Short T.A. Electric Power Distribution Handbook

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Short-Circuit Protection 417

a downstream line recloser. Consider a station recloser R1 and a downstream

line recloser R2 each with one fast curve (A) and two delayed curves (B). If

a permanent fault occurs downstream of R2, R2 will ﬁrst operate on its A

curve. If the fast curves of R1 and R2 are coordinated, R1 will not operate.

After a delay, R2 recloses. The fault is still there, so R2 operates on its delayed

curve (its B curve). Now, R1 does operate because it is on its A curve which

operates before R2’s B curve. After R1 recloses, R2 should then clear the

fault on its B curve, which should operate before R1’s B curve. The fault is

still cleared properly, but customers upstream of R2 have extra momentary

interruptions.

A more advanced form of coordination called sequence coordination removes

this problem. Sequence coordination is available on electronic reclosers and

also on digital relays controlling circuit breakers. With sequence coordina-

tion, the station device detects and counts faults — but does not open — for

a fault cleared by a downstream protector on the fast trip. If the fault current

occurs again (usually because the fault is permanent), the station device

switches to the time-overcurrent element because it counted the ﬁrst as an

operation. Using this form of coordination eliminates the momentary inter-

ruption for the entire feeder for permanent faults downstream of a feeder

recloser. On a relay or recloser that has sequence coordination, if the device

senses current above some minimum trip setting and the current does not

last long enough to trip based on the device’s fast curve, the device advances

its control-sequence counter as if the unit had operated on its fast curve. So

when the downstream device moves to its delayed curve, the upstream

device with sequence coordination also is operating on its delayed curve.

With sequence coordination, for the fast curves, the response curve of the

upstream device must still be slower than the clearing curve of the down-

stream device.

8.7 Fuse Saving vs. Fuse Blowing

Fuse saving is a protection scheme where a circuit breaker or recloser is used

to operate before a lateral tap fuse. A fuse does not have reclosing capability;

a circuit breaker (or recloser) does. Fuse saving is usually implemented with

an instantaneous relay on a breaker (or the fast curve on a recloser). The

instantaneous trip is disabled after the ﬁrst fault, so after the breaker recloses,

if the fault is still there, the system is time coordinated, so the fuse blows.

Because most faults are temporary, fuse saving prevents a number of lateral

fuse operations.

The main disadvantage of fuse saving is that all customers on the circuit

see a momentary interruption for lateral faults. Because of this, many utilities

are switching to a fuse blowing scheme. The instantaneous relay trip is

disabled, and the fuse is always allowed to blow. The fuse blowing scheme

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418 Electric Power Distribution Handbook

is also called trip saving or breaker saving. Figure 8.15 shows a comparison

of the sequence of events of each mode of operation. Fuse saving is primarily

directed at reducing sustained interruptions, and fuse blowing is primarily

aimed at reducing the number of momentary interruptions.

FIGURE 8.15

Comparison of the sequence of events for fuse saving and fuse blowing for a fault on a lateral.

Fault

Breaker or recloser

Lateral Fuse

Fuse Saving

Temporary fault

1. The circuit breaker operates on the instantaneous relay trip (before

the fuse operates).

2. The breaker recloses.

3. The fault is gone, so no other action is necessary.

Permanent fault

l. The circuit breaker operates on the instantaneous relay trip (before

the fuse operates).

2. The breaker recloses.

3. The fault is still there.

4. The instantaneous relay is disabled, so the fuse operates.

5. Crews must be sent out to fix the fault and replace the fuse.

Fuse Blowing

Temporary fault

1. The fuse operates.

2. Crews must be sent out to replace the fuse.

Permanent fault

1. The fuse operates.

2. Crews must be sent out to fix the fault and replace the fuse.

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Short-Circuit Protection 419

8.7.1 Industry Usage

Until the late 1980s, fuse saving was almost universally used. As power

quality concerns grew, some utilities switched to a fuse blowing mode. An

IEEE survey on distribution protection practices that is done periodically

has shown a decrease in the use of fuse saving as shown in Table 8.11.

Another survey done by Power Technologies, Inc., in 1996 showed a mix-

ture of practices at utilities as shown in Table 8.12. A few used fuse blowing

because they indicated that fuse saving was not successful. Many of the

“mixed practices” utilities decided on a case-by-case basis. Many of these

normally used fuse saving but switched to fuse blowing if too many power

quality complaints were received.

8.7.2 Effects on Momentary and Sustained Interruptions

The change in the number of momentary interruptions can be estimated

simply by using the ratio of the length of the mains to the total length of the

circuit including all laterals. For example, if a circuit has 5 mi of mains and

10 mi of laterals, the number of momentaries after switching to fuse blowing

TABLE 8.11

IEEE Survey Results on the Percentage of Utilities

that Use Fuse Saving

Survey Year

Percent of Utilities

Using Fuse Saving

1988 91

1994 71

2000 66

Sources: IEEE Working Group on Distribution Protection,

“Distribution Line Protection Practices — Industry Sur-

vey Results,” IEEE Transactions on Power Delivery, vol. 3,

no. 2, pp. 514–24, April 1988; IEEE Working Group on

Distribution Protection, “Distribution Line Protection

Practices — Industry Survey Results,” IEEE Transactions

on Power Delivery, vol. 10, no. 1, pp. 176–86, January 1995;

Report to the IEEE working group on system perfor-

mance, 2002.

TABLE 8.12

1996 Survey on the Usage of Fuse Saving

Use fuse saving 40%

Use a mixture 33%

Use fuse blowing 27%

Source: Short, T. A., “Fuse Saving and Its Effect

on Power Quality,” EEI Distribution Committee

Meeting, 1999.

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420 Electric Power Distribution Handbook

would be 1/3 of the number of momentaries with fuse saving (5/(5+10) =

1/3). This assumes that the mains and laterals have the same fault rate; if

the fault rate on laterals is higher (which it often is because of less tree

trimming, etc.), the number of momentaries is even less. Note how dramat-

ically we can reduce momentaries by using fuse blowing. No other methods

can so easily eliminate 30 to 70% of momentaries. The effect on reliability of

going to a fuse blowing scheme is more difﬁcult to estimate. Fuse blowing

increases the number of fuse operations by 40 to 500% (Dugan et al., 1996;

Short, 1999; Short and Ammon, 1997). This will increase the average fre-

quency of sustained interruptions by 10 to 60%. Note that there are many

variables that can change the ratios. One example is given in Figure 8.16.

Note that the effect on sustained interruptions is not equally distributed.

Customers on the mains see no difference in the number of permanent

interruptions. Customers on long laterals may have many more sustained

interruptions with a fuse-blowing scheme.

8.7.3 Coordination Limits of Fuse Saving

One of the main reasons that utilities have decided not to use fuse saving is

that it is difﬁcult to make it work. Fuses clear quickly relative to circuit

breakers, so where fault currents are high, the fuse blows before the breaker

trips. This results in a fuse operation and a momentary interruption for all

customers on the circuit. K links, the most common lateral fuses, are fast

fuses. Most distribution circuit breakers take 5 cycles to clear. For fuse saving

to work, the breaker must open before the fuse blows, so the fuse needs to

survive for the time it takes the instantaneous relay to operate (about one

cycle) plus the 5 cycles for the breaker. As an illustration, Figure 8.17 shows

the limit of coordination of a 5-cycle breaker and a 100 K fuse. Fuse saving

only coordinates for faults below 1354 A. Smaller fuses have lower current

limits. Note that the breaker time is coordinated with the damage time of

the fuse.

FIGURE 8.16

Comparison of fuse saving and fuse blowing on a hypothetical circuit. Mains: 10 miles (16.1 km),

fault rate = 0.5/mile/year (0.8/km/year), 75% temporary. Taps: 10 miles (16.1 km) total, 20

laterals, fault rate = 2/mile/year (3.2/km/year), 75% temporary. It also assumes that fuse saving

is 100% successful.

127%

27%

Fuse Blowing

Sustained Interruptions

100%

Fuse Saving

Momentary Interruptions

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Short-Circuit Protection 421

Table 8.13 and Table 8.14 show the limits of coordination of several com-

mon lateral fuses for a standard circuit breaker (5 cycle) and for a fast

breaker/relay combination (3-cycle circuit breaker and 1-cycle relay). Also

shown are translations of these fault currents into distances from the sub-

station at 12.47 kV (assuming an 8-kA fault level at the substation). Note

FIGURE 8.17

Coordination of a 100K lateral fuse with a 5-cycle circuit breaker.

TABLE 8.13

Maximum Fault Currents and Critical Distances for Fuse

Saving Coordination for Several Common Fuse Links for a 5-

Cycle Circuit Breaker and a 1-Cycle Relay Time

Fuse I

, A d

, mi d

, km Fuse I

, A d

, mi d

, km

20 K 254 26.5 42.6 20 T 433 15.5 25.0

25 K 323 20.8 33.5 25 T 552 12.2 19.6

30 K 398 16.9 27.2 30 T 699 9.6 15.5

40 K 520 12.9 20.8 40 T 896 7.5 12.1

50 K 665 10.1 16.3 50 T 1125 6.0 9.7

65 K 816 8.3 13.3 65 T 1428 4.8 7.7

80 K 1078 6.3 10.1 80 T 1790 3.8 6.2

100 K 1354 5.0 8.1 100 T 2277 3.1 4.9

140 K 2162 3.2 5.2 140 T 3447 2.1 3.4

200 K 3401 2.1 3.5 200 T 5436 1.5 2.4

Note: I

= Maximum current where fuse saving works. d

= Distance

from the substation where fuse saving starts to work for 12.47-

kV, 500-kcmil overhead line.

Power Quality Improvement Methodology for Wire Companies. Reprinted

with permission.

100 K

Maximum

coordination

current

Breaker + relay time = 0.1 s

2255

0.01

0.1

0.02

0.2

0.05

Current, amps

Time, seconds

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422 Electric Power Distribution Handbook

that only the larger fuses shown (greater than 100 A) will coordinate for

signiﬁcant portions of the feeder. Smaller fuses used as second and third

level fuses do not coordinate over the length of most feeders. The situation

is even worse at higher voltages. At 24.94 kV, the distances in Table 8.13 and

Table 8.14 are doubled, so fuse saving is more difﬁcult to achieve at higher

voltages. Reclosers are faster than standard 5-cycle breakers — the 4-cycle

total operating time in Table 8.14 is representative of many reclosers.

If smaller K links are used such as 100K and 65K fuses (the most common

lateral fuses), then fuse saving is not going to work very well. In that case,

why use it? There is no sense in having a momentary every time a fuse blows

(which is what will happen since the circuit breaker is not fast enough to

save the fuse).

8.7.4 Long-Duration Faults and Damage with Fuse Blowing

Fuse blowing has drawbacks: faults on the mains can last a long time. With

fuse saving, main-line faults normally clear in 5 to 7 cycles (0.1 sec) on the

ﬁrst shot with the instantaneous element. With fuse blowing, this same fault

may last for 0.5 to 1 sec. Much more damage at the fault location occurs

during this extra time. Some of the problems that have been identiﬁed are

• Conductor burndowns — At the fault, the heat from the fault current

arc burns the conductor enough to break it, dropping it to the

ground.

• Damage of inline equipment — The most common problem has been

with inline hot-line clamps. If the connection is not good, the high-

current fault arc across the contact can burn the connection apart.

TABLE 8.14

Maximum Fault Currents and Critical Distances for Fuse Saving Coordination for

Several Common Fuse Links for a 3-Cycle Circuit Breaker and a 1-Cycle Relay Time

Fuse I

, A d

, mi d

, km Fuse I

, A d

, mi d

, km

20 K 332 20.3 32.6 20 T 565 11.9 19.2

25 K 424 15.9 25.5 25 T 723 9.3 15.0

30 K 522 12.9 20.8 30 T 920 7.4 11.8

40 K 682 9.9 15.9 40 T 1175 5.8 9.3

50 K 875 7.7 12.4 50 T 1479 4.6 7.4

65 K 1070 6.3 10.2 65 T 1878 3.7 5.9

80 K 1407 4.8 7.8 80 T 2346 3.0 4.8

100 K 1763 3.9 6.3 100 T 2975 2.4 3.9

140 K 2823 2.5 4.1 140 T 4522 1.7 2.7

200 K 4409 1.7 2.8 200 T 7122 1.3 2.0

Note: I

= Maximum current where fuse saving works. d

= Distance from the substation

where fuse saving starts to work for 12.47-kV, 500-kcmil overhead line.

Reprinted with permission.

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Short-Circuit Protection 423

• Station transformers — Extra duty on substation transformers

• Evolving faults — Ground faults are more likely to become two- or

three-phase faults.

• Underbuilt — Faults on underbuilt distribution are more likely to

cause faults on the transmission circuit above due to rising arc gases.

A fault current arc will expand after it is initiated. It has been found that

the growth of the arc is generally in the vertical direction, and the growth

is primarily a function of time and not of current or voltage [(Drouet and

Nadeau, 1979) and Chapter 7]. The growth of the arc means that a 0.1-sec

fault on the instantaneous trip (with fuse saving) is less likely to involve

other phases or other circuits than a 0.2 to 1-sec fault on the time-delay trip

(with fuse blowing).

8.7.5 Long-Duration Voltage Sags with Fuse Blowing

With fuse blowing, voltage sags last longer, especially for faults on the three-

phase mains, which have to be cleared by phase or ground time-overcurrent

elements. An example is shown in Figure 8.18 where voltage sag magnitudes

and durations are shown for faults at various distances from a substation

using fuse blowing. For the same circuit with fuse saving, all of the faults

would have cleared in 0.1 sec. For a fault at the substation, the duration

triples. For a fault one mile (1.6 km) from the substation, the duration qua-

FIGURE 8.18

Magnitudes and durations of substation bus voltage sags for ground faults applied at the given

distance with a fuse-blowing scheme. For the same circuit with fuse saving, all of the faults

would clear in 0.1 sec. Assumptions: 12.47 kV, 500 kcmil, all-aluminum conductors. Ground

relay: CO-11, TD = 5, pickup = 300 A.

0 mi (0.0km)

0.5 mi (0.8km)

1.0 mi (1.6km)

2.0 mi (3.3km)

4.0 mi (6.6km)

6.0 mi (9.9km)

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

Sag duration, seconds

Voltage magnitude

[per unit]

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424 Electric Power Distribution Handbook

druples. The situation is worse for phase-to-phase faults and three-phase

faults because they must be cleared by the phase relays which are generally

slower.

8.7.6 Optimal Implementation of Fuse Saving

In order to get a fuse saving scheme to work, it is necessary to get the

substation protective device to open before fuses operate. We can achieve

this in several ways:

• Slow down the fuse — Use big, slow fuses (such as a 140 or 200 T)

near the substation to ensure proper coordination.

• Faster breakers or reclosers — If three-cycle circuit breakers are used

instead of the normal 5-cycle breakers, fuse saving coordination is

more likely. Some reclosers are even faster than 3-cycle breakers.

• Limit fault currents:

• Open station bus ties: An open bus tie will reduce the fault

current on each feeder and make fuse saving easier. This is the

normal operating mode for most utilities.

• Use a transformer neutral reactor: A neutral reactor reduces the

fault current for single-phase faults (all faults on single-phase taps).

• Use line reactors: This reduces the fault current for all types of

faults. This has been an uncommon practice. An added advan-

tage, reactors reduce the impact of voltage sags for faults on

adjacent feeders.

• Specify higher impedance transformers.

We can employ other strategies to limit the impact of momentary inter-

ruptions:

• More downstream reclosers — Extra downstream devices will reduce

the number of momentaries for customers near the substation. It is

important to coordinate reclosers with the upstream device (includ-

ing sequence coordination).

• Single-phase reclosers

• Immediate reclose

• Switch to a fuse blowing scheme on poor feeders — For a feeder with

many momentaries, disable the instantaneous relay for a time

period. Identify poorly performing parts of the circuit during this

time. The blown branch fuses provide a convenient fault location

method. Once the poor performing sections are identiﬁed and

improved, switch the circuit back to fuse saving.

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Short-Circuit Protection 425

8.7.7 Optimal Implementation of Fuse Blowing

Several strategies can optimize a fuse blowing scheme:

• Fast fuses (or current-limiting fuses) — If smaller or faster fuses are

used, faults clear faster, so voltage sag durations are shorter. Current-

limiting fuses also limit the magnitude and duration of the sag. Be

careful not to fuse too small, or fuses will operate unnecessarily due

to loading, inrush, and cold load pickup. Note that if smaller fuses

are used, it is difﬁcult to switch back to a fuse-saving scheme.

• Covered wire or small wire — Watch burndowns on circuits with

covered wire or small wire that is protected by the station circuit

breaker or recloser. If either of these cases exists, use a modiﬁed fuse

blowing scheme with a time-delayed instantaneous element (see the

next section).

• Use single-phase reclosers on longer laterals — A good way of main-

taining some of the reliability of a fuse-saving scheme is to use

single-phase reclosers instead of fuses on longer taps. Then, tempo-

rary faults on these laterals do not cause permanent interruptions

to those customers.

• More fuses — Add more second and third level fuses to segment the

circuit more.

• Track lateral operations — Temporary faults on fused laterals cause

sustained interruptions. In order to minimize the impacts on lateral

customers, track interruptions by lateral. Identify poorly perform-

ing laterals, patrol poor sections, then add tree trimming, animal

guards, etc.

8.8 Other Protection Schemes

8.8.1 Time Delay on the Instantaneous Element (Fuse Blowing)

An alternative implementation of a fuse blowing scheme is to use a time

delay on the instantaneous trip (rather than removing the instantaneous trip;

a deﬁnite-time overcurrent relay also could do the same function) (Engel-

man, 1990). Faults do not last as long as they would if the relay went to a

time-overcurrent element; there is less chance of wire burndowns, and volt-

age sags are of shorter duration for faults on the mains. A common delay is

0.1 sec.

An example implementation is shown in Figure 8.19 where a 0.1-sec delay

is added to the instantaneous. A 100K fuse link is also shown. For the 100K

link, the scheme is actually a mixture of fuse saving and fuse blowing. For

fault currents above roughly 1700 A, it is a fuse blowing scheme (the fuse

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426 Electric Power Distribution Handbook

clears before the instantaneous relay operates). For currents below 1000 A,

the scheme is a fuse saving scheme (the circuit breaker trips before the fuse

is damaged). Between 1000 and 1700 A, one or both devices operate.

Another option sometimes used with this scheme is a high-set instantaneous.

The high-set instantaneous has no time delay and is set to clear faults close to

the station. This removes the most damaging faults quickly (because they are

the most likely to cause damage and cause the most severe voltage sags).

Using a time delay is a better fuse saving scheme than just removing the

instantaneous relay. The disadvantage, and the reason that it is not imple-

mented as much, is that it is usually more difﬁcult and costly to implement.

For electromechanical relays, another timer relay must be added, and the

relay scheme must be engineered. Many digital relays ease the implemen-

tation since they have this time-delay option available.

8.8.2 High-Low Combination Scheme

Another option is to use fuse blowing at the substation and fuse saving at

downstream reclosers (Burke, 1996).

• Substation fuse blowing — Fault currents are high near the substation,

so it is difﬁcult to get fuse saving to work here.

FIGURE 8.19

Example of a delayed instantaneous element used for fuse blowing.

100 K

Breaker + relay time

+ delay = 0.2 s

Relay delay = 0.1 s

Fuse saving

below this current

Fuse blowing

above this current

2255

0.01

0.1

0.02

0.2

0.05

Current, amps

Time, seconds

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