C57.91 WG Meeting

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C57.91 WG Meeting

• Tuesday, October 25, 2005
• Memphis, TN

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Agenda

• Introductions
• Approve Minutes of Previous Meeting
• Chairs Comments
• Discussion of Insulation Aging
• Loading Limits for Power Transformers
• New Business?
• Adjourn

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Patents

• IEEE SA requires that a request be made at each
  WG meeting for disclosure (identification) of any
  patents that may be related to the work of the
  WG.
• The Transformers Committee is covering the
  required discussion of this issue at this meeting
  by posting the IEEE patent requirements on the
  Committee website and sending e-mail notification
  of the posting to all potential attendees through
  the Committee Association Management System. You
  are advised to read them.
• The Minutes of this meeting shall note
• that IEEE Patent disclosure requirements were
  addressed
• that a request was made for disclosure of any
  patents that may be related to the work of the
  WG.
• any responses to the request for disclosure.
• Does anyone have information on any patents that
  may be related to the work of this WG?

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Comments

• Draft 4 nearly ready to be posted. New this
  draft
• Clause 10 (Temperature Calc) cleaned up. Now
  only includes old Annex G model. This is the
  recommended temperature calculation from now on.
  (Thanks to Glenn Swift)
• New Annex for Simplified Thermal Calculation.
  This is essentially the old Clause 7 model for
  those without bottom oil rises, or who wish to
  use the old model. (Again, thanks to Glenn for
  writing this up)
• Minor cleanups and additions based on comments
  from those who volunteered to review particular
  sections (Thanks to Don Platts, Dave Wallach and
  Bob Tillman)

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Comments

• We are now official! (3 years later) PAR was
  approved last spring.
• Last meeting we discussed T. V. Oommens gas
  evolution model, agreed to keep. Add wording to
  clarify issues with moisture content, etc. Glenn
  Swift volunteered to take a cut at it. Another
  volunteer?
• At end of last meeting, I mentioned two
  proposals. We ran out of time to discuss. Will
  discuss at this meeting.
• Goal for this meeting is to produce a feature
  list of things to add/change for this revision.

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Insulation Aging

• One major point of confusion and misunderstanding
  when performing loading calculations and making
  loading decisions is the discussion of percent
  loss-of-life (in percent total life expectancy)
  and in the discussion of the end of life or life
  remaining.
• Aging equations fail to consider condition
  variables.

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End of life

• In rough terms, the end of life for an
  insulation system is the point at which the
  insulation no longer performs reasonably.
• For electrical insulation, this means the point
  at which the insulation system no longer
  maintains a majority of its original dielectric
  strength.
• As insulation ages, the dielectric strength of
  the paper does not decrease significantly until
  well after the paper has become brittle.
• Therefore, the point at which the paper loses
  enough strength to withstand the mechanical
  forces is the practical end of life for the
  insulation system.
• Given this, a precise definition becomes
  difficult because this is dependant upon the
  application of the material, both electrically
  and mechanically.

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End of Life Criteria

• Therefore, if LOL is discussed, the value for a
  given temperature can vary by a factor of three
  for same paper, conditions and temperature.

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What does aging really mean?
This point dictated by thermal aging
Equivalent Thermal Age
10
Insulation Aging

• Current draft discusses age acceleration factor
• aging in equivalent aging hours
• and percent loss-of-life (for compatibility with
  old guides)

Integration of aging rate over time
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Affects of condition

• We know that moisture content of the paper has a
  profound impact on the insulation aging rate
  (roughly, doubling the moisture doubles the aging
  rate)
• Aging tests done at roughly 0.5 moisture.
  Current equations based on this.
• Oxygen also has a profound impact.
• Given the format of the equations in the current
  guide, user are unaware of this. Adding rough
  factors to aging equations makes this difficult
  to miss.

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Include Rough Estimates of Moisture and Oxygen

• Apply multiplying factors to age acceleration
  factor

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Current Power Transformer Limits
Table 10 - Suggested Limits of Temperature and
Load For Loading above Nameplate Power
Transformers with 65 ?C Rise
Table 11 - Suggested maximum limits for the four
types of loading
a 110 ?C on a continuous 24 hour basis. b Gassing
may produce a potential risk to the dielectric
strength of the transformer. This risk should be
considered when this guide is applied refer to
Annex E.
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Problems

• Metallic hot spots other than winding hot spots
  cannot be readily evaluated. This is a design
  criteria, not an operating criteria.
• 200 and 180C are unrealistic.
• Table assumes perfect health.
• LOL requires the contentious issue of life end
  points (and really is ancillary to the intent of
  this limit)

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Proposal
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Condition Ranking
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Condition-Based Limits
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Reasons

• Current loading limits are based on transformers
  in perfect health, although this is not mentioned
  anywhere.
• Highlights the importance of parameters important
  to loading (moisture, oxygen).
• Heating gases provide feedback loop to avoid
  unintended hot spots such as stray flux or
  (maybe) lead heating.

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Add Overload Checklist

• Example
• Gather Information
• Factory Test Report
• Nameplate Drawing
• Outline Drawing (if available)
• Maintenance History
• Perform Field Inspection
• Check and calibrate gauges
• Check gaskets and seals
• Look for discolored paint
• Check pumps and fans for proper operation (and
  rotation if possible)
• Check oil level
• Inspect oil preservation system (N2 pressure,
  check bladder, oil level gauge)
• Check alarm and trip levels on temperature, if
  equipped
• Check radiators or heat exchangers for blockage.
  Clean if necessary.
• Draw sample for DGA and oil quality
• Post Loading
• Draw sample for DGA and oil quality

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Other Items

• Cooling stage switching impact on shorter
  duration overloads/ratings
• Three winding transformers?
• Enhanced sections on risks and increased
  maintenance considerations with higher or more
  frequent overloading (ie. gaskets, DETC or LTC
  contacts, oil)
• DISTRIBUTION TRANSFORMERS
• NEED AT LEAST ONE VOLUNTEER

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New Business

• Comments/Questions/Gripes/Complaints?

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Adjourn