Parallel redundant operation of UPS with robust current minor loop

This paper describes a control method of UPS inverters connected in parallel. The UPS output voltage is controlled by an algorithm with an inverter current. A minor loop using two types of observers. Only the following two points are added to the algorithm for realizing parallel operations: (1) feedback of a common load current instead of the UPS output current; and (2) adjusting the phase of the output voltage to make the reactive component of the circulating currents zero. The proposed method has the following features, compared with conventional methods; (1) it does not need an additional output voltage control for parallel operation; (2) the impedances among UPSs are resistive so that the circulating currents caused by phase difference among UPSs have only reactive components; and (3) increase of software for parallel operation control is minimal.