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?

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?

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?

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.

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.)

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

- 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.

- 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.

- 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.)

- 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.

N-1 Is Met Most of the Time

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:– Docket No. 6823, rebuilding GMP’s 34.5 kV line in

South Burlington to provide first contingency coverage at all load levels

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.