An Optimized Start-up Scheme for Isolated Cascaded AC/DC Power Converters

This paper presents results from an investigation and controls-based solution developed for a soft-start scheme of a cascaded and isolated AC/DC power converter. The problems investigated by the authors consist of inrush currents during pre-charging of the DC bus capacitors, saturation of the isolation transformer magnetics, ability of pre-charging from the medium voltage (MV) AC or low voltage (LV) DC port, and effects of noload and partial load at start-up with or without partial voltage on the output DC bus capacitor. The proposed soft-start modulation scheme is based on phase shift modulation methods for the DC-DC stage. For the control of the AC-DC stage a direct quadrature control is used. This scheme allows the converter system to start under no load or partial load and no voltage or partial voltage at either the MV or LV side. Theoretical analysis and experimental results of the converter are presented to demonstrate the performance of the proposed technique in achieving a smooth start-up.