A Cascaded Dual Four-Leg Inverter for Photovoltaic Systems With Capability to Compensate Unbalanced Distribution Networks

Unbalance or asymmetry in the distribution networks is a well-known power quality issue. This power quality problem is even more important in the context of the microgrids since the presence of unbalanced currents can deteriorate the overall operation of equipment and the grid itself. In this context, this article proposes a multilevel inverter based on a cascaded configuration of a four-leg dual inverter. This multilevel inverter is specially appropriated to be used with a photovoltaic (PV) generator connected to a four-wire low-voltage grid. The power converter scheme is based on two four-leg two-level inverters in which three of the ac-side terminals are connected to a three-phase transformer with the primary in open-end winding arrangement. The dc voltage buses of the inverters are connected to two insulated PV arrays. This configuration accepts the injection of the PV generated power into the grid in a way that provides unbalanced load compensation. Additionally, a control system, a pulsewidth modulation modulator, and a current references scheme for the proposed power converter topology are presented. The proposed PV system generator can balance several load unbalances under different operating conditions. It is concluded that the proposed converter can provide significant ancillary services to an unbalanced grid. The system is validated through experimental tests performed under different operating conditions.