A proportional-resonant controller-based wireless control strategy with a reduced number of sensors for parallel-operated UPSs

In this paper, a novel droop method to control the power sharing of parallel uninterruptible-power-supply (UPS) systems is presented. In a clear-cut contrast to the previously reported works, the controller is a proportional-resonant controller which ensures good transient response and steady-state objectives. Furthermore, the need for sensing the output current for the power-sharing control is removed by implementing it in a software routine. This brings a less complicated and less expensive structure in which the number of feedback sensors is reduced from three to two. The droop paralleling strategy is based on the drop in the inverter output frequency and amplitude. The application of proportional-resonant controllers based on internal model principle is also extended to parallel inverters, and its superior performance over the well-known proportional-integral-derivative controller is presented. In order to show the performance of the proposed system, two parallel-connected UPS systems are analyzed, and two types of linear and nonlinear loads are considered. The nonlinear load is compliant with IEC 62040-3 standard for class I UPS. The results show that the reduction of sensors does not result in any error, and the performance of the control system is excellent. To verify the proposed concept, the proposed strategy is implemented within two-625VA UPS systems. Several test results are provided for linear and nonlinear loads which validate the simulations results. The results indicate that the proposed parallel inverter structure offers a better system in terms of the number of feedback sensors, performance, and complication.