Practical Considerations*

Equipment and support hardware on distribution structures may severely reduce CFO. These “weak-link” structures may greatly increase flashovers from induced voltages. Several situations are described below.

Guy wires. Guy wires may be a major factor in reducing a structure’s CFO.

For mechanical advantage, guy wires are generally attached high on the pole in the general vicinity of the principal insulating elements. Because guy wires provide a path to the ground, their presence will generally reduce the con- figuration’s CFO. The small porcelain guy-strain insulators that are often used provide very little in the way of extra insulation (generally less than 30 kV of the CFO).

A fiberglass-strain insulator may be used to gain considerable insulation strength. A 20-in. (50-cm) fiberglass-strain insulator has a CFO of approxi- mately 250 kV.

Fuse cut-outs. The mounting of fuse cut-outs is a prime example of unpro-

tected equipment which may lower a pole’s CFO. For 15-kV class systems, a fuse cut-out may have a 95-kV BIL. Depending on how the cut-out is mounted, it may reduce the CFO of the entire structure to approximately 95 kV (approximately because the BIL of any insulating system is always less than the CFO of that system).

On wooden poles, the problem of fuse cut-outs may usually be improved by arranging the cut-out so that the attachment bracket is mounted on the pole away from any grounded conductors (guy wires, ground wires, and * This section is from IEEE Std. 1410-1997. Copyright 1997. All rights reserved. The author chaired the IEEE working group on the Lightning Performance of Distribution Lines during the development and approval of this guide.

neutral wires). This is also a concern for switches and other pieces of equip- ment not protected by arresters.

Neutral wire height. On any given line, the neutral wire height may vary

depending on equipment connected. On wooden poles, the closer the neutral wire is to the phase wires, the lower the CFO.

Conducting supports and structures. The use of concrete and steel structures

on overhead distribution lines is increasing, which greatly reduces the CFO. Metal crossarms and metal hardware are also being used on wooden pole structures. If such hardware is grounded, the effect may be the same as that of an all-metal structure. On such structures, the total CFO is supplied by the insulator, and higher CFO insulators should be used to compensate for the loss of wooden insulation. Obviously, trade-offs should be made between lightning performance and other considerations such as mechanical design or economics. It is important to realize that trade-offs exist. The designer should be aware of the negative effects that metal hardware may have on lightning performance and attempt to minimize those effects. On wooden pole and crossarm designs, wooden or fiberglass brackets may be used to maintain good insulation levels.

Multiple circuits. Multiple circuits on a pole often cause reduced insulation.

Tighter phase clearances and less wood in series usually reduces insulation levels. This is especially true for distribution circuits built underneath trans- mission circuits on wooden poles. Transmission circuits will often have a shield wire with a ground lead at each pole. The ground lead may cause reduced insulation. This may be improved by moving the ground lead away from the pole with fiberglass spacers.

Spacer-cable circuits. Spacer-cable circuits are overhead-distribution circuits

with very close spacings. Covered wire and spacers [6 to 15 in. (15 to 40 cm)] hung from a messenger wire provide support and insulating capability. A spacer-cable configuration will have a fixed CFO, generally in the range of 150 to 200 kV. Because of its relatively low insulation level, its lightning performance may be lower than a more traditional open design (Powell et al., 1965). There is little that can be done to increase the CFO of a spacer- cable design.

A spacer-cable design has the advantage of a messenger wire which acts as a shield wire. This may reduce some direct-stroke flashovers. Back flash- overs will likely occur because of the low insulation level. Improved ground- ing will improve lightning performance.

Spark gaps and insulator bonding. Bonding of insulators is sometimes done

to prevent lightning-caused damage to wooden poles or crossarms, or it is done to prevent pole-top fires. Spark gaps are also used to prevent lightning damage to wooden material [this includes Rural Electrification Administra- tion specified pole-protection assemblies (REA Bulletin 50-3, 1983)]. In some parts of the world, spark gaps are also used instead of arresters for equip- ment protection.

Spark gaps and insulator bonds will greatly reduce a structure’s CFO. If possible, spark gaps, insulator bonds, and pole-protection assemblies should

not be used to prevent wood damage. Better solutions for damage to wood and pole fires are local insulator-wood bonds at the base of the insulator.

12.7.3 Shield Wires

Shield wires are effective for transmission lines but are difficult to make work for distribution lines. A shield wire system works by intercepting all lightning strokes and providing a path to ground. If the path to ground is not good enough, a voltage develops on the ground with respect to the phases (called a ground potential rise). If this is high enough, the phase can flashover (it is called a backflashover).

Grounding and insulation are important. Good grounding reduces the ground potential rise. Extra insulation protects against backflashover. As an example, consider Figure 12.25 where a 22-kA stroke (which is on the small size for lightning) is hitting a distribution line. The ground potential rises to 400 kV relative to the phase conductor, enough voltage to flashover most distribution lines.

To keep the insulation high, use fiberglass standoffs to keep the ground wire away from the pole to maximize the wood length. Also, make sure guy wires and other hardware do not compromise insulation.

FIGURE 12.25

Shield-wire lightning protection system.

Phase wire Shield wire 1 kA 1 kA 20 ohms 20 kA = 20 kA(20 ohms) = 400 kV V 22 kA

Ground the shield wire at each pole. Lightning has such fast rise times that if a pole is not grounded, and a lightning strike hits the shield wire it will flash over before the grounds at adjacent poles can provide any help in relieving the voltage stress.

At all poles, obtain a ground that is 20 W or less. Good grounding is vital! Exposed sections of circuit such as at the top of a hill or ridge should have the most attention. Getting adequate grounds may require:

• More than one ground rod. Make sure to keep them spaced further than one ground rod apart.

• Deeper ground rods • Chemical soil treatments

• Counterpoise wires (buried lengths of wire)

Figure 12.26 shows estimates of performance versus grounding for several insulation levels based on the approach of IEEE Std. 1410-1997. In order to ensure that lightning hits the shield wire and not the phase conductors, maintain a shielding angle of 45∞ or less (as defined in Figure 12.27).

12.7.4 Line Protection Arresters

Arresters are normally used to protect equipment. Some utilities are using them to protect lines against faults, interruptions, and voltage sags. To do this, arresters are mounted on poles and attached to each phase. For protec- FIGURE 12.26

Performance of a shield wire depending on grounding and insulation level. 300 kV CFO 200 kV 100 kV 10.0 20. 50. 100.0 200. 500. 0 20 40 60 80 100

Pole footing resistance, ohms

Percentage of direct strikes

tion against direct strikes, arresters must be spaced at every pole (or possibly every other pole on structures with high insulation levels) (McDermott et al., 1994). This is a lot of arresters, and the cost prohibits widespread usage. The cost can only be justified for certain sections of line that affect important customers.

Arresters have been used at wider spacings such as every four to six poles by utilities in the southeast for several years. This grew out of some work done in the 1960s by a task force of eight utilities and the General Electric Company (1969a, 1969b). Anecdotal reports suggest improvement, but there is little hard evidence. Recent field monitoring and modeling suggest that this should not be effective for direct strikes. One of the reasons that this may provide some improvement is that arresters at wider spacings improves protection against induced voltages. Nevertheless, arresters applied at a given spacing is not recommended as the first option. Fixing insulation problems or selectively applying arresters at poles with poor insulation are better options for reducing induced-voltage flashovers.

For direct strike protection, arresters are needed on all poles and on all phases. The amount of protection quickly drops if wider spacings are used. Lead length is not as much of an issue as it is with equipment protection, but it is always good practice to keep lead lengths short. The arrester rating would normally be the same as the existing arresters.

Grounding is normally not an overriding concern if arresters are used on all phases. If grounds are poor, one effect is that surges tend to get pushed out away from the strike location (since there is no good path to ground). One of the implications is if just a section of line is protected (such as an exposed ridge crossing), then grounding at the ends is important. Good grounds at the end provide a path to drain off the surge.

One concern with arresters is that they may have a relatively high failure rate. Direct strikes can cause failures of nearby arresters. Something in the range of 5 to 30% of direct lightning strikes may cause an arrester failure. This is still an undecided (and controversial) subject within the industry. It FIGURE 12.27

Shield wire shielding angle.

Shield wire Phase wire Shielding angle

is recommended that the largest block size available be used (heavy-duty or intermediate-class blocks) to reduce the probability of failure.

Field trials on Long Island, NY, of arresters at various spacings did not show particularly promising results for distribution line protection arresters (Short and Ammon, 1999). LILCO added line arresters to three circuits. One had spacings of 10 to 12 spans between arresters (1300 ft, 400 m), one had spacings of five to six spans (600 ft, 200 m), and one had arresters at every pole (130 ft, 40 m). Arresters were added on all three phases. We also mon- itored two other circuits for comparison. None of the three circuits with line arresters had dramatically better lightning-caused fault rates than the two circuits without arresters. Statistically, we cannot infer much more than this since the data is limited (always a problem with lightning studies). One of the most significant results was that the circuit with arresters on every pole had several lightning-caused interruptions, and theoretically it should have had none. Missing arresters is the most likely reason for most of the light- ning-caused faults. One positive result was that the arrester failure rate was low on the circuit with arresters on every pole.

Automated camera systems captured a few direct flashes to the line during the LILCO study. Figure 12.28 shows a direct stroke almost right at a pole protected by arresters (arresters were every five to six spans on this circuit). Ideally, the arresters on that pole should divert the surge current to ground without a flashover. The arresters prevented a flashover on that pole, but two of the three insulators flashed over one pole span away. An arrester at the struck pole also failed.

Another field study showed more promise for line arresters. Common- wealth Edison added arresters to several rural, open feeders in Illinois (McDaniel, 2001). ComEd uses an arrester spacing of 1200 ft (360 m); as a trial, they tightened the arrester spacing to 600 ft (180 m) on 30 feeders (and all existing arresters that were not metal oxide were replaced). The 30 feeders with the new spacing were compared to 30 other feeders that were left with the old standard. Over three lightning seasons of evaluation, the upgraded circuits showed that circuit interruptions improved 16% (at a 95% confidence level). Note that most of the interruptions were transformer fuse operations.

Another way to apply arresters is to use them on the top phase only. The top phase is turned into a shield wire. When lightning hits the top phase, the arrester conducts and provides a low-impedance path to the pole ground. Just as with a shield-wire system, grounding and insulation are critical. A top-phase arrester application cannot be used on typical three-phase cross- arm designs because there is no top phase.

In areas where grounding is poor and arrester failure is a concern, arresters can be used with a shield wire system. The shield wire takes away most of the energy concerns, and the arresters protect against backflashovers. This provides very good lightning protection (and is very expensive).

FIGURE 12.28

Lightning-caused fault on a Long Island Lighting Company 13-kV circuit. (From Short, T. A. and Ammon, R. H., “Monitoring Results of the Effectiveness of Surge Arrester Spacings on Distribution Line Protection,” IEEE Trans. Power Delivery, 14(3), 1142-1150, July 1999. ©1999 IEEE. With permission.)

FIGURE 12.29