Power Electronic Transformer with Open-End Winding Electric Drive for Wind Energy Conversion Systems

Abstract

Power Electronic Transformers (PETs) provide a reduction in size over line frequency transformers by operating at much higher frequencies than line frequency or grid frequency. Due to their smaller size, they could be useful in renewable energy systems where an interface with the grid is needed. As the name suggests, a power electronic interface is needed to convert line frequency voltages to high frequency voltages before they can be fed to the transformer. A PET topology that has simple control and less number of high voltage devices would be considered desirable due to lower total device cost and easy control implementation. A push-pull based PET topology has been proposed in the past which contains only two high voltage controlled switching devices and the control of those two devices is very simple. This topology could be configured for single stage ac to dc power conversion, to which an open-end winding dc to ac converter could be connected. Alternatively, it could be configured for direct ac to ac power conversion using dual matrix converters. In the first part of this thesis, the aforementioned push-pull based power electronic topology has been studied for power conversion from ac to dc and vice versa. Both single phase ac to dc and three phase ac to dc variants of the topology have been analyzed for power transfer, rms currents and soft switching. They provide attractive features which include single stage ac to dc bidirectional power conversion, unity power factor operation in open loop and control of dc side voltage using simple PI controllers. The other part of this thesis deals with open-end winding drives for suppression of common mode voltages at machine terminals. Switching frequency Common Mode Voltages (CMV) are generated by conventional Pulse Width Modulated (PWM) drives at machine terminals, which cause shaft voltage build up leading to bearing currents. These bearing currents are harmful for the machine and also cause Electromagnetic Interference (EMI). Open-end winding drives consist of one electric drive connected on each end of a three phase electric motor with the stator neutral opened up to give three more terminals. Open-end winding drives can be controlled to suppress switching frequency CMV at machine terminals. In this thesis, open-end winding two level Voltage Source Inverter (VSI) drive and open-end winding two level Matrix Converter (MC) drives have been investigated. Carrier based PWM techniques have been proposed for each of these drives for suppressing CMV. In addition, an improved four step commutation method has been proposed for the open-end winding matrix converter drive to suppress CMV spikes during the commutation process. Finally, a circuit consisting of the reduced switch PET connected with an open-end winding MC drive has been studied for single stage ac to ac power conversion with open loop power factor control.