Short T.A. Electric Power Distribution Handbook

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Electric Power Distribution Handbook

where

is the shielding factor, which is between 0 and 1. In Florida, shield-

ing factors of around 0.7 were found for circuits out of four substations

(Parrish and Kvaltine, 1989). Lines in these areas had moderate to heavy

shielding by nearby objects.

12.3 Traveling Waves

When lightning injects surge currents into power lines, they move down the

conductors as traveling waves. Understanding the nature of traveling waves

helps in predicting the voltage and current levels that occur on power sys-

tems during lightning strikes. Surges travel at 99% the speed of light on

overhead circuits — about 1000 ft/

sec(300 m/

sec). The inductance

and

capacitance

uniformly distributed along the line determine the velocity

and the relationship between voltage and current. The velocity in a unit

length per second is based on

and

per length unit as

The voltage and current are related by

, the

surge impedance

V = ZI

where

is a real value, typically 300 to 400

. The distributed inductance

and capacitance determines the surge impedance:

where

, average conductor height

, conductor radius

When surge voltages of like polarity pass over each other, the voltages

add as shown in Figure 12.7. The currents subtract; current is charge moving

in a direction. Charge of the same polarity ﬂowing in opposite directions

cancels.

NN S

=-()1

==60 60

VV V ZI I

IZI I

FB FB

=+= -

()

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Lightning Protection 599

A surge arriving at an open point in the circuit reﬂects, sending back a

voltage wave equal to the incoming wave (see Figure 12.8). The voltage

doubles. Think of the incoming wave as a stream of electrons. When these

electrons hit the open point, they stop, but the pileup of electrons at the end

forces the electrons back in the direction they came from (like charges repel

each other). While the voltage doubles, the current cancels to zero as the

return wave counters the incoming wave.

Voltage doubling at open points is an important consideration for the

protection of distribution insulation. It is a very important consideration

when protecting underground cables. It also comes into play when placing

arresters to protect a substation.

When a surge hits a short circuit, the voltage drops to zero as we expect.

The ground releases opposing charge that cancels the voltage, creating a

reverse wave traveling back toward the source. Now the current doubles. Two

cases are very close to a short circuit: a conducting surge arrester and a fault

arc. Both of these act like a short. They generate a voltage canceling wave.

Line taps, capacitors, open points, underground taps — any impedance

discontinuities — cause reﬂections. In the general case, the voltage wave

reﬂected from a discontinuity is

where a is the reﬂection coefﬁcient:

FIGURE 12.7

Forward and backward traveling waves passing each other.

FIGURE 12.8

Traveling wave reﬂection at an open point.

+++

Open point

VaV

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600 Electric Power Distribution Handbook

For an open circuit (Z

= •), the reﬂection coefﬁcient, a = 1, and for a short

circuit (Z

= 0), a = –1.

The voltage at the discontinuity is the sum of the incoming wave and the

reﬂected wave:

where b is the refraction coefﬁcient:

The current waves are found from the voltages as for forward

waves and for backward waves. See Figure 12.9.

When a traveling wave hits a split in the line, a portion of the wave goes

down each path and another wave reﬂects back toward the source as shown

in Figures 12.10. Use to calculate the voltage at the split, just use

a new Z

’ as the parallel combination of Z

and Z

. For equal surge impedances (Z

= Z

), the total voltage

is 2/3 of the incoming wave.

To a traveling wave, a capacitor initially appears as a very small imped-

ance, almost a short circuit. Then, as the capacitor charges, the effective

FIGURE 12.9

Traveling wave reﬂection at a surge-impedance discontinuity where Z

< Z

FIGURE 12.10

Traveling wave reﬂections at a split.

Before After

VVVVaV bV

TIRI I I

=+ =+ =

=+=

IVZ

= /

IVZ

=- /

VbV

ZZ Z

BB C

+ZZ Z Z

BC B C

/( )

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Lightning Protection 601

impedance rises until it becomes an open circuit with the capacitor fully

charged. For a traveling wave section terminated in a capacitor, the voltage

ramps up to double the voltage as

An inductance initially appears almost as an open circuit. As the inductor

allows more current to ﬂow, its impedance drops until it is equivalent to a

short circuit. For a traveling wave section terminated in an inductor, the

voltage initially doubles then decays down to zero as

A cable section attached to an overhead circuit behaves similarly to a

capacitor. It is like an open-circuited traveling wave section with a low surge

impedance. In fact, a capacitor can be modeled in a traveling wave program

such as EMTP as a traveling wave section with a surge impedance of Dt/2

C and a travel time of Dt/2, where Dt is small (usually the time step of the

simulation) (Dommel, 1986). Likewise, we may model an inductor as a

traveling wave section shorted at the end with a surge impedance equal to

2L/Dt and a travel time of Dt/2.

Corona impacts traveling waves by modifying the surge impedance. When

lightning strikes an overhead distribution circuit, tremendous voltages are

created. The voltage stress on the conductor surface breaks down the air

surrounding the conductor, releasing charge into the air in many small

streamers. A high-voltage transmission line may have a corona envelope of

several feet (1 m), which increases the capacitance. This decreases the surge

impedance and increases coupling to other conductors and slopes off the

front of the wave. Normally, distribution line insulation ﬂashes over before

signiﬁcant corona develops, so we can neglect corona in most cases.

Predischarge currents, an effect similar to corona, may also form between

conductors. When the voltage between two parallel conductors reaches the

ﬂashover strength of air (about 180 kV/ft or 600 kV/m), a sheet of many

small streamers containing predischarge currents ﬂows between the conduc-

tors. This is a precursor to a complete breakdown. The predischarge current

delays the breakdown and relieves voltage stress between the conductors.

While the predischarge currents are ﬂowing, the resistance across the gap is

400 W/ft or 1310 W/m of line section (Wagner, 1964; Wagner and Hileman,

1963; Wagner and Hileman, 1964). Normally, on overhead distribution cir-

cuits, the weakest insulation is at poles, so in most cases, the circuit ﬂashes

over before the circuit enters the predischarge state. Predischarge currents

do make midspan ﬂashovers less likely and may help with using arresters

to protect lines.

VV e

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602 Electric Power Distribution Handbook

Normally, for modeling lightning in circuits or cables, we can ignore corona

and predischarge and use lossless line sections (wire resistance and damping

is ignored). This is normally sufﬁcient since only short sections need to be

modeled for most situations.

A multiconductor circuit has several modes of propagation; each has dif-

ferent velocity and different surge impedance. In most cases, these multi-

conductor effects are ignored, and we assume each wave travels on each

conductor at 95 to 100% of the speed of light.

For multiconductor circuits, the coupling between circuits is important.

The mutual surge impedance determines the voltage on one conductor for

a current ﬂowing in another as V

= Z

. The mutual surge impedance is

where

, distance between conductor i and the image conductor j

, distance between conductor i and conductor j

Conductors that are closer together couple more tightly. This generally

helps by reducing the voltage stress between the conductors.

12.4 Surge Arresters

Modern surge arresters have metal-oxide blocks, highly nonlinear resistors.

Under normal voltages, a metal-oxide surge arrester is almost an open cir-

cuit, drawing much less than a milliampere (and the watts losses are on the

order of 0.05 W/kV of MCOV rating). Responding to an overvoltage almost

instantly, the impedance on the metal-oxide blocks drops to a few ohms or

less for severe surges. After the surge is done, the metal oxide immediately

returns to its normal high impedance. Metal-oxide blocks are primarily zinc

oxide with other materials added. Boundaries separate conductive zinc-

oxide grains. These boundaries form semiconductor junctions like a semi-

conducting diode. A metal-oxide block is equivalent to millions of series and

parallel combinations of semiconducting diodes. With low voltage across

the boundaries, the resistance is very high, but with high voltage, the resis-

tance of the boundaries becomes very low.

Metal-oxide surge arresters overcame many drawbacks of earlier designs.

The spark gap was one of the earliest devices used to protect insulation

against lightning damage. A spark gap has many of the desirable beneﬁts

of a surge arrester; the gap is an open circuit under normal conditions and

when it sparks over, the gap is virtually a short circuit. The problem is that

after the surge, the gap is still shorted out, so a fuse or circuit breaker must

= 60 ln

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Lightning Protection 603

operate. In some parts of the world, notably Europe, spark gaps are still

widely used to protect transformers. Spark gaps were improved by adding

nonlinear resistors in series. The nonlinear material would clear the power

follow current ﬂowing through the spark gap after the surge was done.

Gapped arresters with silicon carbide blocks were developed in the 1940s

and used for many years. Metal oxide is so nonlinear that a gap is not

required. This allows an arrester design, which is simple and highly reliable,

with excellent “fast-front” response characteristics. Figure 12.11 shows the

voltage-current relationships of a typical metal-oxide arrester. At distribution

voltages, metal oxide was ﬁrst used to provide better protection at riser poles

feeding underground cables (Burke and Sakshaug, 1981).

Distribution surge arresters are primarily for protection against lightning

(not switching surges or other overvoltages). In areas without lightning they

would not be needed. That said, most utilities in low-lightning areas use

arresters even if the cost of arresters exceeds the cost of occasional failures.

If a utility is hit with a 1-in-40-year storm, it can wipe out a signiﬁcant

portion of unprotected transformers, enough that the utility does not have

the inventory to replace them all, and some customers would have very

long interruptions.

Arresters must be placed as close as possible to the equipment being

protected. This means on the same pole because lightning has such high

rates of rise, an arrester one pole span away provides little protection. At

the pole structure, the best place for the arrester is right on the equipment

tank to minimize the inductance of the arrester leads.

Arresters protect any equipment susceptible to permanent damage from

lightning. Figure 12.12 shows examples of arrester applications. Transform-

FIGURE 12.11

Typical characteristics of an 8.4-kV MCOV arrester (typically used on a system with a nominal

voltage of 7.2 or 7.62 kV from line to ground).

Nominal voltage

0 10 20 30 40

Current, kA

Voltage, kV

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604 Electric Power Distribution Handbook

FIGURE 12.12

Example arrester applications.

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Lightning Protection 605

ers should have arresters. Two-terminal devices such as reclosers, regulators,

and vacuum switches should have arresters on both the incoming and out-

going sides. Arresters on reclosers are especially important. When the

recloser is open, a surge hitting the open point will double. Commonly,

reclosers open to clear lightning-caused faults, and if the downstream side

is not protected, a subsequent lightning stroke that followed the one that

caused the fault could fail the open recloser. Cables need special attention

to prevent lightning entry and damage. At equipment with predominantly

air ﬂashover paths — insulators, switches, cutouts — utilities do not nor-

mally use arresters.

Most utilities also protect capacitor banks with arresters. Some believe

that capacitors protect themselves because capacitors have low impedance

to a fast-changing surge. While this is somewhat true, nearby direct light-

ning ﬂashes may still fail capacitors. About 1.9 C, which is a small ﬁrst

stroke, charges a 400-kvar, 7.2-kV capacitor unit to 95 kV (the BIL for 15-

kV equipment).

Several classes of arresters are available for protecting distribution

equipment:

• Riser pole — Designed for use at a riser pole (the junction between

an overhead line and a cable); has better protective characteristics

than heavy-duty arresters (because of voltage doubling, under-

ground protection is more difﬁcult)

• Heavy duty — Used in areas with average or above average lightning

activity

• Normal duty — Higher protective levels and less energy capability.

Used in areas with average or below average lightning activity

• Light duty — Light-duty arrester used for protection of underground

equipment where most of the lightning current is discharged by

another arrester at the junction between an overhead line and the cable

Two protective levels quantify an arrester’s performance (IEEE Std. C62.22-

1997):

• Front-of-wave protective level (FOW) — The crest voltage from a cur-

rent wave causing the voltage to rise to crest in 0.5 msec.

• Lightning impulse protective level (LPL) — The crest voltage for an 8/

20-msec current injection (8 msec is the equivalent time to crest based

on the time from 10 to 90% of the crest, and 20 msec is the time

between the origin of the 10 to 90% virtual front and the half-value

point). These characteristics are produced for current peaks of 1.5,

3, 5, 10, and 20 kA.

Table 12.4 gives common ranges. Lower protective levels protect equip-

ment better.

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606 Electric Power Distribution Handbook

12.4.1 Ratings and Selection

The choice of arrester rating depends on the voltage under normal conditions

and possible temporary overvoltages. Temporary overvoltages during line-

to-ground faults on the unfaulted phases are the main concern, therefore,

system grounding plays a large role in arrester application.

The most important rating of a metal-oxide arrester is the maximum con-

tinuous operating voltage (MCOV). Arresters also have another voltage rat-

ing called the duty-cycle rating (this rating is very commonly used to refer

to a particular arrester). The duty cycle rating is about 20% higher than the

MCOV rating. Within a given rating class, arresters may have different

performance characteristics (such as the discharge voltage and the energy-

handling capability). For example, one 8.4-kV MCOV normal-duty arrester

may have a protective level for a 10-kA, 8/20-msec current injection of 36

kV. A riser-pole arrester of the same rating may have a 27-kV protective level.

Operating voltage is the ﬁrst criteria for applying arresters. The arrester

MCOV rating must be above the normal upper limit on steady-state voltage.

Some engineers use the upper (ANSI C84.1-1995) range B, which is slightly

less than 6% above nominal. Use 10% above the nominal line-to-ground

voltage to be more conservative.

Temporary overvoltages are the second application criteria. An arrester

must withstand the temporary overvoltages that occur during line-to-

ground faults on the unfaulted phases. The overvoltages are due to a neutral

shift. On an ungrounded system, a ground fault shifts the neutral point to

the faulted phase. The arresters connected to the unfaulted phases see line-

to-line voltage. Four-wire multigrounded systems are grounded but still see

TABLE 12.4

Common Ranges of Protective Levels

Duty

Cycle

Rating

kV rms

MCOV

kV rms

Front-of-Wave Protective

Level, kV

Maximum Discharge Voltage,

kV 8/20 mm

sec Current Wave

5 kA

Normal

Duty

10 kA

Heavy

Duty

10 kA

Riser

Pole

5 kA

Normal

Duty

10 kA

Heavy

Duty

10 kA

Riser

Pole

3 2.55 11.2–17 13.5–17 10.4 10.2–16 9.1–16 8.2

6 5.1 22.3–25.5 26.5–35.3 17.4–18 20.3–24 18.2–25 16.2

9 7.65 33.5–36 26.5–35.3 22.5–36 30–33.5 21.7–31.5 20–24.9

10 8.4 36–37.2 29.4–39.2 26–36 31.5–33.8 24.5–35 22.5–26.6

12 10.2 44.7–50 35.3–50 34.8–37.5 40.6–44 32.1–44 30–32.4

15 12.7 54–58.5 42–59 39–54 50.7–52 35.9–52 33–40.2

18 15.3 63–67 51–68 47–63 58–60.9 43.3–61 40–48

21 17.0 73–80 57–81 52–63.1 64–75 47.8–75 44–56.1

24 19.5 89–92 68–93 63–72.5 81.1–83 57.6–83 53–64.7

27 22.0 94–100.5 77–102 71–81.9 87–91.1 65.1–91 60–72.1

30 24.4 107–180 85–109.5 78–85.1 94.5–99 71.8–99 66–79.5

36 29.0 125 99–136 91–102.8 116 83.7–125 77–96

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Lightning Protection 607

some neutral shift. (IEEE C62.92.4-1991) recommends using a 1.35 per unit

overvoltage factor for a multigrounded system. Chapter 13 has more infor-

mation on overvoltages during ground faults.

Arrester temporary overvoltage (TOV) capabilities are time-dependent;

manufacturers publish temporary overvoltage curves for their arresters (see

Figure 12.13). The duration of the overvoltage depends on the relaying and

fusing. Normally, this varies, and since arrester applications are not individ-

ually engineered, a value is normally assumed.

On four-wire multigrounded systems, the MCOV criteria, not the TOV

criteria, determines the arrester application. On ungrounded or impedance-

grounded circuits, TOV capability determines the rating.

Riser-pole arresters have less temporary overvoltage capability than normal

or heavy-duty arresters. Riser-pole arresters normally have the same arrester

block size as heavy-duty arresters; the manufacturer chooses blocks carefully

to ﬁnd those with lower discharge voltages. A lower discharge characteristic

means more current and energy for a given temporary overvoltage.

Utilities typically standardize arrester ratings for a given voltage and

grounding conﬁguration. Table 12.5 shows some common applications. For

the four-wire systems, most are okay, but try to avoid the tight applications

such as the 9-kV duty cycle arrester at 12.47 kV and the 10-kV duty cycle

arrester at 13.8 kV.

For ungrounded circuits, the voltage on the unfaulted phases rises to the

line-to-line voltage. This normally requires an arrester with an MCOV equal

to the line-to-line voltage because the fault current is so low on an

ungrounded system that single line-to-ground faults are difﬁcult to detect.

FIGURE 12.13

Temporary-overvoltage capabilities of several different arrester manufacturers and models

(dashed-lines: heavy-duty arresters, dotted lines: riser-pole arresters, bottom line: lowest TOV

capability expected as published in IEEE Std. C62.22-1997).

0.1 1.0 10.0 100.0 1000.

1.2

1.4

1.6

1.8

Time, seconds

Temporary overvoltage capability

[per unit of MCOV]

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