Asymmetrical converters with medium frequency transformer isolation to interface low voltage DC to a medium voltage three phase grid

This paper presents an asymmetrical converter topology with medium frequency transformer (MFT) isolation to interface a low voltage DC source with a 3-phase medium voltage grid. The topology consists of asymmetrical converters connected in parallel across the DC input, followed by a MFT for isolation and a diode bridge fed current source inverter (CSI) connected to the MFT secondary. The asymmetrical converters connected in parallel share the high input currents and are modulated so as to generate a multi-level stepped waveform across the transformer secondary. The asymmetry is introduced in the MFT turns ratio. The power flow in DC sources like PV being unidirectional, the transformer secondary side is connected to diode bridge and pulse-width modulated (PWM) CSI which interfaces a medium voltage grid. The stepped voltage across the transformer secondary and the use of diode bridge results in a trapezoidal current through the transformer windings eliminating the problem of leakage commutation. The paper also determines analytically the transformer voltages based on the DC link current requirement of the CSI based on the grid active power. The proposed topology is simulated using MATLAB/Simulink.