Generator Differential Relay Trip on an Offshore Platform: Copyright Material IEEE, Paper No. PCIC-2018-09

Low-impedance generator differential protection is commonly used for detecting stator phase and ground faults in medium-voltage applications. An offshore platform started experiencing sporadic generator differential protection trips during start-up of newly commissioned large water injection pump (WIP) motor rated 4100 kW. The main power sources on the platform are three 16.25 MVA gas turbine generators (GTGs). The GTGs are connected to the 11 kV main electrical distribution system switchgear which also supplies the low-inrush WIP motor started direct-online (DOL). The differential relay trips happened under various loading conditions with one or more GTGs supplying the platform power demand. Site checks and subsequent successful motor starts ruled out any internal fault in the differential protection zone or equipment failure. This paper describes the operating conditions, analysis methods, investigation findings, and proposed solution for the differential relay trip events. In addition, it also provides recommendations for new projects and existing facilities that utilize generator differential protection systems.