Transcript What is VELCO? - State of Vermont

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
What is the Vermont
subtransmission system?
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Vermont’s 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
Vermont’s 225 distribution substations.
Why consider subtransmission
reliability in a VELCO context?
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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.
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Docket No. 6839, Tafts Corner substation
Future: Tafts Corner substation expansion?
Future: Southern Loop upgrades?
What is the reliability criteria applied
to Vermont subtransmission?
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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.
Tempered N-1 Standard
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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.)
Why not meet N-1 all of the time?
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The N-1 criteria appears to be tempered by the
following:
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Radial lines
Cost considerations
Load levels
Environmental considerations
- Radial Lines
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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.
- Cost Considerations
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Providing looped subtransmission service to
substations may not be the most cost-effective
use of limited capital dollars.
Example:
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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.
- Load Levels
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Situations exist in which N-1 coverage is
provided most of the time, but is not provided at
the highest load levels.
Example:
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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.)
- Environmental Considerations
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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.)
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In Docket No. 6860, a proposed loop feed to
Vergennes was rejected and replaced by a
subtransmission radial feed to address
aesthetic concerns.
N-1 Is Met Most of the Time
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Despite the impediments described above,
Vermont’s subtransmission systems have
evolved to the point that an N-1 supply criteria
to most substations is met most of the time.
Example:
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Docket No. 6823, rebuilding GMP’s 34.5 kV line in
South Burlington to provide first contingency
coverage at all load levels
Summary
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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.