Alternator Protection for
Emergency Standby
Engine Generators
Kenneth L. Box P.E.
Regional Sales Manager – Power Electronics
Cummins Power Generation
10
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AMPS (TIMES RATED)
TIM
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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
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GENERATOR
THERMAL DAMAGE
CURVE
100
CABLE THERMAL
DAMAGE CURVE
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AMPS (TIMES RATED)
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GENERATOR
THERMAL DAMAGE
CURVE
100
CABLE THERMAL
DAMAGE CURVE
10
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1 103
AMPS (TIMES RATED)
TIM
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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
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0 5 10 15 20
time, sec
%C
urr
en
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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
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0 1 2 3 4 5 6 7 8
time, sec
Perc
en
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urr
en
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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
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120
0 1 2 3 4 5 6 7 8
time, sec
Perc
en
t o
f N
om
inal V
olt
ag
e