Effectiveness of zinc oxide surge arresters on substation equipment probabilities of flashover

Analyses indicate that significant differences can exist between effective probability of flashovers (PFOs) for a given insulation level, depending on whether surge arresters or circuit-breaker closing resistors are used to hold surges to about the same phase-to-ground levels. For phase-to-phase insulation, arresters are significantly less effective than circuit-breaker closing resistors at reducing PFOs. This is true for application of phase-to-ground arresters with or without phase-to-phase arresters. For phase-to-ground insulation, resistors are also more effective than arresters at reducing insulation requirements. This result is particularly applicable for station and line uprating and compact designs. Calculation techniques which consider phase-to-ground surges on all three phases, rather than techniques which simple consider only the phase which has the maximum surge, yield PFOs which more correctly represent the overall PFO of the insulation system