A new medium-voltage energy storage converter topology with medium-frequency transformer isolation

Abstract

In this paper, a new medium-voltage energy storage converter topology with medium-frequency-link transformer isolation is introduced. A medium-voltage (MV) medium frequency (MF) transformer is realized along with several series connected AC-AC full-bridge converters with bi-directional switches. The AC-AC converters transform the low-frequency (50/60 Hz) AC utility voltage into MF (0.4 to 2 kHz) AC voltage by modulating the utility input with MF square wave. The series stacked AC-AC converters share equal voltages, enabling the usage of lower voltage rated IGBTs/MOSFETs. The 3-phase voltage at the secondary winding of the MF transformer is fed to a three-phase Pulse Width Modulation (PWM) rectifier to enable bi-directional power flow. The rectifier output is filtered and interfaced with the energy storage devices. The rectifier gating signals can be generated using hysteresis or PI control of 3-phase transformer currents, forcing the currents to be of the same shape as the transformer voltages. It is shown that the system input currents as well as the battery current are of good quality and at the same time, the transformer voltages are of medium frequency. Overall, the proposed energy storage converter topology results in smaller weight/volume and at the same time, provides unity power factor sinusoidal input currents. Simulation results along with preliminary experimental waveforms are shown to validate the concept.