Low-voltage-ride-through control of a modular multilevel SDBC inverter for utility-scale photovoltaic systems

This paper presents theoretical and experimental discussions on low-voltage-ride-through (LVRT) operation of a modular multilevel single-delta bridge-cell (SDBC) inverter intended for utility-scale photovoltaic (PV) systems. As the penetration of distributed generation from renewable energy sources connected to medium- and high-voltage grids increases, grid-tied inverters progressively need to abide to stricter grid codes, which demand measures to maintain reliability of the grid. The latest grid codes require inverters in generation systems to provide dynamic grid support in the event of grid faults by injection of a reactive current. This paper discusses two unique solutions for an SDBC inverter to enable dynamic grid support. One method is to inject a zero-sequence current through feedforward control within the inverter, and the other is by utilizing power from the distributed dc-dc converters. No negative-sequence current is injected into the grid in both cases. This paper also highlights the issue of voltage-sag transformation through delta-wye transformers and its effect on the LVRT capability of the SDBC inverter. Experimental results on a three-phase 12.6-kVA system prove that the SDBC inverter is capable of seamlessly operating through symmetric and asymmetric voltage sags.