On Circulating Current Mitigation for Modular UPS/Inverters

Abstract

In order to improve reliability, modular inverter configurations with both ac and dc ports connected in parallel are employed. Typically, power sharing between these modules is achieved using droop control. The circulating current flow between these modules is an issue that needs to be addressed. These currents can be categorized into two types, namely, cross and zero-sequence circulating currents, and are caused by differences in output impedance, dead times, or nonsynchronized pulses between the modules. This article focuses on reducing the circulating current arising due to the difference in output impedances and suggests two schemes for curtailing the circulating currents. The ideas are based on varying the virtual impedances adaptively with the output current values. The proposed methods are tested using real-time simulations and through experiments on a laboratory hardware system with different types of loads.