Influence of current measurement errors on parallel-connected UPS inverters

Redundancy is a common approach for ensuring reliability on applications using UPS inverters. However, overall system´s reliability and efficiency is very dependent on the power measurement precision, which if not properly done can lead to system failure. Current transformers, that are widely used to measure the output current of UPS inverters, introduce a phase shift in the output signal that negatively influences the parallelism circuits, leading to a waste in the generated power and therefore reducing system´s efficiency and reliability. This paper analyses the influence of current measurement problems that may occur when two or more uninterruptible power supplies are connected together. Current measurement errors are experimentally obtained with two UPS inverters connected in parallel using the drooping method and the results, presented at the end, suggest that the power measurement needs to be carefully designed.