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In selecting adequate insulators for substations at distribution and transmission voltage levels, special measures may be needed in locations exposed to freezing conditions. This first part of the paper describes the environmental and insulator parameters that influence the risk of flashover on station insulators exposed to freezing conditions. Index Terms--Climate, contamination, flashover, fog, high voltage, ice, insulator coordination, snow, station insulators.

In locations exposed to freezing conditions over the station service life, additional measures for selecting adequate station insulators at distribution and transmission voltage levels should be taken. This second part of the paper describes the selection process and mitigation options, based on the environmental parameters, as discussed in Part I. These include insulator size, shape, surface material, surface quality, electric field improvement, and orientation. Index Terms--Climate, contamination, flashover, fog, high voltage, ice, insulator coordination, snow, station insulators.

Purpose: To find how a spark from a positive or negative rod locates an object to connect to, or finds a weak spot in a cable insulation. Experimental: 1-2-m gap in ambient air between a rod and a horizontal wire suspended 0.25-0.5 m above ground, subjected to fast-rise (1.2 [micro]s) and slow-rise (150 [micro]s) positive and negative impulse voltages, and observed with photomultipliers, streak/framing camera, and open-shutter cameras. Main result: A leader channel from the rod feels its way by spraying a conical volume in front with streamers. When these connect to object(s) the leader follows either the direction of the highest streamer density or the most conductive streamer channel (our cameras cannot decide). If the streamers do not connect the influence of the missed object(s) is small. Additional: Evidence has been found for the slow increase with time of the field-to gas density ratio E/n in streamer channels due to channel heating and subsequent gas density reduction. Index Terms--High-voltage spark breakdown, meeting streamers, spark attraction, streamer channel heating.

This paper reports on the relative vulnerabflities of bare- and insulated-line conductors to hits from discharge strokes coming from sources at voltages too high to prevent flashovers, like lightning. Protection against such strokes must be based on diversion strategies, where the conductor to be protected is made as unattractive as possible compared to alternative, usually grounded conductors nearby. As a first attempt to quantify this discharge-stroke susceptibility for some commonly used line conductors, we have performed measurements where high-voltage impulses, applied to a rod 1-2 m above a ground plane, launch streamer-leader sequences propagating downwards toward a wire or insulated cable or both, suspended horizontally 0.25-0.5 m above the ground plane. The ensuing spark will thus always have a dual choice, either between the wire/cable and ground, or between the wire and the cable. The percentage of spark hits to the wire/cable is counted and plotted as a function of the sideways displacement of the wire/cable from the rod axis, and is a measure of the 'spark appeal' of the wire/cable. The measurements confirm that streamers emitted from an object are especially important for attracting sparks from negative sources. Our results indicate that three-phase overhead lines should be as thick and as closely spaced as possible to minimize lightning-stroke risks. The effect of line insulation is both to increase the effective thickness and to permit closer spacing between the phases. Index Terms--High-voltage spark breakdown, meeting streamers, spark attraction.

Test methods for evaluating flashover voltage of ceramic and nonceramic insulators under ice, snow, and cold-fog conditions are recommended. The paper describes the procedure to be followed at various stages of an evaluation test including preparation of the test object, insulator precontamination, ice, snow, and cold-fog deposits as well as voltage applications and flashover voltage evaluation. Some relevant parameters for ice accretion, flashover voltage evaluation, artificial pollution, and power source requirements are taken tram IEC 60129, IEC 60507, and IEEE Std.-4-1995 on standard techniques for high-voltage testing. Index Terms--Contamination, flashover, fog, high voltage transmission, ice, insulator, snow, statistical interpretation, test methods.