A simple closed-loop interphase power controller for cascaded H-bridge converter-based DG units in islanded microgrids with imbalanced loads

Cascaded H-bridge (CHB) converters have been used to integrate multiple DC sources into grids with only a single-stage power conversion. However, power sharing among parallel DG units and power sharing among power cells of a CHB converter must be satisfied when they are used in an islanded microgrid system. This work presents a comprehensive power sharing scheme. First, the power sharing between parallel DG units is realized using the well-understood droop control. Power allocation among the CHB converter three-phase legs is also obtained with injection of zero-sequence voltage. In contrast with the conventional open-loop zero-sequence voltage injection approaches, where the reference zero-sequence voltage is obtained via an open-loop interphase power adjustment manner that is typically calculated based on the assumption of a well-balanced three-phase converter current, the proposed approach obtains zero steady-state interphase power control errors via a simple closed-loop interphase power controller. This approach can be used in the case of a CHB system with significant imbalanced current presented. Finally, a power sharing approach among the series-connected H-bridge modules within a converter leg is also developed via the re-distribution of series-connected module voltages. Each power cell can actively participate in the power sharing according to the back-stage battery state of charges using the aforementioned approach, but without any interference from the abovementioned triple-layer power control architecture.