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ARIPPA
Technical Symposium
August 28, 2007
What You Should Know
About Your Generators
Life Cycle
W. Howard Moudy
National Electric Coil
Generator Life Cycle
• Having a thorough knowledge of your
generator life cycle can help improve
planning and budgeting
• Effective planning and budgeting can
enhance your generators life cycles while
avoiding unexpected outages and
ensuring the best value of money spent.
Understanding Life Cycle
• The “Bathtub Curve” is typically used as a
visual model to illustrate the three key
periods of product failure
Understanding Life Cycle
• To be clear, this is not
the “Bath Tub” curve or
model for discussion.
• However, please
remember that keeping
your generator clean is
an important element to
help ensure a long and
reliable life cycle.
Understanding Life Cycle
• Discussion will
focus on the
characteristics,
issues, and
options
associated
with:
–
–
–
–
Infant Mortality
Normal Life
End of Life
Life Extension
Understanding Life Cycle
• Depiction of an
entire population –
not one item or
unit
• A more accurate
depiction might
focus on a
particular
generator type or
the fleet of one
specific generator
model.
Infant Mortality
• Significant number of
failures in a short time,
and decreasing over
time
• Failures in this period
are usually caused by
one or a combination
of design, material, or
workmanship
deficiencies that were
built into the generator.
Infant Mortality Period
• Manufacturer warnings and suggestions
» OMM’s; TIL’s; AIB’s; TA’s
• Dealing with technical concerns
• Getting to Know your machine
• Establishing an effective long–term
maintenance program
• Data Collection
» Setting Base Lines
» Trending
Infant Mortality Case Study -A
• Stator Failure – Partial
Discharge / Corona
• Condition initially found
during the first major
outage
Infant Mortality Case Study - A Cont.
• Issues
– Slot Packing – conductive felt
– Limited compression & durability for purpose
– Lacks constant compressive force
– Difficult to remove from bottom of slot
– Poor Outer Corona Protection treatment and
interface in slot with the OCP and Core
– High Volt Per Mil Rating (~68 or 69)
– Poor Strand Configuration – size and
configuration (high design losses)
.
Infant Mortality Case Study - A Cont.
• Optimize Design, Configuration and
Application
• Utilize top and semi-conductive side ripple springs
• Apply coil semi conductive treatment to achieve
desired characteristics
• Install coils insuring proper mechanical fit and
electrical characteristics between the Semi-Con and
Core Iron
• Optimize conductor strand size (reduce losses)
• Incorporate a Roebel inside the slot (reduce losses)
• Clip and Cap the connections
• Decrease Volt Per Mil (lower voltage stress)
Infant Mortality Case Study - A Cont.
Ensure secure contact
between the coil OCP and
the core laminations,
despite changes resulting
from temperature
variations
Prevent loosening of the
slot wedges
Restrain the coil and limit
radial movement in the slot
Apply constant compressive
force
Slot Ripple Springs
Infant Mortality Case Study - A Cont.
Infant Mortality Case Study - B Cont.
What is the Pole to Pole Crossover?
The crossover carries the current from Coil #7, Pole A to Coil #7, Pole B.
Thus the terminology “Pole-to-Pole Crossover.”
Pictured below is the original ”rigid” crossover design.
Infant Mortality Case Study - B Cont.
Pictures of the Original “Rigid” Crossover
Infant Mortality Case Study - B Cont.
Pictures of Omega pole to pole crossovers
Infant Mortality Case Study - B Cont.
• Repairs at site
– Old pole connectors
cut away
– New omega
connectors installed
on all units
– Flux probe test verifies
success of
installations
Normal / Useful Life Period
Normal / Useful Life Period cont.
• This period is normally characterized by a
relatively low and constant failure rate
• Often failures in this period can be caused
by or brought on by outside influences such
as, other equipment failures (transformer,
Isophase, switchgear) weather (lightning), or
operations errors. Inadequate maintenance
can be another failure concern.
• Plants should have a regularly scheduled,
effective maintenance program in place to
monitor and trend machine condition.
Normal / Useful Life Period cont.
• Maintenance Program Review
– Visual Inspection
– Testing
– Keep it clean – “Cleanliness is next to godliness!”
• Minimize risk of forced outage due to a
catastrophic failure
•
•
•
•
Effective Maintenance Program Implemented
Operator Training
Operational Monitoring
Data Banking
Outage Testing
Be Sure All Circuits Are De-Energized
MAINTENANCE ACTIVITY
SHOWS
FREQUENCY
Dielectric Absorption
Polarization Index (PI)
Winding cleanliness
Winding cleanliness/moisture
Power Factor
Partial Discharge (PD)
Insulation integrity
Coil tightness; insulation
integrity
Integrity of Insulation
Major Outage
Major and Minor Outage
Cycles
Major Outage Cycle
On-line or Outage Cycle
Megger
Blackout
Resistance
Flux Probe
Rotor Impedance
Ground Fault
Split Voltage
Voltage Drop
El Cid
Core Loop
Bolt Torque
Ultrasonic
Temperature Monitoring
Dye Penetrant
Eddy Current
Magnetic Particle
Wedge Mapping
Hi-Pot
Vibration
Visual Inspection
Oil Chemistry and Count
Corona suppression integrity
Integrity of joints and
connections
Rotor winding shorts
Rotor winding shorts
Rotor Ground
Location of rotor grounds
Presence of shorted turns
Integrity of stator core
Integrity of stator core
Stator core looseness
Cracks, defects in forgings
Normal/abnormal operation
Cracks, defects in forgings
Cracks, defects in forgings
Cracks, defects in forgings
Stator winding tightness
Insulation integrity
Rotor imbalance
Normal/Abnormal Performance
Bearing oil contamination
Major
Cycles
and
Minor
Outage
Rewind
Major and Minor Outage
Cycles
On-line, Rewind
Rewind
Continuous
As Needed
Major Outage Cycle
Major Outage Cycle
Major Outage Cycle
Major Outage Cycle
Major Outage Cycle
On-line and Continuous
Major Outage Cycle
Major Outage Cycle
Major Outage Cycle
Major Outage Cycle
Major Outage Cycle
Monthly and On-line
As Available
Twice Yearly
Data Banking
• Involves taking unit measurements that
are needed to manufacture replacement
stator windings
• Can be done during major rotor-out
outages
• Makes data available to non-OEM vendors
prior to forced and planned outages
Normal / Useful Life Period
Case Study
Normal / Useful Life Period
Case Study cont.
• Rotor Failure
• Operational Error
Normal / Useful Life Period
Case Study cont.
• Negative
sequence
heating currents
flow over the
surface of the
rotor
• Localized
heating often
occurs at the
body to ring
interface
Normal / Useful Life Period
Case Study cont.
End of Life Period
End of Life Period cont.
• Increasing failure rate
• It is bound to happen, often before most would
like it to, or are prepared for!
• Minimize Risk
• Predicting
• Anticipating
– Coil Ready Kit
– Spare Set of Stator Coils
• Planning
– Decommissioning?
– Life Extension?
– Up – Rate?
End of Life – Case Study
• Family of units considered
• Somewhat unique feature – Skewed
Stator core and coil
• Common feature for machines of this OEM
type and vintage – Asphalt based stator
winding insulation system
End of Life – Case Study cont.
oFailure mode -girth
cracking - common with
older, asphalt windings
oCracks occur in outer
layers of groundwall
insulation, just outside
stator core
oDifferential expansion
between copper coils and
iron core
Girth Cracking
Tape Separation at End of Core
Tape Separation at End of Core
End of Life – Case Study cont.
•Tripped off line 1.5
months later
•Girth cracking problem
known from unit 5 and
the rewind of both 4
and 5 were in the
discussion stages
•Failure occurred
before the planned
rewind
End of Life – Case Study cont.
•Found 2 top bars and
2 bottom bars failed
•Major core damage
on the first two packs
of iron surrounding the
failed bars
•Two iron packs
replaced
End of Life – Case Study cont.
End of Life – Case Study - B
•
•
•
•
Thermal Aging
Repeated Cycling
Shorts/Grounds
Rewind
Life Extension
• A clean slate
– Consider all factors
• Timing can have a dramatic effect on the
cost and duration of the outage.
• Be Proactive - Plan and Prepare!
• Data Banking
• Up Rate?
• Factor in machine inherent Deficiencies such as:
– PD Issues
– Loose Core
– End Winding Vibration
Life Extension – Case Study
• Old hydro electric
plant and 25 cycle
generator
• Generator
components nearing
the end of useful life
• Owner no longer
needs 25 cycle –
desires 60 cycle
generation capability
Life Extension – Case Study cont.
• Significant up-front
Engineering and
Planning
Life Extension – Case Study cont.
Life Extension – Case Study cont.
Stator Rewind
Life Extension – Case Study cont.
Rotor:
• Rim Modifications
• New Poles and Coils
What You Should Know
About Your Generators
Life Cycle
Questions ?
W. Howard Moudy
National Electric Coil