ENR458 Presentation SPRING2013
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Transcript of ENR458 Presentation SPRING2013
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Agenda
What is Distribution System Protection?
System Protection Considerations.
Why do we care?
Commonly used Distribution Protective Devices.
Distribution Protection Basic Concepts.
Information/data required to perform a Study. Review of Hospital Case Study.
Miscoordination Case Study.
Conclusion/Questions?
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Distribution System Protection involves the selection,
arrangement, installation and programming ofprotective devices selectively coordinated to limitthe effects of an overcurrent (short-circuit) situationto the smallest area by clearing a fault in the
minimum amount of time possible, while minimizingthe impact to customers and the electric distributionsystem.
WhatisDistributionSystemProtection?
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SystemProtectionDesignCriteria
Reliability: System operates as designed.
o Security: Dont trip when you shouldnt.
o Dependability: Trip when you should.
o Safety: Preventing hazards to the public by isolatingand removing a faulted section from the system.
Selectivity: Trip the minimal amount to clear the fault orabnormal operating condition.
Speed: Usually the faster the better in terms of minimizingequipment damage and maintaining system integrity.
Economics: Dont break the bank while maintaining ability
to operate correctly under all predictable power systemconditions.
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What isthefirstconsiderationwhen
designingSystemProtection?
Is what results from a short caused by lowimpedance and a three phase bolted,phase-to-phase or phase-to-groundconnection.
Passes through all the components in theaffected circuit path.
Is often several orders of magnitude
greater than normal operating current. Needs to be interrupted or it can destroy
insulation, melt metal, start fires, or causeexplosion if arcing occurs.
FAULT CURRENT
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What isthesecondconsiderationwhen
designingSystemProtection?
TIME
High-speed fault clearance with correctselectivity.
High Sensitivity to faults and insensitivity to
maximum load currents. The more time a protection scheme allows,
the greater the flexibility, if a fuse blowsinstead of a circuit breaker opening, a lotfewer customers are affected.
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Selection of protective devicesrequires compromises: Protection isdesigned to be as inexpensive aspossible. Maximum and Reliableprotection at minimum equipmentcost.
Minimum standards vary. A circuit toa hospital needs greater reliabilitythan a circuit to a shopping center.
Cost of protective devices should bebalanced against risks involved ifprotection is not sufficient and not
enough redundancy is provided.
Whatisthethirdconsiderationwhen
designingSystemProtection?
COST
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WhatisaShortCircuit?
A short circuit orfault current is a
path of lowimpedance whichallows an abnormally
high amount ofcurrent to flow. Thisfault current if notinterrupted, can
cause catastrophicdamage to electricalequipment.
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MostCommonFaultTypesonDistributionSystem
Three-phase
Phase-to-ground
Phase-to-phase
X
X
Z
Z
Z
G
BC
A
X
X
Z
Z
Z
G
BC
A
Z
Z
Z
G
BC
A
X
X
X
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Polesknockeddownduringastorm
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MylarBalloonscaughtinPowerLines
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Groundsleft
connectedinside
switchgearcompartment
atsubstation.
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MostCommonProtectiveDevicesusedon
DistributionCircuits
Feeder Circuit Breaker
Line Recloser
Subsurface Fault Interrupter
Fuses both Overhead and Underground
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Substation12kVCircuitBreaker
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Substation12kVCircuitBreaker
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SubstationFeederRelay
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ElectromechanicalRelays
Have moving parts.
Can get out ofadjustment.
Can wear out.
Have broad tripping
ranges. Have Tap and Lever
settings.
Best approximation ofprotection.
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MicroprocessorbasedRelays
Have multiple functions.
Can be utilized for SCADAoperating.
Provide present and previous faultdata.
Provide fault locating capability.
Can be accessed remotely toprovide fault data/location and orchange settings.
Have very specific trip ranges.
Easy to test.
Easy to install settings and settingchanges.
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SubstationFeederRelays
Device 50/51- ThreeSingle Phase
Time/ InstantaneousPhase Overcurrent
Relays
Device 50G/51GTime/ InstantaneousGround OvercurrentRelay
Device 79Reclosing Relay
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TypicalNumberofCBAutomaticReclosing
Two Tests (Three Shots to Lock-Out): Used for overhead andcombination overhead / underground circuits. With reclosing
interval times of 5 and 20 seconds.
Zero or One Test (One or Two Shots to Lock-Out): Used forexclusively underground circuits. One test (two shots) shouldbe considered on underground circuits with exposure due to
risers. The reclose time can be between 10 and 20 seconds. Zero or one test is a very helpful way of reducing I2T since it is
cumulative.
With a generation facility located on the load side of a line
recloser, the first reclosing time must be equal to or greaterthan 10 seconds.
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Line
RecloserReclosers are overhead pole mounted protective deviceswhich combine:
Three-phase oil or vacuum circuit breaker withcapabilities for closing into or interrupting faults.
Phase relay protection and ground relay protection.
Reclosing capabilities. Typically set for two tests(three shots to lockout) at 5, 10 and 15 second intervals.
A combination of slow and fast curves for fuse savingand other features.
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PoleMountedAutomaticLineRecloser
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PoleMountedAutomaticLineRecloser
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LineRecloser&
Controller
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SubsurfaceFaultInterrupters
Three phase subsurface, underground vacuum oroil insulated device.
Capabilities for testing into and interrupting faults.
One shot to lock out.
Three phase and ground protection.
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SubsurfaceFaultInterrupter
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SubsurfaceFaultInterrupter
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Subsurface
Interrupter
installed
in
Underground
Vault
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Fuses
Fuses are low cost automatic sectionalizing devices.They have fault sensing and interruption capability
but, obviously, lack automatic reclosing capability.
Two fundamental different types of fuses in distributionsystems:
Expulsion for Overhead Applications.
Current limiting for Underground Applications.
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Overhead
Fuse
Types The two typical types of fuses commonly used on
the overhead for line protection, are T Fuses and E
fuses. The decision to use T or E fuses is based onthe asymmetrical fault duty at the fuse location andcoordination.
If an E fuse will coordinate with the source side
device where a T fuse will not, then an E fuse couldbe used.
Other types of Fuses are used for fusing in fire
danger areas, transformer fusing, and capacitorfusing.
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65Evs.65TFuses
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PoleMountedFusedCutoutsProtectingCapacitorBank
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PoleMountedFusedCutoutsProtecting3singleTransformers
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NoteonUndergroundFuseApplication
There are typically two types of fuses used in the undergroundsystem:
Current limiting fuses E fuses.
Existing underground radial and looped taps should be fused tocorrect I2t and service reliability problems.
Ground coordination is essential with these devices because mostinitial faults in the underground are line to ground faults.
It is not a good practice to install fuses of any type on the load side ofcurrent limiting fuses. Although they may appear to coordinate perthe fuse curves there may be miscoordination at the higher fault
currents. If possible, current limiting fuses should be used to minimize cable
damage. However, it is acceptable to use E fuses to obtaincoordination or to accommodate loading.
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SubsurfaceFusedSwitch
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SubsurfaceFusedSwitchshowingFuseWells
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DistributionProtection: BasicConcepts
Minimum Time Coordination Interval
Minimum to TripCurrent and Potential transformer ratio
Time/Current Inverse Curves
Zones of ProtectionMost Commonly used Relays
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Coordination
Time
Between
Protective
DevicesA coordinating time should be allowed betweencharacteristic curves of protective devices installed in
series. This is to allow a margin for any of the following: Breaker time.
Relay over-travel.
Current transformer errors.
Variation from published curves of devices due tomanufacturing Tolerances or actual relayperformance.
Errors of short-circuit current due to small variationsin voltage.
Errors due to tolerance in system data.
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Timeittakestoclearafault
1 second = 60 Cycles
1 cycle = 1/60 = 0.0167 secCircuit Breakers take approx. 5 cycles (0.083sec) from relay sensing to circuit interruption.
Power Fuses typically require no more than 1cycle (.016 sec) for circuit interruption.
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Mi i ti di ti i t l
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Minimumtimecoordinationinterval
BT
I
A
Typical CB Opening Time = 5 Cycles (0.083 sec)
+
Induction Disc Overtravel of EM relays = 6 Cycles(0.1 sec)
Minimum Safety margin = 12 Cycles(0.2 sec)
Mi i T T i
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The Minimum To Trip (MTT) is the minimum amount ofcurrent it takes a relay to trip.
In the illustration below, the fuses on the threephases should trip before the Line Recloser (LR), andCircuit Breaker so the LRs and CBs relay MMT is setat a higher current.
MinimumToTrip
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TimeCurrentCurves(TCC)
Current (Multiples of pick-up)
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ZonesofProtection
Are based on protective device timing.
The closer the fault occurs to a C ircuitBreaker or Line Recloser, the more likelythe protection will trip instantaneously.
Zones of Protection
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CB LR
Phase A
Phase B
Phase C
X
XX
3,000Customers
Lost
1,500Customers
Lost
150Customers
Lost
Zones of ProtectionFuses
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CommonlyUsedRelays
Device 25 Synchronizing or Synch Check Relay
Device 27 Undervoltage Relay
Device 32 Directional or Reverse Power Relay Device 50 AC Phase Instantaneous Overcurrent Relay
Device 50G AC Ground Instantaneous Overcurrent Relay
Device 51 AC Phase Time Overcurrent Relay
Device 51G AC Ground Time Overcurrent Relay
Device 59 Overvoltage Relay
Device 67 AC Directional Overcurrent Relay
Device 79 AC Reclosing Relay Device 81U Underfrequency Relay
Device 81O Overfrequency Relay
Device 87 Differential Relay
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Current Transformers
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Current transformers are used to step down primary systemcurrents to values usable by relays, meters, SCADA, etc.
CT ratios are expressed as primary to secondary; 2000:5, 1200:5,800:5, 600:5, 400:5, 200:5, etc.
A 1200:5 CT has a CT Ratio of 240.
CT Saturation: Select the CT ratio and burden so the CT will notbe saturated because of high currents and/or high secondaryvoltage.
Current Transformers
S d d IEEE CT R l A
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IEEE relay class is defined in terms of the voltage a CT can
deliver at 20 times the nominal current rating withoutexceeding a 10% composite ratio error.
For example, a relay class of C100 on a 1200:5 CT meansthat the CT can develop 100 volts at 24,000 primary amps
(1200*20) without exceeding a 10% ratio error. Maximumburden = 1 ohm.
100 V = 20 * 5 * (1ohm)
200 V = 20 * 5 * (2 ohms)400 V = 20 * 5 * (4 ohms)
800 V = 20 * 5 * (8 ohms)
Standard IEEE CT Relay Accuracy
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CT SaturationCalculationExample
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p
I Asymmetrical 3: 8630 Amps CT Class: C20
I Line-Ground: 4490 Amps CT Ratio: 600:5 = 120
CT Burden: 0.285 Ohms
Wiring Burden: 0.024 Ohms
Relay Burden: 0.0108 Ohms
Total Burden=Relay Burden + Wiring Burden + CT Burden
Total Burden: 0.285 + 0.024 + 0.0108 = 0.3198 Ohms
Maximum Symmetrical Secondary Current = Maximum Fault Duty/CT Ratio
Maximum Symmetrical Secondary Current: 8630/ 120 = 71.92 Amps
Maximum Symmetrical Secondary Voltage Produced = Maximum Sym. Secondary Current x Total Burden
Maximum Symmetrical Secondary Voltage Produced:71.92 x 0.3198
= 23.00 Volts
CT Will Saturate
CT SaturationCalculationExample
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p
AdditionalInformationCT Burden value can usually be found on CT excitation curve or on manufacturer's cut sheet.
Wiring Burden can be calculated if size and length of wire is known.
Example: 20 ft of #10AWG wire in CT circuit (1.21 ohm/1000 ft per NEC) = 0.024 ohm
Relay burden value can usually be found on manufacturer's cut sheet or relay manual.
If Relay burden value is given in VA's convert to Ohms.
Example: SEL-351 relay burden per SEL Manual (0.27VA@5A) = 0.27/5^2 = 0.0108 ohms.
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Hospital Case Study
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HospitalCaseStudy
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ShortCircuitStudy
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y
Provides fault duty information required forproperly setting overcurrent devices.
Ensures adequately sized protectionprevents exceeding the AmpereInterrupting Capacity or (AIC) rating ofelectrical equipment which refers to themaximum level at which the equipmentcan safely interrupt or withstand fault orshort circuit" currents in order to prevent
catastrophic damage to the equipment.
Performed when utilitys available faultduty is increased.
Performed when substantial systemmodifications are planned i.e. new feederor generation is installed.
Assists in conceptual design.
Transmission115kV
Bus12.47kV
BUS712.47kV
BUS612.47kV
BUS012.47kV
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Phase
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PhaseProtectiveDevice
Coordination
1200:5
51
50
Ground
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GroundProtectiveDevice
Coordination
1200:5
51N
50N
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MiscoordinationCaseStudy Electric service interruption to 4,881 customers
due to miscoordination between protectivedevices.
Had devices been selectively coordinated,outage could have been limited to only 420
customers. Minimum time coordination interval between
fuses may have extended outage to 1,005customers.
Relatively low fault duty magnitude of 1,596 AmpsLine-to-Ground contributed to miscoordination.
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Automatic Line Recloser
Operated interrupting service to4,881 customers.
50T Fuse failed to blow andinterrupt fault. 420 customerspast this point in circuit.
Location of section ofunderground cable that failed.Line to ground fault withmagnitude of 1,596 Amps.
65T Fuse failed to blow andinterrupt fault. 1.005 customerspast this point in circuit.
TCC Showing Miscoordination
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TCC Showing Miscoordination
between Fuses and LineRecloser.
Given the fault dutymagnitude of 1,596 Amps
Line to Ground, the LineRecloser was operatingbefore allowing the 50T or 65Tfuses to blow.
Notice the overlap between
the two fuses. Removing theLine Recloser from theequation, the possibility existsthat both fuses could haveblown during the fault.
Conclusions
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The use of TCC Curves as agraphical technique to illustrateproactive device coordination
makes it easy to demonstratewhether or not coordination hasbeen obtained by the devicesettings and whether theyadequately protect thedistribution equipment.
Once you become accustomedto reading these curves, thesystem evaluation can be donerelatively quickly.
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Questions?Thanks for your time!