Unbalanced DC-link voltage regulation in a back-to-back 3-level PWM converter for a doubly-fed induction motor/generator

An unbalanced DC-link voltage regulation strategy is proposed for a three-level voltage source inverter used in the rotor cascade of a double-fed asynchronous motor-generator. The major problem in determining the presence of the unbalanced component is investigated and a solution is proposed using an adaptable filter. This latter is only of a low order and is designed to ensure easy implementation into a (DSP). The regulator is based on the injection of a second order harmonic and its design is presented as well as an investigation of the critical point for correct action of the regulator. Real measurements performed on a low-power test bench after implementation into a DSP validate the accuracy and quality of the filter as well as the control strategy proposed.