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For rational insulation design of a UHV substation, full-scale switching impulse tests of phase-to-ground (shielding ring-to-tower) and phase-to-phase (shielding ring-to-shielding ring) insulating clearance have been carried out in dry and wet conditions. In the tests, shielding rings that will be installed on bushings at actual UHV substations were used. The sparkover voltages depend on not only gap configurations but also the waveform of applied switching impulses; in other words, they show what we refer to as U-characteristics. Critical sparkover voltages are higher than those of rod-rod gaps by 20%-25% in dry conditions and by 7%-10% in wet conditions.

A current waveform at flashover of an arcing horn affects a peak value of a calculated transient overvoltage in lightning surge analysis of a power transmission system.

A new calculation method of leader development which considers predischarge currents is proposed. Calculated leader delopment characteristics with this model are compared with photographs of a image converter camera and close agreement is shown. Further, breakdown voltage-time characteristics of rod-plane and rod-rod gaps with standard and non-standard lightning impulses of both positive and negative polarities are calculated and these values agree with experimental results. This new calculation method is applicable to long gaps of several meters.