Path Rating Catalog 2013 - Western Electricity Coordinating Council

18. Montana - Idaho
Revised January 2013
Accepted Rating
Existing Rating
Other
Location:
Eastern Idaho and Western Montana
Definition: Sum of the flows on the following lines: Meter Location
Dillon Salmon-Big Grassy 161 kV Big Grassy
Peterson Flats-AMPS 230 kV Amps
Transfer Limit: North to South: 337 MW
South to North: 256 MW
Critical
Disturbance
that limits the
transfer
capability:
North-to-South:
Outage of the Bridger-Three-Mile Knoll and Bridger-Populus 345 kV lines
with RAS tripping of two Bridger units.
South-to-North:
Outage of the Brady-Antelope 230 kV line.
When:
System
Conditions:
Study Criteria:
Remedial
Actions
Required:
The 337 MW capability was established by the transmission path owners
(PacifiCorp, NorthWestern Energy, and Idaho Power) December 28, 2009.
Recent studies all confirm that the rating of 337 MW meets the performance
standards the owners have applied to this path since 2004 (see study criteria).
North-to-South limit is achieved with high Montana hydro and thermal
generation and low Idaho generation patterns, and heavy loads in Eastern
Idaho.
The South-to-North limit is achieved with light loads.
The 337 MW North-to-South rating of this path is based on the postcontingency
relative voltage performance at load buses served by both lines.
The basis for the rating is somewhat less conservative than the present WECC
criterion for post-transient voltage performance. However, even with the
relaxed criteria the stated capacity is within the 2.5 percent power margin
prescribed by the WECC voltage stability criterion for this contingency. The
North-to-South ratings are Accepted Ratings. This path rating study was
performed in 2009.
The South-to-North rating of the path is 256 MW. This rating is based on
study work performed August 23, 2007. This study was a part of the
operational transfer capability study process conducted for NOPSG for the
Winter 2007-08 operating season. It is rare for this path to be heavily loaded
South-to-North. The critical contingency is the loss of the Brady-Antelope
230 kV line. This leads to overloading of the Goshen-Antelope 161 kV line.
An automatic Overload Mitigation Scheme has been installed to mitigate this
overload. See operating procedures for additional details.
None
PART VI Item 1-49