power plant horror stories
TRANSCRIPT
Chuck MozinaConsultant
Beckwith Electric Co. Inc.
Power Plant“Horror Stories”
Power Plant“Horror Stories”
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”
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”
Multi-Phase Generator Faults
“Horror Stories”
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”
Time (sec)5 8
GeneratorBreaker Trips
0
Current I Gen. Current Delay
Isystem
Generator Terminal Fault Current
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High Side ofGeneratorBreakerCurrents
GeneratorNeutral
TerminalCurrents
Fault Inception
High Side Breaker Opens
Multi-Phase Generator Fault Oscillograph
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G X
GSU
F1
XF2
UATR
What If Fault is in GSU or UAT?
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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”
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 andUAT to long clearing faults.
“Lessons Learned”
“Horror Stories”
Stator Ground Faults
“Horror Stories”
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”
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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”
XF1
CTRANS.
XVT
Fuse
R 59N VN
CTRANS.
F2
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Generator NeutralVoltage
GeneratorPhase
Currents
Fault Inception
Breaker Opens
Trip Command
Oscillograph of Field Ground Fault
“Horror Stories”
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”
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”
Accidental Off-Line Generator Energizing
“Horror Stories”
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”
“Horror Stories”
“Horror Stories”
Holy Cow!I jumperedthe wrongcontact!!!
52G
Generator Phase Voltage
GeneratorPhase
Currents
Fault Inception
Breaker Opens
Inadvertent Energizing Oscillograph
“Horror Stories”
Inadvertent Energizing Protection Logic
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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”
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”
OverexcitationOverexcitation
“Horror Stories”
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”
GENERATORTRANSFORMER ≈EXCITATION
Voltage V
Freq. Hz
GENERATOR LIMITS (ANSI C 50.13)Full Load V/Hz = 1.05 puNo 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”
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”
TYPICAL CURVES
Overexcitation/Volts per Hertz (24)
“Horror Stories”
CAUSES OF V/HZ PROBLEMSGenerator 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)
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V/Hz Event #1
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TRIPGEN. BREAKER OPENALARMALARM
OverviewVA VB VC IA IB IC
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TRIPGEN. BREAKER OPENALARM
ALARM
VA & IA
“Horror Stories”
TRIPGEN. BREAKER OPEN
ALARMALARM
IA
“Horror Stories”
Open Prior to Synchronizing
AVR V/Hz
Open VT
R
Full Voltage
Voltmeter
Event #2
“Horror Stories”
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”
Loss-of-Field (40)
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Detriments- Generator
Synchronous generator becomes inductionSlip induced eddy currents heat rotor surfaceHigh reactive current drawn by generator overloads stator
Loss-of-Field (40)
“Horror Stories”
Detriments- Power system
Loss of reactive supportCreates a reactive drainCan trigger system/area voltage collapse
Loss-of-Field (40)
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Generator Capability Curve
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Impedance Trajectoryon Loss-of-Field
+X
-X
+R-R
Heavy Load Light Load
Machine Capability
Minimum Exciter Limit
Xd{
-X d́ 2
1.0 pu
Modern Loss-of-Field Using Two-Zone Off-Set mho Method
“Horror Stories”
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”
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
GEN42 MVA
BUSTIE
IncomingBreaker
Gen.Breaker
TTurbine
Field Breaker
Tie Bus
Loss-of-Field (40) - What the operator wanted to do
“Horror Stories”
Utility Tie
BA
GEN42 MVA
BUSTIE
IncomingBreaker
Gen.Breaker
TTurbine
Field Breaker
Tie Bus
Actual sequence ofevents:
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”
Turbine Valves ClosedField Breaker Trip
Relay Trip
Loss-of-Field (40) Oscillograph
“Horror Stories”
R2.0Ω
27.7Ω
26.1(T=0)
X
trip point
Impedance LocusFrom ComTrade Simulation
∞
∞
R-X Diagram Analysis Using ComTrade
“Horror Stories”
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”
Generator Breaker Failure(Open Breaker Flashover)
“Horror Stories”
Note: Tripping of generator breaker does not arrest the event. Need to activate breaker failure.
Vsystem
Vgen
Vgen
Vgen
Vgen
Vgen
Voltage Across Voltage Across Open Breaker Prior Open Breaker Prior
to Synchronizingto Synchronizing
Open Generator Breaker Flashover on Mexican Utility System
“Horror Stories”
Head Flashover Equivalent Circuit
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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”
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”
Conclusions
Generator events are not as rare as some people believeThey 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 happenedThis greatly helps to keep damaged generators off-line and in returning undamaged machines to serviceMany power plant events involve human error—may be the result of downsizingWe learn from our experiences—or we are DOOMED to repeat them
Conclusions
“Horror Stories”
The End
Power Plant“Horror Stories”
Chuck MozinaConsultant
Beckwith Electric Co. Inc.
“Horror Stories”
©2008 Beckwith Electric Co., Inc.