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these zones. An exception is NYISO Zone J
(New York City), but Zone J was retained as a
single node rather than divided.
• For all other markets, generation and load buses
within the same control area were grouped into
nodes using a clustering algorithm to reflect the
impact of power injections and withdrawals at individual
buses on major constrained paths. As
noted above, some nodes were primarily generation
sources, others were dominated by load
rather than generation, and some had both load
and generation.
The existing electrical links between pairs of nodes
were the paths tested for congestion. The nodes for
the Eastern Interconnection are shown in Figure
2-3.
Defining paths
For modeling purposes, a transmission path is defined
as a complex of lines linking two nodes. Paths
may be defined on two levels—at the aggregate
level, a path is defined as one or more lines on the
network between two nodes, while at a detailed
level a path includes a number of related physical
transmission elements connecting one node to another.
A path may extend across transmission owners
or control areas, and may contain one or more
existing transmission system facilities.
Along a path, the model incorporates constraints
that are most limiting for flows between generation
nodes and load nodes. Constraints restrict the flow
on a line, transformer, or a group of related elements,
so that the flows do not exceed the appropriate
thermal or stability-based reliability limit for the
path.
For each hourly time-step in a year-long simulation,
GE-MAPS calculates the least-cost dispatch from
the portfolio of available generation to meet all
loads, recognizing limits on the ability of the paths
to support electrical transfers. The program then
totals the amount and cost of each generator’s production
as well as the loading on each path over
all hours in the study year. For every scenario run,
the model tabulates the transmission congestion
for each path over the hours and year, so that the
congestion results can be compared within and
across scenarios. These results are presented in
Chapter 3.
2.6. The Western
Interconnection
Review of historical information
Information on congestion in the Western Interconnection
was received in two forms. First, over 35
documents from a variety of sources were reviewed;
these documents are listed in Appendixes
H and J. The majority of these documents were prepared
by regional and sub-regional transmission
planning study groups, including Western Governors’
Association (WGA); Seams Steering Group –
Western Interconnection (SSG-WI); Northwest
Transmission Assessment Committee (NTAC);
Rocky Mountain Area Transmission Study
(RMATS); Southwest Transmission Expansion
Plan (STEP); Southwest Area Transmission
Planning Group (SWAT); and Western Electric
Coordinating Council (WECC). One of these, the
Western Governors’ Association Transmission
Task Force report in support of the CDEAC initiative,
is notable because materials developed for that
study were important inputs to this congestion
study’s western region modeling. 16
Second, this study examined historically archived
data collected by WECC, including hourly line
flows. Access to this comprehensive set of historical
information permitted direct calculation of actual
grid congestion, including U90, for the transmission
paths in the west.
The analysts conducting the Western Interconnection
study used the same model and data sources as
used for various recent WECC reliability assessments
and other modeling and represent the same
organizations (and in most cases are the same analysts)
as those conducting other western modeling.
Accordingly, the western team concluded that there
was no need for fresh validation of its model and
data against other sources.
16 http://www.westgov.org/wga/initiatives/cdeac/index.htm.
U.S. Department of Energy / National Electric Transmission Congestion Study / 2006 17