Short T.A. Electric Power Distribution Handbook

Подождите немного. Документ загружается.

628 Electric Power Distribution Handbook

12.6.4 Other Cable Failure Modes

Lightning may also puncture the jackets in jacketed cable. Ros (1993) dem-

onstrated that lightning current into a riser-pole ground created enough

ground potential rise to puncture jackets and leave pin holes. Fifty-mil jackets

punctured with 150 to 160 kV, and 80-mil jackets punctured with 155 to 170

kV. The voltage impressed across the cable jacket is the surge current times

the ground resistance at the riser pole, which may be very high with poor

grounds. Ros’ tests found semiconducting jackets did not allow nearly the

voltage to develop across the jacket.

Lightning can also damage cables from strikes above the ground. EPRI

tests found that rocket-triggered lightning strikes to the ground above buried

cables caused extensive damage (Barker and Short, 1996). Lightning nor-

mally breaks down the soil and arcs to the cable (see Figure 12.22 and Figure

12.23). We tested three cable conﬁgurations: unjacketed direct buried, jack-

eted direct buried, and jacketed cable in conduit. All were single-conductor

220-mil XLPE cables with a full copper neutral. Lightning above the ground

damaged them all to varying degrees. Lightning melted most strands of the

concentric neutral and punctured the jacket and conduit if present. In some

cases, lightning also damaged the insulation shield. What made the damage

particularly bad was that lightning hit the soil surface and continued an

arcing path 3 ft (1 m) underground directly to the cable. Measurements

FIGURE 12.22

Institute. Reprinted with permission.)

1791_book.fm Page 628 Monday, August 4, 2003 3:20 PM

Lightning Protection 629

showed that 15 to 25% of the lightning currents reached the padmounted

transformers on either side of the ﬂash point. This type of cable damage

would not show up immediately. More likely, cable failure would accelerate

from increased water entry and localized neutral heating. Voltages we mea-

sured across the cable insulation were not a signiﬁcant threat (they were less

than 17 kV).

The tests also pointed to secondary voltages as a concern. We found nearly

4 kV on the closest transformer secondary. Surge entry into the secondary

FIGURE 12.23

Cable and conduit damage observed from rocket-triggered lightning ﬂashes above the ground.

1791_book.fm Page 629 Monday, August 4, 2003 3:20 PM

630 Electric Power Distribution Handbook

stresses the transformer insulation and sends possibly damaging surges into

homes. Current entering into the secondary neutral terminal of the trans-

former induces voltages that stress the primary winding. Williams (1988)

presented evidence that padmounted transformers fail from secondary-side

surge entry in similar percentages to overhead transformers.

During the rocket-triggered lightning tests, strokes to the cable attached

from as far away as 15 ft (5 m). Furrows from trees to underground cables

as long as 300 ft (100 m) have been found (Sunde, 1968). Sunde derived a

model for lightning attraction to buried cables based on the lightning current

and the soil resistivity. The number of ﬂashes attracted to cables depends on

ﬂash density, the peak current magnitude, and the ground resistivity. Higher

peak lightning currents make longer jumps to cables. Cables in higher-

resistivity soil attract ﬂashes from farther away. The Sunde model predicts

that cables attract strikes within the following radius:

where

r = attractive radius, m

r = soil resistivity, W-m

= peak lightning current, kA

Using the IEEE distribution of peak ﬁrst strokes, this gives the following

collection rates to cables:

for N in ﬂashes/100 km/year (multiply by 1.609 for results in ﬂashes/100

mi/year) and N

in ﬂashes/km

/year. For resistivities in the region that

Sunde left out (100 to 1000 W◊m), either interpolate the results from each

equation or use the closest equation. For 1000 W◊m soil with N

= 1, cables

attract 2.7 ﬂashes/100 mi/year (1.7 ﬂashes/100 km/year). For areas in trees,

cables collect more ﬂashes; EPRI (1999) recommends doubling the number

of strikes for areas in trees.

These ﬂashes to cables puncture jackets and conduits. The amount of

heating damage to the neutral and to the insulation is primarily a function

of the total charge in the lightning ﬂash (since arcs exhibit a fairly constant

voltage drop at the point of attachment, the energy is a function of ÚIdt).

Burying shield wires above the cables is one way to offer protection against

this type of damage. An AT&T handbook (1985) provides estimates on shield-

wire effectiveness. Note that there is no current power industry practice for

◊£◊

◊≥◊

0 079

0 046

100 m

1000 m

£◊

≥◊

0 092

0 053

100 m

1000 m

1791_book.fm Page 630 Monday, August 4, 2003 3:20 PM

Lightning Protection 631

this type of protection, and the amount of damage (percent of cable faults)

is unknown.

12.7 Line Protection

Line protection is the attempt to reduce the number of lightning-caused

faults. Utilities have increased interest in line protection as one way to

improve reliability and power quality. Since lightning can ﬂash over a 230-

kV or 500-kV transmission line, we should not be surprised that protecting

a 13-kV distribution line is difﬁcult. To protect against direct hits, we need

either a shield wire or arresters to divert the stroke to ground without a

ﬂashover. Lightning strokes close to a line may induce enough voltage to

ﬂash over the line insulation. Induced voltages are easier to contain since

induced voltages have much lower magnitudes than direct strike voltages.

Maintaining enough insulation capability is the normal way to limit induced-

voltage ﬂashovers. Line arresters can also greatly reduce induced voltage

ﬂashovers from nearby strokes.

12.7.1 Induced Voltages

Lightning strikes near a distribution line will induce voltages into the line

from the electric and magnetic ﬁelds produced by the lightning stroke. These

induced voltages are much less severe than direct strikes, but close strikes

can induce enough voltage to ﬂash insulation and damage poorly protected

equipment.

The charge and current ﬂow through the lightning channel creates ﬁelds

near the line. These ﬁelds induce voltages on the line. The vertical electric

ﬁeld is the major component that couples voltages into the line (magnetic

ﬁelds also play a role). As the highly charged leader approaches the ground,

the electric ﬁeld increases greatly; and when the leader connects, the electric

ﬁeld collapses very quickly. The rapidly changing vertical electric ﬁeld

induces a voltage on a conductor, which is proportional to the height of the

conductor above ground. The induced voltage waveform is usually a narrow

pulse, less than 5 or 10 msec wide, and it may be bipolar (negative then

positive polarity).

Most measurements of induced voltages have been less than 300 kV, so

the most common guideline for eliminating problems with induced voltages

is to make sure that the line insulation capability (CFO) is higher than 300

kV. Lines with insulation capabilities less than 150 kV have many more

ﬂashovers due to induced voltages.

A simpliﬁed version of a model developed by Rusck (1958, 1977) approx-

imates the peak voltage induced by nearby lightning. Rusck’s model can be

1791_book.fm Page 631 Monday, August 4, 2003 3:20 PM

632 Electric Power Distribution Handbook

simpliﬁed to estimate the peak voltage developed on a conductor (IEEE Std.

1410-1997):

where

V = peak induced voltage, kV

I = peak stroke current, kA

h = height of the line, usually ft or m

y = distance of the stroke from the line

This equation is for an ungrounded circuit. For a circuit with a grounded

neutral or shield wire, the voltage from the phase to the neutral is less. For

normal distribution line conductor spacings, multiply the answer by 0.75 for

lines with a grounded neutral. So, for a 30-ft (10-m) distribution line, a 40-

kA stroke 200 ft (60 m) from the line induces 165 kV on a line with a grounded

neutral. Most lines are immune from strikes farther than 500 ft (150 m) from

the line.

Models of attraction to distribution lines show that for lines out in the

open, most ﬂashes that do not hit the line are too far away to induce a

particularly high voltage across the insulation. For environmentally shielded

lines — those with nearby trees and buildings — fewer strikes hit the line,

but the line should have higher induced voltages because lightning strokes

could hit closer to the line.

EPRI sponsored rocket triggered lightning tests in 1993 that showed

induced voltages could be higher than predicted by Rusck’s model for some

strikes (Barker et al., 1996). For strikes to the ground 475 ft (145 m) from the

line, voltages were 63% higher than Rusck’s model. Hydro Quebec and New

York State Electric and Gas sponsored another round of tests in 1994 that

were also led by P. P. Barker. Closer strokes, strokes 60 ft (18 m) from the

line, induced less voltage than the Rusck model.

Table 12.14 compares the rocket-triggered lightning measurements with the

Rusck model. High ground resistivity at the Florida test site probably explains

why the 475-ft (145-m) measurements were higher than the Rusck prediction

(Ishii, 1996; Ishii et al., 1994). Why the closer measurements are lower is not

TABLE 12.14

Comparison of Induced Voltage Measurements to Rusck Predictions

Distance

Number of

Data Samples

Rusck Prediction

ind

Rocket-Triggered

Lightning Measurements

ind

60 ft (18 m) 20 12.2 5.25

400 ft (125 m) 8 1.7 2.67

475 ft (145 m) 63 1.4 2.24

V = .

36 5

◊

1791_book.fm Page 632 Monday, August 4, 2003 3:20 PM

Lightning Protection 633

veriﬁed. My best guess is because those strokes were triggered from rockets

launched on a 45-ft (14-m) tower. Since the tower is above ground, the positive

charge on the tower shields some of the negative charge in the downward

leader, so the electric ﬁeld inducing voltages into the line is smaller. Environ-

mentally shielded lines should act similarly to the tall tower. The charge

collected on the tree should shield the line and reduce the induced voltage.

The three induced-voltage data points when normalized for a 30-ft (10-m)

tall distribution line ﬁt the following equation:

Figure 12.24 shows estimates of induced voltages with insulation level for

both open ground and for lines that are environmentally shielded (usually

by trees). One is based on Rusck’s model using the approach from IEEE Std.

1410-1997. Another is based on the curve ﬁt of the triggered lightning results,

which shows more reasonable answers for environmentally shielded lines.

12.7.2 Insulation

High insulation levels on structures help prevent induced voltage ﬂashovers.

Insulation levels are also critical on some types of line protection conﬁgura-

tions with shield wires or arresters. Note that for a normal distribution line,

higher insulation levels may actually stress nearby cables and transformers

more. With high insulation strengths, a higher voltage develops across the

insulation before it ﬂashes over. By allowing a larger magnitude surge on

the line before ﬂashover, damage to nearby equipment is more likely. The

ﬂashover of the insulation acts as an arrester and protects other equipment.

The critical ﬂashover voltage (CFO) of self-restoring insulation (meaning

no damage after a ﬂashover) is the voltage where the insulation has a 50%

probability of ﬂashing over for a standard 1.2/50-msec voltage wave. For

insulators, manufacturers’ catalogs specify the CFO. CFO and BIL are often

used interchangeably, but they have slightly different deﬁnitions. A statistical

BIL is the 10% probability of ﬂashing over for a standard test wave. Normally,

CFO and statistical BIL are within a few percent of each other.

Lightning may ﬂash along several paths: directly between conductors

across an air gap or along the surface of insulators and other hardware at

poles (normally the easiest path to ﬂashover). We need to consider phase-

to-ground and phase-to-phase paths. At a pole structure, the ﬂashover path

may involve several insulating components, the insulator, wood pole or

crossarm, and possibly ﬁberglass. Wood and ﬁberglass greatly increase the

structure insulation.

Table 12.15 shows the critical ﬂashover voltage of common components.

When more than one insulator is in series, the total insulating capability is

V =

121

1791_book.fm Page 633 Monday, August 4, 2003 3:20 PM

634 Electric Power Distribution Handbook

FIGURE 12.24

Induced voltage ﬂashovers vs. insulation level for a line with a grounded neutral.

TABLE 12.15

CFO of Common Distribution

Components (By Themselves)

Component kV/ft kV/m

Air 180 600

Wood pole or crossarm 100 350

Fiberglass standoff 150 500

Shielded lines

Lines in the open

Rusck model

Rocket-triggered model

0 100 200 300 400 500

0.01

0.1

1.0

10.0

100.0

CFO, kV

Flashovers/100 miles/year

for N

=1 flash/km

/year

1791_book.fm Page 634 Monday, August 4, 2003 3:20 PM

Lightning Protection 635

less than the sum of the components. When insulation is subjected to a

voltage surge, the voltage across each component splits based on the capac-

itance between each element. Normally, this voltage division does not split

the voltage by the same ratio as their insulation capability, so one component

ﬂashes ﬁrst leaving more voltage across the rest of the components.

The simplest way to estimate the insulation level of a structure is to take

the CFO of the insulator (usually about 100 kV) and add the wood length at

75 kV/ft (250 kV/m). Often the wood provides more insulation than the

insulator. Estimate the air-to-air gap using 180 kV/ft (600 kV/m). The air gap

between conductors usually has higher insulation than the path along the

insulator and wood. So, most ﬂashovers occur at the poles, the weakest point.

Typical distribution structures generally have CFOs between 150 and 300 kV.

And, to eliminate induced voltage ﬂashovers, we try to have 300 kV of CFO.

Another way to estimate the structure CFO of several components in series

is to take the square root of the sum of the squares of each component.

Another more precise way to estimate structure CFOs is with the extended

CFO added method described by Jacob et al. (1991) and Ross and Grzy-

bowski (1991) (or, see IEEE Std. 1410-1997 for a simpliﬁed version).

12.7.2.1 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

chaired the IEEE working group on the Lightning Performance of Distribution Lines during the

development and approval of this guide.

1791_book.fm Page 635 Monday, August 4, 2003 3:20 PM

636 Electric Power Distribution Handbook

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

1791_book.fm Page 636 Monday, August 4, 2003 3:20 PM

Lightning Protection 637

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

20 ohms

20 kA

= 20 kA(20 ohms)

= 400 kV

22 kA

1791_book.fm Page 637 Monday, August 4, 2003 3:20 PM