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the resistor. The factory setting is for a maximumsensitivity angle of 75° ± 3° current lagging voltage.

2.2 Auto-Transformer

The auto-transformer has three taps on its mainwinding, S, which are numbered 1,2, and 3 on thetap block Figure 2. A tertiary winding M has four tapswhich may be connected additively or subtractively toinversely modify the S setting by any value from -18to +18 percent in steps of 3 percent.

The sign of M is negative when the R lead is abovethe L lead. M is positive when L is in a tap locationwhich is above the tap location of the R lead. The Msetting is determined by the sum of per unit valuesbetween the R and L lead. The actual per unit valueswhich appear on the tap plate between taps are0., .03, .09, and .06.

The auto-transformer makes it possible to expand

the basic range of T ohms by a multiplier of .

Therefore, any relay ohm setting can be made within± 1.5 percent from the maximum value of a givenrange by combining the compensator taps TAB andTBC with the auto-transformer taps SA-MA and SC-MC.

2.3 Phase Shifting Circuit

“Polarization” is the reference against which the“operate” signal is compared. Polarization for thethree-phase unit is obtained by shifting phase 1-2voltage 90°. The phase shifting circuit consists of acenter tapped step-up auto-transformer, XS, whichsupplies voltage to a series connected resistor andcapacitor, RS and CS respectively (Figures 5 and 6).Voltage between the resistor-capacitor junction andthe auto-transformer center tap leads the appliedvoltage by 90°.

2.4 Memory Circuit

The memory circuit consists of a large inductive reac-tance, XL, and a large capacitive reactance, C3C,which are series connected and are tuned veryclosely to 60 hertz. In the event of a close-in faultwhich drops the relay terminal voltage to zero, theenergy trapped in the memory circuit will decay rela-tively slowly, while oscillating at 60 hertz frequency.This will maintain a polarizing signal long enough forthe relay to operate with an accurate reach anddirectional sense.

2.5 Isolating Transformers

Transformers T1, T2, and T3 serve two purposes.First, they isolate the ac circuits from the dc circuit.Secondly, they amplify the clipped ac signal by afactor of 1:8 to make the relay sensitive to low levelinput signals.

2.6 Printed Circuit Board Assembly

The printed circuit board assemblies shown in Fig-ures 3 and 4 contain all the resistors, diodes,transistors, necessary to perform the functions of adual polarized phase angle comparison unit. In Fig-ures 3 and 4, resistors are identified by the letter Rfollowed by a number. The same combination is usedto identify the same resistor in the internal schemat-ics, Figures 5 and 6. Similarly, diodes are identifiedby a D and the cathode (the end out of which con-ventional current flows) is identified by a bar acrossthe point of an arrow. Zener diodes are identified bythe letter Z, transistors are identified by a Q, siliconcontrolled switches by SCR, capacitors by C, andtest points by TP.

When facing the component side with terminals atthe bottom, terminals are numbered from right to leftstarting at 1 and going through number 19. Theseterminal numbers are shown within brackets on theinternal schematic and will be referred to as PrintedCircuit Terminals, PCT, in the trouble shooting sec-tion.

3. OPERATION

The SKD-T and SKD-IT relays utilize identical acinput circuits. Therefore, an explanation for the SKD-T will suffice for both.

Two distinctly different logic systems are used in theSKD-T & SKD-IT relays. One detects all exceptthree-phase faults and is called the phase-to-phaseunit. The other system detects three-phase faultsand is called the three-phase unit. Each of the twosystems present two voltages to the static phaseangle comparison unit which checks the phase anglerelation between the two. A non-trip, or restraint con-dition exists when VYB leads VXB, is referred to inFigure 7(a). A trip condition results when VYB lagsVXB.

The three-phase unit can be blocked by externalmeans to prevent tripping during load conditions ofsystem swing conditions which enter the electricaltrip zone. Blocking is accomplished by supervising

S1 M±--------------

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the three-phase output contacts at terminal 20.

3.1 Phase-to-Phase Unit

For phase-to-phase fault detection, the system moni-tors the three phase-to-phase voltages and com-pares them with the related IZR drops computed bythe relay compensators. The compensators are setto replicate the protected line section as seen by thesecondary side and have the same relative currentflowing through them as flows through the total lineimpedance. If a fault is external to the protected linesection as shown in Figure 7(b) at point 1, it can beseen that the voltage VBC is greater than the permis-sible minimum (IC-IB) ZR computed by compensatorTCB. The difference voltage VYB shown as a heavyline still leads voltage VXB and the phase angle com-parison unit remains in a non-trip or restraint condi-tion.

For a fault within the protected line section at point 2,the voltage VCB is less than the permissible minimumcomputed by the compensator. This results in the dif-ference voltage VYB being flipped 180° so that it lagsVXB and causes the phase angle comparison unit totrip.

Faults behind the relay produce a restraint conditionas shown for a fault at point 3. In this case the faultcurrent polarity is the reverse of that for faults atpoints 1 and 2. The compensator voltage is added toVCB and the resulting voltage VYB leads VXB to main-tain the restraint condition.

For each fault illustrated in Figure 7(c), it is assumedthat the angle of the protected line is 90° and that therelay is set for a maximum sensitivity angle of 90°.

The phase-to-phase unit will not trip for any balancedthree-phase condition whether it be a three-phasefault, an out-of-step or swing condition, or a heavyload. For this reason, an additional circuit is providedwhich will detect and trip for three-phase faults.

3.2 Three Phase Unit

The three-phase unit is basically a single phase unitoperating from delta voltage and current. Since athree-phase fault produces nearly identical condi-tions in all three phases, any set of phase-to-phasevoltage and current may be used to detect a three-phase fault.

Figure 8(a) illustrates the connections which applyvoltages to the static phase angle comparison unit.

Voltage VAB is modified by the compensator outputvoltage (IA IB) ZR to produce voltage VXB. (This isidentical to the phase-to-phase unit). A polarizingvoltage VYB is obtained by advancing VAB 90°through the phase shifting circuit. A restraint condi-tion exists when VYB leads VXB and a trip conditionexists when VYB lags VXB.

A simple diagram in Figure 8(b) shows the protectivezone for the three-phase unit and indicates threelocations for faults which will present a different set ofvoltage conditions to the phase-angle comparisonunit. Figure 8(c) illustrates the three conditions.

For a fault at location 1, external to the protected linesection, VAB is greater than the permissible minimum(IA-IB) ZR computed by the compensator TAB. Thephase shifted polarizing voltage VYB leads the differ-ence voltage VXB and a restraint condition exists.

When a fault occurs within the protected line sectionsuch as at point 2, VAB becomes smaller than thepermissible minimum computed by the compensator.This results in VXB being flipped 180° so that thepolarizing voltage VYB lags VXB and the phase-angle-comparator unit trips.

For a fault behind the relay such as at point 3, currentthrough the relay has a polarity opposite from that forfaults at points 1 and 2. The compensator voltage isadded to VAB so that polarizing voltage VYB leads theresulting VXB to maintain the restraint condition.

In Figure 8(c), it is assumed that the angle of the pro-tected line is 90° and that the relay is set for a maxi-mum sensitivity angle of 90°.

The three-phase unit will trip for some phase-to-phase faults and some ground faults as well as forthree-phase faults. However, it will not overreach thephase-to-phase setting for any type of fault. Note thatthe phase-to-phase unit and the three-phase unit usethe same compensator, TAB. Thus, the two unitshave identical reach settings. The three-phase unitmay trip for an out-of-step or swing condition. Toprevent such tripping, the three-phase unit may beexternally supervised at the three-phase output con-tacts as previously indicated.

3.3 Phase Angle Comparison Unit

Referring to Figure 5, the phase-angle comparisonunit for the phase-to-phase unit trips when current

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flows into the base of transistor Q17 through zenerdiode Z7. Such tripping current must come from the20V bus through either transistor Q12 or Q14 locatedin what might be called the “operate” circuit. Thiscircuit driven by the transformer, T1, is continuallytrying to trip the unit by supplying current throughQ12 or Q14. Each conducts on alternating half-cycles.

When Q14 conducts, a portion of the current goesthrough resistor R9. This current, IR9, may take eitherof the two paths to the negative bus. If SCR1 is in aconducting state, IR9, passes through it directly to thenegative bus. If SCR1 is in a blocking state, IR9

passes through D13 and then through Z7 to transis-tor Q17 to cause tripping. Restraint thyristor SCR1 islocated in what might be called the “polarizing” circuitof the phase-to-phase unit. This circuit is driven bytransformer T2.

To prevent the operate circuit from tripping, the polar-ity-marked terminals of T2 must go positive beforethe polarity terminals of T1 do. This causes Q2 toconduct current through D5, D6 and D10 to drive thebase of Q4. Q4 then conducts current from the 13Vbus through R11 to drive the gate SCR1 into conduc-tion. When SCR1 conducts, it short circuits the cur-rent which might otherwise pass through D13 tocause tripping. Once SCR1 begins to conduct, thegate loses control and it remains in the conductingstate until the current is turned off by Q14. No trip-ping output can develop as long as the T2 voltageleads the T1 voltage.

The operate circuit switches for the next half cycle sothat transistors Q11 and Q12 conduct in an attemptto cause tripping. In the polarizing circuit, Q1, Q5 andSCR2 seek to prevent tripping by short circuiting thecurrent which might otherwise pass through D14, Z7and Q17.

3.3.1 Restraint Squelch

When the operate circuit transistor Q14 conducts,approximately 18V is applied through diode D9 toback bias D10 and prevent Q4 from turning on. Thusa trip signal, initiated because the T1 voltage is lead-ing, cannot be improperly interrupted when the T2voltage goes positive. A full half-cycle tripping outputis therefore produced by Q14. This back-baising con-nection is called the restraint squelch circuit.

3.3.2 Restraint-Signal Detection

If a condition should develop so that no polarizing

voltage appears at transformer T2, then no gatingsignal would be available to switch SCR1 and shortcircuit the Q14 current. This, of course, could causeincorrect tripping. A signal detector circuit preventsthis from happening. When a useful voltage level issupplied by T2, Q1 and Q2, alternately short circuitthe current which flows through R3 from the 20V bus.When the voltage from T2 drops too low to drive Q1and Q2, the R3 current flows through Z2 to switch Q3into conduction. This in turn drives Q4 through D7,R5 and D10 causing Q4 to switch SCR1 into conduc-tion to short circuit D13 and prevent tripping.

The operate circuit, driven by T1, and the polarizing-circuits, driven by T2 and T3, are duals having identi-cal circuits which operate on alternate half cycles.The restraint squelch and the restraint-signal detec-tor both work into each of the duals in the same way.

Transformer T3 receives a polarizing signal from thethree-phase circuit and drives a polarizing circuitwhich is identical to the one supplied by transformerT2. The phase-angle relation between the T1 voltageand the T3 voltage is compared in the same manneras described above, and tripping signals are suppliedthrough D27 and D28, through Z9 and to Q20. Thispolarizing circuit contains a restraint squelch and arestraint-signal detector identical to those describedfor the T2 circuit.

3.3.3 SKD-IT Relay

The SKD-IT relay has a characteristic circle whichincludes the origin. That is, it will trip on current whenthe voltage is zero. This feature is obtained by omit-ting the restraint-signal detector from the T3 circuit.This omission reduces the accuracy of the SKD-ITrelay at low-voltage test levels.

3.3.4 Voltage Detector

Operation of the phase-angle comparison unit isbased upon a comparison of the phase relationbetween two amplified signals. If either the ac inputsignal or the dc amplifying voltage is absent, then nophase relation can be established. Therefore, at theinstant either quantity is applied, the logic does notknow whether it would trip or restrain since it has noprior knowledge of phase relations. The voltagedetector sends a gating pulse from Q15 to allrestraint thyristors SCR1, SCR2, SCR3 and SCR4 toblock tripping long enough for the true phase-anglerelation between input signals to be established.After approximately one-half cycle, Q15 turns on toremove the gating signal unit the relay is

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deenergized again.

The zener diode Z6 monitors the dc voltage level. ifthe dc voltage drops too low for the logic to operateproperly, it will cause Q16 to turn off and therebysend a gate signal to the restraint thyristors. This willblock tripping as long as the dc voltage is at a levelwhich would otherwise cause an incorrect operation.

3.3.5 Output Circuits

The outputs of the SKD-T and SKD-IT are dry tele-phone relay contacts (normally open). The phase-to-phase output for both relays appears at the TR1 con-tact (Terminal 11) while the three-phase output is atthe TR2 contact (Terminal 20).

4. CHARACTERISTICS

4.1 Distance Characteristic

Phase-to-Phase Unit

This unit responds to all phase-to-phase faults andmost two-phase-to-ground faults. It does not respondto load current, synchronizing surges, or out-of-stepconditions. While a characteristic circle can be plot-ted for this unit on the RX diagram as shown in Fig-ure 9, such a characteristic circle has no significanceexcept in the first quadrant where resistance andreactance values are positive. A small portion of thefourth quadrant, involving positive resistance valuesand negative reactance values, could have some sig-nificance in the event that the transmission lineincludes a series capacitor. The portion of the circlein the first quadrant is of interest because it describeswhat the relay will do when arc resistance is involvedin the fault. The phase-to-phase unit operating on anactual transmission system is inherently directional.

An inspection of Figure 9 indicates that the circle ofthe phase-to-phase unit is dependent on sourceimpedance ZS. However, the circle always goesthrough the line balance point impedance. The reachat the compensator (and line) angle is constant,regardless of the system source impedance. Thebroadening out of the characteristic circle with a rela-tively high source impedance gives the phase-to-phase unit the advantageous characteristic that forshort lines, it makes a greater allowance forresistance in the fault. Stated another way, the char-acteristics approach that of a reactance relay moreand more closely as the line being protected becomeshorter and shorter with respect to the source imped-ance back of the relaying location.

4.2 Sensitivity

Phase-to-Phase Unit

A plot of relay reach, in percent of tap block setting,versus relay terminal voltage is shown in Figure 10.The unit will operate with the correct directionalsense for zero voltage phase-to-phase faults. Forthis condition the product of fault current times T set-ting must be not less than 0.40 with rated voltage onthe unfaulted phase.

The relays may be set without regard to possibleoverreach due to dc transients. Compensators basi-cally are insensitive to dc transients which attendfaults on high-angle systems. The long time-constantof a high-angle system provides a minimum rate ofchange in flux-producing transient current withrespect to time, and therefore induces a minimum ofunidirectional voltage in the secondary.

4.3 Distance Characteristic

SKD-T, Three-Phase Unit

The three-phase unit has a characteristic circle whichpasses through the origin as shown in Figure 11.This circle is independent of source impedance. Thethree-phase unit is also inherently directional.

If a solid-three-phase fault should occur right at therelay location, the voltage will collapse to zero to givea balance point condition, as implied by the relaycharacteristic (Figure 11) passing through the origin.When the YB voltage (Figure 13) drops to zero, therelay is unable to determine whether an internal orexternal fault existed. To avoid this condition, a reso-nant circuit causes the Y voltage to collapse gradu-ally at a frequency of 60 hertz. This characteristiccalled memory action provides a reference voltagelong enough to determine whether the fault is insidethe protected line section or behind the relay.

4.4 Sensitivity

Three-Phase Unit

The impedance curve for the three-phase unit isshown in Figure 16. The unit will operate with thecorrect directional sense for zero voltage three-phase faults when normal voltage exists at the relayterminals prior to the fault. This operation occurs dueto memory action as described above. For thiscondition, the product of fault current times T settingmust be not less than 0.40. The minimum steadystate voltage below which the three-phase unit prob-ably will be disabled by the voltage detector circuit is

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1.5VL-L .

4.5 Distance Characteristics

SKD-IT Three-Phase Unit

The three-phase unit of the SKD-IT relays has acharacteristic circle which includes the origin. Thisfeature results from the omission of the restraint-sig-nal detector in the polarizing circuit. The unit will tripfor an “IT” product of 0.40 or more when the inputvoltage is zero.

4.6 General Characteristics

Impedance settings in ohms reach can be made insteps of 3 percent for any range, the .1-3.45 ohmrelay, the .2-4.35 ohm relay, the .73-21 ohm relay,and the 1.1-31.8 ohm relay. The maximum sensitivityangle, which is set for 75 degrees at the factory, maybe set for any value from 60 degrees to 80 degrees.A change in maximum sensitivity angle will produce aslight change in reach for any given setting of therelay. The compensator secondary voltage output Vis larger when V leads the primary current, I, by 90°.This 90° relationship is approached, if the compensa-tor loading resistor (R2A or R2C) shown in Figure 5and 6 is open-circuited. The effect of the loadingresistor, when connected, is to produce an internaldrop in the compensator, which is out-of-phase withthe induced voltage, ITAB or ITAC. Thus, the net volt-age, V, is phase-shifted to change the compensatormaximum sensitivity angle. As a result of this phaseshift the magnitude of V is reduced.

Tap markings in Figure 2 are based upon a 75° com-pensator angle setting. If the resistors R2A and R2Care adjusted for some other maximum sensitivityangle the nominal reach is different than indicated bythe taps. The reach Ζθ varies with the maximum sen-

sitivity angle, θ, as follows:

4.7 TIME CURVES AND BURDEN DATA

4.7.1 Operating Time

The speed of operation for the SKD-T and SKD-ITrelays is shown by the time curves in Figure 12. Thecurves indicate the time in milliseconds required forthe relay to provide an output for tripping after theinception of a fault at any point on a line within therelay setting.

Zθ TS θsin1 M+( ) 75°sin

------------------------------------=

.944 1.23 5.8 8.7 5.0 8.5 9.5 240

.708 .92 4.2 6.1 6.0 10.0 10.0 240

.531 .69 3.0 4.35 8.0 10.0 10.0 240

.354 .537 2.2 3.05 10.0 10.0 10.0 240

.236 .383 1.6 2.18 10.0 10.0 10.0 240

.177 .307 1.16 1.74 10.0 10.0 10.0 240

.118 .23 0.87 1.31 10.0 10.0 10.0 240

“T” TAP SETTINGCONTINUOUS

S = 1 S = 2 S = 31

SEC.

Current Circuit Ratings in Amperes

4.7.2 Burden

The burden which the relays impose upon voltageand current transformers in each phase is listed inTable 1. The potential burden and burden phaseangle are based on 69.3 volts line-to-neutral appliedto the relay terminals.

5. SETTINGS

5.1 Setting Calculations

Relay reach is set on the tap plate shown in Figure 2.The tap markings are:

(TA, TB, and TC)

.1-3.45 Ohms .118 .177 .236 .354 .531 .708 .944

.2-4.35 Ohms .23 .307 .383 .537 .69 .92 1.23

.73-21 Ohms .87 1.16 1.45 2.03 2.9 4.06 5.81.1-31.8 Ohms 1.31 1.74 2.18 3.05 4.35 6.1 8.7

(SA and SC)1 2 3

± Values (MA and MC)between taps .03 .09 .06

Maximum sensitivity angle, θ, is set for 75° (currentlagging voltage) in the factory. This adjustment neednot be disturbed for line angles of 65° or higher. Forline angles below 65° set θ for 60° maximumsensitivity angle, by adjusting R2A and R2C. SetZone 1 reach to be 90% of the line (85% for lineangles of less than 50°).

EQ. ( 1 )

The terms used in this formula and thereafter aredefined as follows:

Calculations for setting the SKD-T and SKD-IT relaysare straightforward and apply familiar principles.

Zθ Zpri 0.9 RcRv

------------------------------=

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Assume a desired balance point which is 90 percentof the total length of line. The general formula for set-ting the ohms reach of the relay is:

Zθ = the ohmic reach of the relay in secondary

ohms when the maximum sensitivity angle isset for θ degrees.

EQ. ( 2 )

T = Compensator tap value

S = Auto-transformer primary tap value

θ = Maximum sensitivity angle setting of therelay (Factory setting of θ = 75°)

± M = Auto-transformer secondary tap value. (Thisis a Per Unit value and is determined by thesum of the values between the “L” and the“R” leads. The sign is positive when “L” isabove “R” and acts to lower the Z setting.The sign is negative when “R” is above “L”and acts to raise the Z setting).

Zpri = Ohms per phase of the total line section.

0.9 = The portion of the total line for which therelay is set to trip.

Rc = Current transformer ratio

Rv = Voltage transformer ratio

The following procedure should be followed in orderto obtain an optimum setting of the relay:

A. Establish the value of Zθ, as above.

B. Determine the tap plate value Z using theformula:

EQ. ( 3 )

Now refer to Table II, Table III, Table IV,and Table V for the optimum tap settings.

C. Locate a Table value for relay reach near-est the desired value Z. (It will always bewithin 1.5% of the desired value.)

D. Select from the Table “S”, “T”, and “M” set-tings. “M” column includes additional infor-mation for “L” and “R” leads setting for the

specified “M” value.

E. Recheck the selected “S”, “T” and “M” set-tings to assure the correct value of Z byusing equation (2).

For Example:

Step (A) Assume the desired reach, Zθ for a .73-21ohm relay is 7 ohms at 60°.

Step (B). The line angle of 60° requires that the relaymaximum sensitivity angle be changedfrom a factory setting of 75° to the newvalue of 60°. Using equation (3), we findthe corrected value for the relay tap set-tings:

Step (C). In Table IV, we find nearest value of 7.8ohms is 7.88. That is

percent of the desired reach

Step (D). From Table IV, read off:

S = 2T = 4.06M = +.03

and “L” lead should be connected over “R” lead, with“L” lead connected to “03” tap and “R” lead to tap “0”.

Step (E). Recheck Settings:

from Eq. (3) which is 101.0 percent of thedesired setting.

5.2 SETTING THE RELAY

The SKD-T and SKD-IT relays require settings forthe two compensator (TAB and TCB), the two auto-transformer primaries (SA and SC) and secondaries(MA and MC). All of these settings are made with taps

Z TS1 M+-------------- the tap plate setting= =

Z Zθ75°sinθ°sin

-----------------=

When θ 75°, Z = Zθ=

Z TS1 M±--------------=

Z = 7 x75°sin60°sin

----------------- 7.8 ohms=

1007.887.8----------× 101.0=

Z TS1 M±--------------

4.06x21 0.03+------------------- 7.88 FromEq. 2( )= = =

Z 60°( )∠ Z 60°sin75°sin

----------------- 7.88 0.896× 7.06= = =

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on the tap plate.

5.3 Compensator (T AB and TCB)

Each set of compensator taps terminates in insertswhich are grouped on a socket and form approxi-mately three quarters of a circle around a centerinsert which is the common connection for all of thetaps. Electrical connections between common insertand tap inserts are made with a link that is held inplace with two connector screws, one in the commonand one in the tap. There are two TB settings to bemade since phase B current is passed through twocompensators. A compensator tap setting is made byloosening the connector screw in the center. Removethe connector screw in the tap end of the link, swingthe link around until it is in position over the insert ofthe desired tap setting, replace the connector screwto bind the link to this insert, and retighten the con-nector screw in the center. Since the link and con-nector screws carry operating current, be sure thatthe screws are turned to bind snugly but not so tightlyas to break the tap screw.

5.4 Auto-Transformer Primary (S A and SC)

Primary tap connections are made through a singlelead for each transformer. The lead comes out of thetap plate through a small hole located just below orabove the taps and is held in place on the tap by aconnector screw (Figure 2).

An “S” setting is made by removing the connectorscrew, placing the connector in position over theinsert of the desired setting, replacing and tighteningthe connector screw. The connector should nevermake electrical contact with more than one tap at atime.

5.5 Auto-Transformer Secondary (M A and MC)

Secondary tap connections are made through twoleads identified as L and R for each transformer.These leads come out of the tap plate each througha small hole, one on each side of the vertical row of“M” tap inserts. The lead connector screws.

Values for which an “M” setting can be made arefrom -.18 to +.18 in steps of .03. The value of a set-ting is the sum of the numbers that are crossed whengoing from the R lead position to the L lead position.The sign of the “M” value is determined by which leadis in the higher position on the tap plate. The sign ispositive (+) if the L is higher and negative (-) if the Rlead is higher.

An “M” setting may be made in the following manner.Remove the connector screws so that the L and Rleads are free. Refer to Table II, Table III, Table IV orTable V to determine the desired “M” value. Neitherlead connector should make electrical contact withmore than one tap at a time.

5.6 Line Angle Adjustment

Maximum sensitivity angle is set for 75° ± 3° (currentlagging voltage) at the factory. This adjustment neednot be disturbed for line angles of 65° or higher. Forline angles below 65° set for a 60° maximum sensi-tivity angle by adjusting the compensator loadingresistor R2A and R2C. Refer to Section 7.3,REPAIR CALIBRATION, steps 7.3.3 a to f and7.3.6 a when a change in maximum sensitivity angleis desired.

6. INSTALLATION

The relays should be mounted on switchboard pan-els or their equivalent in a location free from dirt,moisture, excessive vibration and heat. Mount therelay vertically by means of the rear mounting stud orstuds for the type FT projection case or by means ofthe four mounting holes on the flange for the semi-flush type FT case. Either the stud or the mountingscrews may be utilized for grounding the relay.External toothed washers are provided for use in thelocations shown on the outline and drilling plan tofacilitate making a good electrical connectionbetween the relay case, its mounting screws orstuds, and the relay panel. Ground Wires are affixedto the mounting screws or studs as required forpoorly grounded or insulating panels. Other electricalconnections may be made directly to the terminals bymeans of screws for steel panel mounting or to theterminal stud furnished with the relay for thick panelmounting. The terminal stud may be easily removedor inserted by locking two nuts on the stud and thenturning the proper nut with a wrench.

For detailed information on the FT case refer to I.L.41-076.

6.1 External Connections

Figure 13 shows the connections for three zone pro-tection using an SKD-T and SKD-IT Relay.

7. ADJUSTMENTS AND MAINTENANCE

7.1 Acceptance Test

Acceptance tests consist of a visual inspection tomake sure there are no loose connections, broken

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resistors, or broken resistor wires; and an electricaltest to make certain that the relay measures the bal-ance point impedance accurately.

7.1.1 Recommended Instruments for Testing1. Portable high resistance analog or digital ac volt-

meter.

2. Portable analog ac ammeter.

3. Ohmmeter,Tripping is indicated when the ohmmeter inFigure 14 shows the first indication of contactclosure. For best results use one of the high ohmranges on the ohmmeter.

7.1.2 Distance Units – Electrical Tests1. Check the electrical response of the relay by

using the test connections shown in Figure 14.Set TA, TB and TC for the maximum tap value: SAand SC for 1; MA and MC for +0.15.

2. Connect the relay for a 1-2 fault as indicated forTest #5 in Figure 14 and adjust the voltagebetween PH.1 and 1F and between PH.2 and 2Ffor 45 volts each so that the resultant voltageV1F2F equals 30 volts. (120-45V-45V = 30V.)

3. Supply current necessary to trip the phase-to-phase unit at 45° and swing the phase shifter todetermine the two angles, θ1 and θ2 at which theunit just trips. The maximum sensitivity angle θ is

. This should be 75° ± 3°. Go from non-

trip to trip area.

4. The current required to make the phase-to-phaseunit trip should be within the limits given in TableV at an angle of 75° current lag.

5. Repeat Step 4 while using connections for Test#6 and Test #7. The difference in values of cur-rent that makes the unit trip for each of the threetest connections should not be greater than 5%of the smallest value.

6. Repeat the above test for the three-phase unit.The current required to trip the three-phase unitwould be within the limits given in Table V at themaximum sensitivity angle of 75°.

θ1 θ2+

2------------------

41-495.51B

11

I = 0

69.3

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S =

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3.1

2.38

1.99

02.

982.

38-1

.92

4.98

4.97

0.30

2.27

1.69

1.52

-.18

2.22

1.57

-1.5

73.

683.

680.

131.

571.

131.

09

+.1

81.

470.

80-1

.24

2.27

2.24

-0.3

30.

720.

550.

460

1.33

0.62

-1.1

71.

931.

89-0

.37

0.55

0.42

0.36

-.18

1.24

0.48

-1.1

41.

611.

57-0

.38

0.38

0.28

0.26

+.1

81.

220.

44-1

.14

1.57

1.52

-0.4

10.

330.

250.

210

1.16

0.34

-1.1

11.

441.

38-0

.40

0.25

0.19

0.16

-.18

1.11

0.27

-1.0

81.

311.

24-0

.42

0.18

0.14

0.12

+.1

81.

721.

66-0

.46

3.52

1.60

-3.1

41.

300.

36-1

.25

01.

341.

33-0

.14

2.53

1.26

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90.

830.

27-0

.79

-.18

1.03

1.03

-0.0

71.

680.

89-1

.42

0.45

0.20

-0.4

0

+.1

86.

654.

7-4

.70

10.3

5.75

-10.

15.

001.

30-4

.84

05.

03.

94-3

.08

7.90

2.04

-7.6

03.

760.

91-3

.65

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3.46

3.14

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65.

702.

41-5

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0.65

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2.9

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5.8

.019

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4.0

514

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.153

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8.0

203

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3.0

295

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2.0

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6.0

054

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3.0

177

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4.0

67.1

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6.0

39.0

432

.056

9.0

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30.2

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9.0

379

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1.0

507

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7.0

87.1

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8.0

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3.0

257

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7.0

965

.191

.016

2.0

194

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6.0

294

.047

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.135

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9.0

188

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9.0

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4.0

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41-495.51B

12

.118

.100

.103

.105

.108

.111

.114

.118

1.22

1.26

1.30

1.34

1.39

1.44

.177

.150

.154

.158

.162

.167

.172

.177

.182

.188

.194 – – –

.236

.200

.205

.211

.217

.223

.229

.236

.243

.251

.259

.268

.278

.288

.354

.300

.308

.316

.325

.334

.344

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.365

.376

.389

.402

.416

.432

.531

.450

.462

.474

.487

.501

.516

.531

.547

.565

.584 – – –

.708

.600

.616

.632

.650

.668

.687

.708

.730

.753

.778 – – –

.944

.800

.821

.843

.866

.890

.916

.944

.973

1.00

1.04

1.07

1.11

1.15

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1.20

1.23

1.26

1.30

1.34

1.37

1.42

1.46

1.51

1.56 – – –

.944

1.60

1.64

1.69

1.73

1.78

1.83

1.89

1.95

2.00

2.08

2.14

2.22

2.30

– – – – – – – – – – – – – –

1.23

2.40

2.46

2.53

2.60

2.67

2.75

2.83

2.92

3.00

3.12

3.21

3.33

3.45

+M

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8

+.1

5

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2

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9

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6

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-.06

-.09

-.12

-.15

-.18

.06

.06

.09

.09

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0 .03 0 .03

.09 0 0 .03

.06

.09

.09

.06

.06

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41-495.51B

13

.23

.195 .20

.205

.211

.217

.223 .23

.237

.245

.252 – – –

.307 .26

.267

.274

.281

.289

.298

.307

.316 – – – – –

.383

.324

.333

.342

.352

.362

.372

.383

.395

.407 .42

.435

.

– –

.537

.455

.466 .48

.493

.506

.521

.537

.554

.571 – – – –

.69

.585 .6 .615

.633 .65

.67

.69

.71

.735

.758 – – –

.92

.78 .8 .82

.845

.868

.893 .92

.948

.978

1.01 – – –

1.23

1.04

1.07 1.1

1.13

1.16 1.2

1.23

1.27

1.31

1.35 1.4

1.45 1.5

.92

1.56 1.6

1.64

1.69

1.74

1.79

1.84 1.9

1.96

2.02 – – –

1.23

2.08

2.14 2.2

2.26

2.32

2.39

2.46

2.54

2.62 2.7

2.8

2.89 3.0

.92 – – – – – – – – –

3.03 – – –

1.23

3.13

3.21

3.29

3.38

3.48

3.58

3.69 3.8

3.92

4.05

4.19

4.35 4.5

+M

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8

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5

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2

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9

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6

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3

0

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-.03

-.06

-.09

-.12

-.15

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.06

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.09

.06

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.03 0 .03 0

0 .03 0 .03

.09 0 0 .03

.06

.09

.09

.06

.06

T =

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= 1

“S”

= 2

“S”

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41-495.51B

14

.87

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.755

.775

.800

.820

.845

.870

.897

.925

.955 – – –

1.16 .98

1.01

1.03

1.01

1.09

1.12

1.16

1.20 – – – – –

1.45

1.23

1.26

1.29

1.33

1.37

1.41

1.45

1.49

1.54

1.59

1.65

1.71 –

2.03

1.72

1.76

1.81

1.86

1.91

1.97

2.03

2.09

2.16

2.23

2.31

2.39 –

2.9

2.46

2.52

2.59

2.66

2.74

2.81 2.9

2.99

3.09

3.19

3.29

3.41 –

4.06

3.44

3.53

3.63

3.73

3.83

3.94

4.06

4.18

4.32

4.47

4.62

4.77 –

5.8

4.92

5.04

5.18

5.32

5.48

5.64 5.8

5.98

6.18

6.38

6.60

6.82

7.08

4.06 – –

7.26

7.44

7.65

7.88

8.12

8.36

8.65

8.93

9.13

9.55 –

5.8

9.85

10.1

10.3

10.6

10.9

11.3

11.6

11.9

12.3

12.7

13.2

13.7

14.1

4.06 – – – – – – – – – – – –

14.3

5.8

14.7

15.1

15.5

15.9

16.4

16.9

17.4

18.0

18.6

19.2

19.8

20.5

21.3

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6

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3

0

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-.06

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-.15

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.06

.09

.09

.06

.03 0 0 .09

.03 0 .03 0

0 .03 0 .03

.09 0 0 .03

.06

.09

.09

.06

.06

T =

“S”

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“S”

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“S”

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41-495.51B

15

1.31

1.11

1.14

1.17

1.20

1.23

1.27

1.31

1.35

1.39

1.44 – –

1.74

1.47

1.51

1.55 1.6

1.64

1.69

1.74

1.79 – – –

2.18

1.85

1.89

1.95

2.00

2.06

2.12

2.18

2.25

2.32 2.4

2.48

2.56 –

3.05

2.58

2.65

2.72 2.8

2.87

2.96

3.05

3.14

3.24

3.35

3.46

3.59 –

4.35

3.68

3.78

3.88

3.99

4.10

4.22

4.35

4.49

4.62

4.78

4.94

5.11 –

6.1

5.16

5.30

5.45 5.6

5.75

5.92 6.1

6.29

6.49

6.70

6.93

7.17 –

8.7

7.37

7.56

7.76

7.98

8.20

8.45 8.7

8.96

9.25

9.55

9.88

10.2

10.6

6.1 – –

10.9

11.2

11.5

11.8

12.2

12.6

13.0

13.4

13.9

14.3 –

8.7

14.7

1

15.1

1

15.5

16.0

16.4

16.9

17.4

17.9

18.5

19.1

19.8

20.5

21.2

6.1 – – – – – – – – – – –

2.15 –

8.7

22.1

22.7

23.3

23.9

24.6

25.3

26.1

26.9

27.8

28.7

29.7

3.14

31.8

+M

+.1

8

+.1

5

+.1

2

+.0

9

+.0

6

+.0

3

0

-M 0

-.03

-.06

-.09

-.12

-.15

-.18

.06

.06

.09

.09

.06

.03 0 0 .09

.03 0 .03 0

0 .03 0 .03

.09 0 0 .03

.06

.09

.09

.06

.06

T =

“S”

= 1

“S”

= 2

“S”

= 3

“M”

LEA

DC

ON

NE

CT

ION

S

“L”

“R”

LEA

DLE

AD

TAB

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Rel

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1-31

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ange

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“L” Over “R” “R” Over “L”

41-495.51B

16

If the electrical response is outside the limits, a morecomplete series of tests outlined in Section 7.3 titled“REPAIR CALIBRATION” may be performed todetermine which component is faulty or out of cali-bration.

Indicating Contactor Switch

With either of the distance units tripped, pass suffi-cient dc current through the trip circuit to close thecontact of the ICS. This value of current should benot less than 1.0 ampere nor greater than 1.2amperes for the 1 ampere ICS. The current shouldnot be greater than the particular ICS tap settingbeing used for the 0.2-2.0 ampere ICS. The opera-tion indicator target should drop freely.

The contact gap should be approximately 0.047” forthe 0.2/2.0 ampere unit and 0.070” for the 1.0ampere unit between the bridging moving contactand the adjustable stationary contacts. The bridgingmoving contact should touch both stationary contactssimultaneously.

7.2 ROUTINE MAINTENANCE

The relays should be inspected periodically, at suchtime intervals as may be dictated by experience, toinsure that the relays have retained their calibrationand are in proper operating condition.

CAUTION

Before making “hi-pot” tests, connect togetherterminals 2 and 3 to avoid destroying compo-nents in the static network.

TABLE VI

Test Voltage = 30 VL-LM Setting = +.15S Setting = 1

SettingRange T Ohms Test Unit Amperes to Trip

No. At 75° Lag

.1-3.45 .944 .821 5,6,7 Ø-Ø (100% Z-94.7%Z)Ohms 5 3Ø 18.0-19.0 Amps

.2-4.5 1.23 1.07 5,6,7 Ø-Ø (100% Z-94.7%Z)Ohms 5 3Ø 13.8-14.8 Amps

.73-21 5.8 5.05 5,6,7 Ø-Ø (101% Z-94.5%Z)Ohms 5 3Ø 2.95-3.15 Amps

1.1-31.8 8.7 7.57 5,6,7 Ø-Ø (100% Z-94.5%Z)Ohms 5 3Ø 1.94-2.1 Amps

7.2.1 Distance Units

Use connections for Test #5, #6, and #7 of Figure 14to check the reach of the relay. Note that the imped-ance measured by the 3-phase unit in Tests #5, #6,and #7 is:

where VL-L is the phase-to-phase voltage, and IL isthe phase current. When in service and receivingthree phase currents, the 3-phase unit response is:

7.3 REPAIR CALIBRATION

Use the following procedure for calibrating the relay ifthe relay has been taken apart for repairs or theadjustments disturbed.

Connect the relay for testing as shown in Figure 14.The four-pole double-throw switch shown in the testcircuit selects the type of voltage condition, for aphase-to-phase or a three-phase fault, that will beapplied to the relay voltage terminals. The rotaryswitch switches the fault voltage to various terminalsand thereby simulates any combination of phase-to-phase faults without the tester having to change con-nections or readjust the phase shifter and variableauto-transformers.

7.3.1 Auto-Transformer Check

Auto-transformers may be checked for turns ratioand polarity by using the No. 2 Test connections ofFigure 14, and the procedure outlined below.

a. Set SA and SC on tap number 3. Set the “R”leads of MA and MC on 0.0 and disconnect all the“L” leads. Adjust the voltages V1F2F and V2F3F for90 volts. Measure the voltage from terminals 8 tothe #1 tap of SA. It should be 30 volts. From 8 tothe #2 tap of SA should be 60 volts. The voltageshould read 30 volts from 8 to SC = 1 and 60volts from 8 to SC = 2.

b. Set SA and SC on 1 and adjust V1F2F and V2F3Ffor 100 volts. Measure the voltage drop from ter-minal 8 to each of the MA taps. This voltageshould be equal to 100 (1+ the sum of valuesbetween R and the tap being measured).Example: 100 (1 + .03 + .09) = 112 volts.

ZR

VL L–2 IL

---------------=

ZR

VL L–

3 IL----------------=

41-495.51B

17

Check the taps of MC in the same manner. Trans-formers that have an output different from nominal bymore than 1.0 volt probably have been damaged andshould be replaced.

7.3.2 Distance Unit Calibration

Check to see that the taps on front of the tapblock are set as follows:

TA, TB (twice and TC: Set on the highest tapvalue. (.944, 1.23, 5.8, or 8.7)

SA and SC: Set on 1.

“R” for MA and MC: disconnected

“L” for MA and MC: set for .06 (top position)

7.3.3 Maximum Sensitivity Angle Adjustment

Phase-to-Phase Unit

a. Connect the relay for a 1-2 fault as indicated forTest #5. Connect a high-resistance voltmeter(2000 ohms/volt) between the “R” lead andthe .03 tap position of MA, and adjust voltageV1F2F for 30 volts.

b. Pass the current called for in Table VII for 30volts through TAB compensator with the phaseshifter set for the desired maximum sensitivityangle. Adjust potentiometer R2A for a null, orminimum reading, on the voltmeter.

c. Swing the phase shifter and adjust R2A slightlyto obtain a minimum reading on the voltmeterwhen the phase-angle reading is at the desiredmaximum sensitivity angle. This adjusts the TABcompensator angle.

d. Connect the relay for Test #6. Connect the volt-meter between the “R” lead and the .03 positionof MC, and repeat steps 6 and 7 above exceptadjust R2C to set the TCB compensator angle.

e. The compensator output can be checked by con-necting the voltmeter between the “L” lead of MCand top terminal of resistor RC. With 5 amperesthrough the compensator, the voltage should beas measured above in step d.

f. The compensator output can be checked by con-necting the voltmeter between the “L” lead of MAand top terminal of resistor RA. Pass 5 amperesthrough the compensator. The secondary voltageshould be:

VS VOLTS FOR GIVEN“T” SETTINGS

Ø VS T = 1.23 T = 5.8 T = 8.7

75° 10T 12.3 58 87

60° 8.96T 11.0 52 78

VS = 10.35 T Sin θ ± 1.5%

where

θ = the desired maximum sensitivity angle

T = compensator tap setting

10.35 = a design constant =

1.5% = the allowable variation from nominal

1075°sin

-----------------

7.3.4 Circuit Calibration

a. Connect “R” for MA and MC on “.03” and “L” forMA and MC in the top position so that there is .15between L and R. Connect terminals 7 and 9together, apply 120Vac between terminals 8 and9, and adjust RMC until the Ø-Ø unit just trips.

b. Connect the relay as listed in Table VII. Thephase-to-phase unit should trip within the currentlimits listed for the given test voltages. Check the5.0 volt level first, and adjust RMC further, if nec-essary, to make the current trip level or Test #7fall between the trip levels for Test numbers 5 and6.

To determine the limits of current when θ is notequal to 75°, multiply the nominal values tabu-lated above by the ratio:

7.3.5 Maximum Sensitivity Angle Check

Use the 30 volt condition in Table VII, except use20 volts for short reach voltage. Supply the cur-rent necessary to fully trip at the 45° angle andswing the phase shifter to determine the twoangles, θ1 and θ2, at which the unit just trips.

The maximum sensitivity angle, θ, is

This value should not be more than 5° differentfrom the setting made in steps 5 to 9. The phase-

75°sinθsin

-----------------

θ1 θ2+

2------------------

41-495.51B

18

to-phase unit testing is complete. True maximumsensitivity angle is the angle adjusted by nullmethod.

7.3.6 Maximum Sensitivity Angle Adjustment

Three-Phase Unit

a. Connect the relay for a 1-2 fault as indicated forTest #5. Apply 30 VL-L between relay terminals 7and 8. Supply 103% of the current necessary totrip the three-phase unit, and swing the phaseshifter to determine the two angles, θ1 and θ2, atwhich the unit just trips. The maximum sensitivity

angle,

The maximum sensitivity angle can be set by

adjusting potentiometer RS. The angle for thethree-phase unit should be the same as for thephase-to-phase unit as determined with Test #5in part 7.3.5.

b. Set the phase shifter for the maximum sensitivityangle. Using only Test #5 connections, the three-phase unit should trip within the same limits aslisted for the phase-to-phase unit.

c. The three-phase unit testing is complete.

8. RENEWAL PARTS

Repair work can be done most satisfactorily at thefactory. However, interchangeable parts can be fur-nished to the customers who are equipped for doingrepair work. When ordering parts, always give thecomplete nameplate data.

θ isθ1 θ2+

2------------------,

TABLE VII

Amperes To Trip for θ = 75°

.1-3.45 .2-4.5 .73-21 1.1-31.8Test Volts Range Range Range Range

No. V1F2F Imin Imax Imin Imax Imin Imax Imin Imax

5,6,7 5.0 3.05 3.45 2.34 2.55 0.50 0.54 0.33 0.36

30.0 18.0 19.0 14.0 14.8 2.95 3.15 1.98 2.1

70.0 32.7 33.4 6.9 7.1 4.62 4.72

ABB Inc.4300 Coral Ridge DriveCoral Springs, Florida 33065Telephone: +1 954-752-6700Fax: +1 954-345-5329www.abb.com/substation automation

IL 4

1-49

5.51

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ABB