Voltage transformer failure during routine switching in Drill Site

An extensive 34.5kV Drake overhead line network (OHL) supplies power to a group of Drill Sites scattered across a vast desert area. During routine energization in one of these sites, a bus-side voltage transformer experienced an insulation failure at the primary winding neutral point. This paper summarizes the frequency domain analysis to identify the root cause of the failure, as well as the phasor domain methodology to determine the parameter limits of the neutral voltage displacement (ANSI59) and negative sequence overvoltage (ANSI47) as protection functions against transient overvoltage conditions (TOV) and slow front transients (SFO) caused by switching, fault clearance or ferroresonance. As a lesson learned, installing an additional surge arrester at the main incomer, in addition to the one at the riser pole, must always be evaluated against ground faults close to the supply cable termination.