A simplified droop method implementation in parallel UPS inverters with proportional-resonant controller

In this paper, a simpler implementation of the well-known droop method for the control of parallel Uninterruptible Power Supply (UPS) systems is presented. In this method, in the power-sharing control scheme, the output current is calculated by software without the need for a current sensor, resulting in a simpler and cheaper structure. By doing so, the number of feedback sensors is reduced from three to two. The paralleling strategy uses the droop method in which the control strategy is based on the drop in the inverter output frequency and amplitude. The application of Proportional-Resonant (PR) controllers is also extended to parallel inverter and its superior performance over the well-known Proportional-Integral-Derivative (PID) controller is shown. To show the performance of the proposed system, a system of two-parallel connected UPS is simulated, and two types of linear and non-linear loads are considered. The non-linear load is compliant with the IEC 62040-3 standard for class I UPS. The results show that the reduction of sensors results in no error, and the control system performance is quite satisfactory. To verify the proposed concept, a two-625VA UPS system is implemented. Several tests on both linear and non-linear loads are performed and the results, which are in good agreement with those of the simulations, are provided. The results indicate that the proposed parallel inverter control structure provides a better system in terms of performance parameters.