As the grid codes of wind turbines note, doubly fed induction generator (DFIG)-based wind energy conversion system (WECS) should maintain uninterrupted operation during certain voltage faults. To meet the codes, a novel configuration of DFIG-based WECS is proposed in which a superconducting magnetic energy storage (SMES) and a series transformer are incorporated. The control strategies of the WECS are redesigned to smooth the output power fluctuations and ride through different levels of voltage faults. The grid-side converter (GSC) is controlled to smooth power fluctuations caused by random wind speeds under normal conditions. The GSC and protection circuit are controlled to switch into the series compensation mode to fully compensate the stator voltage during voltage swells and slight voltage sags. While under severe voltage sags, the protection circuit operates in the current limiting mode, which significantly limits the stator and rotor fault overcurrent, improves the stator voltage and supresses fluctuations of the electromagnetic torque. The different faults are applied, the simulation results demonstrate that the presented scheme can effectively smooth output power fluctuations and strengthen the voltage fault ridethrough capability (FRT) of WECS.

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This paper presents a voltage sag compensator (VSC) integrated with superconducting magnetic energy storage (SMES) device. Performance evaluations of the SMES-based VSC for the initial and steady voltage sag compensations in sensitive load are carried out in details. Effects of the time delay in discharge response and initial voltage sag angle on the initial voltage sag compensation are discussed to clarify the transient SMES property. In addition, three definitions for the voltage sag compensation time duration are introduced to evaluate the steady SMES property for short-time and long-time voltage sag compensation operations. Index Terms-power quality, voltage sag compensator, sensitive load, superconducting magnetic energy storage (SMES).