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O

Current practices

A present-day distribution systemtypically consists of a feeder (line)originating from a substation. Severallaterals are tapped off this feeder.Distribution transformers installed onthese laterals supply end users. It is notunusual to see several branch linestapped off a lateral which, in turn,supply power to several end users.

The challenge for a protection engineeris to keep outages caused by overcur-rents confined to a minimum possiblesection of this complex system. So, theusual practice is to install progressivelysmaller protective devices (generallyfused cutouts) as the circuit moves awayfrom the substation. Were there noother complications, this scheme wouldprovide nearly perfect coordination forthe system. However, it is an establishedfact that most faults on a system are

temporary in nature lasting a few cyclesto a few seconds. (While some utilityrecords indicate 70 to 80 per cent of allfaults are temporary, another studysuggests they run as high as 80 to 95 percent.) Therefore, if the simple schemesuggested above is used, as many as 95per cent of all system outages could becaused by temporary faults.

The advent of reclosing circuit breakersand automatic circuit reclosers has at

the smart way to improve distribution system coordination

Figure 1Typical Distribution System

FUSE CURVE (25T)

RECLOSERMINIMUMTRIP

RECLOSERFASTCURVE

COORDINATION

RANGE

TIME

CURRENT

100 800

MAXIMUMCOORDINATIONPOINT

RECLOSERSLOW CURVE

Figure 2Recloser - Fuse

Coordination

least partially solved this problem. Thesedevices differentiate between temporaryand permanent faults with the reclosingdevice clearing the temporary fault,while protective device immediatelydown stream from the recloser willoperate on permanent faults only.

With these developments, the typicalpresent-day distribution circuit lookslike Figure 1. A recloser generally isinstalled at the start of a feeder, thelateral is fused with a sectionalizing-typecutout and a cutout with an appropri-ately-sized fuse link is installed at eachdistribution transformer.

In general, the transformer (or equip-ment) fuse is sized so that it will operateon all selected overcurrents withoutcausing the recloser to operate. Becausethe operation of this fuse affects only alimited number of customers, suchoperations are easily justifiable.

The lateral (sectionalizing) fuse issupposed to coordinate with the recloserin such a way that the recloser fast curveis faster than the fuse minimum-melttime-current characteristic and therecloser slow curve is slower than themaximum-clear time-current character-istic of the fuse. This allows the recloserto clear temporary faults while operatingon its fast curve and lets the fuse clear apermanent fault if the recloser transfers

Recloser

Transformer

LATERAL >

Cutout

LATERAL >

FEEDER

Cutout

Cutout

ptimizing system coordination is a prerequisite for reliable andeconomic operation of a modern distribution system. Over the pastseveral decades, certain system protection philosophies have evolvedutilizing available equipment. While those philosophies have

enhanced distribution system coordination, they do not fully address increas-ingly pressing needs to lower operating costs and raise customer satisfaction.This article reviews current practices and recommends improvements throughuse of Chance Electronic Resettable Sectionalizers (CRS).

CHANCE

ELECTRONICRESETTABLE

SECTIONALIZERS

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trip settings and higher-rated fuse links, the coordination range could be significantlyextended. However, system overload considerations generally will limit such anextension. Such extension will rarely extend the coordination to available short circuitcurrent level.

Fuse/fuse coordination

Most distribution circuits have at least two fuses in series. An essential requirement forcoordinating two or more fuse links in series is that the maximum-clearing time of thedownstream fuse link (protecting fuse link) shall not exceed 75 per cent of the mini-mum-melting time of the upstream fuse link (protected fuse link).Because both theminimum-melting and maximum-clear times are based on the level of fault current,there is a level beyond which a particular fuse link will stop coordinating with anotherlink as shown in Figure 3. This may result in both the upstream and downstream fuselinks operating at the same time. In the example above, a 25T fuse link was used at thestart of the lateral. From Figure 3, it is apparent that a 25T link will not coordinatebeyond 1,500 amps with the smallest of the fuse links (1T).

Therefore, it may be concluded that:

a) Beyond the maximum-coordination point, the recloser will not coordinatewith the sectionalizing (lateral) fuse link.

b) Beyond a certain level, the recloser, sectionalizing fuse link and the equipmentfuse link will not coordinate with each other.

The noteworthy common denominator in these conclusions is the sectionalizing fuselink. So, if it could be either eliminated or replaced by a device to extend the coordina-tion range to the systems maximum available short-circuit current, system coordina-tion would be greatly enhanced.

This can be easily achieved by using a sectionalizer in place of the fuse link at the startof a lateral. A sectionalizer is a protective device which has no time-current characteris-tic and does not interrupt a fault. When subjected to an overcurrent followed by acurrent below its minimum threshold, it merely counts backup recloser operations.After a predetermined number of such operations, it isolates the circuit while thebackup recloser is in the open position. The recloser is then allowed to close, restoringservice to unaffected sections of the system. If the fault is temporary and is cleared

to its slow curve. A typical recloser-fuse coordination scheme is shownin Figure 2.

It is a popular belief that this schemeprovides perfect coordinationbecause a recloser prevents fuseoperations on temporary faults toeliminate nuisance outages. How-ever, it is not uncommon to find autility where the engineeringdepartment believes that the systemis properly coordinated, but opera-tions personnel keep experiencingnuisance fuse blowing for noapparent reason.

In fact, there are two major prob-lems with present coordinationschemes. One involves recloser/

sectionalizing-fuse coordination andthe other, sectionalizing-fuse/equipment-fuse coordination.

Recloser/sectionalizing-fusecoordination

Because the recloser is a mechanicaldevice, it requires a specific time tooperate. This time is generally on theorder of 3 to 4 cycles. Consequently,the recloser fast curve (while inversetime in nature for lower levels of

current) tends to become asymptoticat 3 to 4 cycles. A fuse link, on theother hand, is a fusible element and,depending on the current level, canmelt in as little as 10 milliseconds.Therefore, as shown in Figure 2,beyond a level of current known asthe maximum-coordination point,the fuse will operate before therecloser has a chance to clear atemporary fault. This obviously willresult in a nuisance outage. Themaximum-coordination pointtypically occurs at 6 to 8 times theminimum-trip (pickup) setting ofthe recloser. As Figure 2 shows, withthe minimum trip of the recloser setat 100 amps, a 25T link ceases tocoordinate beyond 800 amps. Mostlikely, short-circuit currents willexceed the 800-amp level, especiallynear substations. Indeed, with higher

Figure 3Coordination Chart for T Links

Source:Chance Fuse Link Application Guide, Bull. 10-7701

Protecting TypeT Fuse Link

Ampere Rating

1

2

3

6

8

10

12

15

20

25

30

40

50

65

80

100

140

200

6

280

280

280

8

390

390

390

10

510

510

510

340

12

690

690

690

690

400

15

920

920

920

920

850

480

20

1150

1150

1150

1150

1150

990

550

25

1500

1500

1500

1500

1500

1500

1190

670

30

1900

1900

1900

1900

1900

1900

1900

1500

890

40

2490

2490

2490

2490

2490

2490

2490

2490

2000

1100

50

3000

3000

3000

3000

3000

3000

3000

3000

3000

2250

1250

65

3900

3900

3900

3900

3900

3900

3900

3900

3900

3900

3000

1700

80

4800

4800

4800

4800

4800

4800

4800

4800

4800

4800

4800

3700

2100

100

6200

6200

6200

6200

6200

6200

6200

6200

6200

6200

6200

6200

5000

2700

140

9500

9500

9500

9500

9500

9500

9500

9500

9500

9500

9500

9500

9500

9500

6600

3900

200

15000

15000

15000

15000

15000

15000

15000

15000

15000

15000

15000

15000

15000

15000

15000

15000

5200

Protected Type T Fuse Link Ampere Rating

Maximum Currents (rms amperes) for safe coordination

2

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before the sectionalizer count reaches the predetermined number, thesectionalizer remains closed and resets to its original state after a predeter-mined time.

If this new approach is adopted, the system shown in Figure 1 becomes theone shown in Figure 4 with the sectionalizer denoted as CRS. Because thesectionalizer has no time-current characteristic, the net effect is that therecloser coordination scheme shown in Figure 2 is greatly simplified, as

shown in Figure 5. With no fuse curve to intersect recloser time-currentcharacteristics, the coordination range is extended to the systems maximum-available short-circuit current. As an added benefit, the transformer fuse linkbecomes an independent entity.

Based on the foregoing analysis, an optimum coordination of a distributionsystem can be achieved by using reclosers on the feeders, sectionalizers on thelateral and fuses to protect such equipment as transformers.

The ChanceElectronic Resettable Sectionalizer (CRS)

The previous section established that replacing a fuse with a sectionalizer atthe start of a lateral greatly enhances system coordination. Although the

sectionalizer has been available for many years, a lack of understanding aboutits application and less than satisfactory performance by some early designshave severely restricted its use on distribution systems. The CRS offersexcellent performance and gives protection engineers the opportunity tomarkedly enhance system coordination.

Construction

Shown in Figure 6, CRS consists of an electronic-logic circuit mountedoutside a copper tube. The logic receives signals from two current transform-ers also mounted on the tube. The tube is fitted with top and bottomcontacts and a trunnion similar to those on a single-vent open-type cutoutand a spring loaded mechanism for the drop out action of the sectionalizer.

This entire assembly fits in a standard Chance

Type C cutout mount.Earlier versions of Chance Electronic Sectionalizers (Type CES) requiredreplacement of the actuators after each operation. Users generally carried aninventory of and equipped each line truck with the actuators. The newChance Electronic Resettable Sectionalizer (Type CRS) uses the springloaded mechanism to actuate the drop out action of the sectionalizer. Themechanism can then be reset with help of an adjustable wrench as shown inFigure 6. This eliminates the need to inventory the actuator and the linecrews can have one less item on their trucks.

Operation

Operational characteristics of the CRS are: Minimum-actuating current,

number of counts and reset time. The operational logic of a two-countsectionalizer can be summarized with reference to Figure 7, which illustratesthe responses to a transient fault and a permanent fault.

For the transient fault, fault current exceeded the CRS actuating current,and subsequently was reduced to less than the hold-off threshold by trippingthe upstream reclosing device. These first steps in the operational logic setthe sectionalizer to a count of one (1). But because the fault was

Recloser

Transformer

LATERAL >

Cutout

CRS

CRS

CRS

LATERAL >

CRS

FEEDER

RECLOSERMINIMUMTRIP

RECLOSERFASTCURVE

TIME

CURRENT

100

RECLOSERSLOW CURVE

COORDINATION

RANGE

Figure 4Distribution system with sectionalizers

Figure 5Recloser coordination

with sectionalizer

Figure 6Resetting Chance

Type CRS Sectionalizer

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The Chance Electronic ResettableSectionalizer (CRS)

Features and benefits

1. Resettable DesignCRS is a resettable design. There is no need to replace theactuator after each operation of the sectionalizer.

2. Low threshold for dead-line detection (0.3amp)This ensures that a return to normal load current followinga temporary fault will not result in an erroneous count bythe sectionalizer.

3. Immunity to inrush currentThe logic circuit is programmed to recognize anovercurrent only if both the negative and positive halfcycles of an overcurrent exceed the CRS minimumactuating current level. Because magnetic inrush isinvariably unidirectional, they are ignored by the logiccircuit.

4. Immunity to temperature variationsThe electronic logic circuit provides accurate, consistentoperation across a wide range of ambient temperatures.

5. Self poweredPower required to drive the logic circuit and the actuator issupplied by two current transformers mounted on thecopper tube. Therefore no external power source isrequired.

6. Protects against cold-load pickupThe low threshold for dead-line detection protects againstan erroneous count from a temporary current rise abovethe actuating-current level due to cold-load pickup.

7. Surge and EMI protection

CRS electronic circuitry is qualified against electromag-netic interference, radio frequency interference and surgecurrent.

8. Reliable dropout actionCRS hardware provides more positive and rapid dropoutoperating action.

9. InterchangeabilityCRS hardware is designed with the user in mind. It fits theChance Type C cutout mounts and the S&C ElectricType XS cutout mounts, simplifying installation andretrofitting.

10. Increased reliability at reduced costs

Because CRS virtually eliminates nuisance outages (and upto 80 per cent of all outages are termed nuisance), usingCRS can significantly reduce system operating costs.

transient, the load current upon reclosing stays below the

CRS actuating current so its operational logic proceeds nofurther. In fact, at the end of the reset time, the CRS countresets to zero (0).

In the case of the permanent fault, the first two logic stepsare identical to the transient fault, but this time the faultcurrent once again exceeded the actuating current andsubsequently was reduced to less than the hold-off thresh-old by a second tripping of the upstream reclosing device.These final two steps in the operational logic are what causethe sectionalizer to proceed to a count of two (2). At thistime, the logic circuit sends an electrical signal to the

actuator, causing the trunnion to unlatch and rotate thesectionalizer electronic module which swings down to itsopen position during the open time of the recloser.

NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice.

210 N. Allen St. Email: hpsliterature@hps.hubbell.comCentralia, MO 65240 USA Web: www.hubbellpowersystems.comPhone: 573-682-5521Fax: 573-682-8714

Copyright 2005 Hubbell Printed in USA 5MRGS6/04Bulletin 10-9801 Rev. 4/05

Figure 7

Operational logictransient fault

Operational logictwo counts

FAULT

LOAD

LineCurrent

OPEN

CLOSE

Recloser

OPEN

CLOSE

CRS

FAULT

LOAD

LineCurrent

OPEN

CLOSE

OPEN

CLOSE

A B D E

CF

A B

D

E

C

Inrush Current

Service Restored

Count 0

Count 1

Count 0

ResetTime

100 ms

Inrush

Current

K

Service Restored

to Unfaulted

Section

100 ms100 ms

100ms

Count 2 - Sectionalizer

Disconnects Faulted SectionCount 1

Recloser

CRS