by Reza Tajali, P.E.

To assure critical power in many industrial and commercial facilities, medium voltage power is often switched between alternate sources. In these power systems, each individual feeder often supplies multiple transformers. The automatic transfer operation energizes all the transformers and their loads in one block, resulting in transformer inrush current on phase relays and ground relays. Where a ground relay tripping occurs, maintenance personnel may “solve” the problem by increasing the relay settings. The problem is that increasing the settings reduces the sensitivity of the relay, which reduces the likelihood of catching a fault online; late detection of faults allows more arcing and other damage.