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For a multi-megawatts doubly-fed induction generator (DFIG), the grid voltage disturbances may affect the stator flux and induce the transient stator flux, due to the direct connection of the stator and the grid. The accumulation of the transient stator flux caused by the variations of the stator voltage may introduce harmful power and torque oscillations to the DFIG, and even lead to rotor overcurrent. For the conventional field oriented vector control (FOVC) strategy, the design of the controller is based on the steady state model of the DFIG, which neglects the dynamic of the stator flux, and therefore it cannot work well during the transient state to decay the transient flux and to suppress the flux accumulation. In this paper, a dual-loop control strategy, which includes the conventional current loop and an additional flux loop, is proposed to not only control the active and reactive power, but also decay the stator transient flux, and avoid the accumulation of the stator transient flux. Moreover, the proposed strategy can obtain nearly constant stator active power and electromagnetic torque, which may prolong the lifetime of the drive train. A case study on a typical 2MW DFIGbased wind turbine demonstrating the effectiveness of the proposed control methods is verified with simulations in Matlab/Simulink. The proposed control methods are also experimentally validated using a scaled-down 7.5kW DFIG. The simulation and experimental results clearly validate the effectiveness and feasibility of the proposed strategy, and show the improved dynamic performances of the DFIG. Index Terms-doubly-fed induction generator (DFIG), grid disturbances, transient stator flux, wind turbine (WT), dual loops.

Large renewable energy systems (RESs), e.g., photovoltaic (PV) and wind turbine (WT) are connected to grid usually weak, with long lines. This degrades power system stability and reliability. Additionally, the magnetic characteristic of conventional power transformer (CPT) implies a slow dynamic and leads to grid current harmonic too high when wind speed is low and irradiance is weak. This paper proposes a parallel connection of a solid-state transformer-based smart transformer and CPT to not only reduce the power rating but also to support the voltage amplitude and improve the grid current quality. The simulation and experimental results clearly verify the correctness and feasibility of the proposed strategy.

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