arippa technical symposium august 28, 2007 w. howard moudy national electric coil what you should...
TRANSCRIPT
ARIPPATechnical Symposium
August 28, 2007
W. Howard MoudyNational Electric Coil
What You Should Know About Your Generators
Life Cycle
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 TestingBe Sure All Circuits Are De-Energized
MAINTENANCE ACTIVITY SHOWS FREQUENCY
Dielectric Absorption Winding cleanliness Major Outage
Polarization I ndex (PI ) Winding cleanliness/moisture Major and Minor Outage Cycles
Power Factor I nsulation integrity Major Outage Cycle
Partial Discharge (PD) Coil tightness; insulation integrity
On- line or Outage Cycle
Megger I ntegrity of I nsulation Major and Minor Outage Cycles
Blackout Corona suppression integrity Rewind
Resistance I ntegrity of joints and connections
Major and Minor Outage Cycles
Flux Probe Rotor winding shorts On- line, Rewind
Rotor I mpedance Rotor winding shorts Rewind
Ground Fault Rotor Ground Continuous
Split Voltage Location of rotor grounds As Needed
Voltage Drop Presence of shorted turns Major Outage Cycle
El Cid I ntegrity of stator core Major Outage Cycle
Core Loop I ntegrity of stator core Major Outage Cycle
Bolt Torque Stator core looseness Major Outage Cycle
Ultrasonic Cracks, defects in forgings Major Outage Cycle
Temperature Monitoring Normal/abnormal operation On- line and Continuous
Dye Penetrant Cracks, defects in forgings Major Outage Cycle
Eddy Current Cracks, defects in forgings Major Outage Cycle
Magnetic Particle Cracks, defects in forgings Major Outage Cycle
Wedge Mapping Stator winding tightness Major Outage Cycle
Hi- Pot I nsulation integrity Major Outage Cycle
Vibration Rotor imbalance Monthly and On- line
Visual I nspection Normal/Abnormal Performance As Available
Oil Chemistry and Count Bearing oil contamination 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.
Girth Cracking
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
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 MoudyNational Electric Coil