A wireless controller for parallel inverters in distributed online UPS systems

In this paper, a novel controller for parallel-connected online-UPS inverters without control wire interconnections is presented. The wireless control technique is based on the well-known droop method, which consists in introducing P-ω and Q-V schemes into the inverters, in order to share properly the power drawn to the loads. The droop method has been widely used in applications of load sharing between different parallel-connected inverters. However, this method has several drawbacks that limited its application, such as a trade-off between output-voltage regulation and power sharing accuracy, slow transient response, and frequency and phase deviation. This last disadvantage makes impracticable the method in online-UPS systems, since in this case every module must be in phase with the utility ac mains. To overcome these limitations, we propose a novel control scheme, endowing to the paralleled-UPS system a proper transient response, strictly frequency and phase synchronization with the ac mains, and excellent power sharing. Simulation and experimental results are reported confirming the validity of the proposed approach.