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Hybrid virtual impedance‐based control strategy for DFIG in hybrid wind farm to disperse negative sequence current during network unbalance

When a hybrid wind farm based on doubly fed induction generator (DFIG) and fixed speed induction generator (FSIG) is connected to an unbalanced network, most of the negative sequence current caused by the unbalanced grid will be centralised at the FSIG for its inherent small negative sequence output impedance (NSOI). The centralised negative sequence current can cause significant torque pulsation in the drive train of FSIG, which may go beyond the pressure endurance of the drive train and cause some damages. Meanwhile, losses and risks of overcurrent and grid code violation can also be increased. This study presents a control strategy for DFIG in hybrid wind farm during network unbalance, which can disperse the negative sequence current and the consequent pressure among the three parallel power sources without communication, i.e. among the DFIG stator, the grid side converter (GSC), and the FSIG, rather than centralising the pressure at any of them. In order to achieve this target, a novel hybrid virtual impedance method is proposed to flexibly control the NSOI of the DFIG stator and the GSC from zero to infinity. Theoretical analysis and simulation results are provided to verify the operation performance of the proposed strategy.