Parallel Connection of Different UPS Units for Operation, Maintenance and Reliability Improvement

Nowadays, the parallel operation of different Uninterruptible Power Supply (UPS) units is not technically permitted. In this case, UPS hot swap and defective UPS succouring are not possible, too. Moreover, power supply interruptions are necessary to carry out maintenance interventions. The main purpose of this work is to study solutions to allow the parallel connection of different UPS units. A mathematical model has been developed to simulate and analyse the parallel operation of different UPSs. First, the model has been validated by means of data from the literature and also personally collected by the authors on the field. In particular, two different hardware configurations were analysed. The proposed solutions mainly limit the circulating currents in parallel operation of different devices. The first solution is based on adding an output compensation inductance for each UPS. The second solution makes use of an output insulation transformer, for each UPS, as a compensation inductance. The technical limits of the parallel operation are discussed. Further analyses have been carried out to formulate a proposal for a new binding technical standard, to be adopted by manufacturers, to ensure a future global compatibility between different UPSs.