2. To dispel the myth that nothing exciting ever
happens in power plants
Many power plant events are not short circuits and
are not easy to analyze (example—loss of field)
Many events involved human error by less
experienced operators
With the high volume of IPP plants coming on-line,
commissioning errors are occurring
“Lessons Learned” are important to avoid repeating
errors in the future
Why I wrote this paper:
“Horror Stories”
3. Multi-Phase Generator Faults
Stator Ground Current
Accidental Off-Line Generator Energizing
Overexcitation
Loss-of-Field
- ComTrade oscillo analysis
Generator Breaker Failure
- Breaker Interrupter Flashover
Power Plant Events That The Paper Addresses:
“Horror Stories”
5. Generator trips: generator breaker is tripped,
field is shut down and turbine tripped.
Current from system stops, current from
generator continues until stored energy is
dissipated.
Multi-Phase Generator Faults
“Horror Stories”
9. If fault is in GSU (F1) or UAT (F2), long
clearing times will result in extensive damage
A significant number of these transformers
have failed catastrophically with tank rupture
and oil fire
Low voltage generator breaker significantly
reduces damage
Sudden pressure relay a good idea because
faults are detected as incipient fault before
high current occurs
What If Fault is in GSU or UAT?
“Horror Stories”
10. There is no effective way to quickly “turn off”
generator fault current.
Long fault decay results in the vast majority of
damage (85%) occurring after tripping.
Make every effort in design to make sure the
only credible fault in the generator is a ground
fault.
Consider the value of including a generator
low voltage breaker in your next generator
addition to reduce exposure of GSU and
UAT to long clearing faults.
“Lessons Learned”
“Horror Stories”
12. Generator Tripping: Generator breaker is
tripped, field is shut down and turbine is tripped.
Current from system stops when generator
breaker trips, current from generator continues
BUT magnitude is reduced to generator
grounding.
Stator Ground Faults
“Horror Stories”
14. 59N pickup generally set down to 5-6V pickup
to “see” faults near the neutral.
59N tripping must be delayed to coordinate
with:
- System ground faults
- Wye-wye generator VT’s fuses
Stator Ground Setting Coordination
“Horror Stories”
17. Because grounding transformer impedance is
very high compared to generation—full line-to-
neutral voltage appears across transformer.
Not uncommon for fault to self-extinguish and
re-start because of low ground current.
Oscillograph of Field Ground Fault
“Horror Stories”
18. Oscillographic records played a key role in
preventing damaged generators from being
returned to service by confirming relaying
operated properly.
Oscillographic records can also speed a
generator’s return to service if relaying operated
improperly.
“Lessons Learned”
“Horror Stories”
20. Event occurred during commissioning of a gas
turbine installation.
Commissioning crew was trying to simulate a
52A contact closer, but jumped the wiring studs
on a terminal block, closing the breaker.
Accidental Off-Line Generator Energizing
“Horror Stories”
25. I= Inadvertent Energizing
Current
X2G=Generator Negative
Sequence Reactance
XT =GSU Transformer
Reactance
XS=System Reactance
EG=Generator Terminal
Voltage
ET=GSU High Side Voltage
Inadvertent Energizing Equivalent Circuit
“Horror Stories”
26. Despite the design of modern interlock
schemes to prevent it—inadvertent energizing
events continue to happen.
Dedicated inadvertent energizing protection is
needed even on new gas turbine plants with
generator low voltage breakers.
Inadvertent energizing schemes need to be in-
service when the generator is out of service.
“Lessons Learned”
“Horror Stories”
28. V/Hz relaying used to protect generator and
power plant transformers from excessive
magnetic flux.
Excess flux level overheats generator and
transformer core steel.
Core flux proportional to voltage and
inversely proportional to frequency—thus
V/Hz protection.
Most V/Hz events occur when generator is
off-line prior to synchronizing.
Overexcitation
“Horror Stories”
29. GENERATOR
TRANSFORMER ≈
EXCITATION
Voltage V
Freq. Hz
GENERATOR LIMITS (ANSI C 50.13)
Full Load V/Hz = 1.05 pu
No Load V/Hz = 1.05 pu
TRANSFORMER LIMITS (ANSI C57.12)
Full Load V/Hz = 1.05 pu (HV Terminals of GSU)
No Load V/Hz = 1.10 pu (HV Terminals of GSU)
Overexcitation/Volts per Hertz (24)
“Horror Stories”
30. PHYSICAL INSIGHTS
As voltage rises above rating leakage flux increases
Leakage flux induces current in transformer support
structure causing rapid localized heating
Overexcitation/Volts per Hertz (24)
“Horror Stories”
32. CAUSES OF V/HZ PROBLEMS
Generator voltage regulator problems
- operating error during off-line manual regulator
operation
- control failure
- loss of VT regulator supply voltage
System problems
- unit load rejection: full load, partial rejection
- power system islanding during major
disturbances
Overexcitation/Volts per Hertz (24)
“Horror Stories”
37. Open Prior to Synchronizing
AVR V/Hz
Open VT
R
Full Voltage
Voltmeter
Event #2
“Horror Stories”
38. Most V/Hz events occur when the generator is
off-line.
VT open circuit to the AVR is a frequent
condition that causes V/HZ events.
V/HZ protection should be in a different VT
circuit than the AVR to prevent a single open
VT from causing a V/Hz condition and at the
same time disabling protection.
“Lessons Learned”
“Horror Stories”
40. Detriments
- Generator
Synchronous generator becomes induction
Slip induced eddy currents heat rotor surface
High reactive current drawn by generator
overloads stator
Loss-of-Field (40)
“Horror Stories”
41. Detriments
- Power system
Loss of reactive support
Creates a reactive drain
Can trigger system/area voltage collapse
Loss-of-Field (40)
“Horror Stories”
44. Causes
- Field open circuit
- Field short circuit
- Accidental tripping of field breaker
- Regulator control failure
- Loss of main exciter
Loss-of-Field (40)
“Horror Stories”
45. Desired sequence
of events:
- Turbine was tripped
due to oil valve
solenoid failure
- Sequential tripping
scheme failed to shut
down unit
- Operator wanted to
trip generator breaker
A and then field
breaker
Utility Tie
BA
GEN
42 MVA
BUS
TIE
Incoming
Breaker
Gen.
Breaker
T
Turbine
Field Breaker
Tie Bus
Loss-of-Field (40) - What the operator
wanted to do
“Horror Stories”
46. Utility Tie
BA
GEN
42 MVA
BUS
TIE
Incoming
Breaker
Gen.
Breaker
T
Turbine
Field Breaker
Tie Bus
Actual sequence of
events:
Then tripped Field
Breaker
Operator mistakenly
opened in-coming
Breaker B
B
RESULT—Generator
connected to system
with no field
Loss-of-Field (40) - What the operator
actually did
“Horror Stories”
49. Operating errors are a significant cause of loss-of-
field events.
Relay oscillographs provide unbiased evidence of
what actually happened.
Use of programmable inputs to monitor turbine
values and generator breaker(s) positions help
document sequence of events.
ComTrade provides a handy tool to verify loss-of-
field relay operation.
“Lessons Learned”
“Horror Stories”
51. Note: Tripping of generator
breaker does not arrest the
event. Need to activate
breaker failure.
Vsystem
Vgen
Vgen
Vgen
Vgen
Vgen
Voltage AcrossVoltage Across
Open Breaker PriorOpen Breaker Prior
to Synchronizingto Synchronizing
Open Generator Breaker Flashover on
Mexican Utility System
“Horror Stories”
53. Key item – current detector (CD) must be set below flashover
current. In Mexico, current detector was set above
flashover current.
Result – Complete Failure of RotorResult – Complete Failure of Rotor
Generator Breaker Failure Logic
One-Line Diagram
Basic Generator Breaker Failure Logic
“Horror Stories”
54. Current Detector (CD) must still be set below
flashover current.
Use of 50N relay speeds up B.F.I.– Mexican utility
made this modification.
One-Line Diagram
Generator Breaker Failure Logic
Modified Breaker Failure For Flashover
Protection
“Horror Stories”
55. Conclusions
Generator events are not as rare as some people believe
They are very disruptive and costly to a utility or IPP
owner. These events immediately cost the generator
owner $
Oscillographs and sequence-of-event data from digital
relays play a key role in determining what really
happened
This greatly helps to keep damaged generators off-line
and in returning undamaged machines to service
Many power plant events involve human error—may be
the result of downsizing
We learn from our experiences—or we are DOOMED to
repeat them
Conclusions
“Horror Stories”
