A Novel Hybrid Pulse Train Control Strategy to Mitigate Voltage Unbalance Rate in Stand-Alone Four-Leg Voltage Source Inverter System

Three-phase four-leg voltage-source inverter (FL-VSI) has the advantages of flexible control and high power density. For FL-VSI with three-phase asymmetric loads, positive-sequence and negative-sequence synchronous rotating dq-based PI controllers are usually applied to realize balanced three-phase output voltages. However, symmetric component decomposition and positive-sequence and negative-sequence rotating coordinate transformation make the digital implementation of the control system complicated. To mitigate voltage unbalance rate (VUR) and to avoid the transformation of rotating coordinates and the calculation of symmetric component decomposition, a hybrid pulse train (PT) control strategy for stand-alone FL-VSI with a three-phase asymmetric load in
$\alpha \beta z$
stationary coordinate system is proposed. The PT control strategy in
$\alpha \beta $
stationary coordinate system is utilized to mitigate negative-sequence VUR, and the zero-sequence differential feedforward strategy in the
$z$
stationary coordinate system is applied to mitigate zero-sequence VUR. The simulation and experimental results of the proposed control schemes are provided, which show that the VUR of the proposed system is mitigated below 2%, which meets IEC Standards.