El acto de habla y su interpretación como unidad comunicativa del discurso

12.8.1 Role of Grounding

With poor grounding, lightning strikes to distribution lines subject more equipment to possibly damaging surges due to ground potential rise. Light- ning current must flow to ground somewhere. If the pole ground near the strike point has high impedance, more of the surge flows in the line, exposing more equipment to possible overvoltages. At the point of a direct stroke to a distribution conductor, the huge voltages flash over the insulation, shorting the phase to the neutral. From there, the lightning travels to the closest ground. If there is no proper pole ground nearby, the next most likely path is down a guy wire. If the path to ground is poor, the phase conductor and neutral wire all rise up in voltage together. This surge (on both the phase and neutral) travels down the circuit. When the surge reaches another ground point, current drains off the neutral wire, which increases the voltage between the phase and the neutral. The voltage across the insulation grows until a surge arrester conducts more current to ground or another insulation flashover occurs. Figure 12.29 shows a drawing describing the ground poten- tial effect, and Figure 12.1 shows a photograph of a direct strike that caused another flashover remote from the flash point.

Good grounding helps confine possible lightning damage to the immediate vicinity of the strike. If the distribution insulation flashes over, the short circuit acts as an arrester and helps protect other equipment on the circuit (as long as grounds are good).

Normally, the ground resistance right at a piece of equipment protected with an arrester does not impact the primary-side protection. The primary surge arrester is between the phase and ground. For a lightning hit to the primary, the arrester conducts the current to ground and limits the voltage across the equipment insulation (even though the potential of all conductors may rise with poor grounding).

Grounding does play a role for transformers, as they are vulnerable to surge current entering through the secondary. Poor grounding pushes more current into the transformer on the secondary side and increases the possi- bility of failure. Poor grounding also forces more current to flow into the secondary system to houses connected to the transformer. And, poor ground- ing may also push more current into telephone and cable television systems.

12.8.2 Burndowns

The lightning arc itself can pass enough charge to burn small primary and secondary wires although sometimes it is hard to tell whether it is the lightning or the fault arc that does most of the damage. Normally, it is the power frequency arc that does the most damage. Chisholm et al. (2001) cites

a damage rate on Hydro-One’s primary distribution system of 0.13/100 mi/ year (0.08/100 km/year) for an area with a ground flash density of about 1 flash/km2_{/year. This is about 1% of the lightning flashes hitting circuits.}

The lightning arc causes damage the same way that a fault current arc (and also an arc welder) does: the heat of the arc melts conductor strands. Damage is mainly a function of the charge in the flash. Lightning tends to be more damaging than the equivalent ac fault arc. A negative direct current is about 50% more damaging than an alternating current passing equal coulombs. Negative flashes are more destructive than positive flashes; it takes about three times the charge for a positive stroke to cause the same damage as a negative stroke. Lightning also tends to stay in one place more than an arc because it does not have the motoring effect.

12.8.3 Crossarm and Pole Damage and Bonding

Although service experience indicates that lightning rarely damages poles or crossarms; in high-lightning areas, concern is warranted. Darveniza (1980) cites a survey by Zastrow published in 1966 that found poles failing from lightning in the range of 0.008 to 0.023% annually (versus an overall failure rate of 0.344 to 1.074% with more than half of these due to decay). Other surveys summarized by Darveniza also found minimal lightning-caused damage. Lightning overvoltages damage and shatter poles and crossarms when the wood breaks down internally rather than along the surface. If the wood is green and wet, internal breakdowns and damage are more likely. Damage within the first year of service is more likely.

If historical records show that wood damage is a problem, bonding the insulators (grounding the base of each insulator) protects the wood, but this shorts out the insulation capability provided by the wood. Better are surface electrodes fitted near the insulator pin, including wire-wraps, bands, or other metal extensions attached near the insulator in the likely direction of flashover. This local bonding encourages breakdown near the surface rather than internally.

Preventative measures for lightning damage to wood also reduce the likelihood of pole-top fires. Leakage current arcs at metal-to-wood inter- faces start fires (Darveniza, 1980; Ross, 1947). Local bonding using wire bands or wraps helps prevent pole damage. Bonding bridges the metal-to- wood contacts where fires are most likely to start. Local bonding is better than completely bonding the insulators because the insulation level is not compromised.

12.8.4 Arc Quenching of Wood

Wood poles and crossarms can prevent faults from forming after lightning flashes across the wood (Armstrong et al., 1967; Darveniza et al., 1967). Whether this arc quenching occurs depends mainly on the power frequency

voltage gradient along the arc at the time of the flashover. If the power- frequency voltage happens to be near a zero crossing when the lightning strikes, the lightning-caused arc is likely to extinguish rather than becoming a power-frequency fault.

The voltage gradient for an arc inside wood is much higher than an arc voltage gradient in the air. Darveniza (1980) found voltage gradients in wood on the order of 1000 V/in. (400 V/cm) through wood compared to about 30 V/in. (12 V/cm) for arcs in air. When dry wood insulation arcs, the path is often just barely below the surface (about a millimeter); and the arc is sur- rounded by wood fibers, so the arc-suppression action occurs. Wood fibers raise the arc voltage primarily by cooling it and absorbing ionized particles (similar to the arc-absorbing action of fuses). The arc voltage gradient is lower for wet wood (flashovers are no longer fully contained and tend to arc on the surface rather than just below the surface). There is considerable variability in the arc voltage, especially for wet wood. To transition from a lightning arc to a power-frequency fault, the power-frequency voltage must be higher than the arc voltage to keep the arc conducting.

Darveniza calculated several probabilities for arc quenching based on the arc voltage gradients (see Figure 12.30). Arc quenching is less likely on wet wood and when multiple phases flash over (even if the flashovers are on the same phase). To achieve significant benefit, voltage gradients must be kept less than 3 kV/ft (10 kV/m) of wood. For a 12.47Y/7.2-kV system, this is at least 2.4 ft (7.2/3 = 2.4 ft or 0.7 m) of wood in the phase-to-ground flashover paths and at least 4.1 ft (1.2 m) of wood in the phase-to-phase

FIGURE 12.30

Probability of a power-frequency fault due to a lightning-caused flashover over wet wood crossarms with different numbers of flashover paths. (Adapted from Darveniza, M., Electrical

Properties of Wood and Line Design, University of Queensland Press, 1980. With permission.)

1f, 1 path 3f, 6 paths _{3f, 3 paths} 1f, 3 paths 0 5 10 15 0.0 0.2 0.4 0.6 0.8 1.0

Voltage gradient, kVrms per foot of wood

Probability of a power arc

flashover paths. And, if we really want to count on this effect, we should at least double these lengths.

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Check the top of your arrester and make sure you have the proper size for the voltage. Some times they look alike. Don't assume, it ain’t good for your Fruit of the Looms.

Anonymous poster, on a near miss when a 3-kV arrester was mistakenly installed instead of a 10-kV arrester (7.2 kV line to ground)