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Research on Control Strategy of Hybrid Energy Storage System of Squirrel Cage Wind Turbine under Unbalanced Fault

When the grid voltage is unbalanced, the lifetime of the drive train is reduced because of the squirrel cage induction generator (SCIG) torque oscillations with double grid frequency. In this paper, the coordinated control strategy of a hybrid energy storage system (HESS) is designed, including the positive-and negative-sequence voltage coordinated control strategy of DC/AC converter, the control strategy of the lithium battery and supercapacitor. The objective of the positive-sequence voltage control strategy is to maintain the voltage of the point of common coupling (PCC) at the rated value, while the objective of the negative-sequence voltage control strategy is to control the negative-sequence voltage to zero. Therefore, the control strategy can eliminate the generator electromagnetic torque oscillations. The control strategies of the lithium battery and supercapacitor in an HESS are designed as well. A lithium battery is employed to stabilize the DC voltage of the DC/AC converter and a supercapacitor is used to absorb power fluctuations on the DC side of the inverter. Finally, the correctness and effectiveness of the control strategy are verified by the simulation work in the real time digital simulator (RTDS).