LIGHTNING FLASHOVERS ON OVERHEAD TRANSMISSION LINES

 Lightning flashovers are segregated into three main types, for stroke locations on a phase conductor, on an overhead shield wire, or to nearby ground.


SHIELDING FAILURE FLASHOVERS
Shielding failure flashovers events result from a lightning stroke terminating directly on a phase conductor. For shielded lines, these events should be very infrequent and of very low stroke current magnitude.

For unshielded lines (i.e., “static less” lines), these events will be much more common and will involve the full distribution of lightning stroke current magnitudes. Arresters can be used to address shielding failure flashovers by applying the arresters on the exposed phases.

The arresters must be installed at every tower or pole to be effective at preventing shielding failure flashovers. For unshielded line applications, arrester energy requirements must be adequately addressed since the stroke currents and durations they will be exposed to are harsher than in shielded line applications.

BACK FLASHOVERS
Back flashovers, events result from a lightning stroke terminating on the ground system (i.e., shield wires, tower tops, and pole tops) causing a potential across the insulation that causes a flashover to occur.

The surge traveling on the shield wire will cause surge voltages to be induced in the phase conductors. The magnitude of the induced voltage is a function of the current magnitude, resistance, and geometry.

Stroke currents exceeding a critical current value will develop sufficient voltage between the structure and the phase conductor to cause an insulator flashover. The phase with the poorest coupling to the shield wire will be the most highly stressed and therefore most likely to flash over. Local grounding conditions have a major impact on back flashover performance.

Arresters can be used to address these types of outages by placing them on the least coupled phases (e.g., bottom phases) or in high footing resistance areas. For applications in high footing resistance areas, it is important to apply the arresters not only in the areas of high footing resistances, but also one or two structures away from the high footing resistance areas.

INDUCED VOLTAGE FLASHOVERS
Induced voltage flashovers events result from nearby lightning strokes inducing voltages on line
conductors. Because the induced overvoltages measured on distribution lines rarely exceed 300 kV, it is common belief that this phenomenon has little effect at transmission voltage levels. However, the induced voltages tend to increase with line height.

There may be some structures used at 34.5 kV through 69 kV (sometimes referred to as “sub-transmission” voltages) that could be susceptible to induced voltage flashovers from nearby lightning strokes.

For lines that are susceptible to induced voltage flashovers, arresters at relatively wide spacing may be used to minimize the effects of these events.

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