Cooper - Fault Indicators - Application Guide - 320-05.pdf

7/27/2019 Cooper - Fault Indicators - Application Guide - 320-05.pdf

http://slidepdf.com/reader/full/cooper-fault-indicators-application-guide-320-05pdf 1/8

ContentsGeneral . . . . . . . . . . . . . . . . . . . . . .1Overview ofCooper Fault Indicators . . . . . . . . .2Fault Indicator Reliability . . . . . . .3System Conditions That AffectFCI Operation . . . . . . . . . . . . . . . . .3

Inrush Current . . . . . . . . . . . . . .3Cable Discharge . . . . . . . . . . . . .3Proximity Effect . . . . . . . . . . . . .4Backfeed Voltages andCurrents . . . . . . . . . . . . . . . . . . .4Temperature Compensation . . .5Cable Preparation: URD . . . . . .5

Simplify Fault IndicatorApplication . . . . . . . . . . . . . . . . . . .5Fault Indicator Inrush Operation .6

Three-Phase System . . . . . . . . .6Current Trace . . . . . . . . . . . . . . .7Single-Phase System withLaterals . . . . . . . . . . . . . . . . . . . .7

GENERAL

Fault indicators are devices whichindicate the passage of fault current.When properly applied, they can reduceoperating costs and reduce serviceinterruptions by identifying the sectionof cable that has failed. At the sametime, fault indicators can increasesafety and reduce equipment damageby reducing the need for hazardousfault chasing procedures. To providethe greatest benefit, the fault indicatormust indicate reliably when faultcurrent passes through the cable towhich the fault indicator is mounted.Misapplication or improper selectionof the fault indicator can reducereliability.

Figure 1 illustrates a typical loopedunderground distribution system. Theunderground cable is looped into andout of each transformer to the open

point. Typically one fault indicator isplaced on each incoming phase of thetransformer. Figure 1 shows the faultindicator target position after a cablefault caused the tap fuse to operate. Ifthe line is followed from the source,the fault would be located betweenthe last tripped indicator and the firstnon-tripped (normal) indicator. Visualinspection of the fault indicator elimi-nates the need for trial and error sec-tionalizing of the system, thus reduc-ing service restoration time.

Fault Indicators

Faulted Circuit Indicator Application Guide

Electrical Apparat

320-05

October 1998 • Supersedes 07/94Printed in U.S.A.

FAULT

FUSEDCUTOUT(OPENED) TRIPPED

FAULT INDICATOR

NORMALFAULT INDICATOR

(NOT TRIPPED)

FUSEDCUTOUT(CLOSED)

Figure 1.Typical looped underground system application.




2

Figure 2 illustrates a typical overheadapplication with a main sectionalizingfuse and unfused laterals. With a faultindicator on each lateral, the lateral onwhich the fault is located can easily bedetermined. This saves time in locatingthe fault, especially on lines locatedon terrain that makes visual inspectionof the line difficult (i.e. swamps,

wooded areas, mountain terrain).Fault indicators can be installed ontransformers, switchgear, sectionalizingcabinets, bushing terminators, over-head lines and underground cable.The quantity and location of the faultindicators should be sufficient to elim-inate the need for the sectionalizingfault chasing method.

OVERVIEW OF COOPERFAULT INDICATORS

Cooper Power Systems offers a widevariety of fault indicators ranging from

basic circuitry models in the DelayedReset style to the more sophisticatedcircuitry of the Test Point Reset andElectrostatic Reset types. The CooperPower Systems S.T.A.R. faulted cir-cuit indicator product line offers fivebasic types of FCI’s and each unit istailored to be the most reliable for theintended application. Each type variesby reset method and the type of sys-tem it connects to.

All Cooper Power Systems S.T.A.R.faulted circuit indicators reset auto-matically upon restoration of systempower or after a predetermined time

period. Automatic resetting fault indi-cators sense either voltage or currentto determine that power has beenrestored to the system. The units thenreset to the normal position to elimi-nate the need for line personnel tomanually reset the units. This savestime, money and makes the fault indi-cation more reliable.

Table 1 shows an overview of theCooper Power Systems faulted circuitindicator product line. It shows the faultindicator types based on reset methodand lists the typical system application,physical mounting location and reset-

ting requirements for each type.

Test Point Reset faulted circuitindicators are mounted to the testpoint of cable terminators (loadbreakelbows, deadbreak elbows, etc.). Theyare economical and simple to applyon underground distribution systems.Test Point Reset fault indicatorsrequire no special cable preparationin order to install and fit all majormanufacturers voltage test points.

Low Voltage Reset faulted circuitindicators are commonly used onunderground systems where a voltagetest point is not available on the cableterminator. It utilizes the secondaryvoltage provided by a distributiontransformer to power the device. Thisfault indicator is ideal for single-phasepadmount transformers where asecondary voltage source is readilyavailable.

Electrostatic Reset faulted circuit

indicators are powered and reset bythe electrostatic field surrounding abare or insulated, non-shieldedconductor. These fault indicators areideal for overhead distribution systems.A minimum line-to-ground voltage of6.9 kV is sufficient to produce thepotential gradient needed to powerthe device.

Current Reset faulted circuit indicatorscan be applied on systems where avoltage source is not readily available.A minimum continuous load current of2.4 A must be present to power thefault indicator. Current reset fault indi-cators can be applied to either under-ground or overhead systems, provid-ed the cable shield does not allow areturn path for current to pass throughthe FCI sensor area.

Delayed Reset Faulted Circuit

Indicators automatically reset back tothe normal position when reset timehas expired. They can also be resetwith a manual testing/reset tool. Thiseconomical unit is ideally suitable fortemporary use as a testing tool, sinceit does not contain all the standardfeatures of the rest of the S.T.A.R. line.The unit is powered by a long lasting,lithium ion battery that provides thepower to indicate the faulted condi-tions via a high intensity LED display.

Figure 2.Typical Overhead Application.

TABLE 1Cooper Power Systems Faulted Circuit Indicator

Type Typical Physical Voltage/CurrentDescription System Application Mounting Location Requirements

Test Point Reset Underground On the test point Min 5kV L-Gof the connector

Low-Voltage Reset Underground On the URD shielded A secondarycable below the voltage sourceconnector (min. 105 volts)

Electrostatic Reset Overhead On bare or insulated Min. 6.9 kV L-Gnon-shielded cable

Current Reset Underground On the URD shielded Min. 2.4 Acable below the continuousconnector

and Overhead and on bare or insulatednon-shielded cable.

Delayed Reset Underground On the URD shielded None (Lithium Ioncable below the battery poweredconnector with timed reset)

and Overhead and on bare or insulatednon-shielded cable

FUSED CUTOUT

TRIPPEDFAULT

INDICATOR

NORMAL(NOT TRIPPED)

FAULT INDICATOR



320-0

FAULT INDICATORRELIABILITY

All Cooper Power Systems faultedcircuit indicators result in reliableoperation when applied properly. Toselect a reliable fault indicator, onemust consider the various operatingconditions that can affect operation.

Reliability can have many differentmeanings. For some, fault indicatorreliability means that the fault indicatoroperates every time a fault occurs onthe distribution circuit. However, faultindicators may operate erroneouslywhen not applied properly. A betterdefinition of fault indicator reliability isthat the faulted circuit indicators showproper indication each time theyoperate. This means that the units mayonly operate 95% of the time, but thatthe fault indicators will operatecorrectly each time they operate.Most operations personnel would

agree that FCI misoperation causesmore problems than FCI non-operation.

Cooper Power Systems has beenproviding fault indicators to the utilitymarket for over 20 years. Experienceshows that in order to achieve reliableindication, the proper fault indicator,with the appropriate features must beapplied to the distribution system. Inmany cases, customers reportinginaccurate operation of fault indicatorsare applying a fault indicator that doesnot have the proper features for theapplication. Furthermore, the primaryreason for the wrong fault indicatorbeing used was the fact that the utilityhad such a large variety of units thatline personnel became confused asto the proper FCI to use for a givenapplication.

In 1991, Cooper Power Systemsbegan a project to better understandwhat system variables can affect faultindicator operation. The goal of thestudy was to determine why faultindicators misoperate and how tocorrect the problems. In conjunctionwith a major customer of CooperPower Systems, several underground

distribution circuits, where knownproblems existed, were modeled.What was found provided a betterunderstanding of what featuresshould be standard on all faultedcircuit indicators.

SYSTEM CONDITIONSTHAT AFFECT FCIOPERATION

Inrush Current

Inrush currents can occur any time anelectrical distribution system is ener-gized. Inrush currents can exceed

normal load currents by 12 to 60 timesthe normal load current. These higherthan normal currents can falsely trip afaulted circuit indicator since faultindicators are typically peak sensingdevices. Fortunately, system energiza-tion occurs in two distinct instances.The first being when the system hasbeen without power for an extendedperiod of time and the second beingduring a recloser operation.

If an automatic resetting faulted circuitindicator is being used, initial systemenergization will not affect the FCIindication, because the FCI will

automatically reset to the normalposition if the system remainsenergized.

The vast majority of FCI inrush opera-tions are due to recloser operations.On most distribution systems, areclosing device is almost always pre-sent somewhere on the system,either at the substation or further outon the feeder circuit. In the case ofthree-phase applications, recloseroperation can cause tripping of faultindicators on phases not involved inthe fault. The best method for dealingwith inrush due to recloser operationsis to inhibit the trip circuit following amomentary interruption of service.Cooper Power Systems fault indicatorsequipped with inrush restraint, willsense the momentary service inter-ruption and keep the fault indicatorsin the normal position (see FaultIndicator Inrush Operation on page6). If the reclose sequence is suc-cessful, the units will automaticallyreset and arm for the next operation.For simplicity of application, it is rec-ommended that inrush restraint bespecified on all faulted circuit indica-tors.

Cable Discharge

Cable discharge is a phenomenonthat occurs at the instant of faultoccurrence and manifests itself bytripping indicators beyond the fault.The 1991 system study showed thata substantial length of cable existsbeyond the fault, the charge on that

cable can discharge to the fault. Thiscable discharge is usually of shortduration, but can be of high enoughmagnitude to trip one or two indica-tors beyond the location of the fault,especially if the fault indicator has arelatively low trip rating.

Since the frequency of the cabledischarge is in the several kilohertzrange, it becomes possible tofilter out the high frequency signals.This effectively eliminates the cabledischarge as a source of FCI operatioThe application of a low pass filter inthe fault indicator sensing circuit will

efficiently perform this function. Thefrequency of the low pass filter is setat a precise level that filters out thecable discharge, while allowing thefault indicator to indicate if a currentlimiting fuse is protecting the circuit.

Figure 3 shows a simplified 25 kV ditribution cable system with a faulthaving a variable amount of cablebehind it. Assuming that the copper,1/0 cable has an L=196.3µH per fooand C=59nF per foot the dischargefrequency can be calculated:

f = 1/(2π√LC)

If there is 1000’ of cable behind thefault then f = 46.77 kHz.

Increasing the length of the cablebehind the fault will not significantlychange the peak outrush current thadischarges into the fault. The frequecy of the transient inrush currentoscillation is however proportional tothe length of the cable behind thefault. For example if 3000’ werebehind the fault the frequency wouldbe reduced to 15.59 kHz.

Figure 3.A simplified 25 kV distribution cablesystem.

25 kV

1/0, 260 mil,copper cable

Distributed Capacitance

of High-Voltage Cable

1000' 3000'



Proximity Effect

Proximity effect is defined as thecurrent from an adjacent cable affect-ing the operation of a faultindicator on the desired cable of oper-ation. Proximity effect has two differ-ent modes in which it can affect faultindicator operation.

First, on multi-phase circuits, conduc-tors of different phases can be in veryclose proximity to one another. This isespecially true on underground appli-cations. Due to the distance betweenthe conductors, it becomes difficult forthe fault indicator to distinguishbetween the magnetic field of thedesired conductor and that of theadjacent conductor. Therefore, a faulton the adjacent conductor can causethe fault indicator on another cable totrip in error.

The second way proximity effect can

influence cause fault indicator opera-tion is when cables of the samephase are in close proximity to oneanother. For example, in a typical sin-gle-phase transformer application,incoming and outgoing primary cablesare usually in close proximity to oneanother. In this case, the fault currentwould be traveling in one directionthrough the incoming cable and goingin nearly the opposite directionthrough the outgoing cable. The cur-rent in the outgoing cable can thengenerate a magnetic field that effec-tively cancels the magnetic field gen-erated in the incoming cable. This is

especially true for low trip level faultindicators, because they are muchmore sensitive to the adjacent mag-netic field.

Proximity effect can be eliminated intwo ways. First, selecting a high triprating can make the fault indicator lesssensitive to adjacent phases. Thesecond and most effective method foreliminating proximity effect is by theuse of a current transformer (eitheropen or closed core) to concentratethe flux from the desired phase andtherefore reduce the sensitivity of thesensor to adjacent phases.

Backfeed Voltages andCurrents

Automatic-resetting fault indicatorsthat use either voltage or current toreset the unit when system power isrestored can be affected by the sys-tem connection. Typically on systemswhere delta connections exist, single-phase sectionalizing can cause volt-age or current backfeed which maycause tripped fault indicators to reset.

It is difficult to quantify the exact mag-nitude of the backfeed current withoutdoing an in-depth system study.However, voltage backfeed can bequantified. Due to typical transformerconnections, the maximum voltagebackfeed that a fault indicator maysee is 60% of the nominal systemvoltage.

This fact provides a means by whichvoltage backfeed can be eliminatedas a cause of FCI resetting. A resetrestraint circuit, with the threshold setabove the 60% level will prevent thefault indicator from resetting in error.


4

Figure 4.Proximity effect illustration.

ELBOWTIE-OFFTAB

ELBOW

LOCATETHE SENSORPORTION OFTHE FAULTEDCIRCUITINDICATOR(in the outlinedarea)

NEUTRALWIRES

TERMINATEDNEUTRALWIRES

a b c d

Figure 5.

Recommended methods of concentric neutral primary cable preparation.



TemperatureCompensation

Temperature compensation is a fea-ture that allows the fault indicators toprovide a more accurate and reliableoutput between -40°C. and +85°C.

S.T.A.R. fault indicators use a currenttransformer design to implement thelow pass filter circuit and to minimizethe affects of adjacent magneticfields. These features provide a sig-nificant performance improvementover the use of a reed switch sensingcircuit.

The copper coil of the current trans-former changes resistance as thetemperature varies, therefore, the out-put will also change due to variabletemperature ranges. Cooper PowerSystems designed and integrated acircuit that compensated for the tem-perature changes, therefore, provid-

ing an accurate output.Cable Preparation: URD

Proper primary cable preparation isnecessary for fault indicators to workreliably. The unit cannot be installeddirectly over the concentric neutral,because the unit monitors current. Ifthe current in the neutral (return path)is high it could cancel the effects ofthe field from the fault current in theconductor. The net result could beless than the trip rating; therefore, thefault indicator would not trip. The linecrew, upon investigation, would be

misled by the false indication. Theunit can, however, be mounted overthe neutral if the neutral is doublebacked as shown in Figure 5 a and b.Doubling the neutral allows the FCI todetect the field from the fault currentin the conductor because it eliminatesa return path. Refer to Figure 5 for therecommended methods of concentricneutral primary cable preparation.

Fault indicators can be used on tapeshield conductor or drain wire cable. Ifthe cable shielding does not providethe return path for the indicator, theindicator can be installed directly tothe cable. If the cable shielding pro-vides the return path for the fault cur-rent, the indicator will not reliablydetect a fault, as described above,

and will require the use of a tapeshield or a drain wire adapter. (Anadapter must be installed approxi-mately four inches below the elbow toallow ample space to mount the faultindicator on the cable.)

SIMPLIFY FAULTINDICATOR APPLICATION

Reliable fault indicator application canbe achieved by selecting the properfaulted circuit indicator for the job, butin addition, the fault indicator must beapplied properly in the field. The bestmethod to assure proper field applica-tion is to make the application rulessimple for field personnel.

Due to the characteristics of a typicaldistribution system, it is possible toachieve a high degree of faultindicator reliability while using justone or two trip ratings for the entiresystem. The application of the faultindicators then becomes a matter ofapplying the right fault indicator forthe given system type.

In 1983, EPRI published a studycalled “Distribution Fault CurrentAnalysis.” This study showed typical

fault current characteristics for across-section of distribution feedersthroughout the United States. Theresults of the study showed that faultstypically have less than two ohms ofimpedance. Furthermore, it showedthat system fault calculations wererelatively accurate when a zero faultimpedance was used to calculateline-to-ground fault current magni-tudes.

This information suggests that sincethe maximum line-to-ground fault curent will flow to the fault, selecting alow trip rating on a fault indicator provides little added sensitivity over afault indicator with a higher trip ratingAs a matter of fact, due to phenomena like proximity effect, a lower triprating can make the fault indicator

less reliable than one with a higherrating. Therefore it makes sense toset the fault indicator trip rating baseon the available fault current levels,not the load current level.

A better method for selecting a trip raing is to analyze the system parameteand select a trip rating based on ageneralized system. For example, atypical 200 A underground systemrarely carries more than 50 to 100 Abut is designed to carry a maximum o200 A. While a trip rating just abovethe 100 A level could be selected,selecting a trip rating greater than the200 A maximum continuous load current will allow the load current to varyanywhere in the acceptable currentrange. This trip setting can then beused for any 200 A underground distribution system. The only criteria thaneeds to be satisfied to make this phlosophy work is that the fault indicatomust respond faster than any protective device on the system.

320-0



As seen in Figure 6, the fault indicatorwill allow the load current to vary withinthe acceptable range without tripping.The FCI will trip when fault currentreaches approximately 400 A, whichis beyond any expected continuousload current, and less than the typicalexpected minimum fault current. And,because response time of the fault

indicator is faster than protectiveequipment, the unit will indicate afaulted condition before any protectivedevices operate.

This philosophy will work for overheaddistribution as well. A system that isprotected by fused cutouts, can havefuse ratings from 1 A through 200 A.Since 200 A tin links can carry 150%rated load, the possible load currentscould vary anywhere from a fewamperes up to a maximum of 300 A.If a faulted circuit indicator with a triprating of greater than 300 A is used,that trip rating could be used anywhereon the overhead system, provided thefault indicator is fast enough to beatout the fuse for any fault occurrence.

When using this application philosophy,one can also ignore any variations inthe fault indicator trip rating due tochanging cable dimensions, becauseas long as the trip rating is substan-tially larger than the maximum loadcurrent, the exact trip rating does notmatter. The maximum line-to-groundfault current will flow to the fault,effectively tripping a fault indicatorthat is slightly higher in rating. This

further allows the fault indicator to beapplied generally to any system.

In general, a fault indicator applied toa 200 A URD loop should have aLOW trip rating (around 400 A) and a600 A system should utilize a HIGHtrip rating (around 800 A).

The last step to making the applicationof the faulted circuit indicator simpleris to make all the features that maybe needed in any application standardon the fault indicator specified. Forexample, it is recommended that allfault indicators have inrush restraint.This means that this fault indicator canbe applied anywhere on the system,even where a recloser is not beingused. Therefore, the line personnelwon’t have to decide between a unitwithout inrush restraint and one withinrush restraint.

FAULT INDICATORINRUSH OPERATION

The following is a description of howa system operates during inrushwhen fault indicators equipped withinrush restraint are applied.

Three-Phase System

(Refer to Figure 7)

Fault occurs on Phase A. Faultcurrent levels exceed trip rating ofindicators and, thus, all indicators(#1 and #2) between recloser andfault trip.

Recloser opens. Indicators, with

inrush restraint, on Phase B and Cdetect loss of power. Deadtime issufficient to fully discharge the tripcapacitors.


6

Figure 7.Typical Three-Phase Application ofFault Indicators with the InrushRestraint Option.

Figure 6.Low Voltage Reset faulted circuit indicator (Low trip rating) response curve asapplied to a typical 200 A underground system.

1 10 100 10,000

CURRENT IN AMPERES

T I M E I N S E C O N D S

.0001

.001

.01

.1

1

10 2 0 0

4 0 0

Typical Rangeof ContinuousCurrent underNormal Load

TypicalRange

of FaultCurrent

ResponseCurve

1,000

AUTO RECLOSEROR BREAKER

THREE-PHASESYSTEM

AØ

#3

#2

#1

#6

#5

#4

#9

#8

#7

BØ CØ

FAULT



Recloser closes back in. Phase Asees fault current again. #1 and #2indicators remain tripped while Band C phases experience inrush.Trip circuits for indicators on Band C phases have already beeninhibited and, thus, will not trip.

The recloser cycles until recloser

lock-out occurs. The only trippedindicators are #1 and #2, thus,locating the fault between #2 and#3 on the A-phase.

Current Trace

(Refer to Figure 8)

Time A - Normal Load Current Allindicators in normal state.

Time B - Fault occurs on Phase A (orA-leg of bifurcated feeder).All indicators on Phase A(or A-leg) between recloserand the fault trip.

Time C - Recloser opens.Indicators with inrushrestraint on B and C-phases(or B-leg and C-leg) detectloss of power.

Time D - Deadtime exceeds timenecessary to trigger inrushrestraint circuitry. (i.e. 300milliseconds, but in somecases 100 milliseconds).Indicators on B andC-phases (or B-leg andC-leg) inhibit their tripfunction in anticipation ofinrush current.

Time E - Recloser closes back-in.A-phase (or A-leg) currentrises to fault level. Previouslytripped indicators on A-phase(or A-leg) remain tripped. Band C-phase (or B-leg andC-leg) indicators experienceinrush, but do not responddue to disabled trip function

at time D.Time F - Recloser cycles until it locks

open or until the fault isisolated by the operation ofa fuse link or sectionalizer.The only tripped indicatorsare on A-phase (or A-leg)between the recloser andthe fault.

Single-Phase Systemwith Laterals

(Refer to Figure 9)

Fault occurs on A-leg of bifurcatedfeeder. Fault current levels exceedtrip rating of indicators and, thus,all indicators (#1 and #2) betweenrecloser and fault trip.

Recloser opens. Indicators, withinrush restraint, on B-leg detectloss of power. Deadtime is sufficientto fully discharge the trip capacitors.

Recloser closes back in. A-legsees fault current again. #1 and #2indicators remain tripped while B-leg experiences inrush. Tripcircuits for indicators in B-leghave already been inhibited and,

thus, will not trip.

The recloser cycles until recloserlock-out occurs. The only trippedindicators are #1 and #2, thus,locating the fault between #2 and#3 on the A-leg.

Current trace: Same as Figure 8.

320-0

Figure 9.Typical Single-Phase Application ofFault Indicators with the InrushRestraint Option.

Figure 8.Current Trace for Three-Phase Application of Fault Indicators with the InrushRestraint.

FTIME

TRIP RATING

FAULT CURRENT

FAULT CURRENT

NORMAL LOAD

ZERO CURRENTA B C D E

CURRENT IN A-PHASE OR A-LEG

FTIME

TRIP RATING

NORMAL LOAD

ZERO CURRENT A B C D E

CURRENT IN B AND C-PHASE OR B-LEG Ø C-LEG

FAULT

A-LEG B-LEG

#3

#2

#1 #4

#5

#6

AUTO RECLOSEROR BREAKER

SINGLE-PHASE SYSTEMWITH LATERALS

(b)

Cooper - Fault Indicators - Application Guide - 320-05.pdf

Documents

Transcript of Cooper - Fault Indicators - Application Guide - 320-05.pdf