Short T.A. Electric Power Distribution Handbook

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618 Electric Power Distribution Handbook

2. Tie the ground lead to the tank — The NESC (IEEE C2-1997) requires

arrester ground leads to be tied to an appropriate ground. To achieve

any protection, the ground lead must be tied to the tank of the equip-

ment being protected. Without attaching the ground lead to the tank,

the transformer or other equipment is left completely unprotected.

Arrester vs. fuse placement is an ongoing industry debate. Tank mounting

an arrester protects the transformer best; but since the transformer is down-

stream of the fuse, the lightning surge current passes through the fuse.

Lightning may blow the fuse unnecessarily, and many utilities have histories

of nuisance fuse operations. Applying the arrester upstream of the fuse keeps

the surge current out of the fuse, but usually results in long lead lengths. I

prefer the tank mounted approach along with using larger fuses or surge

resistant fuses to limit unnecessary fuse operations.

12.5.3 Secondary-Side Transformer Failures

Single-phase residential-type transformers (three-wire 120/240-V service)

may also fail from surge entry into the low-voltage winding. This damage

mode has been extensively discussed within the industry (sparking com-

peting papers from manufacturers) and summarized by an IEEE Task Force

(1992). Lightning current into the neutral winding of the transformer (usu-

ally X2) induces possibly damaging stresses in the high-voltage winding

near the ground and line ends, both turn-to-turn and layer-to-layer. Light-

ning current can enter X2 from strikes to the secondary or strikes to the

primary (where it gets to the neutral through a surge arrester or ﬂashover,

see Figure 12.18).

The most concern with secondary surge entry is on small (< 50 kVA)

overhead transformers with a noninterlaced secondary winding (pad-

FIGURE 12.17

Lead length.

Surge current path

Insulation Arrester

Line side lead length

Ground side lead length

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Lightning Protection 619

mounted transformers are also vulnerable). The primary arrester does not

limit the voltage stresses on the ends of the primary winding. Currents

couple less to the high-voltage winding on transformers with an interlaced

secondary winding, which is used on core-type transformers and some shell-

type transformers. Smaller kVA transformers are most prone to damage from

this surge entry mode.

When the surge current gets to the transformer neutral, it has four places

to go: down the pole ground, along the primary neutral, along the secondary

neutral towards houses, or into the transformer winding. Current through

the secondary neutral creates a voltage drop along the neutral. This voltage

drop will push current through the transformer when load is connected at

the customer (Figure 12.18). The voltage created by ﬂow through the neutral

is partially offset by the mutual inductance between the secondary neutral

FIGURE 12.18

Surge entry via the secondary.

Pole ground

Service entrance ground

Loop voltages induced

No Customer Load

Pole ground

Service entrance ground

With Load (or meter gap sparkover)

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620 Electric Power Distribution Handbook

and the phases (tighter coupling is better). The major factors impacting

secondary-surge severity are:

• Interlacing — For a balanced surge (equal in both windings), the

inductance provided by the transformer is approximately the hot

leg to hot leg short-circuit reactance. Transformers with interlaced

secondaries have signiﬁcantly lower inductances (see Table 12.10).

Higher transformer inductance induces a higher voltage on the pri-

mary-side winding.

• Transformer size — Smaller transformers have higher inductances,

inducing a higher voltage on the primary-side winding.

• Grounding — Better grounding on the transformer helps reduce the

current into the transformer. Poor grounding sends more current

into the transformer and more current into houses connected to the

transformer. Other nearby primary grounds help to reduce the cur-

rent into the transformer.

• Secondary wire — Since triplex has tighter coupling than open-wire

secondaries (and less voltage induced in the loop), a secondary of

triplex has less current into the transformer neutral terminal.

• Multiple secondaries — The worst case is with one secondary drop

from the transformer; several in parallel reduce the current by pro-

viding parallel paths.

• Secondary length — Longer secondaries are more prone to failure

(more voltage induced in the loop).

In normal lightning areas, this surge failure mode is normally neglected.

In high lightning areas, using interlaced transformers leads to a lower failure

rate [by a factor of three for lightning-related failures according to Goedde

et al. (1992)]. Utilities may use secondary arresters or spark gaps to reduce

the transformer failure rate on noninterlaced units. Goedde also reported

that transformer failure rates reduced to 0.05% (from their normal 0.2 to 1%)

on 25,000 transformers that they made with secondary arresters. Either a

metal-oxide secondary arrester or a secondary spark gap effectively protects

TABLE 12.10

Typical Transformer Inductances

Interlaced Transformers Noninterlaced Transformers

10 kVA Core 17.4 mH 10 kVA Shell 204 mH

10 kVA Shell 18.1 mH

15 kVA Shell 17.8 mH 15 kVA Shell 257 mH

25 kVA Shell 22.6 mH 25 kVA Shell 124 mH

Source: EPRI TR-000530, Lightning Protection Design Workstation

Seminar Notes, Electric Power Research Institute, Palo Alto, CA,

1992.

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Lightning Protection 621

transformers from secondary-side failure modes. Economically, secondary-

side surge protection can make sense in higher lightning areas. An IEEE Task

Force report (1992) (based on Ward, 1990) cites that about 1% of a trans-

former’s ﬁrst cost is saved for every 0.1% reduction in failure rate.

12.6 Underground Equipment Protection

Underground equipment is susceptible to lightning damage; in the normal

scenario, lightning hits an overhead line near the cable, and a surge travels

into the cable at the riser pole. The most important item that makes cable

protection difﬁcult is:

• Voltage doubling — when a surge voltage traveling down a cable hits

an end point, the voltage wave reﬂects back, doubling the voltage.

Normally, for analysis of underground protection, we assume no attenu-

ation in the surge and that the voltage doubles at open points. The surge

impedance of cable is approximately 50 W, which is lower than the 400 W

overhead line surge impedance. So, if a lightning current has a choice

between the cable and an overhead line, most of it enters the cable. Normally

though, the cable has a surge arrester at the riser pole, and a conducting

surge arrester has an impedance of less than ﬁve ohms (for an arrester with

LPL = 30 kV and a 10-kA stroke, LPL/I = 30 kV/10 kA = 3 W), so the surge

arrester conducts most of the current. Surges travel more slowly on cables,

roughly one half of the speed of light. Transformers along the cable have

little effect, and a transformer termination is the same as an open circuit to

the surge.

The chopped wave withstand equals the BIL for cable (so we do not have

the extra 15% for the chopped wave protective margin). Since the BIL and

CWW are the same, we only need one protective margin equation:

where V

is the total voltage impressed across the insulation. With an arrester

at the riser pole and no arresters in the underground portion, we must use

twice the voltage at the riser pole, including the lead length:

CWW

BIL

◊1 100

V Ldi dt

=+2( / )FOW

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622 Electric Power Distribution Handbook

Table 12.11 shows protective margins for several voltages using a riser-

pole arrester. All of the margins are unacceptable, except for the 12.47-kV

case with no lead length (the 24.9-kV case is barely acceptable per the stan-

dards). With a heavy-duty arrester (FOW = 36 kV) for the 12.47-kV case, the

protective margin drops to:

While a 20% protective margin is considered acceptable per IEEE C62.22-

1997, there are many reasons why it is prudent to design for larger margins.

First, insulation in cables and transformers can degrade with time. Second,

non-standard lightning waveshapes, 60-Hz cable charging effects, and

extreme rates of rise may increase the voltage above what we have estimated.

Ideally, a 50% margin or better is a good design objective. To have a 50%

margin with the riser-pole arrester, the lead length must be less than 4 in.

(10 cm). Figure 12.19 shows how to obtain the smallest lead length possible.

To obtain minimum lead length, be sure to jumper to the arrester ﬁrst and

tie the ground lead to the cable neutral.

Table 12.11 shows that it was difﬁcult to obtain reasonable protective

margins with just an arrester at the riser pole. In the next section, we will

discuss how to increase margins by adding an open-point arrester to prevent

voltage doubling.

12.6.1 Open Point Arrester

For grounded 15-kV class systems, an arrester just at the riser pole is sufﬁ-

cient, but only if it is a riser-pole type arrester and only if the arrester is right

across the pothead bushing (minimal lead length). For higher-voltage appli-

cations, we need additional arresters. Putting an arrester at the end of the

cable prevents doubling. Although the voltage cannot double, reﬂections

that occur before the endpoint arrester starts full conduction will raise the

voltage in the middle of the cable (see Figure 12.20). To account for the

reﬂected portion, use:

where LPL

riser

is the 10-kA protective level of the riser-pole arrester, and

LPL

openpoint

is the 1.5-kA protective level of the open-point arrester. The lower-

Ldi dt

CWW

BIL

FOW2

1 100

(/)+

◊

CWW

236

1 100 32

()

◊=%

=+LPL LPL

riser openpoint

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Lightning Protection 623

FIGURE 12.19

TABLE 12.11

Typical Underground Protective Margins with a Riser-Pole

Arrester, No Other Arresters, and Using 8 kV/ft (26 kV/m) of

Lead Length

System

Voltage

(kV)

Arrester

MCOV

(kV)

CWW

(= BIL)

(kV)

FOW

(kV)

Ldi/dt

(kV)

(kV) PM

CWW

No Lead Length

12.47Y/7.2 8.4 95 29 0 58 64%

24.9Y/14.4 15.3 125 51 0 102 23%

34.5Y/19.9 22 150 77 0 154 -3%

3-ft (0.9-m) Lead Length

12.47Y/7.2 8.4 95 29 24 106 -10%

24.9Y/14.4 15.3 125 51 24 150 -17%

34.5Y/19.9 22 150 77 24 202 -26%

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624 Electric Power Distribution Handbook

current protective level of the open-point arrester is used since the riser pole

arrester drains off most of the current leaving much less in the cable (with

a surge impedance of 50 W, a 35-kV voltage surge corresponds to 0.7 kA of

current). Note that the protective level of the open-point arrester is higher

(it is a light-duty arrester).

The portion of the wave that is reﬂected before the open-point arrester

starts to conduct adds to the incoming wave. Because of this, surges with

low rise times cause the most severe overvoltage (contrary to most overvolt-

age scenarios). We have neglected the riser-pole lead length for the reﬂected

voltage, primarily because lead length causes a short-duration voltage on

the front of a fast-rising surge. This is just the sort of surge that the open-

point arrester handles well because it immediately starts conducting.

With an open-point arrester, we still need to include the lead length in the

chopped-wave margin calculation (we just do not add any factor for a reﬂec-

tion from the end of the cable). Protective margins in Table 12.12 and Table

12.13 show that we have more leeway with lead length if we have an open-

point arrester.

Several options are available for arresters on cable systems. Elbow arresters

attach to padmounted transformers, switching enclosures, or other equip-

ment with 200-A loadbreak connectors (IEEE Std. 386-1995). A parking-stand

arrester attaches to the enclosure and allows the open, energized cable to be

“parked” on an arrester (with a 200-A elbow connector). A bushing arrester

ﬁts between a cable elbow and an elbow bushing. Under-oil arresters used

in padmounted transformers are another option for protecting underground

equipment; they have good thermal performance and have cost advantages.

FIGURE 12.20

Voltage reﬂections with an open-point arrester.

Open point

with an

arrester

Reflected

wave

Reflected wave

Incoming wave

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Lightning Protection 625

Normally, we neglect attenuation in cables. This is the conservative

approach. Owen and Clinkenbeard (1978) found that a 1/40-msec wave

injected into a cable attenuated less than 5% even at 6000 ft (1800 m).

Attenuation increases signiﬁcantly with frequency. Owen and Clinkenbeard

found that a sparkover voltage wave attenuated by 40% after 6000 ft (1800

m) (the sparkover voltage is a narrow voltage spike — on a gapped arrester,

it is the portion of voltage that occurs before the gap sparks over). Most

signiﬁcantly for cable protection, attenuation softens the lead length induc-

tive kick. The attenuation depends on cable type. Zou and Boggs (2002)

showed that EPR has an order of magnitude higher attenuation than TR-

XLPE, mainly because of higher dielectric losses in the cable. Their EMTP

simulations of cable voltages were signiﬁcantly lower on EPR cable than on

TR-XLPE cable, especially with long lead lengths, because of the difference

in attenuation.

For cable protection, utilities are wise to have high protective margins.

Most of the history verifying distribution overvoltage protection practices

comes from arresters applied with over 50% protective margin. Effective

lightning protection helps limit high failure rates that have been experienced

in the industry (and riser poles expose a signiﬁcant amount of equipment

to overvoltages). Several other factors can cause margins to be less than

they appear:

TABLE 12.12

Typical Underground Protective Margins with a Riser-Pole

Arrester and an Open Point Arrester

System

Voltage

(kV)

Arrester

MCOV

(kV)

BIL

(kV)

LPL

riser

(kV)

LPL

openpoint

(kV)

(kV) PM

BIL

12.47Y/7.2 8.4 95 27 32 43 121%

24.9Y/14.4 15.3 125 48 58 77 62%

34.5Y/19.9 22 150 72 90 117 28%

TABLE 12.13

Typical CWW Underground Protective Margins with a Riser-Pole

Arrester and an Open Point Arrester

System

Voltage

(kV)

Arrester

MCOV

(kV)

CWW

(= BIL)

(kV)

FOW

(kV)

Ldi/dt

(kV)

(kV) PM

CWW

3-ft (0.9-m) Lead Length

12.47Y/7.2 8.4 95 29 24 53 79%

24.9Y/14.4 15.3 125 51 24 75 67%

34.5Y/19.9 22 150 77 24 101 49%

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626 Electric Power Distribution Handbook

• Quadrupling — Barker (1990) showed that voltages can as much as

quadruple for bipolar surges into the riser-pole arrester. While light-

ning currents are not known to have bipolarities, induced voltages

from nearby strikes can induce bipolar waveshapes.

• System voltage — The system voltage or trapped charge can add to

the voltage in the cable. If lightning hits a line at a time when the

system voltage happens to be at a peak of the opposite polarity, the

arrester will not go into complete conduction until the voltage

impulse has compensated for system voltage. On a 12.47Y/7.2-kV

system, this adds another to the traveling wave.

• Insulation deterioration — Transformer and cable insulation degrades

over time, which reduces the margin of protection provided by

arresters.

Application of arresters is both a science and an art and should be applied

based on local conditions. Higher lightning areas justify better protection

(a higher protective margin), especially if a utility has high historical cable

failure rates. As always, we must weigh the cost against the risk.

12.6.2 Scout Arresters

Another option for protecting cables is to use scout arresters, arresters applied

on the overhead line on both sides of the riser pole (Kershaw, 1971). A scout

arrester intercepts and diverts a lightning current that is heading towards

the riser pole. Since most of the current conducts through the closest arrester,

less voltage gets in the cable at the riser pole (unless lightning hits almost

right at the cable). Virginia Power has applied scout arresters to riser poles

to try to reduce high failure rates of certain types of cables at 34.5 kV (Marz

et al., 1994). Transient simulations done by Marz et al. found improved

protective margins with the scout arresters.

Scout arrester effectiveness depends on grounding the scout arresters well.

Without good grounding, the ground potential rises at the scout arrester,

causing high voltage on the phase and neutral wire (but little voltage dif-

ference between them). When the surge arrives at the riser pole, the low-

impedance ground path offered by the cable drops the neutral potential (and

increases the phase-to-neutral voltage). This pulls signiﬁcant current through

the riser-pole arrester (and sends a voltage wave down the cable), which

reduces the effectiveness of the scout arresters. The lower the impedance of

the scout arrester grounds, the less this effect occurs.

12.6.3 Tapped Cables

If cables are tapped, complex reﬂections can cause voltage to more than

double, with voltage at an open point reaching over 2.8 times the peak

voltage at the riser pole [see Figure 12.21 and (Hu and Mashikian, 1990)].

2=(.)72 10.18 kV

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Lightning Protection 627

The voltage varies depending on the lengths of each section beyond the tap

point. For tapped cables with no open point arresters, ﬁnd both the BIL and

chopped protective margins and include a 2.6 multiplier in the BIL protective

margin calculation (this is not the worst case but applies some extra safety):

The lead length voltage is not doubled in the chopped-wave protective

margin since the inductive voltage spike from the lead length does not

coincide with the reﬂections causing the highest overvoltage. On tapped

cables, applying open point arresters to both open points provides the safest

protection. Arresters at the tap points are also an option for protection.

FIGURE 12.21

Transient simulation of traveling waves on a tapped cable (0.5-ms risetime).

No lead length

at the riser pole

tap #1

open point

0 2 4

100

2-ft (0.6-m)

lead length

0 2 4

100

Time, µs

Voltage, kV

Time, µs

Voltage, kV

Riser pole

tap #2

tap #1

800 ft

(245 m)

500 ft

(150 m)

100 ft

(30 m)

PM =

BIL

LPL)

PM =

BIL

FOW)

BIL

CWW

1 100

.( /

◊

Ldi dt

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