Variable DC Grid Wind Turbine Motor-Generator Grid-Connected System with Model-Based MPPT

In order to solve the loss in grid strength with inverter-based grid-connected wind energy systems, this paper proposes a variable DC-grid wind turbine system with a motor-generator grid connection. The system consists of a cluster of wind turbines in the wind farm that is connected to a DC grid via passive diode rectifiers. The DC grid feeds an inverter that drives a motor-generator system that is grid connected; even with no wind power, the motor-generator system stays connected to the grid to provide 24/7 grid strength and reactive power compensation. The DC grid voltage is controlled in a unique way by means of the inverter to ensure maximum power point tracking. This is done by a developed model-based slip-frequency V/Hz control method of the induction motor as prime mover. Transfer function analysis and simulations confirm the stability of the system, although only if the induction motor is controlled in the correct way.