MULTILINGER-2622A

SynchronismCheck

Equipment

GE Power Management

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INTRODUCTION

Synchronism check equipment is that kind ofequipment that is used to check whether or nottwo parts of the same system or two separatesystems are in synchronism with each other. Theequipment covered by this paper is of two classes.The first is comprised of standard speed devices(Type IJS) that will check the presence ofsynchronism in a matter of several seconds andthen produce an output to permit closing orreclosing the associated breaker if synchronismexists. The second class is a high-speed device(Type GXS) that operates in the order of onesecond to check synchronism. In both cases, it isimportant to recognize that synchronism checkdevices are basically permissive devices in thatthey permit or prevent closing that is initiated bysome other device. Synchronism check devices donot initiate reclosing.

When deciding to select either the standard or thehigh-speed device, the user should be aware thatall synchronism check relays can be fooled by acondition where the slip between the two systemsbeing checked is slow enough to appear in therelay characteristic long enough to produce anoutput. In general, the slower the relay, the slowerthe slip required to fool the relay. Thus, the GXSdevice could be more subject to this problem thanthe IJS device. This is discussed in detail insubsequent sections of this paper and should beconsidered carefully before making a selection.

Both types of synchronism check equipmentsrequire single-phase (line-to-line or line-to-neutral)potentials from the same phase(s) on both sides ofthe associated circuit breaker or the equivalentthereof in the case where a delta-wye powertransformer is interposed between the twosources of potential.

GENERAL

There are two kinds of synchronism checkequipment available. The first, and by far themost prevalent, is the slow-speed device TypeIJS. The second, and not so prevalent, is thehigh-speed device Type GXS. The high-speedsynchronism check relay (GXS) finds applicationprimarily in conjunction with high-speedautomatic reclosing, but it may be employedwhenever high-speed synchronism check with alock-out feature is desired. The slow-speeddevice (IJS) is used to supervise manual local,and/or supervisory closing, and/or time delayedautomatic reclosing where the possibility of thelack of synchronism exists.

OPERATING PRINCIPLES OF IJS

The IJS relay is an induction disk device thatreceives single-phase voltages from the samephase(s) on both sides of the breaker. There aretwo electromagnets in the relay, and eachelectromagnet has two coils. In the first, oroperating electromagnet, the coils areconnected in such a way that they receive thevector sum of the two voltages (incoming Vi andrunning Vr). This electromagnet produces atorque on the disk in the direction to close therelay contacts. This operating torque isproportional to the square of the vector sum ofthe two voltages as indicated in Equation (1).

Top = Kop (Vi + Vr)2 Equation (1)

where Kop is a design constant.

The second electromagnet is the restrainingdevice. It receives the vector difference of theincoming and running voltages from the samephase (s). This electromagnet acts on the same

SYNCHRONISM CHECK EQUIPMENTK. Winick

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disk as the operating magnet, but it producestorque in the opposite or restraining direction.That is, it develops torque that tends to hold therelay contacts open. This restraining torque isproportional to the square of the vectordifference of the two voltages as given byEquation (2).

Tr = Kr (Vi – Vr)2 Equation (2)

where Kr is a design constant.

In addition to the electrical torques in the relay,there is also a mechanical restraint in the formof the control spring. Thus, the net torqueoperating torque (T) in the relay is the result ofall three.

T = Kop (Vi + Vr)2 - Kr (Vi - Vr)

2 – Ks

In general, the mechanical restraint (Ks) is smallcompared to the electrical torques so that theabove equation becomes

T = Kop (Vi + Vr)2 - Kr (Vi – Vr)

2 Equation (3)

It should be noted that with equal voltages onthe running and incoming busses, (Vi + Vr) willbe equal to (Vi - Vr) when Vi and Vr are 90 degrees apart. As the two voltages approacheach other in phase angle, (Vi + Vr) will becomegreater than (Vi - Vr) and vice versa. Thus, thenet operating torque tends to increase as thetwo systems tend to be more in phase with eachother. Whether the net torque is in the operatingdirection (T>0) or in the restraining direction(T<0) will also depend on the relativemagnitudes of Kop and Kr. In the case of all theIJS relays, Kr is designed to be greater than Kopso that the net torque goes from restraining tooperating at some angular separation that issmaller than 90 degrees. This angle is called theclosing angle. Some IJS relays have fixedclosing angles while others have closing anglesthat are adjustable.

The operating time of the IJS is determined bytwo factors, one of which is the strength andposition of the permanent drag magnet thatacts on the disk to slow its motion, and theother is the rotary distance that the disk has totravel to close the contacts. The former is abuilt-in characteristic that differs from model tomodel, and is not adjustable in the field, whilethe latter is adjustable and depends on the timedial setting applied to the relay.

Since Kr is greater than Kop, if only one voltage(Vr or Vi) is applied to the relay the torque T willbe negative and the relay contacts will be heldopen. This is apparent from Equation (3). Withneither voltage applied, the net electrical torqueis zero, but the spring will keep the contacts ofthe relay open. Thus, in order for the IJS relay toclose its contacts, there must be voltage presenton both sides of the open breaker, and the phaseangle between these voltages must be withinthe closing angle setting of the relay. Thismeans that the synchronism check unit by itselfwill not permit picking up a dead line. Forapplications where dead line and/or dead busoperation is required, undervoltage detectorsare used to bypass the synchronism checkdevice. Some models of the IJS include thesedead line and dead bus auxiliary devices, whileothers do not.

OPERATING CHARACTERISTICS OF IJS

As indicated above, there are several differentmodels of IJS relays. The most popular standardmodels are listed in Table I along withinformation relating to the differences betweenmodels. While Table I lists only 115-volt 60-Hzrelays, models for operation at other voltagesand frequencies can be made available onrequest.

All the IJS relays covered by Table I comecompletely self-contained in a size S1 case. Theoutline dimensions for this case are given on Fig. 1.

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As may be noted from Fig. 2 (IJS51A), Fig. 3(IJS52A), and Table I, the only differencebetween the IJS51A and the IJS52A relays isthat the IJS51A contains a target seal-in unitwhile the IJS52A does not. Neither modelcontains any dead-line or dead-bus voltagedetecting devices. However, both the IJS52Dand IJS52E models do include these devices,but the manner in which these undervoltagedetectors are internally connected differbetween the two models. These differences canbe observed by referring to Fig. 4 (IJS52D) andFig. 5 (IJS52E). The contacts available on thevoltage units in both relays permit a number ofvariations of dead-line and/or dead-busoperation. In the case of the IJS52D, thisoperation will be instantaneous, but in theIJD52E, the undervoltage units are interlockedwith the synchronism check unit so that timedelay operation is obtained in dead-line - dead-bus schemes. This will be covered in detailunder the section on APPLICATION.

The fourth column in Table I indicates theclosing angles of the relays listed. There are onlytwo different kinds: one that is fixed at 20degrees; and another that is adjustable in therange of 20 to 60 degrees. The only differencebetween the two types is that the drag magnetin the 20 to 60 degree relay is stronger, so thatthe operating time for a 20-degree setting islonger than it is for the 20-degree unit. Actually,the 20-degree relay can be set for greater closingangles; however, it is not generallyrecommended because the pick-up timedecreases below what might be desirablevalues. Figure 6 gives the operating times of the20-degree unit (IJS51A1A, IJS52A1A, IJS52D1A,and IJS52E1A) as a function of the phase anglebetween two suddenly applied rated voltages.Figure 7 provides this same information for the20 to 60 degree unit (IJS51A3A, IJS52A7A,IJS52D3A, and IJS52E3A). Note that both sets ofcurves apply for a No. 10 time dial setting. Forlower time dial settings, the operating times will

be approximately proportionally lower. Forexample, at a No. 5 time dial setting, theoperating times will be approximately half ofthe values given in the curves.

When considering the characteristics of the IJSsynchronism check units it is important torecognize that these units can close theircontacts even when the two voltages appliedare not in synchronism. The reason for this canbe understood by referring to Fig. 8. Assumethat the “Running” system is at 60 Hz but thatthe “incoming” system is at some slightlyhigher frequency, say 60.1 Hz. Thus, the vectorVr is rotating 60 times a second and Vi is rotating60.1 times a second, as indicated in Fig. 8a.Since it is the relative positions of the twovectors that count, assume that the Vr vector isstanding still and Vi is rotating at one revolutionevery 10 seconds as indicated in Fig. 8b.

If the IJS synchronism check unit, set for aclosing angle of 30 degrees, is connected tothese two voltages, the unit will develop closingtorque when Vi lies between A and B in Fig. 8c.As Vi rotates, it enters this region at B movingcounterclockwise. At that instant, the torque onthe disk becomes positive (operating torque)and the disk starts to turn in the contact-closingdirection. All the while that the Vi lies betweenA and B the torque will be in the closingdirection, increasing in magnitude as Viapproaches 0 and then decreasing to zero as itapproaches A. Whether or not the contacts ofthe unit actually close during that time willdepend on the operating time of the unit (timedial setting and drag magnet strength) and thelength of time that Vi is in the operating region.This latter condition depends on the differencein frequency (slip) between the two voltagesand the closing angle setting of the unit. It isobvious that the bigger the angular setting andthe slower the slip, the better is the chance forundesired operation. It should be noted thatsince the closing angle of the relay will always

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be less than 90 degrees, as Vi rotates at constantspeed opening torque will be present for alonger time than closing torque for eachrevolution. This ensures that the disk iscompletely reset each revolution.

In the ultimate selection of a relay and a settingthere must be a compromise between speed ofoperation and security against undesiredclosing operation. The curves on Fig. 9 give themaximum slip for which the IJS51A, IJS52A1A,IJS52D1A, and IJS52E1A relays will close theircontacts as a function of time dial setting forclosing angle settings of 20, 40, and 60 degrees.Figure 10 provides the same information for theIJS51A3A, IJS52A7A, IJS52D3A, and IJS52E3A.

Figure 11 applies to all the relays in Table I. Itgives data on how the closing angle of theserelays will change when the voltages applied areboth not rated values. The data is based on theassumption that rated voltage is applied to onecoil of the relay, while the other coil is suppliedwith a voltage that is other than rated. This isdone for closing angle settings from 20 to 60degrees in 10-degree steps. For example, theuppermost curve applies for a 20-degree relaysetting, while the lowest one applies for a 60-degree setting.

Consider a relay set for a 30-degree closingangle at rated voltage. Checking the secondcurve from the top it will be noted that thecontacts will close at rated voltage (115 volts)applied to both coil circuits when the angles arewithin plus or minus 30 degrees. Now if onevoltage is reduced to about 82 volts, the unitwill close its contacts only when the angle isbetween plus and minus 20 degrees. If onevoltage is reduced to 70 volts, the relay will notoperate at any angular separation. For a 20-degree setting, the relay will not operate belowapproximately 88 volts on one of the coilcircuits. If a 10-degree setting were used, thevoltage limitation would be still more

restrictive. Thus, the range of voltages that maybe present at the relay should be carefullyconsidered when determining a relay setting.

As described previously, the IJS52D and IJS52Erelays include instantaneous dead-line anddead-bus detecting units. These units are allalike. They are not adjustable, but they aretested in the factory to ensure that they pick upsomewhere between 30 and 46 percent of ratedvoltage, and drop-out somewhere in the rangeof 14 to 31 percent.

APPLICATION OF IJS RELAYS

The synchronism check relays Type IJS findapplication wherever slow-speed synchronismcheck for automatic or manual closing of a lineterminal is required.

When a transmission line is de-energized for anyreason and it is to be restored to service viasynchronism check, it is necessary to first closeone end. This energizes the line and permits thesynchronism check relay to supervise closing ofthe second end. It is apparent that there is noneed for synchronism check at the first end toclose, so closing at that end may be completelyunsupervised or it may be supervised by somecombination of line and bus voltage indication.

In many instances, either end of the line may beclosed first. This requires that synchronismcheck devices be present to supervise closing atboth ends of the line. Since the synchronismcheck relay requires an energized line in order tooperate, it will block all attempts to close thefirst end unless it is by-passed by some otherdevice. The by-passing function could be anyone or more of a number of combinations, suchas dead-line - live-bus, or dead-bus - live-line, etc.

The IJS52D and IJS52E relays includeinstantaneous voltage units that are connectedto receive bus and line potentials. Figure 12

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shows a typical external connection diagram for the IJS52D in a scheme that offersinstantaneous voltage units in conjunction withtime delay synchronism check.

This diagram illustrates how a typical selectorswitch may be employed to obtain a variety ofschemes. In general, whether any switch isemployed or not depends on the individualuser’s requirements. In any case, the dead-lineand/or dead-bus relay contacts are in parallelwith the synchronism check unit contacts. Withthis arrangement, the first end of the line toclose will be via the voltage units. The secondend will be via the synchronism check unit.

When applying such a scheme with automaticreclosing, it is important to recognize that onlyone end of the line should be permitted to closevia the voltage units; otherwise, it would bepossible for both ends to close simultaneouslyon, say, dead-line - live-bus when the two partsof the system are out of synchronism.

Figure 13 illustrates typical external connectionsfor an arrangement employing the IJS52E relay.This scheme provides instantaneous dead-lineand/or dead-bus detection as well assynchronism check. However, the voltage unit’scontacts do not by-pass the synchronism checkunit directly, but rather they switch that unit’scoil circuits to live potentials so as to utilize thetime delay operation of this unit. In this way, thedead-line - dead-bus closing is delayed by theoperating time of the synchronism check unit.

Here too, a selector switch may be used toobtain flexibility. However, when using thisscheme with automatic reclosing, only one endof the line should be permitted to close viavoltage indication, otherwise, it would bepossible for both ends to close simultaneouslyon say a dead-line - live-bus condition.

In some applications voltage units may not be

required, or some sort of adjustablecharacteristic may be desired. For such cases,the IJS51A and IJS52A relays are available.Figures 14 and 15 illustrate how these relayswould be connected when dead-line - dead-busvoltage detectors are not required. It should benoted that the only difference between the twodevices is the target seal-in unit that is includedin the IJS51A but not included in the IJS52A.

If external voltage relays are to be used witheither of these two devices, they may beemployed in a manner similar to those used inthe IJS52D and IJS52E as illustrated on Fig. 12and Fig. 13.

The 52/b switch shown in the potential circuitsis required to prevent wear as a result ofvibration that would otherwise occur over longperiods when the associated breaker is closedand both incoming and running potentials are inphase. When required, this 52/b contact may bereplaced by contacts of the closing initiationdevice so that the IJS does not start to operateuntil “requested” to do so.

When using voltage relays in conjunction withsynchronism check schemes, it is important torecognize that lines with shunt reactors willmaintain substantial voltages (generally atfrequencies below rated frequency) for someseconds after the associated circuit breakers areopen. This can affect the operation of thevoltage units. Also, in some cases, due toelectromagnetic or static coupling betweenadjacent live circuits and the dead line, voltageon the “dead” line may be present on a steady-state basis. While this voltage is generally low,magnitudes as high as 10 to 15 percent havebeen reported.

When selecting a synchronism check relay ofthe IJS type, the question arises as to whichcharacteristic to select and what closing angleto set. It is suggested that the second question

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be answered first. When one of a number ofload-carrying interconnections between twoparts of a system is open there will existbetween the two remote buses a phase angledifference. From a knowledge of the system, themaximum phase angle difference for which theopen circuit should be reclosed can be obtained.The closing angle of the IJS should be set forthis angle.

Once the closing angle is established, the nextstep is to settle on the maximum slip betweenthe two parts of the system for which closingwould not be objectionable, (recalling, from theprevious sections, that while the IJS is basicallya synchronism check relay, it can operate on anout-of-synchronism condition if the slip is slowenough). Now, from the curves of Figs. 9 and 10,using the selected closing angle and slip,determine the time dial setting for bothcharacteristics that will do the job. In somecases both characteristics will be suitable, inothers only one. When both are suitable, selecteither one.

It should be noted that the selection of amaximum slip must be a compromise betweensecurity and speed. That is, if the scheme islimited to very slow slips, it will be less likely topermit closing on out-of-synchronism, but it willbe quite slow to operate. With closing permittedon higher slips, the scheme will be faster, but itwill be more likely to permit closing on out-of-synchronism conditions. The user must makethis choice.

OPERATING PRINCIPLES OF GXS12A RELAYS

The GXS12A relay is a high-speed synchronismcheck relay with a lockout feature. This relay iscomprised of three units: A time delay (T)induction disk unit similar to that in the IJS relay,an instantaneous (I) induction cup voltage unit,and a dc-operated auxiliary lockout unit (X).

The general torque equation for the inductiondisk unit is the same as that for the IJS relay, asexpressed by Equation (3) which is repeatedbelow.

T = Kop (Vi + Vr)2 - Kr (Vi - Vr)

2 Equation (3)

The only difference is in the values of the designconstants Kop and Kr. In the GXS relay theseconstants are such that the closing angle of thedisk unit is approximately 85 degrees. See Fig. Aon Fig. 16. The only other difference, notapparent from the equation, is the strength ofthe drag magnet. In the case of the disk unit ofthe GXS, the drag magnet is weak so that theunit operates quite a bit faster than the IJS.

The instantaneous cup unit, like the disk unit, isconnected to receive both the running (bus) andincoming (line) voltages. This unit developselectrical torque (Te) as given by the followingequation:

Te = K Vi Vr Sin φ Equation (4)

where:

K is a design constantVi is magnitude of incoming voltageVr is magnitude of running voltageφ is angle by which Vr leads Vi

Thus, for a condition where Vr leads Vi by say 30degrees, the electrical torque developed is

Te = 0.5 K ViVr

If, on the other hand, Vi were leading Vr by thesame 30 degrees, the magnitude of the electricaltorque would be the same as before, but itwould be negative because φ is now -30 degrees.

Te = -0.5 K ViVr

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With positive (Te) torque the cup tends to rotatein one direction, while with negative (Te) torquethe cup tends to rotate in the opposite direction.Thus, the magnitude of the torque developed atrated voltage will be proportional to the sine ofthe angular separation of the two voltages, butthe direction of the torque will depend on whichvoltage is leading.

Because it is necessary for the cup unit tooperate regardless of which voltage is leading orlagging as long as the angle is within the setlimits, the contact assembly is arranged asshown in Fig. B of Fig. 16. As will be noted, thecup shaft is fastened to the lever arm plate sothat they rotate together.

The lever arms are both fastened to the leverarm plate and move with it so that, regardless ofthe direction in which the cup rotates, either oneor the other lever arm applies torque to themoving contact arm in the direction to close thenormally open (a) contact. The spacing of theselever arms relative to the contact arm pivot andthe cup shaft is such that for a given magnitudeof torque on the cup, equal torque is transmittedto the moving contact arm regardless of thedirection of rotation.

A spring with adjustable tension is providedagainst which the electrical torque on the cupmust operate to open the normally closed (b)contact and close the normally open (a) contact.Thus, the balance point (or operating point) ofthe cup unit occurs when the electrical torque(Te) on the cup is equal to the mechanical torqueof the spring (Ks). Rewriting equation (4), thecup unit operates when

Ks = K Vi Vr Sin φ Equation (5)

It is obvious from this equation that at ratedvoltage the angle (φ) between Vi and Vr forwhich the cup unit operates will be directlyrelated to the spring setting. Thus, the desired

operating angle of this unit is set by the springtension.

Consider, for example, that Ks is set for a 30-degree operating angle. This means that thenormally closed contact (b) will be closed andthe normally open contact (a) will be openwhenever Vi is within plus or minus 30 degreesof Vr. It also means that the same will be truewhenever Vi leads or lags Vr by any anglebetween 150 and 210 degrees. This is illustratedin Fig. A of Fig. 17 and it comes about because

Sin φ = Sin (180° - φ)

Figure B of Fig. 17 is a composite of Fig. A (Figs.16 and 17). It indicates that the normally opencontact of the disk unit and the normally closedcontact of the cup unit are concurrently closedonly in the range where Vi leads or lags Vr by 30degrees, or whatever is the setting of the cupunit in the available range of 10 to 45 degrees. Inthis sense, the disk unit acts as a “directional”unit to block closing in the area of arc GDH ofFig. A on Fig. 17.

At this point it should be explained that the GXSrelay gives permission to close when the phaseangle between the two applied voltages iswithin the setting of the cup unit. For thiscondition, the disk unit is picked up and itscontacts are closed, but the cup unit is in itsreset position where its normally closed contactis in series with the disk unit contact in theclosing circuit to permit closing of the breaker.

It should be recognized from Equation 5 that thecalibration of the instantaneous cup unit isdependent on the applied voltages. Forexample, if the spring (Ks) is set with ratedvoltages applied so that the unit resets at 30degrees, and then one of the applied voltagesdrops by 10 percent, the sine of the reset anglemust increase by 10 percent. Thus, the resetangle φ will increase to 33.4 degrees. In other

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words, the angle at which the relay will permitclosing will increase as the applied voltagedecreases and vice versa.

The effects of voltage variations on the disk unitare slight. The reason for this may be noted fromEquation (3). Since the balance point (T = 0) ofthis unit is about 85 degrees, Kr is only slightlygreater than Kop. (Note that for a balance pointof 90 degrees, Kop must equal Kr since thevector difference between two voltages at thatangle is equal to the vector sum). Under suchconditions, moderate variations in themagnitude of one or both voltages will haveonly little effect on the operating angle of thisunit.

As noted earlier, the GXS12A relay also includesa dc-operated auxiliary unit. This has a timedelay pick-up of about six cycles, and it isincorporated in the relay to provide a lockoutfeature as will be discussed subsequently.

OPERATING CHARACTERISTICS OFGXS12A RELAYS

The GXS12A relay is built in a size M2 case andits outline is indicated on Fig. 18.

It will be noted from the internal connections ofFig. 19 that both the instantaneous and the timedelay units have two sets of potential circuits.The external connections of Fig. 20 indicate thatone set of potential coils is supplied from bus orrunning potential, while the second set receivesline or incoming voltage. The contactarrangement in the relay is such that in order topermit closing of the breaker, the cup unitnormally closed 25/Ib contact, the normally open25/T contact, and the normally closed 25/Xcontact must all be closed simultaneously.

The 25/Ib contact will close immediatelywhenever the angle between the two appliedvoltages is within the setting. Thus, the over- all

operating time will depend only on the timedelay disk unit setting. The 25/T contact willclose only if the two voltages are within about85 degrees of one another and if they staywithin that angular separation for long enough.The length of time required will depend on thetime dial setting and the angle between the twoapplied voltages. The curves of Fig. 21 give thistime as a function of time dial setting and fixedphase angle difference between the two appliedvoltages. The 25/X normally closed contact willbe closed if lock-out has not been set up. Lock-out is set up any time that the breaker is openwith both line and bus side potentials present,and the angle between the two is greater thanthe cup unit setting so that 25/Ia closes. Thispicks up the auxiliary 25/X which seals itself into lock out the scheme. Thus, if the anglebetween the two voltages ever exceeds theclosing angle setting while the breaker is open,lock-out will take place.

As in the case of the IJS relay, the disk unit canclose its contacts (25/T) even during out-of-synchronism if the slip is slow enough. However,in this case, closing will be set up only if thevoltages are applied at an instant when they arewithin the closing angle setting of the cup unit,and the slip is zero or slow enough for the diskunit to close its contacts before the angle slipsbeyond the cup unit setting. This is so becauseonce the angle exceeds the cup unit setting,lock-out takes place.

The maximum slip for which the relay willpermit closing may be approximated from thecurves of Fig. 21 by taking the following steps:

1. For the time dial setting employed on the diskunit, read the operating time of the disk unitfrom the curve of phase angle difference thatis the same as the closing angle setting of thecup unit.

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2. Repeat (1) above for the zero phase anglecurve.

3. Calculate the average time from the results of(1) and (2) above.

4. Divide twice the closing angle setting of thecup unit by 360 times the time calculated in(3) above. The result is the maximum slipcycles per second for which the relay couldpermit closing.

As an example, assume a No. 10 time dialsetting on the disk unit and a plus or minus 30-degree closing angle setting of the cup unit.From the curves of Fig. 21, the average time is

1.64 + 1.4= 1.52 seconds

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The maximum slip for which the relay mightpermit closing is

2 x 30360 x 1.52

= 0.11 slip cycles/second

It should be recognized that smaller time dialsettings, and/or bigger closing angle settingswill permit the possibility of closing at higherslips.

Another important characteristic to consider isthe curves of Fig. 22. These curves indicate thevariation in the calibration of the cup unit withapplied voltages. They indicate that themagnitude of the applied voltages have asignificant effect on the actual closing angle,and that the higher the closing angle setting,the greater is this effect.

APPLICATION OF GXS12A RELAYS

The GXS12A is a high-speed synchronism checkrelay that finds application wherever a high-speed synchronism check plus a lock-out

function is required. It is extremely well suitedfor application in conjunction with high-speedautomatic reclosing on transmission lines.

It is important to recognize that the GXS will notoperate to permit reclosing unless both the lineand the bus are energized. Thus, the first end ofthe line must be closed either without anysupervision or by some combination of bus andline voltage indicating relays. As soon as thefirst end is closed and the line is hot, the GXSwill start to perform. The external diagramillustrates a scheme that employs the GXS(device 25) plus bus and line voltage relays (27Band 27L). A selector switch (43) is indicated toselect any one of a number of possible modes ofoperation. However, such a switch is notnecessary unless the complete flexibility isdesired.

Some transmission lines are equipped withshunt reactors and/or transformers at theirterminals. In such cases, when the associatedbreakers are opened, the line continues tooscillate for some time and a voltage ismaintained on this “de-energized” circuit. Thisvoltage is normally at some frequency otherthan 60 Hz and can appear to the GXS relay asan out-of-step condition. This will cause theGXS to lock-out as soon as the line is de-energized. In some cases, where the naturalfrequency of the “dead” line is significantlydifferent from 60 Hz, a high-speed frequencyrelay connected to the line side potential maybe employed to block lock-out of the GXSexcept when the line frequency is at or near 60 Hz.

Another consideration is the performance of dead-line relays (27L) under thesecircumstances. The oscillating voltage on the“dead” line can keep the dead-line relays fromoperating and so delay closing of the first end ofthe line for some seconds. This may mitigatesome of the advantage of the high-speed

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synchronism check scheme and can be avoidedby permitting the first end of the line to recloseunsupervised.

There are only two settings that can be made onthe GXS12A relay, and these are:

1. Closing Angle setting on the cup unit bymeans of the control spring

2. Time Dial setting on the disk unit.

The closing angle setting should be largeenough to ensure that the GXS does not lock-out for any condition of load transfer andsystem swing after a trip out. As noted in theprevious section under “OPERATINGCHARACTERISTICS OF GXS12A”, the closingangle is voltage sensitive and the curves of Fig. 22 should be consulted before settling on asetting for the closing angle.

The time dial setting on the disk unit will affectthe operating time of the scheme. Lower timedial settings will permit faster reclosing.However, lower settings will permit reclosing on faster slips. This should be evaluated as outlined in the previous section on“OPERATING CHARACTERISTICS OF GXS12A”before a time dial setting is selected.

The 52/b contact shown in the potential circuiton Fig. 20 is required to prevent wear on the diskunit over long periods of time as a result ofvibration when the breaker is closed and therunning and incoming voltages are in phase.When desirable, this contact may be replaced bya contact on the closing device so that the GXSwill not operate until “requested” to do so.

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