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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