Stability analysis of instantaneous average current sharing control strategy for parallel operation of UPS modules

This paper discusses and analyzes the parallel operation of uninterruptible power supplies (UPS) with instantaneous average current sharing control strategy. The analysis demonstrate that the control method introduces a virtual impedance between the inverters. If the impedance configured as resistive, it will simplify the control strategy, the paper analyzes the stability of the control system by changing the control diagram from voltage-source mode to current-source mode, precise stable condition has been obtained considering the varying line inductance. Simulation models and several experimental 220V/2kVA inverters are built and paralleled to verify the theoretical predictions.