Transcript Alternator Protection for Emergency StandbyEngine Generators

Alternator Protection for
Emergency Standby
Engine Generators
Kenneth L. Box P.E.
Regional Sales
Manager – Power
Electronics
Cummins Power
Generation
TIME (SECONDS)
10
1
0.1
0.05
1
3
10
AMPS (TIMES RATED)
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.
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
SW
KW
VM
VM
KWH
SW
SW
PF
HZ
HZ
40
27
46
32
81
25C
GOV
51V
59
86
25
UL listed utility grade generator protection relay
SWITCHGEAR
AVR
SURGE SUPPRESSORS
AM
GENSET
SS
TRIP
CLOSE
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
Is the Alternator Protected?
Generator is required to be
protected
CABLETHERMAL
THERMAL
CABLE
DAMAGECURVE
CURVE
DAMAGE
100
GENERATOR
GENERATOR
THERMAL
THERMAL
DAMAGE
DAMAGE
CURVE
CURVE
10
10
10
TIME (SECONDS)
TIME
TIME(SECONDS)
(SECONDS)
PROTECTIVE RELAY
CURVE
1
1
1
0.1
0.1
0.1
GENERATOR
THERMAL DAMAGE
CURVE
0.05
0.05
11
333
10
10
10
AMPS
AMPS(TIMES
(TIMESRATED)
RATED)
AMPS
(TIMES
RATED)
100
100
100
– 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
GEN FLA
Time
800A
INSULATED
CASE CB
800A
MOLDED
CASE CB
GENSET
DAMAGE
CURVE
Gen Relay
Current
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)
GENSET
GENSET
CONTROL
ENG
,
SHUTDOWN
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.
87
51
86
TRIP
SWITCHGEAR
52
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.
Recommendations
A good standardized design is
superior to an optimized custom
design.
Custom designs breed custom
problems
Questions?
3Φ Fault – Current Regulation
3 Phase L1-L2-L3 Short: AmpSentry Regulation and Shutdown
%Current
500
Peak Current: IR/X”d
400
Alt %Standby Max Line
Current
300
Regulates at 3X Rated
Shuts down before damage
200
100
0
0
5
10
time, sec
15
20
1Φ Fault – Current Regulation
Single Phase L1-N Short and Recovery: Current vs. Time
150kW Quiet Site Genset w/Dominion Control
600
Alt %Standby L1 Current
500
Percent Current
Alt %Standby L2 Current
400
Alt %Standby L3 Current
300
200
100
0
0
1
2
3
4
5
6
7
8
tim e, sec
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
120
Percent of Nominal Voltage
100
80
60
40
20
0
0
1
2
3
4
tim e, sec
5
6
7
8