Power electronic transformers for AC-AC and AC-DC conversion with reduced number of switches

Abstract

In this research, a novel reduced-switch Power Electronic Transformer is proposed for three-phase power conversion (three-phase AC to AC as well as three-phase AC to DC). The goal of this project is to reduce the weight and size of the power converter by replacing the low-frequency transformers with high-frequency transformers (HFTs). The novelty of these proposed topologies is that they have only two controlled switches on the primary side of the high-frequency transformers. Additionally, these switches operate at 50% duty ratio, hence they are easy to control. Pulse Width Modulation (PWM) control is only necessary in the converters on the secondary side of the HFT. The modulation strategies proposed in this work achieve nearly Zero Current Switching (ZCS) for these two primary switches. In the proposed three-phase AC to AC Power Electronic Transformer, a Matrix Converter (MC) is employed on the secondary side of the transformer. Matrix converter with nine four-quadrant switches is a `more-silicon' and nearly capacitor-less solution for AC to AC conversion. This single-stage AC-AC converter without any electrolytic capacitors translates to higher reliability and efficiency. High-frequency transformers have finite leakage inductance, hence, any switching in the primary or secondary of the transformer requires commutation of the transformer inductive currents. This problem of leakage energy commutation and the required protection are studied in detail in this thesis. Other features of this converter are bi-directional power flow and power factor correction. The second Power Electronic Transformer is proposed for three-phase AC to DC power conversion. This converter operates on the Dual Active Bridge (DAB) principle wherein the transformer leakage inductance is used for power transfer. Hence, this converter does not suffer from problems associated with leakage energy commutation and additional snubber circuits are not required for this converter. The proposed modulation provides the advantages of unity power factor on the AC side, galvanic isolation and bi-directional power flow capability. Both these PETs have been analyzed and simulated. Laboratory prototypes have been built and tested to verify the advantages of the proposed PETs.