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4. Congestion and Constraints in the
Western Interconnection
Chapter 4 has the same structure as Chapter 3—first
it reviews the historical transmission constraints in
the Western Interconnection, and then it presents
the results of congestion simulation modeling. The
logic and process of comparing the historical and
modeled congestion results in the West was essentially
parallel to that described in Chapter 3 for the
Eastern Interconnection, so that process is not
re-described here.
4.1. Historical Transmission
Constraints in the Western
Interconnection
The transmission constraints described below were
identified by reviewing recent transmission studies,
expansion plans and reliability assessments conducted
by subregional groups of western utilities,
the Western Electricity Coordinating Council
(WECC), the Seams Steering Group – Western Interconnection
(SSG-WI), and the California Independent
System Operator (CAISO). The studies
covered in this review are listed in Appendix J.
Figure 4-1 shows some of the Western Interconnection’s
principal catalogued transmission paths and
indicates those paths that were identified as congested
in the historical studies. 25 A transmission
constraint (or constraints) inhibiting flows on a
transmission path is represented by a red bar across
the path. The bar also crosses or touches all lines
comprising the path.
The western analysis used significantly larger
nodes (covering wider geographical spans with
much larger generation and load weightings) than
those used in the eastern modeling. The western
path catalog includes 67 WECC paths, plus other
monitored lines, as well as specific unscheduled
flow paths, operating transfer capability group
paths, and nomograms 26 that reflect the effect of
other lines (including smaller lines) upon the modeled
paths. Some of these paths are internal to
nodes, and so were not identified by the modeling
described here, although they are well-known and
studied in sub-regional analyses.
In addition to reviewing existing studies by others,
the western analysis team also examined data on actual
transmission usage for the six-year period between
1999 and 2005. Below, Figure 4-2 shows the
western transmission paths that were most heavily
used. The usage metric shown is U75, the metric
that reflects how many hours in a year the path was
loaded at or above 75% of Operating Transfer Capability
(OTC), the coordinated maximum flow
limit set on actual path transfers reflecting system
operating conditions at the time. 27 Consistent with
other congestion results, this shows that the most
heavily loaded lines include the Bridger West
line, the Southwest of Four Corners-to-Cholla-to-
Pinnacle Peak lines (built to deliver power from
baseload plants to loads), Western Colorado to
Utah, the lines from Wyoming to Colorado, and the
southern New Mexico path to El Paso.
Figure 4-3 shows how heavily various paths within
the West have been used over a recent 18-month period.
Based on the U90 metric (which is the percentage
of time a path is loaded at or above 90% of its
limit), this figure shows that only five lines were at
U90 or above for more than 10% of the hours in this
time period. Of the most heavily loaded lines, note
that the Bridger West line is dedicated to delivering
electricity from the Bridger coal-fired power plants
to loads in Utah and Oregon; this is one-way flow
25 Appendix K lists WECC’s 67 paths.
26 A “nomogram” is a graphic representation that depicts operating relationships between generation, load, voltage, or system stability in a
defined network. (See Glossary.)
27 WECC, Operating Transfer Capability Policy Committee Handbook, May 2006 (http://www.wecc.biz/documents/library/OTC/OTCPC_
HANDBOOK_05-19-06.pdf).
U.S. Department of Energy / National Electric Transmission Congestion Study / 2006 31