Design and implementation of a current controller for the parallel operation of standard UPSs

The parallel operation of static inverters is, in a large amount of cases, the appropriate solution to achieve the high power required by some applications or to improve power system reliability. The limited inverter capacity obliges to parallel the individual units to obtain the nominal load power. In UPS systems, there are situations where a high reliability/availability is required by critical loads. Parallel redundancy appears an immediate solution to satisfy this requirement. This paper presents a control system for parallel operation of nonredundant UPSs based on current control. The relative phase between the inverters is constant. Simulation results as well as experimental studies are presented. The overall control system is implemented on a simple and low cost platform.