Power Decoupling Control of Parallel Converters Based on Negative Virtual Impedance Under Unbalanced Conditions

In islanded microgrid, when multiple converters are operated in parallel under the unbalanced load conditions, it is challenging to obtain both high voltage quality and current sharing, especially including unbalanced current sharing, at the same time. Moreover, when load voltage is controlled to be balanced, the instantaneous double-frequency power ripples caused by negative sequence current will be coupled to DC side, resulting in the ripple of the DC voltage or current, which may create instability of the system, lower its efficiency and shorten the lifetime of DC sources. To address these issues, the improved three-phase converter is adopted as a microgrid converter in this paper. The positive and negative sequence equivalent circuits are introduced. On this basis, the distribution principles of the current sharing and double-frequency pulsating power are investigated. A distribution strategy is proposed to achieve proportional distribution of average power by using positive sequence power droop, and proportional distribution of negative sequence current and double-frequency pulsating power based on the combination of negative sequence virtual impedance and active power decoupling control, where the AC side capacitors of converters are used to proportionally supply the double-frequency pulsating power caused by the unbalanced loads. Therefore, it greatly reduces the ripples on the DC link while high voltage quality and current sharing are obtained. The correctness of the method is verified by simulation and experiment.