Comparison of PR and PI Controllers for Inverter Control in an Unbalanced Microgrid

High penetration of renewable-fed Inverter Based Resources(IBRs) will most likely be an integral part of microgrids in the near future. Since such microgrids will, in most cases, evolve from existing distribution feeders, it will be unbalanced in terms of loads, phases, and feeder impedances. Typical control strategy of a conventional inverter that follows the grid voltage and frequency, while injecting positive-sequence current can lead to undesirable performance, especially in the islanded mode of operation. So, for the proper functioning of these microgrids, there is a need for grid-forming inverters that can enable acceptable performance and coexist with conventional grid-following inverters that supply only positive-sequence currents. In addition to the control objectives, limiting of inverter output during faulted or overload conditions with a current limiter is essential. These control objectives can be implemented in both the synchronous reference frame (dq coordinates) and the natural reference frame (abc coordinates). However, a comparison of inverter response, dynamics, and implementation are needed to understand each controller's merit as relates to this specific topology. This paper compares the control performance of the two approaches for a highly unbalanced microgrid in grid-connected and islanded modes. Acceptable operating conditions with stability against disturbances and faults are the primary focus of this study.