The weakness of low voltage ride-through capacity, power oscillations under asymmetrical fault and the instability of the output power are three major difficulties that need to be urgently addressed in the field of doubly fed induction generator (DFIG). To simultaneously deal with these issues, this paper proposes a cost-effective energy storage device-based unified power quality conditioner (ESD-UPQC) to improve the low voltage ride through capability of the DFIG, suppress voltage oscillations under unbalanced faults and regulate DFIG output power during fluctuated wind speed conditions. A comprehensive protection scheme is carried out that the series side converter of the ESD-UPQC is controlled using dual control to suppress the post-fault voltage oscillations under transient conditions, while three advanced control targets are proposed for the parallel side converter aiming at regulating DFIG output power during wind gust, and suppress power oscillations during symmetrical and asymmetrical grid faults. Simulation results attest the feasibility of the proposed controller. The robustness of the proposed ESD-UPQC controller is validated through a series of comparisons based on a lead-acid battery scheme and an ESD-DFIG scheme for power smoothing and LVRT enhancement respectively.

The shortage of dc protection device limits the development of dc grids. This letter proposes a novel superconducting magnetic energy storage (SMES)-based transformerless series voltage restorer (TLSVR) for dc-load protections. The proposed SMES-TLSVR is made up mainly of a bidirectional-switch-based converter and a superconducting coil, and is interfaced in the series with the dc transmission line. It has the advantages of duplex voltage compensation ability, transformerless structure, and swift compensation response, and has a wider compensation range than traditional voltage-fed transformerless dynamic voltage restorer. The topology, circuit principle, and control strategy are elaborated in this letter. Finally, a kW-class SMES prototype is carried out to verify the feasibility of the proposed SMES-TLSVR.

The limitation of theDC protection device confines the development of MV/LVDC grids. This paper presents a DC dynamic voltage restorer to exploit DC custom power devices for DC distribution networks in principle. It is based on an improved AC/DC dual active bridge and battery energy storage to maintain the voltage profile of sensitive loads in DC networks. The principle of the improved DC-DVR has been analyzed from the perspectives of the range of compensated voltage, current characteristics, and control strategy. Compared with other existing methods, the proposed DC-DVR features a simple structure, an easy control strategy, and the ability to mitigate both voltage sag and swell by injecting forward or reverse voltage. Experiment results demonstrate the correctness of the theoretical analysis and the feasibility of the proposed structure. A case study of DC doubly-fed induction generator (DFIG) interfaced with power grid is carried out, integrated with the proposed DC-DVR to verify the operation principle of the proposed scheme.

Scarce fault ride through capacity and unsteady output power are two vital problems in various in-grid doubly fed induction generators (DFIGs). This paper investigates an integrated DFIG protection scheme with a modified superconducting magnetic energy storage-fault current limiter (SMES-FCL). Based on proper parameter estimation of superconducting coil (SC) inside the SMES-FCL, modified controls of the SC and rotor side converter (RSC) are presented. Simulation results obtained from a 1.5 MW DFIG-based wind turbine case show that the proposed scheme is capable of limiting the peak values of fault rotor current, DC-link voltage and electromagnetic (EM) torque under symmetrical and asymmetrical faults, and thus to protect the RSC, gearbox and mechanical parts of the whole DFIG. With an additional positive-sequence d-q current modification control, this scheme is also discussed to lower the SC current capacity, to shorten the recovery time after grid faults, and to smooth the active power of the DFIG during normal operations.