Voltage Ripple Model and Capacitor Sizing for the Three-Phase Four-Wire Converter used for Power Redistribution

Three-phase unbalance in distribution grids is becoming more time-varying and is set to increase due to the integration of single-phase distributed energy resources (DER’s). Unbalance can be actively remediated using converters with a neutral connection to redistribute real and reactive power across all phases. This study considers the three-phase four-wire (3P-4W) converter, which can be used as a DER grid converter to provide grid services. By modelling the converter without a DC voltage source, it is ensured that power redistribution and reactive power compensation can take place even when the DER is not supplying power. Power redistribution is seen to create 60 Hz and 120 Hz voltage ripples across each of the DC-link capacitors of the converter which result in undesired voltages on the AC-side of the converter. Here, the precise time-domain capacitor voltage ripple is derived for each capacitor and experimentally verified. From this, a concise guide to capacitor sizing is provided to mitigate the undesired effects of power redistribution.