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.

I. Introduction

In recent years, the increasing demand for electricity supply in a continuous basis for systems such as life support, communications, commercial, industrial process control and transportation, among others, has led to an increase in research on parallelism of Uninterruptible Power Supplies (UPSs) [1] – [5] and, in particular, on the parallel connection of such equipment. The parallelism of UPSs offers advantages such as the expandability, modularity, redundancy and, consequently, an increase in reliability [1], [5]. From the point of view of energy conversion, the parallelism of UPSs corresponds to the parallelism of frequency inverters, since those are the output stage of the UPSs. Another area that has gained prominence in recent years is that of microgrids, which are formed by the parallelism of distributed generation systems, such as photovoltaic and wind power. These microgrids behave similarly to the parallelism of frequency inverters when they act isolated from the electric grid [9] – [14].