A neutral line regulator is designed as a plug-in module or new integrated inverter with a lower rating 4th-leg, instead of using a conventional four-leg inverter to supply power to three-phase four-wire unbalanced AC loads or three-phase nonlinear loads without a neutral connection. The neutral line regulator may be designed for controlling only the unbalanced power rather than using a fully rated inverter leg. Since this plug-in module may be separate from the main inverter and may operate at a lower power, the switching frequency may be higher than the main inverter. Thus, the size and weight requirements for providing the neutral line can be significantly reduced. In addition, the plug-in regulator may maintain voltage balance between the center-tapped DC link capacitors for non-linear, unbalanced loads. Moreover, the plug-in module may be used as a retrofit module replacing, for example, delta-wye transformers.