Power Plant Horror Stories

Chuck MozinaConsultant

Beckwith Electric Co. Inc.

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:




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.



Time (sec)5 8

GeneratorBreaker Trips

0

Current I Gen. Current Delay

Isystem

Generator Terminal Fault Current


High Side ofGeneratorBreakerCurrents

GeneratorNeutral

TerminalCurrents

Fault Inception

High Side Breaker Opens

Multi-Phase Generator Fault Oscillograph


G X

GSU

F1

XF2

UATR

What If Fault is in GSU or UAT?


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?


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”


Stator Ground Faults


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



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


XF1

CTRANS.

XVT

Fuse

R 59N VN

CTRANS.

F2


Generator NeutralVoltage

GeneratorPhase

Currents

Fault Inception

Breaker Opens

Trip Command

Oscillograph of Field Ground Fault


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


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.





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.





Holy Cow!I jumperedthe wrongcontact!!!

52G

Generator Phase Voltage

GeneratorPhase

Currents

Fault Inception

Breaker Opens

Inadvertent Energizing Oscillograph


Inadvertent Energizing Protection Logic


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


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.



OverexcitationOverexcitation


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


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)


PHYSICAL INSIGHTS

As voltage rises above rating leakage flux increases

Leakage flux induces current in transformer support structure causing rapid localized heating



TYPICAL CURVES



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



V/Hz Event #1


TRIPGEN. BREAKER OPENALARMALARM

OverviewVA VB VC IA IB IC


TRIPGEN. BREAKER OPENALARM

ALARM

VA & IA


TRIPGEN. BREAKER OPEN

ALARMALARM

IA


Open Prior to Synchronizing

AVR V/Hz

Open VT

R

Full Voltage

Voltmeter

Event #2


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.



Loss-of-Field (40)


Detriments- Generator

Synchronous generator becomes inductionSlip induced eddy currents heat rotor surfaceHigh reactive current drawn by generator overloads stator

Loss-of-Field (40)


Detriments- Power system

Loss of reactive supportCreates a reactive drainCan trigger system/area voltage collapse

Loss-of-Field (40)


Generator Capability Curve


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


Causes- Field open circuit

- Field short circuit

- Accidental tripping of field breaker

- Regulator control failure

- Loss of main exciter

Loss-of-Field (40)


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


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


Turbine Valves ClosedField Breaker Trip

Relay Trip

Loss-of-Field (40) Oscillograph


R2.0Ω

27.7Ω

26.1(T=0)

X

trip point

Impedance LocusFrom ComTrade Simulation

∞

∞

R-X Diagram Analysis Using ComTrade


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.



Generator Breaker Failure(Open Breaker Flashover)


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


Head Flashover Equivalent Circuit


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


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


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


The End


Chuck MozinaConsultant

Beckwith Electric Co. Inc.


©2008 Beckwith Electric Co., Inc.

Engineering

Power Plant Horror Stories