Power Management Performance Analysis of a Photovoltaic/Battery Unit Supplying Unbalanced Loads in Islanded Microgrid

Abstract

Since photovoltaic (PV) represents a vast part of microgrid generation, its no power reserve feature requires battery energy storage systems (BESS) to provide frequency and voltage support in islanded mode. Control strategies have been developed to allow it, however, tests have not considered the impact of unbalanced loads, even less for microgrids sourced by only one generation unit, where generation curtailment can be necessary. This paper analyzes the performance of grid-forming droop control in a three-phase microgrid under these circumstances. Tests were carried out in normal operation, where BESS absorbs or injects power to keep the power balance between generation and load, while the PV unit delivers the maximum power; further, for when the BESS reaches charging power or state-of-charge limit, and PV unit must reduce power to maintain the power balance. It was observed that 5% of load unbalance is enough to cause harmonic distortions higher than the allowed limits, and 10% makes the power delivered to the load drop. High load unbalances (above 25%) make the PV unit and the BESS inject their maximum power to compensate for power losses provoked by high harmonic distortions. It means that PV unit does not perform power curtailment in those situations even when it would normally be required to, and BESS abnormally discharge to keep the power balance. The results suggest the need for harmonic control. They were obtained from Matlab/Simulink simulations.