National Electric Transmission Congestion Study - W2agz.com

Refining congestion metrics
As indicated in this study, there are no standard
metrics for measuring congestion and its impacts;
perhaps the only thing that is clear is that no single
metric is sufficient to capture all relevant aspects of
congestion. The metrics used here were developed
specifically for this study, and as with most tools,
they are subject to future refinement. The Department
welcomes further dialogue about congestion
metrics with the industry, regional transmission
planners, market monitors, and the academic community.
Data collection and improvements
As this and previous studies have shown, 67 there is
very little systematic data available on existing and
planned transmission facilities and investments.
Transmission congestion within ISO and RTO areas
is closely measured and tracked, but little comparable
data is collected outside the boundaries of
the ISOs and RTOs. DOE will work with EIA,
FERC, NERC and industry members to determine
whether data collection requirements should be
modified. By making its data base and analytic assumptions
publicly available, the Western Congestion
Assessment Task Force (WCATF) has set important
precedents in this area that the Department
hopes will be continued in the West and adopted in
the East.
Granularity versus aggregation
In the West, transmission expansion planning and
reliability analyses have been conducted chiefly by
sub-regional groups, and the results have been
rolled up at the west-wide level. West-wide regional
planning occurs at a very general analytical level,
compared to the more granular level modeled in the
East. Future western analyses may need to examine
whether it is possible and useful to develop a more
detailed set of models and data, to better understand
the nuances of congestion, reliability and cost variations
occurring within the zones connected by the
West’s 67 major transmission paths. This would allow
western regional planners to more consistently
model and address significant congestion problems
that are now buried inside very large western nodes.
Two examples of such granularity problems are the
congestion on the Seattle-to-Portland transmission
path, and the question of how to provide transmission
for wind generation out of the Tehachapi
Mountains in southern California.
Modeling improvements
One of the important technical challenges to congestion
modeling is that the current DC models do
not address voltage problems. Determining the effects
of a proposed transmission enhancement on
such problems requires separate analysis with an
AC model, to ensure that voltage and transient stability
are properly addressed. As a related issue,
more work is needed to model effectively marginal,
rather than average transmission system losses.
Marginal losses more closely parallel actual power
system physics, but average losses are easier to simulate.
Much of the congestion seen today results from the
practice of adhering to reliability limits imposed so
as to be prepared to withstand contingencies. Without
questioning the need for such adherence, there
are nonetheless legitimate questions about whether
we have adequate tools to represent and analyze the
complex relationship between contingencies and
congestion. This relationship needs to be more fully
understood. Similarly, some congestion and flow
restrictions are due to scheduling practices and
transmission rights rather than reliability and operational
capabilities per se.
DOE will consult with those who performed analyses
related to this study and with other modeling experts,
analysts, and sources of data to determine
what refinements are feasible before undertaking
modeling for the next congestion study.
In the East, as discussed on the preceding page,
there is a need for more systematic and coordinated
analyses and responses regarding congestion problems
that cross regional boundaries.
67 Energy Information Administration, Electricity Transmission in a Restructured Industry: Data Needs for Public Policy Analysis, December,
2004; Hirst, Eric, “U.S. Transmission Capacity: Present Status and Future Prospects,” Edison Electric Institute and U.S. Department of Energy,
August 2004; and Energy Security Analysis, Inc., “Meeting U.S. Transmission Needs,” Edison Electric Institute, June 2005.
U.S. Department of Energy / National Electric Transmission Congestion Study / 2006 65