Two-level backward operation of a VSMC for PMSG grid-connected variable speed wind turbine systems

Commercial wind driven electrical generation turbines are well engineered systems that economically extract power from the wind. These systems often restrict the range of turbine rotational speeds so as to facilitate connection of the generator to the electrical power grid. Recently research attention is turning to the possibility of operating the turbine and generator with a wide range of speeds so as to simplify gear-box design and harvest a higher proportion of the available wind energy. This approach benefits from the use of power electronics to convert the variable frequency and variable voltage amplitude of the generator output into the constant frequency fixed amplitude electrical power required for grid connection. Proposed in this paper is a sensorless real-time maximum power extraction control algorithm for a wind turbine system employing a variable speed permanent magnet synchronous generator which is connected to the electrical grid with a backward very sparse matrix converter consisting of 12 IGBT devices. Power flows backward through the very sparse matrix converter from the generator to the inverter to the rectifier and then to the grid. The proposed technique is based on storing wind energy in the turbine system inertia, then releasing the energy to the grid without need for a mechanical sensor and without exceeding the system ratings. The controller estimates a virtual wind speed which is related to the generator speed and produces a virtual generator reference torque. The reference torque is calculated by simulating wind turbine operation at the maximum power coefficient and optimal tip speed ratio. As the generator speed changes, the estimated reference torque is updated until it settles to the maximum allowable torque at the given wind speed. Soft-switching hysteresis current control is used with space vector modulation and field oriented control to produce sinusoidal ac currents for the grid and the generator. Simulation results are presented for a 2.5 MW wind turbine system.