non-linear time-domain simulations confirm the good performance of the WECS-based SSR damping controller.