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 (DERs). 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 modeling 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- 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.