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Optimal control of a grid connected variable speed wind energy conversion system based on squirrel cage induction generator

This paper presents the design of a Linear Quadratic Regulator (LQR) with Integral action (LQRI) applied to a grid connected wind energy conversion system (WECS) based on squirrel cage induction generator (SCIG) with full-capacity power converter (WECS-SCIG). The controller is used to achieve speed regulation for maximum power extraction from the wind turbine, DC bus voltage regulation and to keep the power factor at unity for the grid-side converter. The model of this latter is set in the d-q rotating reference frame synchronized with phase a of grid voltages while rotor flux oriented vector control is used for the machine-side converter. Knowing that the standard LQR provide essentially proportional gains, the system dynamic is augmented with the integral of the q component of the grid currents, the rotor flux, the DC bus voltage and the generator speed in order to cancel steady-state errors. The WECS-SCIG is controlled as a whole i.e. a multi-input-multi-output (MIMO) system and a fixed PWM at 2.7 kHz is used to generate the gating signals of the two back to back converters. The system is tested for both steady-state and in transient for a sudden variation in wind speed. The simulation results obtained with SimPowerSystems (SPS) and Simulink of Matlab implementation of the controller proved the effectiveness of the control strategy.