Short T.A. Electric Power Distribution Handbook

Подождите немного. Документ загружается.

Short-Circuit Protection 397

Cutouts are rated on a symmetrical basis. Cutouts are tested at X/R ratios

of 8 to 12, so if the X/R ratio at the application point is higher than the test

value, the cutout should be derated. The fuse line holder determines the

interrupting capability, not the fuse link.

Most cutouts are of the open variety with a removable fuse holder that is

placed in a cutout with a porcelain-bushing type support. We also ﬁnd

enclosed cutouts and open-link cutouts. Open links have a fuse link sus-

pended between contacts. Open links have a much lower interrupting capa-

bility (1.2 kA symmetrical).

Many cutouts available are full-rated cutouts that can be used on any type

of system where the maximum line-to-line voltage is less than the cutout

rating. Cutouts are also available that have slant voltage ratings, which

provide two ratings such as 7.8/15 kV that are meant for application on

grounded circuits (IEEE Std. C37.48-1997). One cutout will interrupt any

current up to its interrupting rating and up to the lower voltage rating. On

a grounded distribution system, in most situations, any cutout can be applied

that has the lower slant rating voltage greater than the maximum line-to-

ground voltage. On a 12.47Y/7.2-kV grounded distribution system, a 7.8/

15-kV cutout could be used. If the system were ungrounded, a full-rated 15-

kV cutout must be used. For three-phase grounded circuits, the recovery

voltage is the line-to-line voltage for a line-to-line fault rather than the line-

to-ground voltage (requiring a higher voltage rating). In this case, the slant

rated cutouts are designed and tested so that two cutouts in series will

interrupt a current up to its interrupting rating and up to the higher voltage

rating. The two cutouts share the recovery voltage (even considering the

differences in the melting times of the two fuses). On grounded systems,

there are cases where slant-rated cutouts are “under-rated” — any time that

a phase-to-phase fault could happen that would only be cleared by one

TABLE 8.4

Fuse Minimum-Melt I

t (A

-s)

Rating, A K Links T Links

6 534 1,490

8 1,030 2,770

10 1,790 5,190

12 3,000 8,810

15 5,020 15,100

20 8,500 24,500

25 13,800 40,200

30 21,200 65,500

40 36,200 107,000

50 58,700 173,000

65 90,000 271,000

80 155,000 425,000

100 243,000 699,000

140 614,000 1,570,000

200 1,490,000 3,960,000

1791_book.fm Page 397 Monday, August 4, 2003 3:20 PM

398 Electric Power Distribution Handbook

cutout. This includes constructions where multiple circuits share a pole or

cases where cutouts are applied on different poles.

Most cutouts used on distribution systems do not have load break capa-

bility. If the cutout is opened under load, it can draw an arc that will not

clear. It is not an uncommon practice for crews to open cutouts under load

(if it draws an arc, they slam it back in). Cutouts with load-break capability

are available, usually capable of interrupting 100 to 300 A. Cutouts with

load-break capability usually use an arc chute. A spring pulls the arc quickly

through the arc chute where the arc is stretched, cooled, and interrupted. A

load-break tool is available that can open standard cutouts (with no load

break capability of its own) under load up to 600 to 900 A. Utilities also

sometimes use solid blades in cutouts instead of a fuse holders; then crews

can use the cutout as a switch.

8.2.6 Current-Limiting Fuses

Current limiting fuses (CLFs) are another interrupter having the unique

ability to reduce the magnitude of the fault current. CLFs consist of fusible

elements in silicon sand (see Figure 8.10). When fault current melts the

fusible elements, the sand melts and creates a narrow tube of glass called a

fulgerite. The voltage across the arc in the fulgerite greatly increases. The

fulgerite constricts the arc. The sand helps cool the arc (which means it takes

energy from the arc). The sand does not give off ionizable gas when it melts,

and it absorbs electrons, so the arc has very little ionizable air to use as a

conductor. Without ionizable air, the arc is choked off, and the arc resistance

becomes very high. This causes a back voltage that quickly reduces the

current. The increase in resistance also lowers the X/R ratio of the circuit,

causing a premature current zero. At the current zero, the arc extinguishes.

Since the X/R ratio is low, the voltage zero and current zero occur very close

together, so there will be very little transient recovery voltage (the high arc

voltage comes just after the element melts). Because the current-limiting fuse

FIGURE 8.10

Example backup current-limiting fuse. (From Hi-Tech Fuses, Inc. With permission.)

1791_book.fm Page 398 Monday, August 4, 2003 3:20 PM

Short-Circuit Protection 399

forces an early current zero, the fuse can clear the short circuit in much less

than one half of a cycle.

Current-limiting fuses are noted for their very high fault-clearing capabil-

ity. CLFs have symmetrical maximum interrupt ratings to 50 kA; contrast

that to expulsion fuses which may have typical maximum interrupt ratings

of 3.5 kA in oil and 13 kA in a cutout. Current-limiting fuses also completely

contain the arc during operation and are noiseless with no pressure buildup.

Current-limiting fuses are widely used for protection of equipment in high

fault current areas. Table 8.5 shows the percentages of utilities that use CLFs.

The major reason given for the use of CLFs is safety, and the second most

common reason is high fault currents in excess of expulsion fuse ratings.

There are three types of current-limiting fuses (IEEE Std. C37.40-1993):

• Backup: A fuse capable of interrupting all currents from the maxi-

mum rated interrupting current down to the rated minimum inter-

rupting current.

• General Purpose: A fuse capable of interrupting all currents from the

maximum rated interrupting current down to the current that causes

melting of the fusible element in one hour.

• Full Range: A fuse capable of interrupting all currents from the rated

interrupting current down to the minimum continuous current that

causes melting of the fusible element(s), with the fuse applied at the

maximum ambient temperature speciﬁed by the manufacturer.

Current-limiting fuses are very good at clearing high-current faults. They

have a much harder time with low-current faults or overloads. For a low-

level fault, the fusible element will not melt, but it will get very hot and can

melt the fuse hardware resulting in failure. This is why the most common

CLF application is as a backup in series with an expulsion fuse. The expulsion

fuse clears low-level faults, and the CLF clears high-current faults. Current-

limiting fuses have very steep melting and clearing curves, much steeper

than expulsion links. Many current-limiting fuses have steeper characteris-

TABLE 8.5

Use of Current-Limiting Fuses as Reported in a 1995

IEEE Survey

5 kV 15 kV 25 kV 35 kV

General Purpose 15% 29% 30% 18%

Backup 15% 38% 43% 30%

On OH Line Laterals 5% 6% 9% 3%

On UG Line Laterals 7% 18% 20% 18%

Source: 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.

1791_book.fm Page 399 Monday, August 4, 2003 3:20 PM

400 Electric Power Distribution Handbook

tics than I

t. At low currents, heat from the notches transfers to the un-

notched portion; at high currents, the element melts faster because heat

cannot escape from the notched areas fast enough to delay melting.

General-purpose fuses usually use two elements in series — one for the

high-current faults and one for the low-current faults. General-purpose fuses

could fail for overloads, so restrict their application to situations where

overloads are not present or are protected by some other device (such as a

secondary circuit breaker on a transformer).

Full-range fuses provide even better low-current capability and can handle

overloads and low-level faults without failing (as long as the temperature

is within rating).

Current-limiting fuses can be applied in several ways including:

• Backup current-limiting fuse in series with an expulsion fuse in a

cutout

• Full-range current-limiting fuse in a cutout

• Backup CLF under oil

• Full-range (or general-purpose) fuse under oil

• CLF in a dry-well canister or insulator

The best locations for use on distribution systems are close to the substa-

tion. This is where they are most appropriate for limiting damage due to

high fault currents and where they are most useful for reducing the magni-

tude and duration of a voltage sag.

Some of the drawbacks of current-limiting fuses are summarized as

• Voltage kick — When a CLF operates, the rapidly changing current

causes a voltage spike (V = Ldi/dt). Usually, this is not severe enough

to cause problems for the fuse or for customer equipment.

• Limited overload capability — A backup or general-purpose fuse does

not do well for overloads or low-current faults. A full-range fuse

performs better but could still have problems with a transient over-

current that partially melts the fuse.

• Coordination issues — A current-limiting fuse may be difﬁcult to

coordinate with expulsion fuses or reclosers or other distribution

protective devices. CLFs are fast enough that they almost have to be

used in a fuse-blowing scheme (fuse saving will not work because

the fuse will be faster than the circuit breaker).

• Cost — High cost relative to an expulsion fuse.

Current-limiting fuses limit the energy at the location of the fault. This

provides safety to workers and the public. Arc damage to life and property

occurs in several ways:

1791_book.fm Page 400 Monday, August 4, 2003 3:20 PM

Short-Circuit Protection 401

• Pressure wave — The fault arc pressure wave damages equipment

and personnel.

• Heat — The fault arc heat burns personnel and can start ﬁres.

• Pressure buildup in equipment — An arc in oil causes pressure buildup

that can rupture equipment.

All of these effects are related to the arc energy and all are greatly reduced

with current-limiting fuses. Distribution transformers are a common appli-

cation of current-limiting fuses to prevent them from failing violently due

to internal failures.

8.3 Transformer Fusing

The primary purpose of a transformer fuse is to disconnect the transformer

from the circuit if it fails. Some argue that the fuse should also protect for

secondary faults. The fuse cannot effectively protect the transformer against

overloads.

Engineers most commonly pick fuse sizes for distribution transformers

from a fusing table developed by the utility, transformer manufacturer, or

fuse manufacturer. These tables are developed based on criteria for applying

a fuse such that the fuse should not have false operations from inrush and

cold-load pickup.

One way to pick a fuse is to plot cold-load pickup and inrush points on

a time-current coordination graph and pick a fuse with a minimum melt or

damage curve that is above the cold-load and inrush points. Most fusing

tables are developed this way. A fuse should withstand the cold-load and

inrush points given in Table 8.6. The inrush points are almost universal, but

the cold-load pickup points are more variable (and they should be since

cold-load pickup characteristics change with predominant load types). An

example application of the points given in Table 8.6 for a 50-kVA, 7.2-kV

single-phase transformer which has a full-load current of 6.94 A is shown

in Figure 8.11. The cold-load pickup and inrush points are plotted along

with K links. The minimum melt time and the damage time (75% of the

minimum melt time) are shown. Use the damage curve to coordinate. For

this example, a 12-A K link would be selected; the 1-sec cold-load pickup

point determines the fuse size. Since this point lies between the damage and

minimum melt time of the 10K link, some engineers would pick the 10K

link (not recommended).

Some utilities have major problems from nuisance fuse operations (espe-

cially utilities in high-lightning areas). A nuisance operation means that the

fuse must be replaced, but the transformer was not permanently damaged.

Nuisance fuse operations can be over 1% annually. Some utilities have thou-

1791_book.fm Page 401 Monday, August 4, 2003 3:20 PM

402 Electric Power Distribution Handbook

sands of nuisance fuse operations per year. A utility in Florida had a region

with 57% of total service interruptions due to transformer interruptions, and

63% of the storm-related interruptions required only re-fusing (Plummer et

al., 1995). During a storm, multiple transformer fuses can operate on the

same circuit. There are differences of opinion as to what is causing the

nuisance operations. Some of the possibilities are

• Inrush — Transformer inrush may cause fuse operations even though

the inrush points are used in the fuse selection criteria. Reclosing

sequences during storms can cause multiple inrush events that can

heat up the fuse. In addition, voltage sags can cause inrush (any

sudden change in the voltage magnitude or phase angle can cause

the transformer to draw inrush).

• Cold-load pickup — This is the obvious culprit after an extended

interruption (many of the nuisance fuse operations have occurred

when there is not an extended interruption).

• Secondary-side transformer faults — Secondary-side faults that self-

clear can cause some nuisance fuse events.

• Lightning current — Lightning current itself can melt small fuses.

Arrester placement is key here since the lightning current ﬂows to

the low-impedance provided by a conducting arrester. If the fuse is

upstream of the arrester (which would be the case on a tank-

mounted arrester), the lightning surge current ﬂows through the

fuse link. If the fuse is downstream, then little current should ﬂow

through the fuse.

TABLE 8.6

Inrush and Cold-Load Pickup Withstand Points

for Transformer Fusing

Full-Load Current

Multiplier

Duration,

sec

Cold-load pickup 2 100

310

Inrush points 12 0.1

25 0.01

Source: Amundson, R. H., “High Voltage Fuse Protec-

tion Theory & Considerations,” IEEE Tutorial Course

on Application and Coordination of Reclosers, Section-

alizers, and Fuses, 1980. Publication 80 EHO157–8-

PWR; Cook, C. J. and Niemira, J. K., “Overcurrent Pro-

tection of Transformers—Traditional and New Philos-

ophies for Small and Large Transformers,” IEEE/PES

Transmission & Distribution Conference and Exposi-

tion, 1996. Presented at the training session on “Distri-

bution Overcurrent Protection Philosophies.”

1791_book.fm Page 402 Monday, August 4, 2003 3:20 PM

Short-Circuit Protection 403

• Power-follow current through gapped arresters — Following operation

of a gapped arrester, a few hundred amps of power follow current

ﬂows in a gapped silicon carbide arrester until the gap clears (usually

just for a half cycle if the gap is in good shape).

• Transformer saturation from lightning currents — Lightning can contain

multiple strokes and long-duration components that last from 0.1 to

FIGURE 8.11

Transformer inrush and cold-load pickup points for a single-phase, 50-kVA, 7.2-kV transformer.

The minimum-melt curves and damage curves (dotted lines) for K-link fuses are also shown.

10.0 100.0

0.01

0.1

1.0

10.0

100.0

Current, amps

Time, seconds

1791_book.fm Page 403 Monday, August 4, 2003 3:20 PM

404 Electric Power Distribution Handbook

2 sec. These currents can saturate distribution transformers. Follow-

ing saturation, the transformer becomes a low impedance and draws

high current from the system through the fuse (Hamel et al., 1990).

• Animal faults — Across transformer bushings or arresters.

Several of these causes may add to the total. Nuisance fuse operations

have occurred when circuits were out of service. This means that lightning

is the cause since any type of inrush would require the system to be ener-

gized. Detroit Edison found that 70 to 80% of fuse operations were due to

lightning (Gabrois et al., 1973). Lightning and inrush events are the most

likely cause of nuisance fuse operations. Heavily loaded transformers are

more susceptible to nuisance fuse operations because of the preheating of

the fuse (a heavily loaded transformer is more susceptible to cold-load

pickup as well).

Another method of choosing the transformer fuse size that gives “looser”

fusing is the ¥2 method (Burke, 1996). Choose a fuse size larger than twice

the transformer full load current. A 50-kVA, 7.2-kV single-phase transformer,

which has a full-load current of 6.94 A, should have a fuse bigger than 14 A

(the next biggest standard size is a 15-A fuse). This applies for any type of

fuse (K, T, or other). The factor of two provides a safety margin so that

transformer fuses do not operate for inrush or cold-load pickup, and it helps

with lightning.

The fusing ratio is the ratio of the fuse minimum melt current to the

transformer full-load current (some sources also deﬁne a fusing ratio as the

ratio of fuse rated current to transformer rated current which is different

from this deﬁnition by a factor of two). Tight fusing means the fuse ratio is

low. Relatively low fusing ratios have been historically used which has led

to the nuisance fuse problems. The tighter fusing given using the Table 8.6

approach results in fusing ratios of 2 to 4. The looser ¥2 method gives a

fusing ratio of at least 4 (since the fuse rating is multiplied by two, and the

minimum melting current at 300 sec is twice the fuse rating). The fusing

ratio for the 50-kVA, 7.2-kV transformer with the 12-K fuse is 3.46, and it is

4.32 with the 15-K fuse.

Another strategy that is especially useful in high lightning areas: use a

standard fuse size for all transformers up to a certain size. This also helps

ensure that the wrong fuse is not applied on a given transformer. A standard

fuse size of at least 15T or 20K results in few nuisance fuse operations (IEEE

Std. C62.22-1997). At 12.5 kV, a 20K fuse should protect a 5-kVA transformer

almost as well as it protects a 50-kVA transformer. It may lose some second-

ary protection relative to a smaller fuse, and a small portion of evolving

faults will not be detected as soon; but other than that, there should not be

much difference. If fuses get too big, they may start to bump up against tap

fuse sizes and limit fuse options for lateral taps.

If looser fusing is used, some argue that overload protection of transform-

ers is lost. Countering that argument, overload protection with fuses is not

1791_book.fm Page 404 Monday, August 4, 2003 3:20 PM

Short-Circuit Protection 405

really possible if the transformer is used for its most economic performance

(which means overloading a transformer at peak periods). To avoid nuisance

fuse operations from load, we must use a fuse big enough so that thermal

overload protection is impossible. It is also argued that most transformer

failures start as failures between turns or layers and that a smaller, faster

fuse detects this more quickly. Tests have indicated that a smaller fuse might

not be much better than a larger fuse at detecting interwinding failures

(Lunsford and Tobin, 1997) (pressure-relief valves limit tank pressures very

well during this type of failure). All together, the arguments for a smaller

fuse are not enough to overcome the concerns with nuisance fuse operations.

If overload protection must be used, use a surge resistant fuse (it has a slower

characteristic for high-magnitude, short-duration currents).

A few utilities practice group fusing where a lateral fuse provides protec-

tion to all of the transformers on the tap. If the transformer failure rate

(including bushing faults) is low enough, then this practice will not degrade

the overall frequency of interruptions signiﬁcantly. One of the major disad-

vantages of this approach is that an internal transformer failure on a tap

may be very hard to ﬁnd. This drives up repair time (so the duration reli-

ability numbers suffer but not necessarily the frequency indices). Also, the

beneﬁcial feature of being able to switch the transformer with the fused

cutout is lost if group fusing is used.

Widely used, completely self-protected transformers (CSPs) have an inter-

nal weak-link fuse; an external fuse is not needed (although they may need

an external current-limiting fuse to supplement the weak link).

Transformer bushing faults often caused by animals can have different

impacts depending on fusing practices. A fault across a primary bushing

operates an external transformer fuse. If the transformer is a CSP or group

fusing is used, the upstream tap fuse operates (so more customers are

affected).

Current-limiting fuses are regularly used on transformers in high fault-

current areas to provide protection against violent transformer failure.

NEMA established tests which were later adopted by ANSI (ANSI C57.12.20-

1988) for distribution transformers to be able to withstand internal arcs.

Transformers with external fuses are subjected to a test where an internal

arcing fault with an arc length of 1 in. (2.54 cm) is maintained for 1/2 to 1

cycle. It was thought that 1 in. (2.54 cm) was representative of the length

that arcs could typically achieve. The current is 8000 A. Under this fault

condition, the transformer must not rupture or expel excessive oil. Note that

this test does not include all of the possible failure modes and is no guarantee

that a transformer will not fail with lower current. For example, a failure

with an arc longer than 1 in. has more energy and ruptures the transformer

at a lower level of current.

Table 8.7 shows rupture limits for several types of transformers based on

tests for the Canadian Electrical Association. If fault current values exceed

those given in this table, consider using current-limiting fuses to reduce the

chance of violent failures (the CEA report considers the limits provisional

1791_book.fm Page 405 Monday, August 4, 2003 3:20 PM

406 Electric Power Distribution Handbook

and suggests that more tests are needed). At arc energies within this range,

the failure probability is on the order of 15 to 35%. Note that the 2.5-kA limit

for pole-mounted transformers is much less than the ANSI test limit of 8 kA.

Based on a series of tests with internal 2-in. (5 cm) arcs, Hamel et al. (2003)

recommend considering current-limiting fuses for pole-type transformers

when the short-circuit current exceeds 1.7 kA.

For transformers with an internal fuse, completely self-protected (CSP)

transformers or padmounted transformers, the arcing test is done to the

rating of the fuse which is generally much lower than 8000 A. Table 8.8 shows

the maximum fault current ratings based on the ANSI tests. If the available

line-to-ground fault current exceeds these values, then consider current-

limiting fuses to reduce the possibility of violent failures. Not all utilities use

current-limiting fuses in these situations, and in such instances, internal

faults have failed transformers violently, blowing the cover.

If a transformer is applied in a location where the available line-to-ground

fault current is higher than shown in Table 8.8, use current-limiting fuses.

TABLE 8.7

Transformer Rupture Limits for Internal Faults

Transformer Type

I ◊◊

◊◊

t, A-s, or

Coulombs

Current Limit for a

1 Cycle Clearing

Time, kA

Pole mounted 1f 41 2.5

Pad mounted 1f 150 9

Pad mounted/subway 3f 180 11

Network with switch compartment 3f 90 5.4

Submersible and vault 1f 41 2.5

Source: CEA 288 D 747, Application Guide for Distribution Fusing, Canadian Elec-

trical Association, 1998.

TABLE 8.8

Maximum 1/2- to 1-Cycle Fault Current Rating on Distribution

Transformers Based on the Test in ANSI C57.12.20-1988

Transformer

Maximum Tested

Symmetrical Current

ÚIdt

in the ANSI Test

Overhead transformer 8000 A 66.7 A-s

Under-oil expulsion fuse

(based on typical fuse ratings)

Up to 8.3 kV

3500 A 29.2 A-s

Up to 14.4 kV

2500 A 20.8 A-s

Up to 25 kV

1000 A 8.3 A-s

Source: ANSI C57.12.20-1988, American National Standard Requirements for Over-

head-Type Distribution Transformers, 500 kVA and Smaller: High-Voltage, 67 000

Volts and Below; Low-Voltage, 15 000 Volts and Below, American National Stan-

dards Institute.

1791_book.fm Page 406 Monday, August 4, 2003 3:20 PM