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In this paper, an improved efficiency control scheme for wind energy conversion systems (WECSs) with squirrel cage induction generators is proposed. Thus, the power harvesting from the WECS is increased and additionally expansion of the exploitable wind speed region toward the lower speed range is accomplished. The generator is connected to the power grid by means of two space-vector-controlled back-to-back converters. A minimum ohmic loss (MOL) controller is introduced in order to minimize the generator resistive loss that is accomplished by adjusting the d-axis stator current according to torque conditions. The implementation of the proposed controller is easy and cost effective because neither additional control signals nor the knowledge of the generator loss model is required. The effectiveness of MOL controller and its successful cooperation with two types of maximum power point tracking (MPPT) controllers, which are employed to maximize the wind turbine output power, are experimentally verified. The MPPT controller is implemented by using an adaptive search control and a fuzzy-logic-based control technique, since both are independent of wind turbine characteristics and widely used. Selective experimental results are presented to demonstrate the resulting improvements of the suggested control scheme.
Date of Publication: Dec. 2012