National Electric Transmission Congestion Study - W2agz.com

Wyoming, Montana, Nevada and Utah. The Seams
Steering Group – Western Interconnection (SSG-
WI) Transmission Subgroup proposed 11 transmission
projects and about 3,903 miles of new lines
with costs of almost $7.0 billion.
Line losses
The energy lost in power delivery from the power
plant to the customer’s meter affects grid congestion
and electricity costs. Both the eastern and
western analyses used simulation models that assume
average line losses in a decoupled network
representation. The western analysis used one sensitivity
case to determine the impact of generation
commitment and dispatch based on marginal, rather
than average, line loss calculations and found that
line losses assumptions may have a significant effect
upon congestion findings. This shows that further
analysis is needed to better understand the implications
of line losses for congestion.
Resource assumptions
The modeling results, including projected congestion,
are very dependent upon assumptions about
which specific new transmission elements and
power plants are included in the projected grid and
resource set. Analyses for near-term operational
purposes are essentially snapshots of current conditions.
They look ahead at most for a single year, and
include only those resources currently operating or
nearly operational.
By comparison, longer-term projections of the patterns
and levels of future power flows and congestion
can be strongly affected by assumptions about
whether specific new transmission lines or major
power plants are included in the base resource
set—for example, whether major new coal resources
are assumed on-line in the Powder River
Basin, or new merchant DC cables in the New York
and New England regions are assumed to be constructed.
Such factors are not significant for the
2008 model year, for which transmission and generation
resources can be predicted with relative confidence,
but the impacts of assumed new transmission
and generation resources are very important for
the 2011 and 2015 analyses. A hypothesized new
transmission line can “assume away” an otherwise
significant new congestion problem.
2.4. Estimating and Evaluating
Congestion
In order to assess the magnitude of congestion
across the transmission paths modeled in the two
interconnections, the congestion study team developed
and applied five metrics. Those metrics are:
1. Binding hours: Number of hours (or % of time
annually) that a constrained path is loaded to its
limit.
2. U90: Number of hours (or % of time annually)
that a constrained path is loaded above 90% of
its limit.
3. All-hours shadow price: 9 Shadow price averaged
over all hours in a year.
4. Binding hours shadow price: Average shadow
price over only those hours during which the
constraint was binding (shadow price is zero
when constraint is not binding).
5. Congestion rent: Shadow price multiplied by
flow summed over all hours the constraint is
binding.
Usage metrics
Both the number of hours that a path is loaded to its
limit (the binding hours metric) and the number of
hours that it is loaded close to its limit (the U90 metric)
indicate how heavily that path is used. A path
that is highly loaded for much of the time is likely to
result in significant, costly congestion. In addition,
since the limit on each path is set by operational reliability
considerations—thermal, transient stability
or voltage limits, either singly or in combination
with other paths and elements—a path that hits its
usage limit has also reached its reliability limit.
Transmission path usage is described using a composite
index such as U90. In each hour, the analysts
identified the element that is the most limiting with
respect to incremental transfer of power between
end nodes of the path. The usage indicator (e.g. percent
loading with respect to the flow limit) for the
9 See Glossary for definition of “shadow price.”
U.S. Department of Energy / National Electric Transmission Congestion Study / 2006 13