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CH APT ER 10 Developments in Bench Testing Facilitiesfor Protective Gear

BYF. L. HAMILTON, AND N. S ELLIS.

INTRODUCTIONThe testing of protective-gear systems a nd their con-stituent components calls for test-equipment of a ratherspecialised nature. The requirements of modernprotective-gear systems have increased the complexityand cost of such equipment and the amou nt of testing tobe done has necessitated a speeding up in the procedureof tests.

The tests which may be necessary on protective-gearequipment are somewhat varied but, in general, will fallinto one or more of the following categories.Investigatory

These include those tests which m ay be essential oncircuits and components during development projects.The scope of these test may be large, as they oftenexplore a variety of effects, design factors, parameterchanges, etc. Tests of this type may also include investig-ations into more fundamental problems, such as thetransient response of current-transformers and theeffect of this on variou s protective-sys tems.

The information obtained from such investigations isoften used to check the soundness of new ideas andprovide p ractical design-data upon which new experi-mental equipment may be based o r by which existingdesigns may be modified to improve their performance.

Performance TestingThe overall performance of protective-systems,

relays, and the like is an important aspect of protective-gear testing. Tests of this type may be concerned withexperimental and production prototypes or with the cer-tificiation and type-testing of new protective-gearequipment or relays.

In the past, a large am ount of the testing referred toabove has required heavy-current rigs, which are costlyand limited in flexibility. Such heavy-current rigs requireextensive machine supplies, the demands upon whichare so great that they often form a considerable limita-tion to the numb er of investigations which may beundertaken.

The protective-gear test-bench described in this arti-cle was developed in order to replace the conventionalheavy-current and secondary-injection equipment inmany types of testing particularly those concerned withinvestigatory work on experimental projects and pro-totypes. The main requirements borne in mind in thedesign of the equipment are as follows:

(a) Extreme flexibility of test circuit and conditions.b) Rapid setting up of equipment.

(c) Use of ac. mains as the source of power.

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Provision of facilities for close control of thetest-conditions and parameters.Rapid rate of testing.

General purpose Test bench for Protective gearGeneral

An overall view of the test-bench is shown in fig. 1.The main primary circuits for the test bench are suppliedfrom the 440-volt 3-phase mains, with alternativearrangements for taking this supply from a three-phasevariable-frequency machine when necessary. The max-imum current which is taken by the primary circuits is ofthe order of 120 amperes but, as tests at these currentsare of a limited time-duration, the supply need not berated continuously at this current. When the primarycurrents are of the order of 10 amperes these may beused on a continuous basis.

The main primary circuits are shown d iagrammati-cally in fig. 2, from which it can be seen that the three-phase supply is applied, by a fault-making switch, totwo sets of variable impedances per phase, one of whichrepresents a generating source impedance and the otherthe impedance of a line. The current-transformers, ofwhich there may be up to four per phase, can be con-nected in various com binations according to the particu-lar fault distribution which it is required to reproduce.

The(0)

(b)

primary circuit has two maln functions:To provide primary currents in various combina-tions of current-transformers, with control ofoverall time constant, the point of wave at whichthe fault is applied, the type of fault, and theduration of fault. This function is required whenthe bench is used for tests where only current is ofsignificance.To provide variable current and voltage condi-tions at a relaying point with control of timeconstant, duration of fault, point-of-wave, andtype of fault. This function is required when thebench is used for the dynamic testing of relaysand protective-systems which require both cur-rent and voltage, e.g. distance protection.

The equipment required for the above basic functionslends itself to many other test applications which requirecontrolled current and/or voltage conditions. The vari-ous units which make up the complete test-bench aredescribed in more detail overleaf.Source and Line Impedances

One of the main requirements of test-equipment ofthis type is to obtain current and voltage transients of the

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order of those which may occur in practice. The sourceimpedances are designed with maxim um X/R values of practice. The line impedances are designed with a max-the order of 30 and these produ ce current-transients imum X /R of about 10 which corresponds to that of awhich are large enough to be comparable to those typical 275-kV line. Resistance can be inserted toobtained w ith generator and transformer impedances in reduced this X/R value to about 2,6 which correspondsto that of a typical 132-k V line.

CONNECTION REACTANCE AT 50 C/SUNIT A UNIT 6

3.0 6.0

n 12.0 24.0

48-o

/R O) FOR ALL CONNECTIONS z 35(0)

96.0

- 3.0 12.0 48-O

=-iz 6.0 9.0 160 54.0

24-O 27.0 36.0 72.0

96.0 j 99.0 1 108.0 144-o

(h)

I I ol/ol OI0 2.0 4.0 5.34

0 2.56 8.0 I b-0

0 2.9 10.6 32.0I I I I I

CC)FIG. 3. SOURCEREACTORS,SHOWING(U) CONNECTIONS,b) RANGEOFSOURCEIMPEDANCEWITHUNITSINSERIES,AND(C) RANGEOFSOURCEIMPEDANCEWITHUNITSIN

PARALLEL.

The reactors in both source an d line impedances areair-cored so that they are completely linear.

Two source-reactors are provided for each phase,each having fou r sections. These two units are providedwith switches so that their sections can be connected in anumber of ways and the resulting impedances connectedsingly, in series, or in parallel to give a total source-impedance per phase, variable in relatively close stepsbetween 2 ohms and 144 ohms. This arrangement isshown in fig. 3. The X/R values can be made constantover this range of source-impedance. An earlier versionof the bench uses the simpler method of a tapped reactorwith man ual plug-selection but this has the disadvantageof a varying value of X/R over the range of impedances.Source-impedance is not normally included in theneutral connection, but where tests require a highervalue of zero-sequence impedance in the source, thesource-impedance of one phase may be connected in theneutral connection. This is permissible, since such testswill invariably be concerned with single or doublephase-to-earth faults where one phase is not in use. Aneutral link is provided so that resistance may be con-nected to simulate systems which have a resistance-earthed neutral.

Three line reactors are provided in each phase and inthe neutral. These represent, when all are in series, a lineof approximately 3.5 ohms, the sections being 0.5, 1.0,and 2.0 ohms respectively (at 275 kV). By shorting outvarious sections, the impedance between the relayingpoint and the fault can be varied from O-3 .5 ohms insteps of 0.5 ohm.

The neutral line impedances are half the value of thephase line impedances, giving a typical value of ZO/Z 1 =2.5 for the line. The arrangement of the line reactors areshown in fig. 4.

Impedance ofllne units:Phase 0 10 3.5 ohm\ in O-5 ohm wzps \~~th phase-angles of70 10 nsNeutral 0 to l-75 ohms in 0.25 ohm step, with phase-angle5of ho or 70FIG. 4. ARRANGEMENTO FLINEIMPEDANC ES

Make Switch and Main ContactorThe function of the main contactor is to connect the

voltage-supply to the test-bench a short time (about fsec.) before application of the fault and to interrupt thefault-current after the required duration of the fault.With this arrangement, the main primary circuit is nor-mally d ead and is only made alive for the minimumrequired time. The main contactor is of the heavy indus-trial type and is capable of interrupting repeatedly themaxim um currents at very low power-factors.

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The make switch is required to apply the fault at aparticular point-on-wave , so that high speed and con-sistency of operation are essential. Experience hasshown that telephone-type relays with heavy-duty con-tacts can perform this duty with comparative ease. Thearrangement adopted is shown in fig. 5. One telephone-type relay with two parallel heavy-duy contacts is usedper phase, and two auxiliary relays, energised in parallelwith these, provide synch ronised contacts for timing,interlocking, etc. The standard contacts have been mod-ified by the addition of a mome ntum transfer-devicewith practically eliminates contact-bounce. Very consis-tent operation is thus obtained and wear on these con-tacts is negligible. The relays are energised from themaster control-unit as will be described later.

Current-transformersUp to twelve current-transformers can be energised

from the primary circuit in various com binations. Theyare of typical bar-primary design having normal 300 /land 300 1.5 secondaries. Four primary windings are pro-vided to enable the overall transformation-ratio (andthus secondary current level) to be varied over a widerange in close steps. These windings are in the propor-tion 1 : 3 : 9 : 27 giving primary turns by addition orsubtraction of 2-80 in steps of two. Selection of theprimary turns is by means of a man ual plug board which

FIG. 5. MAKE SWITCH ASS~MBLI

also provides the facilities for interlinking the primarycircuits of the current-transformers to form the variousarrangements required. A tapped section of 10 per centof the secondary turns enables the ratio of some of thecurrent-transformers to be controlled by t10 per cent insteps of 2 per cent. The arrangemen t permits tests up to acurrent which is equivalent to 30 times the current-transformer rating at fairly high values of X/R. Theamoun t of influence which the secondary burdeningexerts on both the magnitude and time constant of theprimary current is relatively small with modern low-VAprotection and the arrangement is a fairly close approx-imation to a current source over the whole range ofcurrents.

Typical arrangements of the current-transformer cir-cuits are shown in figs 6a, 6b, and 6c.

It should be noted that the reversing-switch in theprimary circuits of some current-transformers enablesrapid change-over between a single-end-fed internalfault, a double-end-fed internal fault, and a through-fault, when balanced-current systems of protection arebeing tested.

The provision of both l-ampere and 5-ampere secon-daries on the current-transformers enables relays andprotection for either rating to be tested. Alternatively,one secondary can be used as a search coil while theother is in use, or can be used for the injection of d.c. ora.c. ampere-turns into the current-transformer to simu-late certain conditions.

Voltage-transformersThese are provided so that the voltage windings of

relays may be connected to the relaying point, i e bet-ween the source and line impedances. They a re of nor-mal accuracy, suitable for burdens up to 75 VA, and areof ratio 440/l 10 volts, open-delta windings being pro-vided for relays requiring residual voltage connection.The primary windings of these voltage-transformersmay be connected via a selector-switch to a numbe r ofpositions as follows:(1) Continuous 440-volt supply. This is of use duringthose tests and adjustments where a continuousvoltage-supply is necessary.(2) Th e source side of the make switch, so that therelays are energised at normal voltage prior to the appli-cation of the fault-current. This represents the condi-tion, in practice, of a fault occu rring on a line which is inservice.(3) The line side of the make switch, so that the relaysare energised by the fault-voltage simultaneously withapplication of the fault-current. This represents the con-dition, in practice, of a faulted line being switched intoservice.

The general arrangements of the voltage-transformerconnections is shown in fig. 7. It can be seen from thisthat the current taken by the voltage-transformers is fedthrough the current-transformers. This is not completelydesirable, but with the normal voltage-transformer bur-dens use in practice the reflected impedance of the

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PLUG CONNECTIONS

REVERSINGSWITCHI AMP

IL REVERSING/SWITCH

MP

MP

FIG. 6. CURRENT-TRANSFORMERSSHOWING a) ARRANGE-MENTOFTYPE-A UNIT,(b) ARRANGEMENT OFTYPE-B UNIT,

AND(c) A VIEWOFTYPE A AND B UNIT.44

voltage-circuits is so large compared with the line impe-dance tha t the resulting errors are negligible. This con-nection is preferable to that in which the current-transformers are on the line side of the voltage-transformers, since in this case the small voltage-dropacross the current-transformers is imposed on thevoltage-circuits. This voltage-drop becomes significantwhen testing with terminal faults close to the relayingpoint with heavy currents.

M IN MAKECONTACTOR SWJTCH

VOLTAGE zTRANSFORMER c

jjjc-

FIG. 7. SELECTIONOFVOLTAGE-TRANSFORMERCONNECTION.

Master Control unitThe master control-unit provides the following fea-

tures:(1) A safety time-lag of about f second between closingof the main contactor and operation of the makeswitch. Interlocking contacts on the contactor and make switch ensure that the circuit is always brok enby the contactor.(2) 360 control through a selsyn and thyratron cir-cuit for selecting the point-on-wave at which the makeswitch closes its contacts. This is accurate and consistentwithin about 1 or 2.(3) A timing circuit which controls the duration of theEault-current. Extreme accuracy of interruption of thecircuit is not possible but the duration of fault ma y beadjusted from short faults of the order of 3 loops up tolong faults of the order of seconds.(4) A variable pulse which can be used for triggering anoscilloscope at any point in the fault-sequence. Thismeans that the whole of the fault may be observed on aslow time base or that an y part of the fault or associatedphenomen a may be recorded in an expanded form byusing a fast time base. This feature is extremely valuable,espeically when used with an oscilloscope equipped witha long persistence tube. Full advantage may be taken ofthe rapid testing-rate as photographic records may bekept down to a minimum .

The genera1 arrange ment of the lastest type ofcontrol-unit which has been developed is shown in blockform in fig. 8. The u nit is shown withdrawn from thepane1 in fig. 9 which also shows the unit construction ofthe Dekatron counter stages. These are made withdraw-able so that units c an be easily replaced or interchanged.The selsyn unit provides a variable-phase supply to anelectronic squaring circuit from which a pulse-formingcircuit is energised. The 51-cycle oscillator may be usedas an alternative to the phase-shifter and this provides


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AUXILIARY CIRCUITS

SELECTOR

cR0. TRIGGERFIG 8 BLOCK DIAGRAM OF CONIXOL-UNIT.

the facilities for random point-on-wave switching whichis sometimes required. The pulse-forming circuitdevelops a train of pulses at 10 ms intervals which, whentaken from the phase-shifter, are locked to a selectedpoint-on-wave of the main supply-voltage. The pulsetrain actuates a 3-decade dekatron counter unit whichgives a maxim um overall time of 10 seconds. Thecounter system is arranged to start always on a pulsecorresponding to the negative going half-cycle ofvoltageout of the phase-shifter. Thus the firing unit for the makeswitch, which is selected to a fixed 500-millisecond pointon the second decade. always fires at the correct point-on-wave which is indicated by the phase-shifter. Thefiring unit of the make switch contains a thyratron whichis triggered at the correct point, discharging a condenserthrough the make switch coil. Fast consistent opcra-tion of the make switch is thus obtained.

The trigger control for the oscilloscope can be selectedto any point in the pulse train thus enabling it to be up to1 second in advance or 9; seconds later than the firingpoint of the make switch. Th is enables the beginning ofthe fault to be observed or any point after to beexpanded on a fast time base. For complete conveniencein this respect. a variable IO 20 milliseconds delay isfitted to the trigger circuit to enable triggering to beeffected at points between the pulses of the main pulsetrain.

The internal stop selector is also capable of beingselected to any pulse in the train and this is used to trip

the main contactor. The duration of fault may thereforebe controlled in IO-millisecond steps from about 2 -3loops u p to 9Q seconds. Facilities are also provided forstopping the sequence, i.e. tripping the main contactor,from a protection relay under test.

The whole test-sequence is automa tically controlledfrom on2 push-button. the equipment automaticallyresetting when this button is released.

FIG c). Vlt:U 01. ON IKOL:UNIT


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Ancillary EquipmentApart from the facilities offered by the main primarycircuit, current-transformers, voltage-transformers,control-unit etc., certain auxiliary equipment has beenincluded in the bench in order to extend its use andprovide greater flexibility. The main items are describedbelow:Phase shifters

A variable-phase supply is a frequent necessity in bothprotective-systems and relay-testing and this facility hasbeen incorporated in the test-bench. The normal rotaryphase-shifters have limitations which make them unsuit-able for such use and a special static phase-shifting trans-former has been built. The arrangement is as shown infig. 10a and is basically a tapped three-phase trans-former energised from the same power-supply as themain primary circuits. The taps are so arranged that theman ual plug-selection of the output circuits on thephase-shifter panel provides a 240-volt supply adjust-able in 10 steps through 360 . The plug-selection pro-vides a visual indication of the phase-angle selected. Theturns on the various secondary taps are so arranged thatoutput-voltage remains constant independent of itsphase-angle. Variac transformers can be inserted in theoutput to give control of the output-voltage. Two plug-selector systems are provided so that two variable-phasesupplies may be obtained. The phase-shifter is capableof delivering currents of the order of 10 amperes withoutsignificant phase-shift so that there is no zero-correctionnecessary and the selected phase-angle may be referredto the main supply-voltage. Practice has shown that con-trol in 10 steps is adequate for most tests. Where closercontrol of phase-angle is necessary an external auto-transformer unit has been designed, as shown in fig. lob,which gives control between the 10 positions in steps of1.

Voltage SimulatorsIt is sometimes advantageou s to use the main current

circuits of the bench to energise the current circuits of arelay, but to use voltages which change in a predeter-mined way (independent of the bench circuits) on appli-cation of the fault-current. A voltage-simulator unit hasbeen designed for this purpose. Potentiometers in eachphase-to-neutral voltage enable the voltages on eachphase to be adjusted to a particular value to which theywill fall from n ormal voltage when the current is applied.

Primary Shunts and MetersThe primary circuits are provided with current-shunts

so that the primary current-transient may be observedby oscilloscope. A multi-range ammeter is providedtogether with current-transformers so that the steady-state current in any phase or the neutral may be meas-ured.

An operations-counter is provided so that the numb erof operations may be logged. This is useful both from thetest and maintenance aspects.

SECONDARV

I I440V MAINSINPUT

250 vOUTPUTh)

FIG. 10. PHASE-SHIFTINGTRANSFORMER,SHOWINC a)ARRANGEMENTFORlo'STEPS,AND b) THEATTACHMENT

FOR 1 STEPS.

Auxiliary A.C. and D.C. SuppliesIn view of the large num ber of electronic instruments

in use which require mains supplies, a numb er of mainssockets are provided on the bench. These can also beused for auxiliary voltage supplies of fixed phase-angle.

A 1 lo-volt d.c. supply which can be used for repeat-contactors is provided on the bench. A repeat-contactorwith seal-in contacts and lamp-indication is a built-infeature for relays with no repeat-contactor incorpo-rated.

TimingA portable electronic timer is normally used in con-

junction with the bench. The provision of contacts sync-hronised with those on the make switch ensure easymeasureme nt of overall operating-times of relays andprotection. Auxiliary contacts on the main contactorprovide similar facilities for measuring the release timesof relays on de-energisation.

Oscillographic WorkThe facilities for triggering oscilloscopes on single-

stroke operation enable transient p henomena to bereadily investigated on the bench. The ability to repeatshots under controlled test-conditions in conjunction

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with long persistence tubes reduces photography to aminimum . It is usually only necessary to photographtraces when permanent records of a particular trace arerequired.Typical ApplicationSome of the applications for which the test-bench hasbeen used are described below.Differential Protection

Most of the protective-systems based on current-balance of current-transformers can be explored.Protective-systems with up to four terminals can betested on a three-phase basis, and those w ith up totwelve terminals on a single-phase basis. The steady-state and transient balance of current-transformers arereadily checked by the equipment. The examples ofoscillograms shown in fig. 11 illustrate how clearly theproblems of transient balance of current-transformersmay be demonstrated. These records also show the con-siderable time constants obtainable and the consistencyof point-on-wave switching.Distance Protection

lndividual distance-relays or complete distance-protection schemes may be tested in a very realisticmanner with extreme rapidity. The provision of line andsource impedances affords a realistic relaying point fordistance-protection, the source-impedance being variedto simulate the system plant conditions and the lineimpedance varied to simulate the fault-position. Finecontrol of the impedance relay setting is effected bymeans of primary and secondary adjustments on thecurrent-transformers, thus enabling the accuracy of therelays to be determined under switched conditions inaddition to ordinary static bench-test. The effects of theprimary transients on accuracy is important with high-speed systems and this may be readily explored. Also,

the overall time of operation for faults within the zonemay be determined for various source conditions and forvarying degrees of transients.

With suitable interconnections it has been possible toexplore the effect of reversal of current-flow when aswitch opens under fault. The effect of the zero-sequence impedance of transformers and the efficacy ofearth-fault compensation has also been investigated.Relays

Tests on individual types of relays may be made withor without the effects of current-transformers. Someexamples are as follows:

(a) The dynamic characteristics of instantaneous-relays with off-set current-inputs.

(b) Timing characteristics of overcurrent relays andovershoot measurements. The effect of current-transformer saturation on time of operation ofovercurrent-relays.

(c) Dynam ic tests on directional-relays.CONCLUSIONThe illustrations in this article apply to a new design oftest-bench just nearing com pletion. A previous pro-totype design has been in use for three years and hasproved invaluable. It is of interest to note that nore than100 ,000 operations have been done on the earlier test-bench with practically no maintenance.

The principles developed in these test-benches havebeen applied to the testing of production-equipment,and a bench of similar type is being supplied to theC.E.A. It is also of interest to note that the equipmen tdeveloped and the methods used are finding applicationto University research. For example, a bench of this typeis being constructed at Manchester College of Technol-ogy for use in post-graduate research and as demonstra-tion equipment, the major items being supplied byReyrolle.

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Primary current add ing to rcmanence i

IP

i

IP

i

IP

i

P

i

IP

FIG. 11. ZER O-SEQUENCETEST SONBALANCEDEA RTHFAULTPROTECTIONWITHLOW-IMPEDANCERELAY.

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