A simplified Small-Signal Model for Analyzing the Parallel Operation of Frequency Inverters in UPS

This paper deals with a small-signal model for analyzing the parallel operation of frequency inverters used in Uninterruptible Power Supplies - UPSs. The UPSs operate without communication with each other and is controlled by the traditional droop method. Initially, small-signal models of a single unit connected to an infinite bus and those referring to several ones connected in parallel are revisited. Then, considering a set of common parameters, comparisons are made between the eigenvalues and the dynamic responses found for two systems: (i) an inverter connected to an infinite bus and (ii) two inverters operating themselves in parallel. Simulations and experimental results have shown the possibility of simplifying the small-signal model to represent the parallelism of frequency inverters operating in several situations, making their analysis and design simpler.