Control of distributed generation systems under unbalanced voltage conditions

Managing power delivery from distributed generation systems is challenging when the grid voltages are unbalanced, since the negative sequence voltage causes power oscillations at twice the fundamental grid frequency. Current regulation using sequence components can be used to control these real and reactive power oscillations, and thus help mitigate the unbalanced network voltages. This paper presents a flexible control scheme using double sequence frame current regulators that can readily adjust between eliminating real or reactive power oscillations created by unbalanced grid voltages, or alternatively simply balancing the three phase currents. The mitigation influence of these alternative control strategies on unbalanced grid voltages is then examined for resistive and reactive distribution networks. Simulation results are presented to validate the proposed control strategies and illustrate their influence on unbalanced network voltages.