Unbalanced Load Modeling and Control in Microgrid with Isolation Transformer

Renewables and energy storage rely heavily on power electronics. The control of power electronics in a microgrid with renewables and energy storages become more challenging under unbalanced load operation condition. The unbalance in current magnitude and phase will cause ripples in normal dq0 conversion, further causing voltage unbalance in the power converter current control loop. In this paper, a simulation test microgrid is studied as a good representation of a practical microgrid with local ancillary single loads. The double second-order generalized integrator-phase lock loop (DSOGI-PLL) technique for frequency and phase angle detection is modified to address the unbalanced current issue due to local single-phase load connected through an isolation transformer. The proposed method is verified through software simulation in both grid-connected and islanded modes. From the result, the modified DSOGI method proves to be a viable technique for improving power converter current control loop performance under single load or unbalanced load.