Alternator Protection for

Emergency Standby

Engine Generators

Kenneth L. Box P.E.

Regional Sales Manager – Power Electronics

Cummins Power Generation

10

1

0.1

0.05

1 103

AMPS (TIMES RATED)

TIM

E (

SE

CO

ND

S)

1. SOLID RED LINE IS ALTERNATOR THERMAL DAMAGE CURVE

2. DASHED BLUE LINE IS LINE TO NEUTRAL FAULT.

3. DASHED GREEN LINE IS LINE TO LINE FAULT.

4. DASHED RED LINE IS 3-PHASE LINE TO LINE FAULT.

Engine Generators

Control

Monitoring & Alarms

Engine Protection

System Protection –

Paralleling

Applications

Alternator Protection

IEEE/ANSI Standards 141 & 242

Recommended

Practice for Protection

& Coordination of

Industrial &

Commercial Power

Systems

Recommended

Practice for Electric

Power Distribution for

Industrial Plants

Under & Over Voltage Protection

Protects against a severe overload condition (27)

Initiates the starting of an emergency standby genset (27)

Load shed shut down in the event of AVR failure (27)

Protect against dangerous over-voltages (59)

Backup to internal V/Hz limiters

Commonly combined 27/59

Devices

27

59

Reverse Power Protection

Provides backup protection for the prime mover.

It detects reverse power flow (kW) should the prime mover lose it’s input energy without tripping its generator feeder breaker

Prevents motoring, drawing real power from the system

Device

32

Loss of Field Protection

Senses when the generator’s excitation system has been lost.

Important for paralleling generator applications or when paralleling with the utility.

When generator loses excitation it will steal excitation from other gensets & quickly overheat the rotor due to induced slip-frequency currents

Reverse VAR protection

Device

40

Phase Balance Current Protection

Unbalanced loads

Unbalanced system

faults

Open conductors

Unbalanced I2

currents induce 2X

system frequency

currents in the rotor

causing overheating

Device

46

Backup Overcurrent Protection

The function of

generator backup

protection is to

disconnect the

generator if a system

has not been cleared

by the primary

protective device

Time delays

Device

51V

Ground Overcurrent Protection

Provides backup

protection for all ground

relays in the system at

the generator voltage

level

Provides protection

against internal generator

ground faults

Commonly provided as

GF alarm.

Device

51G

Voltage Balance Relay

Monitors the

availability of PT

voltage.

Blocks improper

operation of

protective relays and

control devices in the

event of a blown PT

fuse

Device

60

Differential Protection

For rapid detection of

generator Φ to Φ or

Φ-G faults.

When NGR’s are

used, 87G should be

used.

Used for protection of

larger generators

Zone protection

Device

87

Temperature Protection

Resistance

temperature detectors

are used to sense

winding temperatures.

A long term

monitoring philosophy

that is not readily

detected by other

protective devices

RTD’s

IEEE Recommended Protection

Schemes

SMALL MACHINES

– Up to 1000kVA, 600V

maximum

MEDIUM MACHINES

– 1000kW to 12,500

kVA regardless of

voltage

LARGE MACHINES

– Up to 50,000 kVA

regardless of voltage

Any recommendation

based entirely on

machine size is not

entirely adequate.

The importance of the

machine to the

system or process it

serves & the reliability

required are the

important factors

Small Generators – 1000kVA

Device 51V – Backup overcurrent

Device 51G - GFP

Device 32 – Reverse Power

Device 40 – Loss of Field

Device 87 - Differential

Medium Size Generators – 1 to

12.5 mVA

Device 51V – Backup overcurrent

Device 51G - GFP

Device 32 – Reverse Power

Device 40 – Loss of Field

Device 87 - Differential

Device 46 – Negative phase sequence for paralleling or utility paralleling

My 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 25 86

SS UL listed utility grade generator protection relay

SWITCHGEAR TRIP

CLOSE

TRIP

NFPA70 - NEC

445.12(A) Overload Protection

– Generators, except AC generator exciters, shall be protected from overloads by inherent design, circuit breakers, fuses, or other acceptable overcurrent protective means suitable for the conditions of use.

240.15(A)

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

240.21(G) Conductors from Generator Terminals

– Conductors from generator terminals that meet the size requirements of 445.13 shall be permitted to be protected against overload by the generator overload devices) 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 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

DAMAGE CURVE

PROTECTIVE RELAY

CURVE

100

100

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

assumed protected by same device protecting the genset.

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

breakers don’t fully protect alternator

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

100% Rated Electronic Trip

Breaker is an Improvement

800A

MOLDED

CASE CB

GENSET

DAMAGE

CURVE

Gen Relay

800A

INSULATED

CASE CBG

EN

FLA

Current

Tim

e

Some Generator Mfrs offer self

contained alternator protection

Is it UL listed as a

generator protection

relay?

Does it provide O/L

protection for the

alternator and O/L and

short circuit protection for

the feeder?

Can it protect its transfer

switch on the emergency

side?

Differential Protection (87)

Rarely selected for LV

machines smaller

than 1.5 mW.

How do you mount

the CT’s?

Cost vs. benefit?

Differential Protection (87)

The value of differential protection is that it is very fast in detecting faults in a circuit.

High current levels that pass through both sets of CT’s will not cause a trip on common events like motor starting, or even on downstream faults that are intended to be cleared by other means.

The high speed of operation for faults sensed within the operating zone makes it possible limit damage inside an alternator stator when a fault inside the machine occurs.

The device would also operate on a feeder fault, but in general, once a fault is sensed in a feeder, the feeder will be replaced,

Differential Protection (87)

A key point to remember is that differential relays don’t prevent damage, they LIMIT damage.

If a relay is properly operating it won’t trip until there is actually a line to ground fault somewhere in its zone of protection.

By limiting the duration of a fault, it is often possible to limit damage, but there is STILL damage. Eventually, you will have to deal with it.

Some mfrs. have high speed internal single phase protection

Differential Protection (87)

The protective devices selected for a specific application should always be selected based on an understanding of the balance between reliability and protection.

The more protection used in the system the lower the reliability, because of the higher probability of failing the system due to a nuisance trip.

52

51

86

TRIP

ENG

GENSET

SWITCHGEAR

87

GENSET

CONTROL ,

SH

UT

DO

WN

Recommendations

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

Always carefully consider the balance of protection versus reliability, especially when the protection is for equipment that is operating for very few hours.

With some mfrs. the alternator current sensing function monitors faults inside the machine. When the machine incorporates protection for the alternator from overcurrent conditions based on an I2t function, and regulates single phase faults differential protection is optional.

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 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 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 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, preferably in the terminal cabinet. The differential relay can be mounted in the vicinity of the generator set or in the switchgear.

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

Custom designs breed custom problems

Recommendations

Questions?

3Φ Fault – Current Regulation

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

0

100

200

300

400

500

0 5 10 15 20

time, sec

%C

urr

en

t

Alt %Standby Max LineCurrent

Regulates at 3X Rated Shuts down before damage

Peak Current: IR/X”d

1Φ Fault – Current Regulation

Single Phase L1-N Short and Recovery: Current vs. Time

150kW Quiet Site Genset w/Dominion Control

0

100

200

300

400

500

600

0 1 2 3 4 5 6 7 8

time, sec

Perc

en

t C

urr

en

t

Alt %Standby L1 Current

Alt %Standby L2 Current

Alt %Standby L3 Current

NOTE: THIS CURVE SHOWS FAULT CLEARED BEFORE SHUTDOWN.

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

150kW Quiet Site Genset w/Dominion Control

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8

time, sec

Perc

en

t o

f N

om

inal V

olt

ag

e