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August, 1989Supersedes DB 41-332pages 1-8, dated April, 1969Mailed to: E, D, C/41-300A

Westinghouse ABB Power T&D CompanyRelay DivisionCoral Springs, FL 33065

Device Number: 87

Descriptive Bu lIeti n41-332

Page 1

TransformerDifferential RelayType CA Single Phase

ApplicationDesigned for 2-winding transformer protec-tion, the Westinghouse CA transformer dif-ferential relay has a constant percentagedifferential characteristic and inverse timing.It is a single phase unit, and three relays areused for three-phase transformer protection.

The CA relay is equipped with two restrain-ing circuits and one operating circuit in thedifferential unit. All three circuits producetorque on the induction disc; the restrain-ing circuits producing contact-opening tor-que, the operating circuit producing contactclosing torque.

The amount of operating or contact-closingtorque required to trip the relay is a fixedpercentage of the restraining torque.

Taps are provided to adjust the relay formismatch between current transformers.

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OperationTypical external connections for the CA re-lay are shown in Figure 1.

With the relay connected as shown in Figure1a, a through fault causes currents to flowthrough two restraint windings in the samedirection. If the current transformers operateproperly, these restraining currents are equal,or effectively equal if appropriate autobal-ance taps are used to compensate for mis-match in current transformer ratios, and noeffective current flows in the operating coilwinding; thus only contact-opening torqueis produced.

If the currents in the two restraining wind-ings are effectively unequal, the effectivedifference must flow in the operating coil.The operating coil current required to over- 1acome the restraining torque and close therelay contacts is a function of the restrain-ing current.

In the case of a heavy internal fault, whenan external source feeds current into thefault, the restraining currents are in oppositedirections and the restraining torque tendsto cancel out, as illustrated in Figure 1b.

When the currents fed from the two sidesare equal, or effectively equal because ofthe taps used, the restraint is totally can-celled.

When effectively unequal currents flow infrom the two sides, the restraint is equiva-lent to the difference in the two effectivecurrents divided by two, but since the moresensitive operating coil is energized by thesum of the two currents the restraint in thiscase is inconsequential, and a large amountof contact-closing torque is produced.

Protected Equipment 1b Protected Equipment

---- ---~~ L I li- External

Fault

-b

15=Ot

InR R5 7 9

-2- L-- Ig---..

--- -..- ~ -~ L

Internal r li-Fault

~

15=I7+I9t (RR R<

5 7 9

17 Ig-- -Figure 1: Typical External Connections for Type CA Relay. 1 B2ABB5

Descriptive Bulletin41-332

Page 2

ConstructionThe CA relay consists basically of a percent-age differential unit and an Indicating Con-tactor Switch (ICS).

Percentage Differential UnitInduction disc type, with an electromagnetthat has poles above and below the disc.Two restraint coils are located on the lowerleft-hand pole (front view), and an oper-ating coil is wound on the lower right-handpole.Transformer windings on both left- hand andright-hand poles are connected in parallelto supply current to the upper pole wind-ings. The upper pole current generates a fluxwhich is in quadrature with the lower poleresultant flux, and the reaction of the twofluxes produces a torque on the disc. If theoperating winding is energized, this torqueis in the contact-closing direction. If currentflows through the two restraining windingsin the same direction, contact openingtorque results.

Indicating Contactor Switch (ICS)The dc Indicating Contactor Switch is aclapper type device in which a magneticarmature, with spring leaf contacts attached,is attracted to a magnetic core upon ener-gization of the unit. When the switch closes,the moving contacts bridge two stationarycontacts, thereby completing the trip circuit.Also during this operation, two fingers onthe armature deflect a spring located on thefront of the switch which allows a target todrop to indicate that operation has occurred.Resetting of the target is by means of apushrod located outside the case at thebottom of the cover.No setting is required on the ICS unit ex-cept selecting the 0.2 or 2.0 ampere tap.When the relay energizes a 125 or 250 voltdc type WL switch or equivalent, the 0.2 tapis used. For 48 volt dc applications, the 2,0ampere tap should be utilized.

.c:>

•Tap Block \

StationaryContact

'----DampingMagnet

Figure 2

August, 1989

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Descriptive Bulletin41-332• ..ll.l.,.'1'1'

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Internal Wiring (Front View)CA Transformer Relayn FT -21 FlexitestG:> Case

Notes:1. Connect Terminal 7 to High

Current, Terminal 9 to LowCurrent, and Terminal 5 toCommon.

2. Terminals 5, 6, and 8 are tobe jumpered at relay case.

',-"CharacteristicsConstant percentage differential.Single phase, 60 hertz.Spst-cc contacts.Two restraint, one operating coil.

Restraint Circuits: 10 amperes, contin-uous (the untapped winding should belimited to 5 amperes, to prevent overloadingof the operating winding)_

Operating Circuit: 5 amperes, continuous.

Sensitivity: 50% unbalance.

Ratio Taps: 5-5, 5-5.5, 5-6, 5-6,6, 5-7,3,5-8, 5-9, 5-10,

Minimum Trip: On 5-5 tap, terminals 9and 5, 2.7 to 2,8 amperes.On 5-5 tap, terminals 7 and 5, 2.9 to 3.2amperes.

Operating CharacteristicsThe operating characteristics of the CA re-lay for normal through load current andthrough fault current are shown in Figures4 and 5. When the currents flowing into

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August, 1989

Page 3

Case Terminals

IndicatingContactor Switch

Restraining Coils

res

Operating Coil

,--III

IIL__

o 0Current Test Jack

Flexitest Switch

Chassis OperatedShorting Switcho o

Figure 3 57D4554

close to the curve value I7 =43.5.

Figure 6 shows the operating curves for therelay with the restraining currents 180' outof phase. These curves also apply wherecurrent flows in only one restraining wind-ing and the operating coil.

(2)

Relay SettingsTo determine the correct tap setting (seeCharacteristics for available taps), calculatethe currents delivered to the relay at full loadon the transformer bank; taking into con-sideration not only the current transformerratios but any delta connections which maybe used, as well.These currents will be in a certain ratio, andthe taps on the relay should be chosen tomatch this ratio as closely as possible. Forexample, assume that the currents are 7.8and 4.6 amperes, with the relay properlyconnected so that the higher current (7.8amperes) flows in the tapped restrainingwinding. The ratio 4.67/7.8 is equal to5/8.47. The nearest tap ratio of the relay is5/8, and this is the pair of taps that shouldbe used. The time dial should be set on thenumber 1 position.

and out of the relay are plotted on thesecurves, if the point falls outside of the in-operative area, the relay will close its con-tacts.

In Figures 4 and 5 the two curves goingwith the 5-5 tap are tied together with abracket to indicate that they go together.Similarly, the two curves for the 5-10 tapare also tied together. The center lines be-tween pairs of curves are shown for all taps.The paired curves, bounding the inoperativeareas, are not shown for all taps. Thesecurves may be determined approximately bymeans of the following formulas:

7.517 (1)For the upper curve: 19=-T-

For the lower curve: 17=.3119

In these formulas, T is the larger number ofthe tap pair. For example, if the relay is usedon the 5-7.3 tap, then T=7.3.

As an example of the degree of accuracy ofthe formula consider the point 17=43.5, and19=30; read from the lower curve for the5-5 tap, Figure 4. Applying the formula,equation (2), the calculated value of 17 isfound to be 45 amperes, which is fairly

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Descriptive Bulletin41-332

Page 4

Performance CurvesTypical Operating Curves, Low Current Values

12

~U0-c:'c'ev;&!-g0.0.oC=>,S

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10

8

6

4

2

00 2 4 6 817 Amperes inTapped RestrainingCoil

Figure 4

10 12 14 16 18

183A161

Typical Operating Curves, High Current Values60

50'0U0-c:'c.~

&!-g~eC=>,S

40

30

20

5-5 Ratio Topir,operot:ve Area'"i

E<!oS'

10

00 10 20 30 4017 Amperes inTopped RestrainingCoil

Figure 5

50 60 70 80 90

183A162

Standard Ratings (Single Phase, 60 Hertz)Trip I Amperes \ IndicatingContacts Minimum Contactor

Trip Switch

Sensitivity

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Typical Operating Curves, Restraint Currents 180·Out-Of-Phase

'0U0>C'c"e;;;

'"co"0G)a.a.

""c"''"~0-E<!.!;

2

00 .5 1.0 1.5 2.019 Amperes in Untapped Restraining Coil

Figure 6

CaseSize

Spst-ccDpst-cc

2.75 0.2/2.0 dc 50%

Further InformationApplication: Application Data 41-300Prices: Price List 41 -020Flexitest Case Dimensions: Descriptive Bulletin

41-075Instructions: Instruction Leaflet 41-332.2Other Protective Relays: Application/Selector Guide TO 41-010

FT-21

Westinghouse ABB Power T&D CompanyRelay DivisionCoral Springs, FL 33065

Printed in U.S.A.

August, 1989

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