Subtransmission Reliability Criteria

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Subtransmission Reliability Criteria

• Informational Workshop
• Investigation into Least-Cost Integrated Resource
  Planning for the Vermont Electric Power Company
• Steve Litkovitz
• Department of Public Service
• September 19, 2005

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What is the Vermont subtransmission system?

• Vermonts medium voltage transmission lines and
  devices, most commonly at 34.5 kV, 46 kV,
  and 69 kV, primarily supplied by VELCO high
  voltage substations.
• Most subtransmission systems are owned and
  operated by CVPS, GMP, and VEC.
• Primary function is to provide reliable supply to
  Vermonts 225 distribution substations.

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Why consider subtransmission reliability in a
VELCO context?

• The subtransmission systems are supplied by VELCO
  substations.
• VELCO planning occurs in step with its
  subtransmission customers.
• Subtransmission system reliability needs can, at
  times, be met by VELCO upgrades.
• Docket No. 6839, Tafts Corner substation
• Future Tafts Corner substation expansion?
• Future Southern Loop upgrades?

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What is the reliability criteria applied to
Vermont subtransmission?

• No Board Order, Board Rule, or state-wide utility
  standard that establishes a single
  subtransmission system reliability criteria.
• However, examination of existing utility
  practices, recent upgrades, and Board Orders
  reveal a tempered N-1 standard for
  subtransmission reliability.

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Tempered N-1 Standard

• Under normal situations, lines are not
  overloaded, voltages hold steady, and loads are
  served. (This criteria is met virtually all of
  the time.)
• After a first contingency (N-1), the remaining
  lines are not overloaded above their emergency
  ratings, voltages are adequate, and substation
  loads are served following required switching.
  (This criteria is met most of the time.)

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Why not meet N-1 all of the time?

• The N-1 criteria appears to be tempered by the
  following
• Radial lines
• Cost considerations
• Load levels
• Environmental considerations

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- Radial Lines

• Ideally, substations are supplied by at least two
  subtransmission feeds in a looped configuration.
  If one feed is lost to contingency, supply to the
  substation can be maintained by the alternate
  feed.
• However, approximately 30 of Vermont substations
  are supplied by only one subtransmission line,
  i.e., supplied radially.

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- Cost Considerations

• Providing looped subtransmission service to
  substations may not be the most cost-effective
  use of limited capital dollars.
• Example
• WEC Maple Corner substation in Calais has a
  radial subtransmission supply and serves 800
  customers.
• Looping would require at least 12 miles of new
  34.5 kV subtransmission costing approximately 2
  million.
• This results in a cost of 2,500/customer to
  cover just the subtransmission contingency.

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- Load Levels

• Situations exist in which N-1 coverage is
  provided most of the time, but is not provided at
  the highest load levels.
• Example
• Docket No. 6794, CVPS St. Albans area upgrades
  N-1 coverage is significantly enhanced through
  relatively modest investments in the 34.5 kV
  subtransmission system.
• Total N-1 coverage, available with a 115 kV
  option, was rejected as being unlikely to be
  cost-effective in terms of its incremental cost
  and benefits. (Order at page 4.)

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- Environmental Considerations

• Transmission projects can have environmental
  impacts.
• The Board stated that 30 V.S.A. Sec. 248 is a
  strong statement that the natural environment
  is of great importance and that it should be
  impaired only if absolutely necessary.
• (Docket No. 4782, Order at page 41.)
• In Docket No. 6860, a proposed loop feed to
  Vergennes was rejected and replaced by a
  subtransmission radial feed to address aesthetic
  concerns.

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N-1 Is Met Most of the Time

• Despite the impediments described above,
  Vermonts subtransmission systems have evolved to
  the point that an N-1 supply criteria to most
  substations is met most of the time.
• Example
• Docket No. 6823, rebuilding GMPs 34.5 kV line in
  South Burlington to provide first contingency
  coverage at all load levels

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Summary

• VELCO planning is conducted in concert with the
  requirements of the underlying subtransmission
  systems.
• Examination reveals a tempered N-1
  subtransmission reliability criteria.
• Despite impediments resulting from configuration,
  cost, load, and environmental considerations, an
  N-1 supply criteria is met for most substations
  most of the time.