New medium-voltage Adjustable Speed Drive (ASD) topologies with medium-frequency transformer isolation

In this paper, two new medium-voltage Adjustable Speed Drive (ASD) topologies with medium-frequency-link transformer isolation are introduced. A medium-voltage (MV) medium frequency (MF) transformer is realized along with several series connected AC-AC converters. Operating the multi-winding transformer at MF contributes to weight and volume reduction without sacrificing efficiency. For this application, it is shown that a unidirectional AC-AC converters with diode front end are sufficient to power the MF transformer. The AC-AC converter gating signals are generated by a convolution of a square wave and a PWM sine wave. This modulation is adjusted in such a way that the input current harmonic frequencies are of higher order, thus introducing power factor correction (PFC). The proposed modulation strategy is flexible, allows for high switching frequency operation while ensuring that the transformer core is operated at constant MF over the entire range. Further, the rectified DC-link voltage is of high quality. Two MV-ASD configurations are explored for generating variable voltage variable frequency (VVVF) - (i) a three-phase NPC inverter with medium-voltage IGBTs and (ii) Multi-level single-phase inverters. Overall, the proposed topologies make the ASD size smaller in weight/volume and at the same time, provide unity power factor sinusoidal currents at the utility input.