Mitigation of SSR by embedding subsynchronous notch filters into DFIG converter controllers

The interaction between controllers of doubly fed induction generators (DFIGs) and fixed series compensations may cause a new type of subsynchronous resonance (SSR), namely subsynchronous control interaction. To mitigate this emerging issue, a novel suppression method, embedding subsynchronous notch filters (SNFs) into DFIG converter controllers, is proposed in this study. By using the impedance-model-based analysis and a quantitative location-dependent performance index, the best location is identified to insert SNFs into the controllers of both rotor-side and grid-side converters. Two specific SNF schemes are recommended for practical use, and a design procedure is developed to tune their parameters. As a case study, they are then applied to a practical series-compensated wind-farm system that suffered from SSR. Both impedance analysis and time-domain simulations have been conducted to investigate their performance. The results verified that they can defuse the interaction between DFIG controllers and series compensation; therefore, successfully eliminating the risk of unstable SSR under all possible operating conditions. The proposed SNF schemes are easy to design and implement, robust to changeable operating conditions and would not affect the normal dynamics of DFIGs. So, they are of great potential in addressing practical SSR issues.