Short T.A. Electric Power Distribution Handbook

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Grounding and Safety

659

•

Ground rod resistance and number of ground rods

— More ground rods

and lower rod resistances reduce overvoltages (see Figure 13.7). But

the results are not particularly sensitive to ground rod resistance

values.

•

Neutral reactor

— A neutral reactor on the substation transformer

causes a higher overvoltage, mainly for faults near the station.

Hydro Quebec staged faults on rural circuits and measured the neutral

voltages and currents (Rajotte et al., 1995). The measured neutral offsets

compared very well with simulations done with Lat’s modeling approach.

But, they compared well only if customer’s grounds were modeled. On this

circuit, the customer grounds contributed more to grounding the line than

the utility ground rods. There were three times more customer ground rods,

and their average resistance was almost eight times lower. It is not unusual

for customer grounds to be better than utility grounds, as customer grounds

are often tied to water pipes and wells. More extensive measurements of

ground rods and customer grounds on 16 rural feeders found an equivalent

of 47

for ground rods and 16

for customer ground connections (these

equivalents are the inverse of the average admittance) (Rajotte et al., 1995).

If the neutral conductor breaks, overvoltages are much higher. Mancao et

al. (1993) found overvoltages as high as 1.58 per unit on simulated systems

for faults downstream of an open point in the neutral. The worst case is for

a fault just past the break, but even a fault several miles from the open point

causes higher than normal overvoltages.

FIGURE 13.6

Overvoltages for different fault locations on feeders of different lengths. 3/0 ACSR phase

conductors, 1/0 ACSR neutral, 4-foot (1.2-m) spacings, four 25-

grounds/mile (normally, there

are many more grounds than this). (Adapted from Lat, M. V., “Determining Temporary Over-

voltage Levels for Application of Metal Oxide Surge Arresters on Multigrounded Distribution

Systems,”

IEEE Trans. Power Delivery

0 2 4 6

0.0

0.5

1.0

1.5

Distance from the station, miles

Maximum overvoltage in per unit

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660

Electric Power Distribution Handbook

Figure 13.8 shows simulated divisions of current for several neutral sizes

for a fault near the end of the circuit. Near a line-to-ground fault, most of

the current is in the neutral. In the middle, more ﬂows through the earth.

Near the station, additional current returns to the neutral. And, some current

returns via grounds beyond the fault location. Larger neutrals carry more

return current.

FIGURE 13.7

Effect of ground rod resistance and spacings. Each curve is labeled with the number of grounds

per mile. (Adapted from Lat, M. V., “Determining Temporary Overvoltage Levels for Applica-

tion of Metal Oxide Surge Arresters on Multigrounded Distribution Systems,”

IEEE Trans. Power

Delivery

FIGURE 13.8

Division of fault current between the neutral and earth. Armless construction, 336-kcmil phase

conductors, four 25-

grounds per mile. (Adapted from Mancao, R. T., Burke, J. J., and Myers,

A., “The Effect of Distribution System Grounding on MOV Selection,”

IEEE Trans. Power Delivery

1.0 10.0 100.0 1000.

1.2

1.3

1.4

Ground electrode resistance, ohms

Maximum overvoltage, per unit

Neutral size:

A: 336.4 kcmil

B: 3/0

C: 1/0

0 2 4 6 8 10

100

Distance from the station, miles

Fault current in the neutral, percent

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Grounding and Safety

661

An IEEE Working Group (1972) stated that transformer saturation holds

down overvoltages based on transient network analyzer evaluations. But

others disagree. Field tests found that saturation did not lower the peak

voltage (Rajotte et al., 1995). A transformer core saturates when the ﬂux is

highest. The ﬂux is 90

∞

out of phase with the voltage. The peak ﬂux occurs

when the voltage crosses through zero. So, when a transformer saturates, it

distorts the voltage but does not appreciably limit the peak voltage.

Normally, the highest overvoltages are for single line-to-ground faults, but

in some cases slightly higher overvoltages occur for double line-to-ground

faults. These cases normally involve large conductors (high

X/R

Spacer cable systems have higher overvoltages because of the messenger

wire (not because of the tighter phase and neutral spacings). An IEEE Work-

ing Group (1972) found overvoltages between 1.36 and 1.45 per unit with

spacer cable having a 5/16-in. or 3/8-in. copperweld messenger with a 30%

conductivity. If a spacer cable uses a lower-resistivity messenger or has a

separate neutral, overvoltages are similar to those of open-wire conﬁgura-

tions. ESEERCO (1992) simulated different spacer cable conﬁgurations and

found overvoltages of about 1.5 per unit on conﬁgurations with just a 3/8-

in. copperweld messenger. With a 1/0 ACSR neutral added to the spacer

cable, the overvoltages dropped to 1.3 per unit.

Underground cables normally have lower overvoltages. Rajotte et. al.

(1990) simulated overhead and underground systems and found that the

neutral shift was a factor of ten lower on underground systems. Urban

locations, whether overhead or underground, have many more grounds, so

overvoltages and other grounding issues such as stray voltage are less likely.

However, circuits of cables with tape shields or lead sheaths can have

signiﬁcantly higher overvoltages. On these cables, the tape shields have very

high resistance. Figure 13.9 shows how the overvoltage on the unfaulted

phases increases with distance of the fault from the substation for an example

scenario. This example assumes that the sheath is perfectly grounded. If

poorly grounded, more current will return through the high-resistance sheath

and increase the overvoltage. Utilities normally use tape shield or lead

sheathed power cables in urban areas where distances are fortunately short.

The high resistance also contributes to transients; the

X/R

ratio is low enough

that faults often spark and cause transients at each zero crossing. In a resistive

circuit, when the arc clears at a zero crossing, the voltage builds up slowly,

which delays the arc breakdown. When the arc does restrike, a transient can

occur because of the trapped voltage just prior to the arc restrike.

13.2.2 Neutral Reactor

A reactor connected between the substation transformer neutral point and

the substation ground reduces ground fault currents. This limits duty on

equipment, but it reduces the effectiveness of the grounding system. During

line-to-ground faults, the voltages on the unfaulted phases are higher. A

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662

Electric Power Distribution Handbook

neutral reactor of

adds 3

to the zero-sequence impedance. The

overvoltage is most pronounced for faults near the substation. Beyond a few

miles, the line impedance dominates.

Figure 13.10 shows an example of the tradeoff between fault current at the

substation bus and neutral reactor impedance on a 12.47-kV system. A 0.5

reactor increases the

ratio to 3.1, which reduces the fault current to

under 6 kA at the substation bus while still keeping the overvoltage below

130%. A reactor larger than this must be considered carefully to avoid sub-

jecting metal-oxide arresters and customer’s loads to undue overvoltages.

This graph was developed from a simpliﬁed equation for overvoltages.

With all circuit resistances equal to zero, the voltage magnitude in per unit

on the worst unfaulted phase for a bolted single line-to-ground fault sim-

pliﬁes (Westinghouse Electric Corporation, 1950) to

where

,circuit

+ 3

reactor

13.2.3 Overvoltages on Ungrounded Systems

Many older distribution systems are ungrounded, typically operating at 2400

or 4800 V. Although ungrounded systems have no intentional ground, they

FIGURE 13.9

Voltage on the unfaulted phases for a system with 500-kcmil, triplex, copper-conductor, tape-

shield cables. 12.47 kV,

= 0.0292 +

0.0355

/1000 feet,

= 0.3765 +

0.2882

/1000 feet,

src

0.7

0 1 2 3 4

1.0

1.2

1.4

1.6

Distance of the fault from the station, miles

Overvoltage on the unfaulted phases

at the fault location in percent

XXXX

1 732

01 1

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Grounding and Safety

663

are grounded through the line’s capacitance. Under normal conditions, the

phase-to-ground voltage equals the phase-to-phase voltage divided by

if the total line-to-ground capacitance on each phase is the same.

Ungrounded systems may have extra-high overvoltages under certain

conditions:

•

Resonance

— The line-to-ground capacitance can resonate with the

system reactance.

• Arcing ground faults — A restriking ground fault can create signiﬁcant

line-to-ground voltage.

FIGURE 13.10

Tradeoff between overvoltage on the unfaulted phases and fault current for a single line-to-

ground fault at the substation bus for different size neutral reactors. (12.47-kV circuit with a

20-MVA, 9% transformer, subtransmission impedance neglected)

0 1 2 3 4

100

120

140

160

Current for a line-to-ground fault, kA

Reactor reactance, ohms

Overvoltage on the unfaulted phases

at the substation, percent

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664 Electric Power Distribution Handbook

The main impact of these overvoltages is stress on insulation and impact

on arresters (higher rated arresters are necessary). A bolted line-to-ground

fault on one phase draws very little current on an ungrounded system. The

current is normally much less than 10 A, so the system may continue sup-

plying power. This is an operational advantage of the ungrounded system.

Because it can operate through line-to-ground faults, ungrounded systems

may have higher reliability. Most lower-voltage ungrounded systems (2400

or 4800 V) are very resistant to trees.

Figure 13.11 shows a simpliﬁcation of an ungrounded system that shows

how resonances can create overvoltages.* For a bolted single line-to-ground

fault on a phase, the voltages on phases B and C can become extremely high

if the system capacitance resonates with the inductance, which occurs when

X = X

where

X = reactance of the system, W

= line-to-ground capacitance, W

The overvoltage reaches a maximum when at the resonant point and can

reach signiﬁcant voltages for ratios of X

/X of up to 15. Normal line capac-

itances and reactances are well away from the resonant point; even cable

capacitances are too low to cause resonance. For example, 10 mi (16 km) of

cable with a capacitance of 0.5 mF/mi (0.9 mF/km) has an impedance of 530

W, and line impedances are less than 1 W/mi. Even with several circuits in

FIGURE 13.11

Ungrounded system schematic.

* The simpliﬁed circuit shows how resonance occurs, but the circuit is actually more complicated

than shown. The capacitance is not necessarily lumped on the source side of the line impedance,

it is distributed along the line (although circuit taps may concentrate the capacitance at points

on the line). The ﬁgure also ignores the line-to-line capacitance and the system load.

Equivalent circuit referred to one phase

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Grounding and Safety 665

parallel (more capacitance), such resonances are unlikely. For a more thor-

ough analysis, see Peterson (1951).

Resonances can occur when high reactances are accidentally connected to

ground. Figure 13.12 shows ground fault scenarios that may cause overvolt-

ages when three-phase distribution transformers become connected to ground.

Ungrounded systems are also susceptible to arcing ground faults. An

intermittent, restriking fault from ground to a phase conductor may spark

high voltages, possibly as high as ﬁve per unit. Consider the equivalent

single-phase circuit in Figure 13.11. This series circuit is a classic restriking

circuit often studied for switching capacitive loads; but now, the restriking

is in an arc to ground instead of in the circuit interrupter. The voltage can

ratchet up as the restriking charges the system capacitance. Since the inter-

mittent ground fault may last for hours, this overvoltage mode subjects

equipment to continued overvoltages. The overvoltages are likely to be

chaotic and random because of the random restriking times.

Fortunately, end users do not see the overvoltages from neutral shifts or

from arcing ground faults. The overvoltage is line to ground, and all loads

are connected line to line. But utility equipment insulation from the primary

to the equipment case does see the voltage, especially surge arresters, which

are sensitive to overvoltages.

13.3 Equipment/Customer Grounding

The National Electrical Safety Code (NESC) (IEEE C2-1997) places several

requirements on grounding systems. The NESC requires effective grounds on

• Surge arresters

• Primary, secondary, and common neutrals

• Unguarded metal or metal-reinforced supporting structures

FIGURE 13.12

Accidental connections of reactances to ground that may cause high overvoltages on unground-

ed systems.

Blown fuse

Ground fault

Open conductor

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666 Electric Power Distribution Handbook

• Other intentionally grounded wires

• Instrument transformers

• Cable concentric neutrals

• Most equipment including transformers, capacitors, regulators,

reclosers, and sectionalizers

• Uninsulated guy and messenger wires

• Customer meters

Some items that may be ungrounded on overhead circuits are insulators,

cutouts, and some open-air switches. For multigrounded systems, the

grounded conductor may serve as the ground for many applications. On a

multigrounded system, the NESC requires ground electrodes at transformer

locations and at customer meters. For an effectively grounded neutral (from

an NESC safety perspective) there must be at least four grounds in each

mile (1.6 km) of the entire line. Customer meter grounds do not count (they

are not necessarily under the control of the utility). Also, the neutral must

be continuous.

The NESC does not allow the earth as the sole conductor for any part of

the circuit.

On systems without a multigrounded neutral, where a grounding elec-

trode is required, it must be less than 25 W. If it is higher than that, another

grounding electrode must be made. The second electrode must be at least 6

ft (1.8 m) from the ﬁrst. Note that there is no requirement on the ﬁnal

resistance (it can still be greater than 25 W and meet this rule). It should be

low enough so that a protective device will clear for a fault from the primary

through the grounding electrode.

The NESC does not require a speciﬁc grounding resistance for multi-

grounded systems. Multigrounded systems achieve their performance

mainly by having many grounds in parallel, so the resistance of individual

electrodes is less signiﬁcant. Still, many utilities use the ungrounded NESC

approach for all grounding points: drive one rod, if it is more than 25 W,

drive another.

At least 20 ft (6.1 m) must separate grounding electrodes of different

systems (Clapp, 1997; IEEE C2-1997). The primary and secondary are “dif-

ferent systems.” Grounding electrodes of different systems may not be

bonded above or below ground.

The NESC requires grounding conductors for surge arresters to be at least

#6 copper or #4 aluminum. Other grounding electrodes may be smaller, but

they must be able to handle the fault current they will see. For multigrounded

systems, ground down leads from the neutral to the grounding electrode

will normally not carry signiﬁcant current. The critical connection is between

the equipment frame and the neutral. First and most importantly, make sure

there is such a connection. The connection between the frame and the neutral

must be designed to take the entire fault current for a line-to-ground fault

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Grounding and Safety 667

from the line to the equipment case. Since the fault current duration plays

a role in whether the wire will handle the fault, we must consider the fault

interrupter. For most applications where normal-sized fuses are used, a #4

or #6 copper bonding conductor is sufﬁcient. For situations with high fault

currents and where a station breaker clears the fault, a larger conductor or

parallel conductors may be needed.

Since the tank-to-neutral connection is critical on multigrounded systems,

on transformers, utilities typically tie two connections from the transformer

tank. One runs from the tank to the pole ground lead, which is attached to

the primary neutral. Another runs from the secondary neutral terminal of

the transformer (X2) to the neutral.

13.3.1 Special Considerations on Ungrounded Systems

Systems that do not have a multigrounded neutral — ungrounded or uni-

grounded systems — have special grounding considerations. Without the

multigrounded neutral, the secondary neutral grounding system is the whole

grounding system. If a transformer fails from the primary to the secondary

(a high-to-low fault), the secondary grounding system must be effective

enough to prevent unsafe voltages on customer wiring. Crews should make

sure that distribution transformers are grounded as well as possible and that

customer meters are grounded as well as possible.

On systems without a multigrounded neutral, the NESC requires the pri-

mary surge arrester to be separate from the secondary ground. The NESC

does allow interconnection of the primary and secondary grounds through

a spark gap as long as the 60-Hz breakdown value of the gap is at least twice

the primary circuit voltage but not necessarily more than 10 kV. If a spark

gap is used, an additional ground electrode must be attached, not including

the customer’s grounds; and the extra ground must be at least 20 ft (6.1 m)

from the arrester’s grounding electrode.

13.3.2 Secondary Grounding Problems

Many complaints are related to grounding problems on the secondary. Some

common problems are shown in Figure 13.13. One of the most common

power quality complaints is due to an open secondary neutral. If the neutral

is open or has a poor connection, the unbalanced current must return through

the grounding electrodes (these can have high impedance). The customer

may see this as light ﬂicker, and high voltage may occur on the less loaded

of the two secondary legs.

Another common problem on secondary systems is when load is connected

to the equipment safety ground (the green wire) instead of the neutral wire,

or the ground wire is wrongly connected to the neutral wire somewhere

within the facility. Both situations violate the National Electrical Code (NFPA

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668 Electric Power Distribution Handbook

70, 2002). Currents ﬂowing in the grounding conductor wire increase shock

hazards. Other secondary problems that can occur are isolated grounds,

ground loops, and missing safety grounds.

13.4 Ground Rods and Other Grounding Electrodes

The earth, the soil, has very low resistance, primarily because there is so

much of it. The trick is getting from the concentrated grounding electrode

to the soil beneath that is almost inﬁnite by comparison.

FIGURE 13.13

Common secondary problems.

Open Secondary Neutral

Customer’s ground

at the meter

House load

Transformer

ground

Unbalanced

current

High neutral-to-earth

voltage

High or low voltage

(depending on load balance)

Neutral and Safety Ground Reversed in the Facility

120 V 240 V

Service

panel

Load current on the ground wire

120 V

Hot

Neutral

Safety ground

Neutral and Safety Ground Connected in the Facility

Service

panel

Improper neutral/ground

interconnection

120 V

Hot

Neutral

Safety ground

High voltage from

the equipment case

to earth

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