Variable Speed Wind Turbines Based on Electromechanical Differential Systems

Abstract

This paper introduces a new electromechanical energy conversion concept for use in wind-based power generation systems. Modern wind turbines use frequency converters to meet the speed control requirement. But voltage source-based frequency converters typically have limited overload capability. In addition, they fully or partially decouple the rotating masses of wind energy converter systems, thereby removing altogether or reducing the inertia as a medium of energy storage and modulation from the perspective of the grid during transient conditions. In the proposed concept, a drive system based on electromechanical differential gear including a servo motor is used for generator torque control, while using a synchronous machine as the main generator. The system thus provides the speed variability required for optimal utilization of wind energy combined with the advantages of a directly grid-connected synchronous generator. It as a result permits the supply of high currents during faults to the grid to provide more effective voltage support as there are no electronic circuits in the main power path, which would limit the overload capability. Furthermore, the inertia of the system comes fully to bear, thus contributing to the overall grid inertia. The performance of the system has been studied experimentally on a prototype system and the results of the conceptual analysis have been verified. Using the mathematical model of the system, a grid fault simulation has been performed, and the response during fault and the damping behavior has been studied. The results show the excellent damping behavior due to the active damping control by the differential drive.