Alternator Protection for Alternator Protection for Emergency Standby Emergency Standby Engine GeneratorsEngine Generators

Kenneth L. Box P.E.Kenneth L. Box P.E.Regional Sales Regional Sales Manager – Power Manager – Power ElectronicsElectronicsCummins Power Cummins Power GenerationGeneration

Engine GeneratorsEngine Generators

ControlControl

Monitoring & AlarmsMonitoring & Alarms

Engine ProtectionEngine Protection

System Protection – System Protection – Paralleling Paralleling ApplicationsApplications

Alternator ProtectionAlternator Protection

IEEE/ANSI Standards 141 & 242IEEE/ANSI Standards 141 & 242

Recommended Recommended Practice for Protection Practice for Protection & Coordination of & Coordination of Industrial & Industrial & Commercial Power Commercial Power SystemsSystems

Recommended Recommended Practice for Electric Practice for Electric Power Distribution for Power Distribution for Industrial PlantsIndustrial Plants

Under & Over Voltage ProtectionUnder & Over Voltage Protection

Protects against a severe Protects against a severe overload condition (27)overload condition (27)Initiates the starting of an Initiates the starting of an emergency standby genset emergency standby genset (27)(27)Load shed shut down in the Load shed shut down in the event of AVR failure (27)event of AVR failure (27)Protect against dangerous Protect against dangerous over-voltages (59)over-voltages (59)Backup to internal V/Hz Backup to internal V/Hz limiterslimitersCommonly combined 27/59Commonly combined 27/59

DevicesDevices27275959

Reverse Power ProtectionReverse Power Protection

Provides backup Provides backup protection for the prime protection for the prime mover.mover.It detects reverse power It detects reverse power flow (kW) should the flow (kW) should the prime mover lose it’s prime mover lose it’s input energy without input energy without tripping its generator tripping its generator feeder breakerfeeder breakerPrevents Prevents motoring,motoring, drawing real power from drawing real power from the systemthe system

DeviceDevice3232

Loss of Field ProtectionLoss of Field Protection

Senses when the generator’s Senses when the generator’s excitation system has been excitation system has been lost.lost.Important for paralleling Important for paralleling generator applications or when generator applications or when paralleling with the utility.paralleling with the utility.When generator loses When generator loses excitation it will excitation it will stealsteal excitation excitation from other gensets & quickly from other gensets & quickly overheat the rotor due to overheat the rotor due to induced slip-frequency induced slip-frequency currentscurrentsReverse VAR protectionReverse VAR protection

DeviceDevice4040

Phase Balance Current ProtectionPhase Balance Current Protection

Unbalanced loadsUnbalanced loads

Unbalanced system Unbalanced system faultsfaults

Open conductorsOpen conductors

Unbalanced IUnbalanced I22

currents currents induce 2X induce 2X

system frequency system frequency currents in the rotor currents in the rotor causing overheatingcausing overheating

DeviceDevice

4646

Backup Overcurrent ProtectionBackup Overcurrent Protection

The function of The function of generator backup generator backup protection is to protection is to disconnect the disconnect the generator if a system generator if a system has not been cleared has not been cleared by the primary by the primary protective deviceprotective device

Time delaysTime delays

DeviceDevice

51V51V

Ground Overcurrent ProtectionGround Overcurrent Protection

Provides backup Provides backup protection for all ground protection for all ground relays in the system at relays in the system at the generator voltage the generator voltage levellevel

Provides protection Provides protection against internal generator against internal generator ground faultsground faults

Commonly provided as Commonly provided as GF alarm.GF alarm.

DeviceDevice

51G51G

Voltage Balance RelayVoltage Balance Relay

Monitors the Monitors the availability of PT availability of PT voltage.voltage.

Blocks improper Blocks improper operation of operation of protective relays and protective relays and control devices in the control devices in the event of a blown PT event of a blown PT fuse fuse

DeviceDevice

6060

Differential ProtectionDifferential Protection

For rapid detection of For rapid detection of generator generator ΦΦ to to ΦΦ or or ΦΦ-G faults.-G faults.

When NGR’s are When NGR’s are used, 87G should be used, 87G should be used.used.

Used for protection of Used for protection of larger generatorslarger generators

Zone protectionZone protection

DeviceDevice

8787

Temperature ProtectionTemperature Protection

Resistance Resistance temperature detectors temperature detectors are used to sense are used to sense winding temperatures.winding temperatures.

A long term A long term monitoring philosophy monitoring philosophy that is not readily that is not readily detected by other detected by other protective devicesprotective devices

RTD’sRTD’s

IEEE Recommended Protection IEEE Recommended Protection SchemesSchemes

SMALL MACHINESSMALL MACHINES– Up to 1000kVA, 600V Up to 1000kVA, 600V

maximummaximum

MEDIUM MACHINESMEDIUM MACHINES– 1000kW to 12,500 1000kW to 12,500

kVA regardless of kVA regardless of voltagevoltage

LARGE MACHINESLARGE MACHINES– Up to 50,000 kVA Up to 50,000 kVA

regardless of voltageregardless of voltage

Any recommendation Any recommendation based entirely on based entirely on machine size is not machine size is not entirely adequate.entirely adequate.

The importance of the The importance of the machine to the machine to the system or process it system or process it serves & the reliability serves & the reliability required are the required are the important factorsimportant factors

Small Generators – 1000kVASmall Generators – 1000kVA

Device 51V – Backup overcurrentDevice 51V – Backup overcurrent

Device 51G - GFPDevice 51G - GFP

Device 32 – Reverse PowerDevice 32 – Reverse Power

Device 40 – Loss of FieldDevice 40 – Loss of Field

Device 87 - DifferentialDevice 87 - Differential

Medium Size Generators – 1 to Medium Size Generators – 1 to 12.5 mVA12.5 mVA

Device 51V – Backup overcurrentDevice 51V – Backup overcurrent

Device 51G - GFPDevice 51G - GFP

Device 32 – Reverse PowerDevice 32 – Reverse Power

Device 40 – Loss of FieldDevice 40 – Loss of Field

Device 87 - DifferentialDevice 87 - Differential

Device 46 – Negative phase sequence Device 46 – Negative phase sequence for paralleling or utility parallelingfor paralleling or utility paralleling

My Opinion – 3mW and lessMy Opinion – 3mW and less GENSET

AM SW

VM SW

KW KWH PF 40 32 GOV AVR 51V

HZ 27 81 59

SU

RG

E S

UP

PR

ES

SO

RS

VM SW HZ 46 25C 2586

SSUL listed utility grade generator protection relay

SWITCHGEAR TRIP

CLOSE

TRIP

NFPA70 - NECNFPA70 - NEC

445.12(A)445.12(A) Overload ProtectionOverload Protection– Generators, except AC generator exciters, shall be protected from Generators, except AC generator exciters, shall be protected from

overloads by overloads by inherent designinherent design, , circuit breakerscircuit breakers, , fusesfuses, or , or other other acceptable overcurrent protective meansacceptable overcurrent protective means suitable for the conditions of suitable for the conditions of use.use.

240.15(A)240.15(A)– Overcurrent Device RequiredOvercurrent Device Required. A fuse or an overcurrent trip unit of a . A fuse or an overcurrent trip unit of a

circuit breaker shall be connected in series with each ungrounded circuit breaker shall be connected in series with each ungrounded conductor. A combination of a current transformer and overcurrent conductor. A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit.relay shall be considered equivalent to an overcurrent trip unit.

240.21(G) Conductors from Generator Terminals240.21(G) Conductors from Generator Terminals– Conductors from generator terminals that meet the size requirements of Conductors from generator terminals that meet the size requirements of

445.13 shall be permitted to be 445.13 shall be permitted to be protected against overload by the protected against overload by the generator overload devices)generator overload devices) required by 445.12 required by 445.12

10

1

0.1

0.05

1 103

AMPS (TIMES RATED)

TIM

E (

SE

CO

ND

S)

GENERATOR THERMAL DAMAGE CURVE

100

CABLE THERMAL DAMAGE CURVE

10

1

0.1

0.05

1 103

AMPS (TIMES RATED)

TIM

E (

SE

CO

ND

S)

GENERATOR THERMAL DAMAGECURVE

100

CABLE THERMAL DAMAGE CURVE

10

1

0.1

0.05

1 103

AMPS (TIMES RATED)

TIM

E (

SE

CO

ND

S)

GENERATOR THERMALDAMAGE CURVE

PROTECTIVE RELAY CURVE

100

100

Is the Alternator Protected?Is the Alternator Protected?Generator is required to be Generator is required to be protectedprotected– Generator conductors are Generator conductors are

assumed protected by same assumed protected by same device protecting the genset.device protecting the genset.

Most common protection is Most common protection is molded case breaker with molded case breaker with thermal/magnetic tripthermal/magnetic trip– 100% rated thermal magnetic 100% rated thermal magnetic

breakers don’t fully protect breakers don’t fully protect alternatoralternator

Generator Protective Relay Generator Protective Relay provides the best protection provides the best protection & superior coordination for & superior coordination for downstream devicesdownstream devices

100% Rated Electronic Trip 100% Rated Electronic Trip Breaker is an ImprovementBreaker is an Improvement

CurrentCurrent

Time

Time

Some Generator Mfrs offer self Some Generator Mfrs offer self contained alternator protectioncontained alternator protection

Is it UL listed as a Is it UL listed as a generator protection generator protection relay?relay?

Does it provide O/L Does it provide O/L protection for the protection for the alternator and O/L and alternator and O/L and short circuit protection for short circuit protection for the feeder?the feeder?

Can it protect its transfer Can it protect its transfer switch on the emergency switch on the emergency side?side?

Differential Protection (87)Differential Protection (87)

Rarely selected for LV Rarely selected for LV machines smaller machines smaller than 1.5 mW.than 1.5 mW.

How do you mount How do you mount the CT’s?the CT’s?

Cost vs. benefit?Cost vs. benefit?

Differential Protection (87)Differential Protection (87)The value of differential protection is The value of differential protection is that it is very fast in detecting faults in that it is very fast in detecting faults in a circuit. a circuit. High current levels that pass through High current levels that pass through both sets of CT’s will not cause a trip both sets of CT’s will not cause a trip on common events like motor starting, on common events like motor starting, or even on downstream faults that are or even on downstream faults that are intended to be cleared by other intended to be cleared by other means. means. The high speed of operation for faults The high speed of operation for faults sensed within the operating zone sensed within the operating zone makes it possible makes it possible limit limit damage inside damage inside an alternator stator when a fault inside an alternator stator when a fault inside the machine occurs. the machine occurs. The device would also operate on a The device would also operate on a feeder fault, but in general, once a feeder fault, but in general, once a fault is sensed in a feeder, the feeder fault is sensed in a feeder, the feeder will be replaced, will be replaced,

Differential Protection (87)Differential Protection (87)

A key point to remember is that differential A key point to remember is that differential relays don’t prevent damage, they LIMIT relays don’t prevent damage, they LIMIT damage. damage. If a relay is properly operating it won’t trip until If a relay is properly operating it won’t trip until there is actually a line to ground fault there is actually a line to ground fault somewhere in its zone of protection. somewhere in its zone of protection. By limiting the duration of a fault, it is often By limiting the duration of a fault, it is often possible to limit damage, possible to limit damage, but there is STILL but there is STILL damage. damage. Eventually, you will have to deal with it.Eventually, you will have to deal with it.Some mfrs. have high speed internal single Some mfrs. have high speed internal single phase protectionphase protection

Differential Protection (87)Differential Protection (87)

The protective devices The protective devices selected for a specific selected for a specific application should always application should always be selected based on an be selected based on an understanding of the understanding of the balance between balance between reliability and protection. reliability and protection. The more protection used The more protection used in the system the lower in the system the lower the reliability, because of the reliability, because of the higher probability of the higher probability of failing the system due to failing the system due to a nuisance trip.a nuisance trip.

52

51

86

TRIP

ENG

GENSET

SWITCHGEAR

87

GENSET CONTROL ,

SH

UT

DO

WN

RecommendationsRecommendationsUse the IEEE Recommended protection schemes with a dose of common sense.Use the IEEE Recommended protection schemes with a dose of common sense.

Always carefully consider the balance of protection versus reliability, especially when Always carefully consider the balance of protection versus reliability, especially when the protection is for equipment that is operating for very few hours. the protection is for equipment that is operating for very few hours. With some mfrs. the alternator current sensing function monitors faults inside the With some mfrs. the alternator current sensing function monitors faults inside the machine. When the machine incorporates protection for the alternator from machine. When the machine incorporates protection for the alternator from overcurrent conditions based on an Iovercurrent conditions based on an I22t function, and regulates single phase faults t function, and regulates single phase faults differential protection is optional.differential protection is optional.On 15kV class machines, the alternator stator is expensive enough that it would On 15kV class machines, the alternator stator is expensive enough that it would probably be repaired rather than replaced, so it will make more sense to try to limit probably be repaired rather than replaced, so it will make more sense to try to limit damage in the machine and have it repaired, in the general case.damage in the machine and have it repaired, in the general case.In cases where it is decided to use differential protection, it is desirable to minimize In cases where it is decided to use differential protection, it is desirable to minimize the zone of protection and use properly sized and matched CT’s so that the the zone of protection and use properly sized and matched CT’s so that the probability of nuisance tripping is reduced. Since the generator set provides probability of nuisance tripping is reduced. Since the generator set provides overcurrent protection from the alternator “out”, differential protection can be applied overcurrent protection from the alternator “out”, differential protection can be applied with matched CT’s provided and mounted at the wye side and alternator output, with matched CT’s provided and mounted at the wye side and alternator output, preferably in the terminal cabinet. The differential relay can be mounted in the vicinity preferably in the terminal cabinet. The differential relay can be mounted in the vicinity of the generator set or in the switchgear.of the generator set or in the switchgear.

A good standardized design is A good standardized design is superior to an optimized custom superior to an optimized custom design.design.

Custom designs breed custom Custom designs breed custom problemsproblems

RecommendationsRecommendations

Questions?Questions?

33ΦΦ Fault – Current Regulation Fault – Current Regulation

3 Phase L1-L2-L3 Short: AmpSentry Regulation and Shutdown

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0 5 10 15 20time, sec

%C

urr

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Alt %Standby Max LineCurrentRegulates at 3X Rated

Shuts down before damage

Peak Current: IR/X”d

11ΦΦ Fault – Current Regulation Fault – Current Regulation

Single Phase L1-N Short and Recovery: Current vs. Time150kW Quiet Site Genset w/Dominion Control

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NOTE: THIS CURVE SHOWS FAULT CLEARED BEFORE SHUTDOWN.

Single Phase FaultSingle Phase FaultSingle Phase L1-N Short and Recovery: Line-Neutral Voltage vs. Time

150kW Quiet Site Genset w/Dominion Control

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