Application Guide AC Pilot Wire Relaying System

SPD and SPA Relays

GET-6462A

CONTENTS

Page

Summary of Features.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Description & Operating Principles-SPD Relay ....................... 4

Settings & Characteristics .......................................... 6Application ........................................................ 10Calculation of Settings ............................................. 11Ground Sensor Connection ........................................ 13

Description & Operating Principles-SPA Relay ....................... 13Pilot Monitoring ................................................... 14Transfer Tripping .................................................. 15Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Protection of Pilot Wires ............................................. 17Ground Potential Rise ............................................. 17Induced Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Lightning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Elements of Pilot Wire System ....................................... 20

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

AC PILOT WIRE RELAYING SYSTEMSPD AND SPA RELAYS

SUMMARY OF FEATURES

Type SPD Relay -

1. High-speed Static Pilot Wire Relay- Protection for phase or ground faults.- No ac potential source required.- Insulating transformer internal.

2. Applicable on Two- or Three-Terminal Lines

3. Excellent Sensitivity on Short Lines.

4. Provision for Sensitive Ground Fault Detection.- Input from ground sensor CT.- Ideal for resistance grounded systems.

5. High Speed- 15 m.s. on heavy faults.- 25 m.s. or less on light to moderate faults.

6. Permits High-speed Reclosing.

7. Neutralizing Reactor and Drainage Reactor Available.

8. Case Fully Drawout.

Type SPA Relay -

1. Complete Monitoring of Pilot Circuit.- Detects open, shorted, grounded or reversed pilot.- LED indication.

2. Transfer-trip Feature Available.- One-way or two-way.

3. Complete Isolation of DC Supply.- Prevents SPD misoperation from battery ground.- Prevents grounded pilot from operating station dc ground monitor.

4. Case Fully Drawout.

3

APPLICATION GUIDEAC PILOT WIRE RELAYING SYSTEM

SPD AND SPA RELAYS

INTRODUCTIONThe Type SPD relay provides selective high-speed settings of other relays or by changes in the system.

clearing of all faults on a protected line, using a The scheme operates on current alone and requirespilot wire circuit to compare line currents at all ter- no potential transformers or channel equipment. Aminals of the line. Simultaneous clearing at all separate input to the sequence network can be fedterminals minimizes damage, permits high-speed from a ground sensor current transformer to providereclosing and improves the transient stability of sensitive ground fault protection on resistance-the system. grounded systems.

Since the pilot wire system circulates a currentthat is a function of the line current, it is similar tocurrent differential relaying. The scheme is easilyapplied, requires a minimum of system data, andbeing inherently selective is not affected by the

The Type SPA static auxiliary relays provide con-tinuous monitoring of the pilot wire circuit to detectopen, shorted, reversed or grounded pilot wires.Models are also available to provide for transfer tripin addition to the monitoring function.

DESCRIPTION AND OPERATING PRINCIPLESTYPE SPD RELAY

The Type SPD relay is a high-speed static pilotwire relay which provides both phase and groundfault protection for two- or three-terminal trans-mission or distribution lines. The relaying scheme,which operates on the circulating current prin-ciple, is represented by the simplified schematicdiagram in Figure 1 for a two-terminal line. Theprincipal functions included in the Type SPD relay,

and shown in the schematic diagram, are a phasesequence network, a voltage limiting circuit, a re-straint circuit, an operate circuit, and an insulatingtransformer. In the following paragraphs each ofthese functions is discussed, and the operatingprinciples of the circulating current scheme aredescribed.

--F------PHASE SENETWORK I

IQUENCE I 1 PHASE SEQlJiiCE

I

I IRESTRAINT PILOT WIRE RESTRAINT---?=

C I R C U I T I - - - - - - -

SATURATING C--_______D SATURATINGTRANSFORMER INSULATING TRANSFORMERS TRANSFORMER

Figure 1. Simplified Functional Diagram of the SPD Relay System

PHASE SEQUENCE NETWORKThe phase sequence network is connected to the

current transformers in the three phases as illus-trated in Figure 1. This network converts the threephase currents from the line current transformersinto a single-phase voltage which is applied to therelay circuits via the saturating transformer.

VOLTAGE LIMITING CIRCUIT

The voltage limiting circuit, which consists of thesaturating transformer and zener diodes, limits therelay output voltage that is applied to the pilot circuitvia the insulating transformer. This circuit limits theinstantaneous peak value of the sequence filter out-put voltage to 15 volts, and the pilot wire voltage to60 volts. Thus, for light currents the output of thevoltage limiting circuit will be a sine wave, for mod-erate currents on the order of five times the open-ended pickup of the relay the output will be a clippedsine wave, and for heavy fault currents the outputwill approach a square wave.

OPERATING AND RESTRAINING CIRCUITS

The sensing circuit responds to a voltage pro-portional to the current flowing into the pilot wires asa restraining quantity, and to the resulting voltageacross the pilot wire pair as an operating quantity.The relative position in the circuit of these restrain-ing and operating functions is shown in the simpli-fied diagram of Figure 1. Pickup for the open pilotwire condition is 3.25 volts across the operate cir-cuit, or 13 volts on the pilot wire side of the insulatingtransformer.

For the two-terminal line application shown inFigure 1, the relay circuits at the two ends are con-nected so that during an external fault the voltages

applied to the pilot pair at opposite ends are equalin magnitude but reversed in polarity. The voltagesare thus additive around the pilot loop, a relativelylarge current will circulate, and the voltage acrossthe operate circuit will below. The relays will restrainwith this condition of high pilot wire current and lowpilot wire voltage.

During an internal fault the voltages applied to thepilot pair, although not necessarily equal in mag-nitude, are essentially in phase and oppose currentflow around the pilot loop. Consequently there willbe a relatively large voltage and a small current; thestatic circuitry is designed to operate for this con-dition.

INSULATING TRANSFORMERThe insulating transformer, which is mounted in-

side the SPD relay case, acts as an impedancematching device between the relay circuit and thepilot wires. The primary winding is connected tothe restraint and operate circuits; the two secondarywindings are connected in series across the pilotwires as shown in the functional diagram in Figure 1.The turns ratio is one to four from the relay side ofthe transformer to the pilot wire side.

STATIC LEVEL DETECTOR LOGIC

Also included, but not shown in Figure 1, is a leveldetector logic which is part of a printed circuit card.The voltage and current measuring functions arealso on this printed circuit card.

The level detector logic consists of a summationamplifier, level detector and timer. The output volt-age signals from the restraint and operate circuitsare applied to the summation amplifier, as shown inthe functional block diagram in Figure 2. The sum-

WIRE CURRENT)

SUMMATION LEVELDETECTOR - - O U T P U T

(DERIVED FROM PILOTWIRE VOLTAGE)

Figure 2. Functional Block Diagram of SPD Pilot Wire Scheme

mation amplifier output is applied to the level de-tector, the operating point of which is determinedby the pickup biassetting. Thesummation amplifier,level detector and timer determine whether theoperate quantity exceeds the restraint quantity bymore than the pickup bias setting for a time intervallonger than the timer setting. When this occurs the5/5 timer will initiate a trip output. The timer desig-nation 5/5 defines operate and reset times of 5milliseconds.

Assuming ideal conditions, the output of therestraint circuit always predominates during ex-ternal faults, whereas during internal faults theoutput of the operate circuit always predominates.

The 5 milliseconds time-delay following the leveldetector provides added security during externalfaults, where conditions might not be ideal becauseof unequal saturation of the CT’s at opposite endsof the line, or because of a phase angle shift betweenthe operate and restraint quantities caused bycharging of the shunt capacitance in the pilot wires.

GENERAL

Additional functions included in the Type SPDrelay are a regulated dc power supply, an electro-mechanical output relay, and a target seal-in unit.These and the other functions noted above areshown on the functional block diagram in Figure 3.

_-_-_--

i

ON PRINTED CIRCUIT CARD

n-

leTO

CURRENTTRANSFORMERS ,c

THREEPHASEPICKUP

AMPERES

VOLTAGEMEASURING

UNIT

CURRENT I ISOLATIONMEASURING TRANSFORMER

UNIT

RESTRAINTTAP

l o - - -HI LO II I

i_- , RESTRAINT

-TO

PILOTWIRE

t,-

TARGETAND

IIIL____

SUMMINGAMPLIFIER DETECTOR

Figure 3. SPD Relay Block Diagram

SETTINGS AND CHARACTERISTICS

CURRENT TAPS

The sensitivity of the relay for the various types of The three-phase pickup taps, which are 4,5,6,7,8,faults is determined by three tap block settings, as 10 or 12 amperes, determine the basic pickup level ofwell as by the pilot wire length and the number of the relay. Relay pickup values expressed as mul-line terminals. The three tap settings, which can be tiples of the three-phase pickup tap setting are listedmade from the front of the relay as is apparent from in Table 1 forthevarious phase-to-phaseand groundthe photograph in Figure 4, are the three-phase tap settings with three-phase, phase-to-phase orpickup tap, the phase-to-phase tap and the ground phase-to-ground faults. The values in the tabletap. apply for the theoretical condition of single-end feed

6

and the pilot wires disconnected at the relay ter-minals. With single-end feed and pilot wires con-nected to relays at both ends of a two terminal line,the factors in the table would be doubled, neglectingthe effect of the pilot wires. The information in TableI, as well as the effect of the pilot wires on relaypickup, is discussed in more detail in the section onAPPLICATION.

Table I shows that the phase-to-phase tap posi-tion C provides the best sensitivity (lowest pickup)for phase faults, and also the closest agreementbetween the pickup currents for three-phase andphase-to-phase faults. Since it is true in most casesthat the positiveand negativesequence impedancesare equal, fault current for a phase-to-phase faultwill be 86 percent of the current for a three-phasefault at the same location and with the same systemconditions. Thus, use of the C tap assures that therelay will operate for the minimum expected phase-to-phase fault if it operates for the minimum three-phase fault at the same location.

If the three-phase pickup tap is set below maxi-mum possible load with the C tap setting, the relaymay operate on some load conditions if the pilot Figure 4. Type SPD Relay, Front

View, Removed, From Case

Table 1Relay Pickup in Multiples of Three-Phase Pickup

Tap Setting*

Phase-Phase

*Available three-phase pickup taps are 4, 5,6, 7,8, 10 and 12 amperes. Relay pickup forsingle-end feed with the pilot wires discon-nected from the relay terminals can be found by multiplying the three-phase current tap setting in amperes times the factor in thetable for the particular fault type and the phase-to-phase & ground tap settings.

**When the “A” tap is used the relay will not respond to balanced 3-phase faults.

#When the “F” ground tap is used the relay will not respond to the zero sequence component of fault current, but will respond to thepositive and negative sequence components, resulting in the pickup multiple shown in the table.

Refer to Instruction Book GEK-49794 and Apparatus Handbook Section 7278 for further information on the SPD relay.

7

wires are open circuited. This may be an acceptablerisk if the desired three-phase fault sensitivity is tobe realized, since the monitoring equipment de-scribed in a later section will sound an alarm on anopen pilot.

Another approach is to use the B phase-to-phasetap setting, which doubles the pickup for three-phase faults while retaining nearly the same phase-to-phase and ground fault sensitivity, and to utilizeother relaying to insure that all three-phase faultsare cleared. One possibility is a zone-packagedfirst-zone distance relay at each end of the line,connected to trip the local breaker and transfer tripthe remote breaker via the transferred trip schemeto be described later in the section on the SPA relay.For this scheme to be acceptable, the protected linemust be long enough so that the first zone distanceunits can be set to underreach the remote terminalbut overlap the reach setting of the remote distancerelay, and an ac potential source must be available.

The three taps F, G and H determine the relaysensitivity to ground faults. On the H tap, which isthe most sensitive, pickup is approximately one-eighth the three-phase pickup tap setting. On theG tap, pickup is approximately one quarter thethree-phase pickup tap setting. On the F tap therelay is insensitive to zero sequence current, but

GROUND

will respond to the positive and negative sequencecomponents, resulting in the pickup listed in Table I.

PROVISION FOR SENSITIVE GROUNDFAULT PROTECTION

On some resistance grounded systems, neitherthe G or H tap will provide adequate sensitivity. Forsuch cases an additional input to the sequence net-work is provided which can be fed from a groundsensor current transformer, as illustrated in Figure 5.With the F tap setting the current pickup with thissupplementary ground connection is approximately0.5 ampere into the relay for the single-end feedcondition and an open pilot. The turns ratio of theground sensor CT must then be selected to providethe desired sensitivity without causing excessivecurrent transformer saturation. This is discussed inmore detail in the section on APPLICATION.

DC CONTROL VOLTAGE TAPS

A movable dual link is provided on the upper tapblock. It can be set for either 48 or 125 volts dc.

RESTRAINT TAPS

Restraint taps, identified as HI and LO, are pro-vided in the middle of the lower top block, as shown

GROUNDSENSOR CTSENSOR CT

PROTECTED LINE_-----~

NETWORK 1

SATURATING ’ c________s SATURATINGTRANSFORMER INSULATING TRANSFORMERS TRANSFORMER

Figure 5. Simplified Functional Diagram of the SPD Relay System With GroundSensor CT Connection for Resistance Grounded Systems

8

in Figure 4. The normal setting is the HI tap and all OPERATING TIMES

discussion in this bulletin are based on the use of Pickup time of the Type SPD relay, expressed as a

this tap. The LO tap is available for special applica-

tions requiring increased sensitivity. Use of the LO

multiple of pickup, is shown in Figure 6. With heavyfault currents of four times pickup or more, thepickup time will be 15 milliseconds or less.

tap may result in lower overall security because the Dropout time, expressed in terms of fault currentratio of the restraining quantity to the operating prior to the clearing of a fault as a multiple of relayquantity will be lower for any system condition. pickup, is shown in Figure 7.

10 J I I I I I I I

1 1.5 2 3 4 5 6 7 6 9

CURRENT PER UNIT OF PICKUP

Figure 6. Pickup Time of the SPD Relay Vs. Multiple of Pickup Current

50

40 -

B

50 30-

#5d5

FtF5 20 -

0

B

5

10-1 3 4 5 6 7 8 9 10

CURRENT PER UNIT OF PICKUP

Figure 7. Dropout Time of SPD Relay

APPLICATION

The ac pilot wire system using the type SPD relayprovides selective high-speed protection for phaseand ground faults on two- or three-terminal lines ofshort to medium length. The basic scheme requiresone SPD relay at each terminal of the protectedline and an interconnecting pilot wire pair. Applica-tion limitations and determination of relay settingsare discussed in the following paragraphs.

THE PILOT WIRES

The pilot is a twisted pair of wires connected be-tween the two (or three) terminals of the protectedtransmission line. The SPD circuit will impose nomore than 60 volts peak on the pilot pair, so theinsulation capability of the pilot should be based onthe maximum induced voltages that may occur inthe pilot circuit, with the proper protection installed.(See section on protection of the pilot).

The limitation on pilot length is determined bythe acceptable magnitudes of the loop resistanceand the shunt capacitance. For two-terminal lines itis recommended that the series resistance of thepilot loop, including the neutralizing reactor wind-ings if present, not exceed 2000 ohms and that theshunt capacitance be less than 1.5 microfarads. Forthree-terminal lines the pilot circuit must be in theform of a T connection as shown in Figure 8. It isrecommended that the two-way resistance of each

leg of the T not exceed 500 ohms, including theneutralizing reactors if present. The resistance ofthe three legs should be matched as closely aspossible by padding one or two of the legs withadditional resistance. Resistance matching withinone percent or less is desirable. Any mismatch willreduce the security of theschemefor external faults.For example, a mismatch of five percent can pro-duce a signal in one or more of the relays which is25 percent of that required to cause a misoperation.It is recommended that the total shunt capacitanceof the pilot circuit for three-terminal applicationsbe less than 1.8 microfarads. If the value of pilotwire resistance or shunt capacitance exceeds therecommended limitsforeithertwo-orthree-terminallines, application of the SPD relay may still be pos-sible but the factory should be consulted.

Pilot loop resistance can be based on informationsupplied by the communication utility if the pilot isleased, or it can be calculated knowing the wire sizeand length of the pilot run, using dc resistance datapublished in wire tables. Pilot loop resistance canalso be determined by direct measurement from oneend with the remote end shorted for two terminallines, or from each terminal to a short at the junctionof the T for three terminal lines. This should be adc measurement since the shunt capacitance willinfluence an ac measurement.

STATION A CENTERRELAYS TAP

STATION BRELAYS

1STATIONC RELAYS

Figure 8. Pilot Wire Connections for Three Terminal Lines

10

The shunt capacitance can be determined eitherby calculation or measurement. However, measure-ment is preferred unless the pilot is under the com-plete control of the user, since it is sometimes thepractice of communication utilities to tap an existingpilot wire. This practice can result in an open sectionof pilot wire beyond one or both stations that willadd to the shunt capacitance, as illustrated in Figure9. If the value of shunt capacitance is supplied bythe communication utility it must be the total shuntcapacitance, including the effects of any opensections beyond either terminal.

Pilot wire resistance in excess of the values notedabove will reduce the current flowing in the pilot,and hence in the restraint circuit of the SPD relays.In the limit, the condition of an open-circulated pilotwould be approached where the relays will operateon external faults, or heavy load if it is above the3-phase pickupsetting. Shunt capacitance in excessof the listed values tends to reduce the relay sensi-tivity, and in the limit would approach the short-circuited condition where the relays will not operate.

RELAY SENSITIVITY AND SETTINGS

The sensitivity of the SPD relay for the varioustypes of faults is determined by the three-phasepickup tap setting, phase-to-phase tap setting andground tap setting, as well as by the pilot wire lengthand number of line terminals. The relay pickup in per

TE

PILOT WIRESECTION WHOSECAPACITANCE MAY

unit of the three-phase tap setting is given in Table Ifor the different phase and ground tap settings andvarious types of faults, with single-end feed and thepilot wires disconnected from the relay terminals.

In the following discussion it is assumed thatoperation of the relay on load flow with an open pilotis not acceptable, and that the scheme should detectall three-phase faults on the protected line. If theSPD relay is to be secure against operation with anopen pilot and maximum load flow, with margin, therelay three-phase current pickup from Table I shouldbe at least 1.25 times the maximum expected bal-anced load current. If the SPD relay is to operatefor any fault on the protected line, with margin, theminimum fault current for each type of fault shouldbe at least 1.33 times the relay pickup for the particu-lar type of fault. The following discussion is basedon the use of the high-restraint (HI) tap setting. Aspreviously noted, a low-restraint tap is also availablefor special applications requiring increased sensi-tivity, but use of the “LO” tap may result in loweroverall security.

The following setting procedure is suggested:

A. If it is assumed that it is desired to trip for allthree-phase faults as well as all unbalanced faultson the protected line, the C phase-to-phase tapshould be selected.

B. Determine the value of IT from equation 2,

NOT BE INCLUDED IN

POWER LINE

PILOTWIRE

TAP TO COMPLETETAP TO COMPLETE

PILOT WIRE

PILOT WIRE SECTIONWHOSE CAPACITANCE MAYNOT BE INCLUDED INCALCULATION

TION

Figure 9. Typical Situation Where Pilot Wire CapacitanceMay Be Greater Than Calculated Value.

11

assuming it is desired that the relay not trip withan open pilot and maximum load:

IT =

where:IT =

IL =

M =

1.25 lL[ ]M

minimum theoret ical three-phasepickup tap; use next higher availablethree-phase pickup tap (T).

maximum load current

multiplier for phase-to-phase tap (1.0for C tap from Table 1)

C. Check that the three-phase pickup tap Tdetermined in B satisfies equation (3):

T I 0.75lF[_]MNP (3)

where:

T =

lF =

P =

M =

N =

three-phase pickup tap setting (deter-mined in B)

minimum 3-phase fault current at faultlocation, sum of contributions from allterminals

factor from Figure 10 for total capaci-tance of pilot wire (“near-end” curve)

multiple for phase-to-phase tap (1.0 for

C tap)

number of line terminals

If the current tap T determined in B satisfies equa-tion (3), proceed to step D below. If the selectedcurrent tap T does not satisfy equation (3), the relayas set may not detect the minimum three-phase faulton the protected line. The user then has two options:

(a) Reduce the T tap setting until equation (3)is satisfied, recognizing that an open pilot maynow cause the relay to trip with load flow nearmaximum.

(b) LeaveT at the setting determined in B, andresort to other relaying to protect for the pos-sible minimum three-phase fault.

As suggested previously, if the line impedance issufficient and a potential source is available, zone-ldistance relays could be used as the other relayingin conjunction with the transferred trip feature ofthe SPA relays. In either case, proceed to step D,using the T tap finally selected.

D. Check that the T tapequation (4), assumingsetting is used:

T I 0.75

where:

T =

IS =

finally selected satisfiesthat the G ground tap

(4)

three-phase pickup tap setting

minimum internal single-phase-to-ground fault current, sum of contribu-tions from all terminals

2.0

PICK UPPER UNIT 1.0

PER RELAY

EQUALS 1333 TIMESCAPACITANCE IN MFD)

.5

PILOT WIRE SHUNT CAPACITANCE (MFD)

Figure 10. Effect of Pilot Wire Shunt Capacitance On SPD Pickup

12

B =

P =

N =

multiplier from Table I for the phase andground tap positions, and B0/ fault

factor from Figure 10 for actual capaci-tance of pilot wire (use “near-end”curve)

number of line terminals

If equation (4) is not satisfied with the G groundtap, check again using the H tap. If equation (4) isstill not satisfied, refer to the following section onuse of the ground sensor connection.

The three-phase pickup amperes tap, the phase-to-phase tap, the ground tap and the restraint tapmust be set in the same position in the relays at allterminals of the protected line.

GROUND SENSOR CONNECTION

On applications where the required sensitivity onsingle-phase-to-ground faults is not realized, theground sensor CT connection shown in Figure 5should be considered. This condition will typicallybe encountered on resistance grounded systems,where the ground fault current is primarily deter-mined by the grounding resistor. In such cases theground fault current is substantially less than the

phase fault current, and there will be little differencebetween the minimum and maximum ground faultcurrents.

A current transformer of relay accuracy may beused as the ground sensor. The turns ratio shouldbe selected so that the current into the relay is atleast 2 amperes with minimum ground fault currentand no more than 12 amperes with maximum groundfault current. The SPD static relay is well suited forapplication with CTs typically used as the groundsensor. For example, calculations have shown thatwith the 50/5ground sensor used in General ElectricPower/Vac switchgear, the SPD relaying system willperform correctly for secondary ground fault cur-rents within the 2 to 12 ampere limits noted above.Other CTs from available ratios of 100/5, 200/5400/5 or 800/5 may also be used as the groundsensor to provide a secondary current in the 2 to 12ampere range.

Relay pickup with this ground sensor input will beapproximately 0.5 amperes into the relay, for the4 ampere three-phase pickup tap position and theopen pilot condition noted in Table I. When theground sensor is used the ground tap must be inthe F position.

DESCRIPTION AND OPERATING PRINCIPLESTYPE SPA PILOT MONITORING RELAYS

The Type SPA relays are static pilot wire auxiliary the other terminal, or terminals, of the pilot circuit.relays designed primarily for use with the SPD pilot Models of the SPA are also available to provide awire system to provide continuous monitoring of the direct transfer trip feature, either unidirectional orpilot wire circuit for the detection of open, shorted, bidirectional, in addition to the pilot monitoringreversed or grounded pilot wires. The basic pilot functions. The four available SPA models are listedmonitoring installation requires a master, or trans- in Table II. Descriptions of each model and themitting, relay at one terminal and a receiving relay at schemes are included in the following paragraphs.

Table II

SPA1 28 X X X

*Refer to Instruction Book GEK-65512 and Apparatus Handbook Section 7278 for further information on the SPA relays.

13

FEATURES

In the transmitting or sending end relays, solidstate detection circuits operate undercurrent (UC),overcurrent (OC) and ground (GND) units, each withoutput contacts and associated light emitting diode(LED). In the receiving end relays the solid statecircuit operates an undercurrent unit with alarmcontacts and LED. Powerforthesolidstatedetectioncircuits and monitoring current is provided by anisolated dc power supply. This isolation of the dcsource prevents a ground in the battery supply frombeingreflectedintothepilotwirecircuitandpossiblycausing a misoperation of the SPD scheme.

PILOT MONITORING

A typical installation of the pilot monitoringscheme is illustrated by the simplified functionaldiagram in Figure 11. This scheme requires a trans-mitting end relay, model SPA11A, and a receivingend relay, model SPA12A. The scheme operates bycirculating a low-level current of approximately0.75 milliamperes dc through the pilot wire circuit.This current is initiated by the power supply in thetransmitting end relay and circulates through therelay circuit at the receiving end. It provides thereference to determine whether the pilot circuit is

SPD, 4 NEUTRALIZING

open or shorted. With normal pilot wires the under-current units in both the transmitting end and re-ceiving end relays will be held in the operated posi-tion by the 0.75 ma circulating current. This normalcirculating current is not sufficient to cause opera-tion of the overcurrent unit in the transmitting endrelay.

Since the transmitting and receiving end moni-toring relays are effectively in series with the pilotwire loop, it is necessary to provide a low-impedancebypass for the ac current circulated between theSPD relays at each end. This system-frequencybypass is provided by capacitors mounted internallyin the SPA relays and connected in parallel with therelay circuit, as shown in Figure 11.

OPEN PILOT

An open circuited pilot wire will cause the mon-itoring current to fall to zero. The undercurrent units(UC) in both the transmitting and receiving endrelays will drop out, the UC contacts will close, andthe LED indicators for the UC function will light.

SHORTED PILOT

A shorted pilot loop will result in an increase inthe monitoring current at the transmitting end, and a

NEUTRALIZING SPD, 1

REACTOR REACTOR

I

+r -_-----mmJ;--l

_ _ _ _I ISOLATION 7r

+ I ,I

_------(IF LINE IS

THREE-TERMINAL)SPAlPA i 13-l U C 7

RECEIVING END ! ?-I

ALARM BLOCKSPD

SPAl2A

SPD

SPA11ASENDING END

RECEIVING END

Figure 11. Simplified Functional Diagram - SPA Relays, Monitoring Only

14

decrease in the current at the receiving end. Theincrease in current at the transmitting end will resultin operation of the overcurrent unit and the associ-ated LED. The current at the receiving end will de-crease to zero for a short circuited pilot, causing theundercurrent unit to drop out. A short circuit on thepilot is the extreme case for the overcurrent con-dition. The overcurrent unit in the transmitting endrelay will pickup for circulating currents of 1.35milliamperes or more.

GROUNDED PILOT

The ground indicating unit in the transmitting endrelay is normally dropped out. A ground on eitherpilot wire, below 10,000 ohms, will causethegroundunit to operate and the ground LED to light.

REVERSED PILOT

A polarity sensitive circuit in the receiving endrelays provides indication of a reversed pilot wire.For the reversed pilot condition sufficient currentis bypassed around the undercurrent detection cir-cuit to cause the UC unit to drop out. Furthermore,as a result of the decrease in resistance at the re-ceiving end, the monitoring current increases suf-ficiently to operate the overcurrent function at the

transmitting end. These indications are the same aswould occur during a shorted pilot.

TRIP BLOCKING

The SPA relays for both the transmitting and re-ceiving ends include contact circuits for the optionalsupervision of the SPD trip circuit. At the trans-mitting end, this consists of normally closed con-tacts of the OC and ground units, and a normallyopen contact of the UC unit, all connected in series.At the receiving end a normally open contact of theUC unit is used. At user discretion, these contactscan be connected in series with the SPD trip circuitto block that circuit when an abnormal pilot con-dition is detected.

TRANSFER TRIPPING

On some applications the user may wish to use thepilot circuit to send a transfer trip signal in one orboth directions. Table II lists the SPA models re-quired when transfer tripping is involved. Theserelays include all the monitoring functions pre-viously described for the transmitting and receivingend relays. In addition they include a transfer tripinitiating auxiliary unit (TTA) and a receiving aux-iliary unit (TT).

NEUTRALIZING NEUTRALIZINGREACTOR

r r

i ISOLATIONI u 1 I

1 l-l STATIC i l- L---L_I,CTCPTlnU I mSAME CONNS. ASI

’ R I G H T H A N D ’I IT E R M I N A L

(IF LINE ISTHREE-TERMINAL)

\INITIATE BL;ICKTTRIPTRANS. SPD

TRIPTRIP

SPA110 SPA12B

Figure 12. Simplified Functional Diagram - SPA Relays,Monitoring and Two-Way Transferred Trip

15

A typical installation of the pilot monitoringscheme, with two-way transfertripping, is illustratedby the simplified functional diagram in Figure 12.The transfer trip signal is initiated by energizing thecoil of the TTA auxiliary unit. Closure of the TTAcontacts reverses the polarity of the dc voltage ap-plied to the pilot wires. Reversal of thevoltage polar-ity causes operation of the receiving auxiliary unit(TT) in the SPA relays at both the local and remoteterminals of the pilot wire.

The same SPA models are used in a one-waytransfer trip application. Referring to Figure 12, theTT unit contacts are not used at the initiating endand the TTA unit coil circuit is not connected at thereceiving end.

THREE-TERMINAL LINES

Application of the monitoring/transfer tripschemes to three-terminal lines is represented bythe dashed connections in Figures 11 and 12, whichshow the addition of the third terminal. The onlydifference in the circuit is that a link in the receivingend relays must be placed in the “three-terminal”position.

CHARACTERISTICS

Detection levels of the undercurrent units (UC) inthe transmit and receiving end relays are listed inTable III:

Table IllUC Unit Detection Levels

RelayLine

Terminals UC Level

relays to realize these operating characteristics.The only settings required at the time of installa-tion are the dc voltage selectors, 48 or 125 volts,the target seal-in unit tap setting when transfertripping is involved, and the selector in the receivingend relays for two- or three-terminal lines.

OPERATING TIMES

The operating times of the undercurrent, over-current, and ground detection circuits are intention-ally made slow to override transient disturbances.The times will vary depending on conditions on theprimary system and the severity of the abnormalcondition on the pilot wire. Thevalues listed in TableIV are approximate:

Table IVOperating Times

In Table IV detection time of each unit is the timeto close its contact after the occurrence of the pilotcondition listed in the table. The restore time is thetime to reopen the contact after the condition listedin the table is removed and normal circuit conditionsare restored.

For transfer trip installations, the operating time isdefined as the time between energizing theTTA unitat one end and the closure of the TT unit contact atthe other end. This time will vary slightly with thelength of the Pilot wire, as listed in Table V.

SPA1 1A or -B 2

SPA11AA o r - B 3

SPA1 2A or -B 2

SPA12AA o r - B 3

0.53 ma or less

0.43 ma or less

0.53 ma or less

0.21 ma or less

Operating current of the overcurrent unit (OC) inthe transmit end relay is 1.35 ma.

The ground unit in the transmit end relay willoperate, and the GND LED will light, when theresistance to ground from either conductor of thepilot pair falls below 10,000 ohms.

No field adjustments are necessary in the SPA

Table VTransfer Trip Times

Pilot Operate* Reset*Resistance Time Time

0 Ohms 4 cyc. 6 Cyc.

2000 Ohms 5 cyc. 6 Cyc.

*60 Hz base.

The transfer trip operating times are virtuallyindependent of station battery voltage variations.

16

PROTECTION OF THE PILOT WIRESAND ASSOCIATED EQUIPMENT

The most likely source of difficulty in an ac pilotwire relaying scheme is the pilot circuit itself. Be-cause the pilot circuit extends over a considerabledistance it is exposed to a number of hazards whichcan interfere with the proper operation of thescheme, or can cause damage to the pilot circuit andthe associated equipment. The more commonhazards are:

1. Rise in station ground potential.

2. Induced voltages.

3. Lightning.

The sources of thesevoltage disturbancesand thevarious devices available to minimize their effectshave been described in a continuing series of tech-nical papers, guides and standards, listed in thebibliography. The user should review these refer-ences for a more comprehensive discussion of pilotwire protection.

GROUND POTENTIAL RISE

A substantial voltage rise can occur at the sub-station ground mat relative to the remote groundpotential. This voltage rise can cause excessiveinsulation stresses on the pilot wire relay, the pilotwire monitoring relay, the pilot wires, and betweenthe pilot wires and other conductors in the samecable.

As illustrated in Figure 13, the typical conditionresulting in ground potential rise is a single-phase-to-ground fault on the power system. With groundcurrent flowing through the Station A ground mat,there will be a voltage difference between this matand the remote ground potential. The magnitude ofthis voltage will depend on the magnitude of thecurrent and on the impedance between the stationground and the remote ground. The value of theground current is usually available from systemfault studies. The determination of the impedancebetween the mat and the remote ground has beenthe subject of a number of papers listed in the bibli-ography which describe means of determining theimpedance by either calculation or test.

The diagram in Figure 14 represents a typicalvoltage profile of the potential difference betweenthe pilot cable and ground during a single-phase-to-ground fault. As is apparent from the plot, the po-tential of earth near the pilot wire cable tends to beat the remote ground potential for most of the lengthof the pilot run. Hence, if the cable is not groundedit will assume a potential near that of the remoteground because of the capacitive coupling. Thus thepotential rise of the station ground mat results in asignificant voltage difference between the stationground mat and the pilot cable with its connectedstation equipment. If this voltage difference be-

STATION A RELAYJ CASE

r - - - - - - - lI I

PILOTSHEATH

I RELAY

IrCIRCUIT

II

PILOTWIRES

3 IO (TOTAL FAULT CURRENTFROM REMOTE GROUND FAULT)

Figure 13. Situation Causing Station Ground Mat Potential Rise

17

4 \I

POTENTliL OF

I \

VOLTAGE PROFILE IN

STATION GROUND MAT/ VICINITY OF SUBSTATION

A______&_c c REMOTE GROUNDPOTENTIAL

PILOT WIRES & RELAYCIRCUIT AT THISPOTENTIAL

Figure 14. Typical Ground Potential ProfileDuring A Single-Phase-to-Ground Fault

comes too great, there is danger to personnel andthe possibility of damage to the pilot wire relays, theconnected equipment and the pilot wire itself. If thisvoltage difference exceeds 600 volts, protection ofthe pilot wires is usually necessary since this is thecontinuous voltage insulation level of theconnectedrelays, terminal boards, panels and standard tele-phone cables.

The usual method of protecting against excessivestation ground potential rise is to install a neutral-izing reactor in the pilot wire circuit as shown inFigure 15. The windings of the neutralizing reactorpresent a low impedance to the normal circulatingcurrent of the wire pilot scheme (i.e., transversevoltage signals) since the currents in the two wind-ings will flow in opposite directions. Consequently,

the transverse signal is opposed only by the leakagereactance of the neutralizing reactor windings.

On the other hand, a common mode voltage dif-ference, as a the result of the station ground po-tential rise, will attempt to force current throughthe two windings in the same direction and willtherefore be opposed by the magnetizing imped-ance of the neutralizing reactor. The capacitorsconnected to ground at the midpoint of the SPDinsulating transformer and the stray capacitance toground of the pilot wires complete the path for thereactor magnetizing current resulting from the po-tential difference between station ground and re-mote ground. Since most of the voltage drop fromthis circulating magnetizing current will appearacross the reactor windings, the relays and pilotconnections on the station side of the reactor willbe elevated in potential to essentially the same levelas the station ground mat, as illustrated by the volt-age profile in Figure 15.

The voltage profile also reveals that the neutraliz-ing reactor itself and a section of the pilot wire cableadjacent to the station may still be subjected toexcessive voltage stresses during a station groundpotential rise. For this reason a neutralizing reactorthat can withstand the full ground potential rise

NOTGROUNDED

NEUTRALIZINGREACTOR

l--PSPA

, I

,STATlON GROUND POTENTIAL DURING GROUND FAULT

POTENTIAL OF RELAY AND \

CONNECTIONS TO.

NEUTRALIZING REACTOR I__‘.

NEUTRALIZINGREMOTE GROUND

REACTORPOTENTIAL

VOLTAGE PROFILE

Figure 15. Typical Application of Neutralizing Reactors, With Voltage Profile

18

should be selected. A neutralizing reactor with a4kV withstand capability, including the capacitorsand resistors for the ground connection, and anungrounded gaseous discharge tube, is availablefrom General Electric. The purpose of the gaseousdischarge tube is explained in a following sectionon lightning effects. If the expected potential riseexceeds the withstand rating of standard pilot wirecable, then a pilot wire with a higher voltage ratingshould be installed from the neutralizing reactor to asplice point outside the zone of influence of stationground potential.

INDUCED VOLTAGES

Disturbances from induced voltages usually havetheir source in the power circuits themselves, eitherduring faults on the line being protected by the pilotwire scheme, or on adjacent lines. This inductivecoupling can result in induced voltages in the trans-verse (or differential) mode, or in the common (orlongitudinal) mode.

If the magnetic flux of the faulted line links onewire of the pilot pair less than the other, then aninduced voltage in the transverse mode will appearas a voltage between the two wires. If the flux linksboth wires of the pilot pair and not theground returnpath, then the induced voltage is known ascommonmode and results in a voltage from both wires to thecommon ground reference plane.

MUTUALDRAINAGEREACTOR

NEUTRALIZINGREACTORSPD

q

TRANSVERSE MODE INDUCED VOLTAGESSince a voltage resulting from the transverse mode

of induction is between the two wires of the pilotpair, it appears as an operating voltage to the SPDrelays. If it exceeds the voltage required to operatethe SPD, which is 13 volts with no restraint currentpresent, an incorrect trip can result. Consequentlyit is essential that transverse mode induced voltagesbe held to a minimum. An effective means of mini-mizing transverse mode induced voltage betweenthe wires is to use a twisted pair for the pilot.

COMMON MODE INDUCED VOLTAGESCoupling between the pilot pair and an adjacent

power line carrying fault current may result in acommon mode (or longitudinal) induced voltage.Generally this induced voltage results from a groundfault and can usually be determined with sufficientaccuracy by multiplying the maximum ground faultcurrent by the zero sequence mutual impedancebetween the power circuit and the pilot circuit. Theneutralizing reactor, described in the section onground potential rise and shown in Figure 15, willprotect the relays, terminal blocks, and pilot wiresup to the reactor from damage by such inducedvoltages. If it is determined that the common modeinduced voltage on the other side of the neutralizingreactor can exceed the insulation capability of thepilot wires, a mutual drainage reactor with gaseousdischarge tube can be installed as shown in Figure16. If it is deemed necessary to provide a mutual

GAS TUBE\ PILOT WIRE

It-REMOTE TO OTHERGROUND TERMINAL

STATION GROUND MAT

Figure 16. Typical Use of Mutual Drainage Reactor and Gas Tube

19

drainage reactor, as well as the neutralizing reactor,then the gas tube associated with the drainagereactor should be remotely grounded at a pointoutside the zone of influence.

If the pilot pair is in the same cable as other com-munication circuits, care must be taken that theintercircuit withstand voltages are not exceeded.An effective method of minimizing intercircuit volt-ages is to protect each circuit in a similar manner atthe same location, thus assuring that at no point willthere be significant voltage differences.

LIGHTNING

needed as typified by Figure 17. The protection pro-vided by the arresters should coordinate with theinsulation withstand capability of the neutralizingreactor and the pilot pair. It is essential that thearresters reseal with nominal ac and dc currentsflowing in the pilot pair. It is for this reason thatlightning arresters rather than carbon blocks arerecommended. The arresters should be remotelygrounded outside the zone of influence so that theydo not operate on a ground potential rise, since theneutralizing reactors protect against this condition.

Control equipment in substations is generally sowell shielded against severe voltage surges causedby lightning that direct protection may not be nec-essary. The pilot wire system differs from othercontrol equipment in that it runs over a considerabledistance from one substation to another and henceis exposed to lightning. Lightning induced voltagesmay exceed the pilot insulation capability, and if thisis possible, lightning arrestors may be applied as

As shown in Figure 17, a gas tube is usually con-nected from wire to wire of the pilot pair when arrest-ers are used. The purpose of this gas tube, which issupplied with the neutralizing reactor, is to shortthe two wires together should one arrester fire andnot the other. This will prevent a high voltage be-tween the pilot pair which could cause a false tripby the SPD relay during arrester discharge. It isessential that the gas tube be left ungrounded sothat it does not provide a path to the ground.

STATION STATIONGROUND GROUNDSHIELD SHIELD

1GAS TUBE

r (NO GROUND) , J

NEUTRALIZINGREACTOR

PILOTWIRES

/

NEUTRALIZINGREACTORrm

I SPD r

LIGHTNINGARRESTERS

ElSPAL

Figure 17. An Illustration of Possible Surge Protection

ELEMENTS OF THEPILOT WIRE SYSTEM

SPDTLThe elements of the ac wire pilot scheme and

their functions, which were discussed in detail inearlier sections. are summarized in Table VI. As isapparent from the table the major components ofthe system, some of which are supplied by the user,are the following:

1. Pilot wire relay, type SPD, and associatedequipment.

2. Pilot pair and surge limiting equipement.

3. Pilot monitoring relay, Type SPA

4. Neutralizing reactor and associated equip-ment.

5. Mutual drainage reactor.

Of these five elements the first two, the pilot wirerelay and pilot pair, are of course essential to the

20

operation of the basic scheme. The pilot monitoringrelays, while not essential to the operation of thescheme, are usually included because of their valuein detecting open, shorted, grounded or reversedpilot pairs. The neutralizing reactor should be usedwhen data is not available to verify that groundpotential rise and/or common mode induced volt-ages on the pilot pair are within the insulation capa-bility of the pilot wires and connected equipment.The mutual drainage reactor is used when pro-tection beyond that provided by the neutralizingreactor is required to protect against common modeinduced voltages.

The SPD-SPA pilot wire relaying system is de-signed to keep to a minimum the number of unitsthat must be ordered and mounted. The SPD11Arelay includes, in addition to the relay itself, an ex-ternal test switch, ammeter, and meter range-changing transformer, thus providing the facilitiesrequired for the basic pilot wire relaying schemeand for the ac testing of the system. The pilot wirepair, and lightning arresters for surge protection,are supplied by the user.

The SPA relays, one per line terminal, providethe transmitting or receiving functions for pilotmonitoring, and can be supplied with or withouttransferred trip capability. The relay type numbers

for these various functions are listed in Table II.Each of these relays includes the auxiliary units andresistors to provide the specified functions, as wellas an internally mounted capacitor to bypass the accirculating pilot current around the dc circuits. Inprevious schemes this ac bypass capacitor, as wellas a number of resistors and contacts for transfertrip, required separate mounting.

The neutralizing reactor available from GeneralElectric includes the gas tube and mounting, as wellas the resistors and capacitors, with mounting plate,to provide the return path for the reactor magnetiz-ing current. The mutual drainage reactor availablefrom General Electric includes the gas tube withmounting plate.

These various components, which are shown inthe photographs in Figure 18 and 19, can be com-bined to provide a number of schemes which differin whether or not the sensitive ground connection isused, whether pilot monitoring is used alone, orwith transferred trip, and whether two-terminal orthree-terminal lines are involved. The schematicdiagrams in Figures 1 and 5 describe the basicscheme, while the elementary diagrams in Figures11 and 12 cover the monitoring and transferredtripping applications.

Table VIItems Included in SPD Pilot Wire Scheme

ORDERED ITEM

SPDllA

SPA11A

:AllB

INCLUDED

Test SwitchAmmeter & Transformer

FUNCTION

Pilot wire relayCheck of AC connectionsin Pilot wire circuit.

Pilot Monitor

Pilot Monitor &Transfer Trip

REMARKS

Transmit End

Transmit End

REQUIRED OR OPTIONAL

Required

Optional

Optional

SPAIZAOrSPA128

Pilot Monitor

Pilot Monitor &Transfer Trip

Receive End Optional

Receive End Optional

NeutralizingReactor

(0257A9787G3)Capacitor & ResistorAssem.Ungrounded Gas Tube

Neutralizes ground potentialrise or common modeinduced voltageProvides exciting currentpath for neut. reactor

Required if calculationsindicate insulationwithstand capabilitywill be exceeded.

Mutual DrainageReactor

(0257A9787G4)Grounded Gas Tube

Limits common-mode inducedvoltage, pilot wires toground

Optional

!

21

SPDRelay

Milliameter MeterRange-Changing

CT

Test SPA RelaySwitch (See Table II)

Figure 18

MutualDrainageReactor

NeutralizingReactor

ExcitationAssembly

Figure 19

22

BIBLIOGRAPHY

1. Committee Report, “Application and Protection of Pilot Wire Circuits for ProtectiveRelaying.” Sponsored by the Pilot Wire Subcommittee, AIEE Relays Committee, July,1960 (Publication S-117).

2. Committee Report, “The Isolation Concept for Protection of Wire Line Facilities EnteringElectric Power Stations.” Sponsored by the Power System Communications Committee,IEEE Power Engineering Society. IEEE Trans. (Power Apparatus & Systems) 1976,pages 1216-33.

3. Committee Report, “The Neutralizing Transformer Concept for Protection of Wire LineFacilities Entering Electric Power Stations.” Sponsored by Power System Communica-tion Committee, IEEE Power Engineering Society. IEEE Trans. (Power Apparatus &Systems) 1977, pages 1256-79.

4. “A Guide for the Protection of Wire Line Communications Facilities Serving ElectricPower Stations,” IEEE Standard 468 published 1980.

5. J. F. Laidig and F. P. Zuper, “A Practical Ground Potential Rise Prediction Technique forPower Stations.” Presented at IEEE-PES Winter Meeting, 1979.

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