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