DIFFERENTIAL VOLTAGE RELAYS

P H I L A D E L P H I A , P A .

I N S T R U C T I O N S GEH-1770BS U P E R S E D E S GEH - 1770 A

H A N D B O O K R E F E R E N C E 7270

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TypesPVD11CPYD11D

POWER SYSTEMS MANAGEMENT DEPARTMENT

ELECTRICGENERAL

, Differential Voltage Relays Type

GEH-1770 PVD

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COVERCASE

CRADLELATCH

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OUTER BLOCK RESET BUTTON

INNER BLOCK

CONNECTING PLUG

i (8009512)Fig. Type PVD11C Relay Disassembled

2

Voltage Relays Type PVD

Pig. 2 Volt-Ampere Characteristic Series Disks in the Relay

two stacks of four disks, connected inparallel. It is intended for application in the fewspecial instances where 10 ampere are

operating have a single closing contact. These contacts and the contact ofthe seal-in unit are connected parallel.

SEAL-IN UNIT

A seal-in unit is mounted in the upper leftcorner of the relay (see Fig. 8). This unit has itscoil in series and its contacts in parallel with themain contacts, so that, when the main contactsclose, the unit operates and seals tn. When theseal-in unit picks up, it raises a target view.The target latches up and remains exposed until itis released by a manual operation of the reset button,which is located at the lower left corner of thecover.

CASE

The case is suitable for either surface orsemi-flush panel mounting and an assortment ofhardware is provided for either mounting. Therover attaches to the case and also the resetmechanism when one is required. Each coverscrew has provision for a sealing wire.

The case has studs or screw connections atthe bottom for the external connections. Theelectrical connections between the relay units and

the case studs are made through spring backed contact fingers mounted in stationary innerand outer blocks which nests a removableconnecting plug which completes the circuits. Theouter blocks, attached to the case, have the studsfor the external connections, and the inner blockshave the terminals for the’ internal connections.

The relay mechanism is mounted in a steel called the cradle and is a complete unit

with all leads being terminated at the block.This cradle is held firmly in the case with a latchat the top and the bottom by a guide pin at theback of the case. The cases and cradles are soconstructed that the relay cannot be inserted in thecase upside down. The connecting plug, besidesmaking the electrical connections between the re-spective blocks of the cradle and case, also locksthe latch in place. The cover, which is fastenedto the case by thumbscrews, holds the connectingplug in place.

To draw out the relay unit the cover removed, and the plug drawn out. Shorting bars areprovided the case to short the current trans-former circuits. The latches are then released,and the relay unit can be easily drawn out. Toreplace the relay unit, the order followed.

separate testing plug can be inserted placeof the connecting plug to test the relay in place onthe panel either from its own source of current andvoltage, or from other sources. the relay can be drawn out and replaced by another which hasbeen tested the laboratory.

GEH-1770 Differential

V O L T A M P E R E C H A R A C T E R I S T I C S ° V C T H T R I T E S T A C K i F O U R O I S K I N S E R I E S, M U L T I P L YC U R R E N T B Y T f O *»•E N T *0 S T A C K S A R £ I N P A R A L L E L. )

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INSTALLATION

AND MOUNTING

The relay should mounted on a verticalsurface a location reasonable free from ex-cessive heat, moisture, dust and vibration. Therelay case may be grounded, desired, atleast No. 12 S gage copper The outline andpanel drilling diagram for the Type PVD relays shown in Fig. 14.

CONNECTIONS

The internal connection diagrams for the Type and Type relays are shown

Figs. 3 and 4, respectively. The of the external connections for a typical applicationis shown Fig. 5.

Note in Fig. 5, that when the relay is installed,a connecting jumper should be placed betweenterminals 4 and 5, and that 5 and arethen connected across the differential A B of the several A shorting bar isprovided between 5 and so that if the connection

the differential

The midpoint between the unit is connected to terminal 3.

Fig. 4 Type

it possible to test or calibrate unit without thenecessity of passing high current through the It also it possible to short-out the

its operation is not desired.

. The external connections in 5 indicate thatthe differential junction, points A and B, should belocated in the switchyard. For outdoor installationswhere the distance between the breaker relaypanel may be great, this may be important resistance through the fault CT loop may be too large. It is satisfactory the junctionpoints at the panel providing that necessaryrelay setting gives the desired

There should be only one ground connection inthe secondary circuit. When the junction arelocated the switchyard, the ground connectionshould be made there rather than at the panel.

‘The voltage short-time ratedand to protect it the contacts of the auxiliary 88short circuits the differential circuit.

Differential Voltage Relays Type PVD UfiH-l'I 'IU

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(K-63T582 -2) internal ConnectionsPVD11D Relay (Front View)3 (K-6375823-6) Internal Connections

(Front View) forType PVD11C

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junction pointsCAUTION: Under no circumstances should the relaybe placed in service without the Thyrite voltage-limiting circuit connected; i.e., without a jumperbetween terminals 4 and 5. Otherwise, the relayand secondary wiring will not be protected from thehigh crest voltages which result from an internalfault.

CTs.plug of the relay is withdrawn,circuit will not be opened.

and6

Thyrite stack andal 3. This makes87H

5

Voltage Relays PVD

UNIT

The’ unit can be adjusted at any voltagewithin the range shown on its calibration plate.Four specific calibration values are shown on theplate, which correspond four on thecalibration tube. These markings indicate the

of the lower edge of the armature in its position, to give the specified pick up

points.

The vertical position of the armature can bechanged by turning it on the threaded plunger rod.The armature is held in position by an internallocking spring which requires no adjustment. The

unit, unless otherwise specified on the re-quisition, will be set at the factory operate atits minimum pick up voltage. If the unit is to beset at some other point, as determined from Table Ior by calculation, the marks should beused as a guide in making a rough adjustment andthe test circuit shown in Fig. 6 should then be usedto make an exact setting.

Note in Fig. 6, that when the test plug is serted in the relay, the current transformer sec-ondaries are shorted by means of the link between

outer terminals 5 and 6. The adjustable test is applied across terminals 5 and 6 of the

that is, across the resonant circuit whichincludes the unit. Since the continuous voltagerating of the resonant circuit is only 150 volts, it is

6

Fig. 6 (K-6507935-5)

recommended a hand-reset lockout relay beused the test set-up, if desired setting isto be above this figure.

The following procedure should be followed checking pickup of unit. Startwith a test voltageconsiderably higher than the expected operatingpoint. Lower the test voltage by successive smallincrements, closing the test switch at each point.The lockout relay will operate each time, protectingthe resonant circuit. Eventually, a point will bereached where the unit will just fail to operate.The preceding voltage value can be taken as the pick-up value of the unit with reasonable accuracy.At the point where the unit fails to pick up,the test voltage must be removed at once to preventdamage to the relay.

unit setting is to be less than the150 volt continuous rating, it will not be necessaryto use the lockout relay. The volt-meter used musthave high internal impedance.

The operating coil of the unit is to provide three separate pick up ranges.desired range can be selected by means of links onthe molded base at each side of the coil. Thecalibration plate indicates the correct position oftbe links for each range. For each link

the pick up current can be varied over a 1range by means of the armature. The right-handlink is shown-in Fig. 8.

GEH- 1770 Differential Type

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Fig. 5 (650793 -5) Test Connectionsfor Type PVD11C and Type PVTllD Relays

Typical External Connectionsof Type PVD11C Relay Shoving One Phase Only

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. Differential Voltage Relays Type

Assuming that is an infinite impedance,consider the effect of an external fault on one of thefeeders. Each of the in the source circuits wiilproduce the secondary voltage necessary to driveits secondary current, as dictated by ratio,through its winding and leads, while the CT in thefaulted feeder will produce the secondary voltagenecessary to force the total secondary fault currentthrough its winding and leads. Neglecting the effectof load current, the in unfaulted feeders willproduce no secondary voltage. If all performas desired, there will be zero voltage between thejunction points c and d.

Unfortunately, during fault conditions, donot always perform ideally, since core saturationcan cause a breakdown in CT ratio. Such coresaturation results from a d-c transient in theprimary fault and may be aggravated byresidual flux left in the core by a previous fault.

the example of Fig. ‘7, the worst conditionpossible would be realized if the CT in the faultedfeeder saturated completely, losing all its abilityto produce a voltage in its secondary. The source

would then have to produce enough voltage toforce their currents through their own windings andthrough the winding and leads of the fault CT, un-assisted. As a result, acrossthe junction points c and d. Since the fault CT isassumed to be completely saturated, this voltage isequal. to the total of secondary currents from

8

Fig. in cradle

the source multiplied by resistance of thefault CT secondary including leads. This relativelysmall voltage (on the order of 100 volts) is, for thefault magnitude in question, the maximum voltagewhich could possibly appear between points c and dunder any condition. Obviously, the fault CT willnot lose all its ability toproduceanassisting voltage,and the source will tend to saturate. Hence, inpractice, junction point voltage be some-thing less than the drop mentioned above.

Consider effect of an internal fault. Asbefore the source will attempt to producesufficient voltage to drive their currents throughthe secondaries of the feeder In this case,however, the in effect have open-circuitedprimaries and their secondary impedances areaccordingly very high. The voltage appearing acrossc and d will therefore be high (on the order of 1000volts), its RMS value being limited by saturationof the source Even for a very moderate in-ternal fault, this voltage will be well in excess of thepessimistic maximum external-fault voltage pre-viously mentioned.

Thus, if a voltage relay having a high im-pedance were connected across junction points cand d, it could be calibrated so it would notpick up on the highest possible external fault but would pick up at a voltage less than that causedby the minimum internal fault. Such a relay wouldbe selective between internal faults and externalfaults or load current.

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T U B E

9(8009510) Type PVD11C Belay Unit(Hear Viev'i

Fig. 5 (5:i3-;5) Type PVD11C Belay Unit in Cradle(Front View'

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The preceding paragraphs describe in simplifiedform, the operating principles of the Type PVDhigh-impedance differential-voltage relay. prac-tice the relay circuit has been further refined secure better operating characteristics. For ex-ample, it is desirable that the differential circuitbe insensitive to the d-c component and to

other than the fundamental in the differentialvoltage. Consequently, the high-impedance operatingcoil is fed through a series-resonant circuit tunedto the fundamental frequency. It is desirable thatsuch a relay should wide adjustment range.The resulting change in inductive reactance in theunit itself would disturb tuning of the resonantcircuit. This is prevented if the operating coil ofthe adjustable high-impedance unit is isolated by afull wave rectifier.

During an internal fault saturation of the source as previously noted, limits the RMS value of

the secondary voltage. However, crest voltagessufficiently high to overstress insulation may occurif some form of voltage limiter is not provided.The resistor stack connected in parallelwith the high-impedance circuit serves thispurpose.

The introduction of the resonant circuit inseries with the high-impedance operating coil slightlyincreases the operating time of this unit. Althoughthis increases (to about 3 to 6 cycles, dependingupon severity of the fault) is not objectionable formoderate faults, it is desirable to shorten the timeon severe faults. This is accomplished by addition of an instantaneous overcurrent unit inseries with the stack, which is set tooperate on the current which flows onsevere faults.

APPLICATION CONSIDERATIONS

GENERALThe Type PM relay can be applied for bus

protection most cases where bushing-type cur-rent transformers are in use, and metal-cladequipment where current transformers with toridallywound cores having their windings completely dis-tributed are employed. The following points shouldbe considered or should be known on any proposedType PVD relay application:

1. All in the bus differential circuit musthave the same ratio. Small auxiliary currenttransformers for ratio matching cannotbeused.

2. must be operated on their full winding;do not use tap connections unless the

section between the taps used is fullydistributed.

3. Determine both maximum phase fault and maxi-mum ground fault.

4. Determine minimum ground fault.5. Lead resistance from differential junction

points to the most distant CT.6. Secondary excitation curves of the in-

volved.

Differential Voltage Relays Type

RELAY TYPE

applications involving bushing the Type relay is used in the great majority of cases.

There are two operating ranges available for the unit, and The

range is satisfactory for the vast majority of ap-plications; whereas,, the range is avail-able for the few where the unitsetting, as determined by calculation, is greater

3oov.

The Type PVDllD relay is intended for appli-cation in the few special instances where thecharacteristic of the CT exceeds 600 volts, orthe maximum secondary fault current exceeds 100amps. The PVDllD relay has two stackspermitting 200 amps maximum. For fault currentsin excess of 200 amps it is recommended that suchapplications be referred to the factory.

DISABLING THE RELAY

If tripping must be prevented because of by-passing a breaker, or other known reasons forfalse tripping, the relay may be burned out bysimply opening its contact circuit. This may beavoided by removing the connection plugs; or, ifexternal means are required, by short circuitingstuds and by different contacts of the sameswitch that is provided for the contact circuit.Opening the contact circuit then serves only toprevent false tripping manual operation of relay contacts.

If the generators feeding the protected busare operated at subnormal frequency, as during a

studsperiod, it is recommended that the

studs 3-6) be shorted during this period. Besure to remove the short when normal conditionsare restored.

RELAY SETTINGS

When the Type PVD relay is applied with bush-ing regardless of manufacturer, the correct

of the and units can be determinedby a simple calculation.

The formulae and procedures described in thefollowing paragraphs for determining relay settingsassume that relay is connected to the full windingof the differentially connected bushing or totan sections which are distributed on the core.

Electric with tap leads tagged X4 and X5 will have fully distrib-

uted tap The winding of any bushingCT as produced by a manufacturer can be assumedto be distributed.

SETTING HIGH-IMPEDANCE UNIT

The secondary winding of a bushing CT, whenevenly distributed around the core, has

reactance. Consequently, if it is assumed an external fault causes complete saturation of

the CT in the faulted feeder, the current forcedthrough this secondary by the source will beimpeded only by the resistance of the winding and

9

PVD GEH-1770

CTs,InInto PVD11C

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CTs CTs

GEH-1770 Differential Relays Type

Fig. (~-6507936-i) Used to Obtain the Factor of Type Relay

leads. The resulting drop will thus be the maxi-mum possible voltage which can appear across thedifferential junction for that external fault.

The setting of high-impedance unit forapplication any bushing CT can thus be ex-pressed as follows:

VR IF

Where:

V pickup setting of unit

d-c resistance of bushing CT secondary, leads to housing terminal (at

75

RL resistance of leads from junction pointto most distant CT (one-way for phasefaults, two-way for ground faults).

IF external fault current, separatevalues for phase and ground faults.

N = Number of secondary turns (ratio ofbushing CT).

Calculate separately for phase fault withme way lead for ground faults with twoway lead resistance higher of values of so obtained.

10

The multiplier 2 is used to provide a reasonablesafety factor. In cases where maximum ex-ternal fault current is not known, the maximuminterrupting rating of the circuit breaker should beused.

As previously noted (see PRINCIPLES OFOPERATION), the pessimistic value of the junctionpoint voltage determined by equation (1) is neverrealized in practice. The CT in faulted feederwill not saturate to point where it produces noassisting voltage. Furthermore, the conditionwhichcaused the fault CT core to saturate also tends tosaturate the cores of source which resultsin a further decrease in junctionvoltage.

These effects, of course, are not readily cal-culated. However, extensive tests on bushing used in General Electric circuit breakers, undersimulated fault conditions, have resulted in theestablishment of a so-called “performance factor”which can be determined for each application. The

for pick-up voltage for unit isthen written:

V =

where:

K performance factor.

This performance factor K is not a constantfor a given bushing CT, but varies for each in-stallation depending on the value of + It is readily determined from the curve in Fig. 10which is based on test data. The use of this curveis explained in the sample calculation whichfollows.

MINIMUM FAULT TO TRIP After the pick-up setting of has

lished for an application, a check should be made todetermine the minimum internal fault current whichwill cause the unit to operate. If this value is lessthan the minimum internal fault current the relay is suitable for the application.

The following expression can be used:

I

where:

minimum internal fault to trip

number of breakers connected to the bus(i.e., number of per phase).

secondary excitation current of the CTat a voltage equal to pickup of 87L.

current in relay unit at pick-upvoltage.

I1 current in limiter circuit atpick-up voltage.

N CT secondary turns (i.e., ratio).

Voltage PVD

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11

Type PVDDifferential Voltagei

VOLT- AMPEAE CHARACTERISTIC, PVO THTRITE STACK, FOUR DISKS IH SERIES.ABSCISSA SNOWS RMS AMPERES IH THTRITE IP THE RMS SYMMETRICAL SINE MAVIvotTi SHOWN ON OROINATE WERE FULLY OFFSET.

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(2TF87H ThanCurveFig.11

87* 11«T SETTING VS SECONDARY FAULT CURRENT -OR VARIOUS FAULT CT LOOPRESISTANCES. INDICATED SETTING iS FOR FULLY OFFSET SECONDARY CURRENTWHOSE SYVICTRICAL RMS VALUE WOULD BE AS SWM ON OROINATE.

400CT SECONDARY LOOPRESISTANCE (Rj «* ROIF/ H

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Differential Voltage Relays Type PVD

The value of is obtained from the secondaryexcitation characteristic curve for the current

former in use. the curve is not but thevalues of ES and are known for the bushingCT involved, an excitation curve can be plotted by thetemplate method described in GET-1225. It isundesirable for the pick-up voltage of the unitto be beyond the knee of the excitation curve, asthis results in poor sensitivity due to inconsistenciesbetween and causes large variations in voltage.

ES is the CT saturation voltage defined as theoint on the excitation curve where the slope is 45 see Fig. 13). is the excitation current at

voltage The relay current can be determined from the

resistance of the resonant circuit, assumed to beconstant at 2600 ohms. That is:

=

The current drawn by the limiter, be obtained from the volt-amperecharacteristic

of the unit as shown in Fig. 2.

SETTING CURRENT UNIT

The purpose of the unit is to provide in-stantaneous operation (less than two cycles) forextremely severe internal faults. It should thus beset for as low a pickup as possible without the of false operation on an external fault.

During an external fault the circuit, forthe worst condition, is subjected to the voltage re-sulting from a fully offset current flowing throughthe resistance of the secondary leadsof the saturated fault CT. Even though the d-ccomponent of the offset secondary current bedecaying, the unit is so fast that the effect ofthe first cycle of offset should be considered.

The correct setting of the unit can be de-termined from one of the curves (Fig. 11 or Fig. 12).The curve in Fig. 11 shows the RMS current whichwill flow in the Type relay stackwhen a fully-offset voltage is applied. For con-venience, the ordinate is plotted in terms of sym-metrical RMS VOLTS. The abscissa shows the

value of current which wouldflow if the ordinate voltage were fully offset. The voltage,read on the ordinate, should be the value of VRdetermined from equation (1). Under no circum-stances should the performance factor K be usedin determining the unit setting.

The curve in Fig. 11 assumes that the appliedvoltage is a perfect, fully-offset sine wave, resultingfrom the flow of an offset current through a re-sistance. The pulse of current drawn of course subtracts from the offset current. Con-sequently, as the magnitude of current

increases, the applied voltage will deviate fromoffset sine wave configuration. The curve in Fig. 11will thus be inaccurate the pessimistic directionat the higher voltage values. It should not be usedfor values of above 350 volts.

12

A second set of curves (Fig. 12) is furnishedfor use in cases where VP becomes excessive.This figure shows a curves for variousvalues of the total resistance of thefault CT and leadresistance.These show of the current atwhich the unit should bs set if a fully-offsetcurrent from the source enter the junction ofthe circuit and the fault CT loop. Selectthe curve for the resistance closest to the actualfault CT loop resistance or interpolate betweencurves for greater accuracy. The curves 12should be used when from equation exceeds350 volts.

Note that when the Type relay is used,there are two stacks in parallel. Hence

unit setting, determined by either of theforegoing methods, should be doubled.

SAMPLE CALCULATION

The various steps in checking a Type PVD relayapplication will be better understood a typicalexample is given. Referring to the one-line diagramin Fig. 5, assume that the breakers are TypeFK-439, 60 KV, 1500 1200 amperes, with amaximum of 14,500 amperes.The excitation curve for the bushing CT in thesebreakers is shown in Fig. 13. Similar data can beobtained from the Handbook and Instruction Bookon other current transformers.

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Thyrite

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VR

VOLTAGE UNIT The unit setting will first be determined

by means of equation factor K. The value of R from Fig. 13 is 0.80 ohm.

the measured value of lead resistance. Substituting the various in

equation we have:

2 0.80 + 0.90 ) 205 volts

The relay sensitivity using this setting of would be obtained from (3) as follows:

I (5 0.05 + + 0.06) 240 = 93 amps

If the installation is checked on the basis ofequation (2) using the performance factor the resultswould be as follows:

+ +

14500 (0.80 + 0.90)

this value is checked on the ordinate of thecurve in Fig. 10, the value of K is found to beapproximately 0.77. Using this value in equation the setting of the unit is found to be:

2 (0.77) (0.80 + 0.90) 158 volts

The sensitivity this setting be:

I + + 0.02) 240 67 amps

It obvious that in this example very is by using the performance factor K. many

applications especially those where ground faultcurrent is limited, it may be necessary to make useof the performance factor in order to realize. thedesired sensitivity. Also, many cases it will befound that the value of Vequation will be above

bushing CT in use; whereas if equation (2) isused, involving the performance factor, the valueof will be below the knee of the curve andhence the application will be feasible.

The performance factor curve in Fig. 10 wasof course determined for the bushing inGeneral Electric circuit breakers. When the Type

rela is applied with of other manufacture,equation 1) should be the unit setting. If the resulting value of VR is too high,it is suggested that application be referred tothe factory.

Voltage Relays Type PVD GEH-1770

CURRENT UNIT

of VR from equation(1) is 205 volts, andhencethecurve in Fig. 11 shouldbe used in setting This curve indicates thata 1.3 ampere setting of the unit would be safe.Since the minimum setting of the unit is 2amperes, the unit would be set at this pickup.

If the value of VR had exceeded 350 volts inthis example, the curves in Fig. 12 would have beenused. Since R L) is 1.70 the 1.5 ohm curvewould be used, or an interpolation made between the1.5 and 2.0 ohm curves.

MAINTENANCE

The relays are adjusted at the factory and itis advisable not to disturb the adjustments. If forany reason they have been disturbed, the section

should be followed in restoring

INSPECTION

A mechanical inspection of the relay should bemade at least once every six months.

CONTACT CLEANING

For cleaning fine silver contacts, a flexibleburnishing tool should be used. This consists ofa flexible strip of metal with an etched roughenedsurface, resembling in effect a superfine file. Thepolishing action is so delicate that no scratches areleft, yet corroded material will be removed rapidlyand thoroughly. The flexibility of the tool insuresthe cleaning of the actual points of contact. Some-times an ordinary cannot reach the of contact because of some obstruction from someother part of the relay.

Fine silver contacts should not be cleaned withknives, files or abrasive paper or cloth. Knivesor may leave scratches which increase arcingand deterioration of the contacts. Abrasive paperor cloth may leave minute particles of insulatingabrasive material in the contacts and thus prevent

The burnishing tool described above can beobtained from the factory.

RENEWAL PARTS

It is recommended that sufficient quantities ofrenewal parts be’ carried in stock to enable

of any that are worn, broken,

When ordering renewal parts,. address thenearest Sales Office of the General Electric Com-pany, specifying the quantity requiredanddescribingthe parts by numbers as shown in Re-newal Parts Bulletin No. GEF-3798.

13

Differential

87H87L

Inthisexample,thevalue87L(1), neglecting the performance

87H.87HAssume that

is 0.90 ohm. 87Hquantities(1 )

14500VR = < 240 (Rs +

87Lequation

205min' x 2600

ADJUSTMENTSthem.

(Rs Hl) Ip/N= 0 34

300(240)ES

If(2),

87L

14500VR = 240

with would actualpointsfile

158= (5 x 0.04min 2600I

littleis filesOngained

closing.inp, as determined fromthe saturation point of(1),

the

VRthe

prompt replacementor damaged.CTs used

PVD CTsl 87Lused in determining

the catalogue