Double trouble

This article presents the modeling, simulation, and practical application of a double-fed asynchronous motor-generator for pumped-storage plants equipped with a three-level VSI cascade. The problem of DC unbalance control is studied in this particular application. A solution is proposed and tested in practice. This solution uses a specific adaptable filter and is based on the injection of the second order harmonic. A method for determining the critical point has been developed. It ensures the correct action of the regulator over the whole operating range. This important aspect has also been validated. A sensorless method is presented for the determination of the rotor angle, which is needed in the control strategy. Measurements performed on a low-power system validate the method. Finally, comparisons between simulations and measurements performed on a low-power system validate not only the complete control strategy but also the DC unbalance control, the sensorless method, and the modeling of the overall system. This study provides a useful basis for an optimized design of a large variable speed motor-generator equipped with a three-level VSI cascade.