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30th AnnualHands-On Relay School

Jon F. DaumeBPA (Retired)

March 12, 2013

Introduction to System Protection

Protection Basics and Terminology

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A Referencefor the

Rest of Us! Hands-On Relay SchoolMarch 12, 2013

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Purpose of Protective RelaysTransmission line fault protectionDetect and isolate equipment failuresImprove system stabilityProtect against overloadingProtect against abnormal conditions

Voltage, frequency, currentProtect public

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Fault CausesLightningWind and iceVandalismContaminationExternal forces

Cars, tractors, balloons, airplanes, trees, critters, flying saucers, etc.

Equipment failuresSystem disturbances

Overloads, system swings

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Fault TypesOne line to ground (most common)Three phase (rare but most severe)Phase to phasePhase to phase to ground

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7

Single Line to Ground Fault

Va

Vc

Vb Ia

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Three Phase Fault

Va

Vc

Vb

Ia

Ic

Ib

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Phase to Phase Fault

Va

Vc

Vb

Ic

Ib

10

Two Phase to Ground Fault

Va

Vc

Vb

Ic

Ib

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Balanced & Unbalanced Systems

A

C

BBalancedSystem

A

C

BUnbalanced System

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Balanced & Unbalanced Systems

Balanced System:3 Phase load3 Phase fault

Unbalanced System:Phase to phase faultOne line to ground

faultPhase to phase to

ground faultOpen pole or

conductorUnbalanced load

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Sequence Quantities(Symmetrical Components)

Condition + - 03 Phase load - -3 Phase fault - -Phase to phase fault -One line to ground fault

Two phase to ground fault

Open pole or conductor

Unbalanced load

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Relay TypesNon-directional

Detect fault in any directionOperate when quantity exceeds pickup valueUsed on distribution linesCan be used on radial systems

DirectionalOnly trip for faults in front of relay (on line)

Use voltages, currents, angles to determine fault direction

Operate when quantities exceed pickup value and correct direction is determined

Relay of choice for HV and EHV transmission

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Relay TypesCurrent, voltage, frequency

Operates if input meets settingDistance

Uses voltage and current to measure impedance to fault

DifferentialLooks at imbalance between inputsCommon for power transformers and

generatorsCan be used for transmission lines

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Relay TypesRecloser

Relay to automatically reclose circuit breaker following a relay operation to restore circuit

Pilot schemeUses communications to transmit relay

information or trip to remote terminalProvides high speed tripping for entire

protection zoneRadio, fiber optics, hard wire, carrier current

can be used for pilot channelMost common on HV, EHV lines

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Relay TypesPhase relay

Relay measures phase current or voltage quantities

Ground relay Relay measures ground current or voltage

quantity (zero sequence values)Protects for one line to ground and phase to

phase to ground faultsSequence relay

Relay measures symmetrical component sequence quantity (+, -, 0)

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Relay Trip TimesInstantaneous

Relay operates as soon as operating value is metTime delay

Relay operating time is delayedFixed delay determined by separate timing element (62)

Inverse delay determined by magnitude of operating quantity and relay operating curveDelay decreases as operating value increases

Actual clearing time includes relay operate time plus circuit breaker opening time

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Relay ConstructionElectromechanical

Several individual relays required for complete fault protection

Static or electronicOne or more relays required for complete

fault protectionDigital or microprocessor

Single device provides complete fault protection

Device may include additional features not available with electromechanical or electronic relays

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Relay BasicsComponent relay

Individual boxes that provide phase or ground protection, reclosing, etc.

Relay systemBunch of single components designed to do a taskA multifunction device to do the same task or

several tasks

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Digital RelaysDigital relays were introduced in early 1980’sAdditional digital relay features

Fault information and locationVoltage and current inputs required to locate fault

Remote communicationsSelf testingCircuit breaker history and monitoringMeteringTime tagging (GPS clock input)

ConcernsComplicated to apply (many elements)Single point of failureLimited life expectancy

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IEEE Device NumbersNumbers 1 - 97 used21 Distance relay25 Synchronizing or synchronism check

device27 Undervoltage relay32 Directional power relay43 Manual transfer or selector device46 Reverse or phase balance current relay50 Instantaneous overcurrent or rate of rise

relay (fixed time overcurrent)(IEEE C37.2)

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51 AC time overcurrent relay52 AC circuit breaker59 Overvoltage relay62 Time delay stopping or opening relay63 Pressure switch67 AC directional overcurrent relay 79 AC reclosing relay81 Frequency relay86 Lock out relay87 Differential relay

(IEEE C37.2)

IEEE Device Numbers

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Overlapping & Redundant Systems

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Relay ReliabilityOverlapping protection

Relay systems are designed with a high level of dependability

This includes redundant relaysOverlapping protection zones

We will trip no line before its timeRelay system security is also very importantEvery effort is made to avoid false trips

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Relay ReliabilityRelay dependability (trip when required)

Redundant relaysRemote backupDual trip coils in circuit breakerDual batteriesDigital relay self testingThorough installation testingRoutine testing and maintenanceReview of relay operations

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Relay ReliabilityRelay security (no false trip)

Relay security failures have increased the impact of numerous system disturbances

Careful evaluation before purchaseRight relay for right applicationVoting

2 of 3 relays must agree before a tripThorough installation testing Routine testing and maintenanceReview of relay operations

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Protection ZonePortion of system protected by relayUsually determined by location of current

transformersCommon protection zones

Substation busTransmission line

May have multiple protection zonesPower transformerGenerator

Common to have backup protection for zone

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Instrument TransformersUsed to transform line currents and voltages to

relay valuesVoltage and current transformers

Transformer typesMagneticCapacitive

Capacitor voltage divider to measure voltages

Optical

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Instrument TransformersTransmission Lines

Zsecondary = Zprimary x CTR / VTRFor distance relays

The PT location determines the point from which impedance is measured.

The CT location determines the fault direction.CT location generally determines zone of

protection

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CT SelectionC800 Current Transformer

Will support 800 volts @ 100 amps on CT secondary before saturation (20 times rated secondary current)

Consider burden of relays and cable impedanceCT Accuracy decreases when less than full

winding usedAt half ratio, CT is C400

CT Saturation:Saturation most severe with high magnitude faults

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Saturated Current

-100

-50

0

50

100

150

-0.017 0.000 0.017 0.033 0.050 0.067

Overcurrent Relays

Distribution Protection

(50/51)

Protective DevicesCircuit Breaker with relaysPole mounted reclosersFuses

Overcurrent Relays (50/51)Non directional3 Phase and ground relaysTime overcurrent elements (51)Instantaneous elements (50)

Ia

Ib

Ic

Ir=Ia+Ib+Ic

50/51 Relays

Time Overcurrent (51)Operate time inversely proportional to currentSelect operating characteristic

Inverse, very inverse, extremely inverseSelect pickup current (tap)Select operate time (time lever or time dial)Usually coordinated with next downstream device

51 Time Overcurrent Curves

0.1

1.0

10.0

1.0 10.0 100.0

Multiples of PU Current

Tim

e, S

econ

ds

Med. Inv. Very Inv. Ext. Inv.

Substation Bus Protection

Simple Bus Differential Scheme

CTs in parallelMust have common CTRExtremely inverse 50/51 relay used

50 Element may have to be disabledCT Saturation for external faults can cause

misoperationNeed GOOD CTs! (C800)Used for main & auxiliary bus, breaker & half

configurations

Bus Protection Zone

5121

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Line Protection

Bus Protection

Line Protection

50/51

50/51

I = 0

External Fault

Internal Fault

Bus Differential Scheme with OvercurrentRelays

86LOR

TripClose Block

86LOR

TripClose Block

Power Transformer Protection

Fault ProtectionPrevent thermal damage

I2 t = kPrevent mechanical damage

Core shifts for through faultsNo trip for normal or emergency loads

NERC Loading criteria150% of Nameplate rating115% of Emergency rating

Coordinate with other relays, fuses

Overcurrent Protection

A

B

C

115 kV / 13.8 kVA

B

C

Relay(s)Relay(s)

50/51 50/5150/51N50/51N

Transformer Differentials (87)

Detect faults within differential zoneBetween CTs (within protective zone)Immune to external faults

Will not see turn to turn faultCT Connections

Delta for wye windingWye for delta windingCompensate for transformer phase shift

Use good quality CTs

External Fault

R1 R2

Op

I1

I2'I1'

I2

I Op ~ 0

Internal Fault

R1 R2

Op

I1

I1'

I Op = I1' + I2'

I2'

I2

Transmission Line Protection

Typical Line Protection

Distance Relays(21, 21G)

Typical Reaches

21 Zone 1 85-90%

21 Zone 2 125-180%, Time Delay Trip

21 Zone 3 150-200%, Time Delay Trip

Typical Relay Protection Zones

67 Ground Instantaneous Overcurrent

67 Ground Time Overcurrent

67 Ground Time Permissive Transfer Trip Overcurrent

Ground Fault Protection(21G, 67N)

Ground FaultsGround distance relays (21G)Directional ground overcurrent relays (67N)Ground overcurrent relays

Time overcurrent ground (51)Instantaneous overcurrent (50)

Measure zero sequence currents

Automatic Reclosing (79)First reclose ~ 80% success rateSecond reclose ~ 5% success rateMust delay long enough for arc to

deionizet = 10.5 + kV/34.5 cycles

14 cycles for 115 kV; 25 cycles for 500 kVMust delay long enough for remote

terminal to clear1LG Faults have a higher success rate

than 3 phase faults

Automatic Reclosing (79)Most often single shot for line protectionMultishot most often used on distributionDelay of 30 to 60 cycles following line trip is

commonChecking:

Hot bus & dead lineHot line & dead busSync check

Generator Protection

Generator ProtectionAmount of protection based on generator size

and typeCommon protection for larger units:

Differential relaysReverse power relaysFrequencyGround fault Over voltageLos of excitationNegative sequence

Differential Protection (87)Generator differentialStep up transformer differentialOverall differential

Generator and transformer

~

87 87

Frequency Protection (81)Overfrequency/overspeedUnderfrequency

Must coordinate with load sheddingWSCC Requirement

~

81

Ground Fault Protection Voltage relay across generator neutral

(59N)Generators usually grounded through a

resistor to limit ground fault currents

~

59N

Unbalanced Fault ProtectionNegative sequence overcurrent relay (46)I2 Generates twice-rated frequency rotor

currents; generates lots of heat

~

46

Balanced Fault ProtectionDistance relay (21)Voltage restrained overcurrent (51V)

~

21

Reverse Power (32)Generators don’t make good motors

Similar to watt meter

~

32

Loss of ExcitationLoss of Excitation (40)

Similar to distance relayMho characteristic @ -90 degrees

Alarm and trip levelsMultiple zones available

~

40

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UFOs vs. Power Outages

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Jon F. DaumeBonneville Power Administration

Electrical Engineer (retired)

bettsoh@embarqmail.comMarch 12, 2013