enr458 presentation spring2013

8/13/2019 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


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


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!

enr458 presentation spring2013

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