Post on 07-Feb-2018
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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