Mitigation of oscillations in DFIG-based WECS operating in unbalanced networks

Recent developments show that renewable energy sources are considerably contributing to the electrical power generation and offer a future-oriented, sustainable and environmental-friendly alternative to fossil energy sources. Wind energy in particular has experienced a rapid growth over the past decades, so that wind energy conversion systems (WECS) will occupy an important role in the power generation having to reliably supply electric energy. Especially in remote rural areas or in weak distribution grids, unbalanced grid voltage conditions are common and need to be addressed by appropriate control measures. They evoke negative sequence grid voltages resulting in torque and active/reactive power oscillations at twice the grid frequency, thus inducing heavy mechanical stress in the WECS drive-train and increasing grid load respectively. An improved space vector control methodology which can simultaneously regulate positive and negative sequence components based on proportional-integral and resonant controllers is presented to effectively mitigate the impacts of grid voltage unbalance.