Short T.A. Electric Power Distribution Handbook

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648 Electric Power Distribution Handbook

Parrish, D. E. and Kvaltine, D. J., “Lightning Faults on Distribution Lines,” IEEE

Transactions on Power Delivery, vol. 4, no. 4, pp. 2179–86, October 1989.

Powell, R. W., Thwaites, H. L., and Stys, R. D., “Estimating Lightning Performance

of Spacer-Cable Systems,” IEEE Transactions on Power Apparatus and Systems,

vol. 84, pp. 315–9, April 1965.

REA Bulletin 50-3, Speciﬁcations and Drawings for 12.5/7.2-kV Line Construction, United

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Ringler, K. G., Kirkby, P., Erven, C. C., Lat, M. V., and Malkiewicz, T. A., “The Energy

Absorption Capability and Time-to-Failure of Varistors Used in Station-Class

Metal-Oxide Surge Arresters,” IEEE Transactions on Power Delivery, vol. 12, no.

1, pp. 203–12, January 1997.

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Ross, E. R. and Grzybowski, S., “Application of the Extended CFO Added Method

to Overhead Distribution Conﬁgurations,” IEEE Transactions on Power Delivery,

vol. 6, no. 4, pp. 1573–8, October 1991.

Ross, P. M., “Burning of Wood Structures by Leakage Currents,” AIEE Transactions,

vol. 66, pp. 279–87, 1947.

Rusck, S., “Induced Lightning Overvoltages on Power Transmission Lines With Spe-

cial Reference to the Overvoltage Protection of Low Voltage Networks,” Trans-

actions of the Royal Institute of Technology, no. 120, 1958.

Rusck, S., “Protection of Distribution Lines,” in Lightning, R. H. Golde, ed., London,

Academic Press, 1977.

Short, T. A. and Ammon, R. H., “Monitoring Results of the Effectiveness of Surge

Arrester Spacings on Distribution Line Protection,” IEEE Transactions on Power

Delivery, vol. 14, no. 3, pp. 1142–50, July 1999.

Short, T. A., Burke, J. J., and Mancao, R. T., “Application of MOVs in the Distribution

Environment,” IEEE Transactions on Power Delivery, vol. 9, no. 1, pp. 293–305,

January 1994.

Sunde, E. D., Earth Conduction Effects in Transmission Systems, Dover Publications,

New York, 1968.

Task force of eight utility companies and the General Electric Company, “Investiga-

tion and Evaluation of Lightning Protective Methods for Distribution Circuit.

I. Model Study and Analysis,” IEEE Transactions on Power Apparatus and Systems,

vol. PAS-88, no. 8, pp. 1232–8, August 1969a.

Task force of eight utility companies and the General Electric Company, “Investiga-

tion and Evaluation of Lightning Protective Methods for Distribution Circuits.

II. Application and Evaluation,” IEEE Transactions on Power Apparatus and Sys-

tems, vol. PAS-88, no. 8, pp. 1239–47, August 1969b.

Thottappillil, R., Rakov, V. A., Uman, M. A., Beasley, W. H., Master, M. J., and

Shelukhin, D. V., “Lightning Subsequent-Stroke Electric Field Peak Greater

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Geophysical Research, vol. 97, no. D7, pp. 7503–9, May 20, 1992.

Wagner, C. F., “Application of Predischarge Currents of Parallel Electrode Gaps,”

IEEE Transactions on Power Apparatus and Systems, vol. 83, pp. 931–44, September

1964.

Wagner, C. F. and Hileman, A. R., “Effect of Predischarge Currents Upon Line Per-

formance,” IEEE Transactions on Power Apparatus and Systems, vol. 82, pp.

117–31, April 1963.

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Lightning Protection 649

Wagner, C. F. and Hileman, A. R., “Predischarge Current Characteristics of Parallel

Electrode Gaps,” IEEE Transactions on Power Apparatus and Systems, vol. 83, pp.

1236–42, December 1964.

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Delivery, vol. 17, no. 2, pp. 569–574, April 2002.

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)

www.powerlineman.com

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Grounding and Safety

Grounding is one of the main defenses against hazardous electric shocks

and hazardous overvoltages. Good equipment grounding helps reduce the

chances that line workers and the public get shocks from internal failures of

the equipment. System grounding determines how loads are connected and

how line-to-ground faults are cleared. Most North American distribution

systems have effective grounding; they have a neutral that acts as a return

conductor and as an equipment safety ground.

13.1 System Grounding Conﬁgurations

13.1.1 Four-Wire Multigrounded Systems

There are several grounding conﬁgurations for three-phase power distribu-

tion systems. On distribution systems in North America, the four-wire, mul-

tigrounded neutral system predominates. Figure 13.1 shows how loads

normally connect to the four-wire system. One phase and the neutral supply

single-phase loads. The neutral carries unbalanced current and provides a

safety ground for equipment. Low cost for supplying single-phase loads is

a major reason the four-wire system evolved in North America. More than

half of most distribution systems consist of single-phase circuits, and most

customers are single phase. On a multigrounded neutral system, the earth

serves as a return conductor for part of the unbalanced current during

normal and fault conditions.

Four-wire multigrounded systems have several advantages over three-

wire systems. Four-wire systems provide low cost for serving single-phase

loads:

• Single cables for underground single-phase load (and potheads and

elbows and padmounted transformer bushings)

• Single-phase overhead lines are less expensive

• Single-bushing transformers

• One arrester and one fuse for a single-phase transformer

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Electric Power Distribution Handbook

• The neutral can be located lower on the pole

• Lower rated arresters

• Lower insulation required; transformer insulation can be graded

Extensive use of single-phase lines provides signiﬁcant cost savings. The

primary neutral may be shared with the secondary neutral for further cost

savings. And ﬂexibility increases: upgrading a single-phase line to two- or

three-phase service on overhead lines is a cost-effective way to upgrade a

circuit following load growth.

Safety. The multigrounded neutral provides signiﬁcant safety beneﬁts for

crews and end users. If a transformer pole-ground down lead gets broken,

little safety margin is lost. With the transformer still connected to the mul-

tigrounded neutral, there is still a low-impedance path to earth, which pro-

tects against internal faults in the transformer.

The underbuilt neutral also partially blocks access to the phase conductors

(to ladders or bucket trucks). It warns crews that high-voltage phase con-

ductors are overhead. And, if a phase wire breaks, there is a good chance

that it will contact the neutral as it falls (if the neutral does not catch it, it

might result in a high-impedance fault).

The substation transformer normally provides the grounding source, the

low-impedance path to zero sequence. On circuits converted from an

ungrounded conﬁguration to a grounded conﬁguration, a grounding trans-

former, usually a zig-zag transformer, may provide the zero-sequence source,

so a neutral can be added. A grounding transformer must not be discon-

nected during operation of the distribution line. If the grounding transformer

becomes disconnected, a single line-to-ground fault causes high overvoltages

FIGURE 13.1

A four-wire multigrounded distribution system.

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Grounding and Safety

653

on the unfaulted phases. Load unbalances may also create overvoltages if

the grounding transformer is disconnected.

13.1.2 Other Grounding Conﬁgurations

Most other distribution conﬁgurations are three-wire systems, either

grounded or ungrounded. Figure 13.2 shows a three-wire unigrounded ser-

vice. A unigrounded is grounded at one place, normally the substation. Many

older distribution systems are three-wire systems, including 2400-, 4160-,

and 4800-V systems. Many utilities have portions of these circuits that are

holdovers from installations in the 1920s to the 1940s. Whether unigrounded

or ungrounded, three-wire systems have the following advantages:

• Lower cost for three-phase loads

• Lower stray voltage

• Easier detection of high-impedance faults

• Lower magnetic ﬁelds

European distribution designs are normally three-wire systems. All load

is three phase or phase-to-phase connected, so there is no need for a neutral.

But without the neutral, these systems must do without the multigrounded

safety ground. Most European designs make up for the lack of a multi-

grounded primary ground by having a multigrounded secondary neutral,

which is grounded at equipment and at each customer. Since European

designs tend to have extensive secondary systems and many houses on each

transformer, the secondary ground provides a safe grounding conductor.

Some parts of the world use a

single-wire earth-return

(SWER) distribution

system. The earth can carry all of the return current. On earth-return systems,

FIGURE 13.2

A three-wire unigrounded distribution system

single-phase

lateral

single-phase

transformer

three-phase

transformer

three-phase

lateral

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Electric Power Distribution Handbook

the resistance of ground electrodes is critical. Poor ground connections can

cause low end-use voltage and create very dangerous step-and-touch poten-

tials to the transformer tank. The single-wire system is used extensively in

Australia to serve remote rural customers with single-phase service; a high

primary voltage (33 kV) is normally used, which keeps currents low. Note

that the National Electrical Safety Code (IEEE C2-1997) does not allow an

earth-return system; it prohibits using the earth as the sole conductor for

any part of the circuit.

Other types of grounding such as high-resistance grounding or high-reac-

tance grounding are rarely used on distribution circuits. European systems

occasionally use a three-wire

resonant

-grounded system, where the substa-

tion transformer neutral is grounded through a tapped reactor (Peterson

coil). The coil is tuned to cancel the line-to-ground capacitance. It is an

ungrounded system. The main advantage is that it can operate with one

phase faulted. On overhead distribution circuits, the main advantage is that

temporary faults clear by themselves without opening a breaker or recloser.

Without the tuning reactor, the capacitance creates enough current to main-

tain the fault arc (especially with cables). In the past, it was hard to keep the

reactor in tune with the system capacitance. Clerfeuille et al. (1997) describe

a resonant grounding distribution system that uses an adaptively tuned

reactor that continuously maintains proper tuning.

The ﬁve-wire design is a new approach that may reduce stray voltage and

magnetic ﬁelds and also make high-impedance faults more easily detectable.

The ﬁfth wire is an isolated neutral that carries all of the unbalanced return

current. Under normal conditions, the 5-wire design operates very similar

to the 4-wire design with one major exception: all the return current is

conﬁned to the neutral that is isolated from ground. The multi-grounded

ground wire continues to perform the safety functions associated with a

multi-grounded system. New York State Electric and Gas (NYSEG) con-

verted a section of circuit to a ﬁve-wire conﬁguration as a research project,

and monitoring results showed reduction in stray voltage and magnetic

ﬁelds along with promising results that high-impedance faults could be

detected (EPRI 1000074, 2000; Short et al., 2002). Challenges found during

the project were: interfacing between the standard four-wire section and the

ﬁve-wire section, how to insulate the ﬁfth wire, surge protection on the ﬁfth

wire, how to protect against an open neutral, and crew acceptance. Increased

cost of the ﬁve-wire system would also factor in any real installation.

13.2 System Grounding and Neutral Shifts During

Ground Faults

The system grounding conﬁguration determines the overvoltages that can

occur during a line-to-ground fault. A single line-to-ground fault shifts the

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Grounding and Safety

655

ground potential at the fault location. The severity of this ground reference

shift depends on the grounding conﬁguration (see Figure 13.3). On a solidly

grounded system with a good return path to the grounding source, little

reference shift occurs. On an ungrounded system, a full offset occurs — the

line-to-ground voltage on the unfaulted phases rises to the line-to-line volt-

age, 1.73 per unit. On a multigrounded distribution system with a solidly

grounded station transformer, overvoltages above 1.3 per unit are rare.

Two factors relate the overvoltage to the system voltage:

Coefﬁcient of grounding:

COG =

Earth fault factor:

EFF =

where

= maximum line-to-ground voltage on the unfaulted phases dur-

ing a fault from one or more phases to ground

= nominal line-to-neutral and line-to-line voltages

FIGURE 13.3

Ground potential shifts and overvoltages depending on the grounding conﬁguration.

Normal conditions

1.73 per unit overvoltage

Total ground reference shift for a

fault from ground to phase A

A = GroundB

Neutral = Ground

Ungrounded System

No overvoltage

Phase A collapses to the neutral

(and ground) voltage

A = Neutral = Ground

Perfectly Grounded System

1.2 per unit overvoltage

Partial ground reference shift for a

fault from ground to phase A

A = Ground at the fault point

Station neutral

Typical Multigrounded System

Line-to-ground

overvoltage

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Electric Power Distribution Handbook

A system is “effectively grounded” if the coefﬁcient of grounding is less

than or equal to 80% (the earth-fault factor is less than 138%) (ANSI/IEEE

C62.92-1987). This is met approximately with the following conditions:

•

< 3

•

For a single line-to-ground fault on phase A, the voltages on phases B and

C are

where

= positive-sequence impedance

= zero-sequence impedance

= 1

–

120

∞

= fault resistance

= line-to-neutral voltage magnitude prior to the fault

For a double line-to-ground fault, the voltage on the unfaulted phase is

In some cases, the double line-to-ground fault causes overvoltages that are

slightly higher than the single line-to-ground fault. But since single line-to-

ground faults are so much more common, we often design for the single line-

to-ground fault. For single line-to-ground faults, the voltage is always worse

with the fault impedance

= 0. For double line-to-ground faults, it may not

always be worse with

= 0. Figure 13.4 shows overvoltage charts as a

function of

and

. This includes overvoltages due to single line-

to-ground faults and for double line-to-ground faults (assuming that

= 0).

IEEE suggests the overvoltage multiplier factors for different systems

shown in Table 13.1 (IEEE C62.92.4-1991). The multipliers include the neutral

shift during line-to-ground faults at a voltage of 105% (so, the ungrounded

system has an overvoltage of 1.73

1.05 = 1.82).

The higher overvoltage factor of 1.35 for multigrounded systems with

metal-oxide arresters was identiﬁed as a more conservative factor for four-

wire systems because of the reduced saturation of newer transformers and

ZZ R

ZZR

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Grounding and Safety

657

use of metal-oxide arresters (they are always connected, so they are more

sensitive to overvoltages than older arresters, which have an isolating gap).

13.2.1 Neutral Shifts on Multigrounded Systems

On four-wire systems with a multigrounded neutral, standard short-circuit

calculation programs calculate overvoltages incorrectly. The problem is not

symmetrical components; the problem is with the impedance of the return

FIGURE 13.4

Maximum overvoltages in per unit for line-to-ground faults based on

and

(the

contours mark the threshold of voltage).

1.25

1.4

1.5

1.6

1.73

0 2 4 6

1.25

1.4

1.5

0 2 4 6

1.25

1.4

1.5

1.6

0 2 4 6

1.25

1.4

0 2 4 6

0 5

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Electric Power Distribution Handbook

path, the zero-sequence impedance. When the impedance of the neutral is

rolled into the zero-sequence impedance, the neutral is modeled as being

perfectly grounded; ground resistances are ignored.

Assuming the neutral is perfectly grounded incorrectly models overvolt-

ages. Lat (1990) analyzed overvoltages with several techniques and found

that the traditional calculation method has serious shortcomings vs. model-

ing each ground and line section separately. We need to model each line

section between grounds as shown in Figure 13.5. Standard short-circuit

programs would not model this detail; we need a ﬂexible steady-state anal-

ysis program such as EMTP.

Lat’s analysis and additional work by Mancao et al. (1993) showed several

interesting ﬁndings:

•

Fault location

— Overvoltages are highest for faults near the substa-

tion. This contradicts the common belief that the highest overvolt-

ages are for faults at the end of the circuit (based on the standard

symmetrical components method).

•

Feeder length

— Overvoltages are higher on shorter feeders (assum-

ing the same number of grounds per unit length, see Figure 13.6).

The ground return path has a higher impedance on shorter lines

because there are fewer grounds beyond the fault location.

•

Neutral size

— A larger neutral provides a better return path.

TABLE 13.1

Overvoltage Factors for Different Grounding Systems

System

Overvoltage

Factor

Ungrounded system 1.82

Four-wire multigrounded system (spacer cable) 1.5

Three- or four-wire unigrounded system (open wire) 1.4

Four-wire multigrounded system (open wire; gapped arrester) 1.25

Four-wire multigrounded system (open wire; metal-oxide arrester) 1.35

Source:

IEEE C62.92.4-1991,

IEEE Guide for the Application of Neutral Grounding in

Electrical Utility Systems, Part IV — Distribution

FIGURE 13.5

Fault current paths for a line-to-ground fault.

Fault

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