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This paper presents a novel type of variable speed wind turbine with a new drive train different from the variable speed wind turbine commonly used nowadays. In this concept, a synchronous generator is directly coupled with the grid, therefore, the wind turbine transient overload capability and grid voltage support capability can be significantly improved. An electromagnetic coupling speed regulating device (EMCD) is used to connect the gearbox high speed shaft and synchronous generator rotor shaft, transmitting torque to the synchronous generator, while decoupling the gearbox side and the synchronous generator, so the synchronous generator torque oscillations during a grid fault are not transmitted to the gearbox. The EMCD is composed of an electromagnetic coupler and a one quadrant operation converter with reduced capability and low cost. A control strategy for the new wind turbine is proposed and a 2 MW wind turbine model is built to study the wind turbine fault ride-through capability. An integrated simulation environment based on the aeroelastic code HAWC2 and software Matlab/Simulink is used to study its fault ride-through capability and the impact on the structural loads during grid three phase and two phase short circuit faults.

This paper proposes a virtual synchronous control strategy for Doubly-Fed Induction Generator (DFIG)-based wind turbines. On the basis of the proposed excitation control model orientated by rotor flux, and the P-f, Q-V droop control models which emulate the governor and exciter of synchronous machine, this paper established the basic virtual synchronous control structure for DFIG. This control strategy can provide DFIGbased wind turbines with intrinsic regulation ability of frequency and voltage without using the phase-locked loop (PLL) of the grid. Considering the power limitation under certain wind speed, the proposed adaptive droop control method realized the maximum power point tracking (MPPT) of wind turbines in steady state. Simulation results demonstrate the feasibility and effectiveness of the presented virtual synchronous control for grid-connected DFIG. This control strategy can not only provide the possibility of unifying the grid-connected wind turbines as the standard synchronous power, but also decrease the negative effect on the grid frequency and voltage with high penetration of wind energy.

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