DC bus voltage balancing controllers for split DC-link four-wire inverters and their impact on the quality of the injected currents

The increasing share of single-phase distributed generation units in low voltage grids causes voltage unbalance problems and overvoltages. Therefore, the need for power quality improving control strategies of grid-connected inverters emerges. Some control strategies require three-phase four-wire inverter topologies. The simplest way to connect the fourth wire is by connecting it to the mid-point of the dc-bus. This sometimes causes challenges in the stabilisation of the mid-point. In this article two algorithms for stabilising the midpoint of a three-phase four-wire inverter are proposed. Both algorithms are described in detail and validated experimentally. The results showed that both algorithms perform well under perturbations and are able to maintain the midpoint potential close to zero while the quality of the injected currents is not deteriorated.

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