1. ARIPPA Technical Symposium August 28, 2007 W. Howard Moudy National Electric Coil What You Should Know About Your Generators Life Cycle

2. 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.

3. Understanding Life Cycle The “Bathtub Curve” is typically used as a visual model to illustrate the three key periods of product failure

4. 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.

5. Understanding Life Cycle Discussion will focus on the characteristics, issues, and options associated with: –Infant Mortality –Normal Life –End of Life –Life Extension

6. 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.

7. 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.

8. 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

9. Infant Mortality Case Study -A Stator Failure – Partial Discharge / Corona Condition initially found during the first major outage

10. 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)

11. . 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)

12. 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

13. Infant Mortality Case Study - A Cont.

14. 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.

15. Infant Mortality Case Study - B Cont. Pictures of the Original “Rigid” Crossover

16. Infant Mortality Case Study - B Cont. Pictures of Omega pole to pole crossovers

17. 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

19. Normal / Useful Life Period

20. 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.

21. 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

22. Outage Testing Be Sure All Circuits Are De-Energized

23. 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

25. Normal / Useful Life Period Case Study

26. Normal / Useful Life Period Case Study cont. Rotor Failure Operational Error

27. 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

28. Normal / Useful Life Period Case Study cont.

29. End of Life Period

30. 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?

33. 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

34. 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

35. Tape Separation at End of Core

37. 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

38. 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

39. End of Life – Case Study cont.

40. End of Life – Case Study - B Thermal Aging Repeated Cycling Shorts/Grounds Rewind

41. 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

42. 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

43. Life Extension – Case Study cont. Significant up-front Engineering and Planning

44. Life Extension – Case Study cont.

45. Stator Rewind

46. Life Extension – Case Study cont. Rotor: Rim Modifications New Poles and Coils

47. What You Should Know About Your Generators Life Cycle Questions ? W. Howard Moudy National Electric Coil