A Case History of Shore Power Transformer Taps Arcing during No-Load Condition

The on-board generators of ships use low-grade fuel for power generation. The process of switching off the on-board generators of berthing ships and connecting berthing ships to shore power is called shore-to-ship power supplies (STSPS). This STSPS operation ships minimizes air pollution. To comply with mandatory air-pollution requirements, in 2014, Port of Oakland installed STSPS projects at various berths. In 2015, at Berth 37, a 7.5 MVA, cast-coil transformer within an outside weatherproof enclosure experienced arcing of no-load primary tap connections at 12.47 kV. Installation and testing of all STSPS, including Berth 37, complied with NETA and LECZIEEE Std 80005-1. At Berth 37, a very sensitive current protection relay scheme cleared the taps arcing ground fault by tipping a 12.47 kV breaker. Arcing occurred when the transformer was energized without any load. No signs of transient overvoltage were observed on the power supply system during transformer taps arcing. Transformer design was per the industry standards. This paper describes the case history of transformer taps arcing and provides recommendations to avoid such incidents, which may cause damage if the arcing ground fault was not cleared quickly by the sensitive protection relay.