Application of MOV and gapped arresters on noneffectively grounded distribution systems

Conditions under which the substitution of a metal-oxide varistor (MOV) arrester for a gapped silicon carbide arrester of similar rating would have been inappropriate are examined with reference to the 33 kV Staten Island system, fed from the Fresh Kills and Fox Hills Substation, in New York City. The investigation indicates that steady-state overvoltages greater than 1.73 per unit are possible on backfed systems. Saturation of transformers cannot be relied upon to decrease steady-state overvoltages even when these voltages approach 2.0 per unit. Unigrounded systems are found to have high steady-state overvoltages regardless of neutral impedance. It is demonstrated that the use of an MOV on a unigrounded system having backfeeds can result in very high arrester ratings which are costly, large, and provide greatly reduced margins. It is concluded that, under conditions of high steady-stage overvoltages during fault conditions, a gapped arrester may prove to be the best solution