National Electric Transmission Congestion Study - W2agz.com

2. Study Approach and Methods
This chapter describes the process and methods
used to study congestion in the Eastern and Western
Interconnections. The process involved two parallel
and coordinated analyses, one for each interconnection.
The analyses of the Eastern and Western Interconnections
are discussed separately.
The methods, which were common to both analyses,
included:
1. Review of available information on historical
congestion and previously documented transmission-related
studies;
2. Simulation modeling to estimate future economic
congestion, using common economic assumptions
and analytical approaches; and
3. Comparison and assessment of the historical information
with the simulation findings.
Although this study reports on transmission congestion
in the United States, the analyses of the Eastern
and Western Interconnections incorporated appropriate
data concerning Canadian electricity generation,
transmission, demand, cross-border flows, etc.
into the simulations.
2.1. Review of Historical
Transmission Studies
For each interconnection, analysts collected and reviewed
recent regional transmission studies, which
in most cases were transmission expansion plans
and reliability assessments. The results of these
reviews are discussed in Chapters 3 and 4. Using
these studies and other information concerning recent
grid transmission flows and curtailments, the
analysts identified existing transmission constraints
within the interconnection, as determined by other
parties, and took into account upgrades already approved
by regulators or under construction to alleviate
specific constraints.
2.2. Simulations
The simulations of Eastern and Western Interconnection
congestion used 2008 as the base year to estimate
congestion on the transmission grid. Each
analysis also simulated congestion for a later year,
but because of differences in data availability, the
eastern analysis focused on 2011 and the western
analysis focused on 2015.
Both eastern and western modeling used simulation
tools that use optimal power flow modeling on a decoupled
network system (i.e., DC power flow with
linear loss estimates) to minimize production costs
across the grid while delivering all needed power
from generators to loads in each hour of the model
year. Each simulation model incorporated average
system line losses. Each model conducts an internal
reliability assessment, optimizing flows while respecting
grid constraints that would limit flows
within or between regions, and redispatching generation
as necessary to ensure that load is served reliably.
Each simulation calculated the location, duration
and cost of congestion across the grid to
identify those elements on the grid that are expected
to experience the greatest congestion (as defined
below). Thus, congestion in the simulations reflects
underlying economic forces, constrained by the
physical limits of the power systems. 8
8 Time series simulations based on optimal power flow models over a defined study period can estimate total system production costs for that
period. These models can be used to compare production costs in base and change case simulations—the base case represents the transmission
system as it exists today or in the near future, with known and quantifiable modifications. The change case represents discrete transmission
enhancements that are being tested to determine their impact on production costs, reliability, or other performance indicators. The difference
between the base and change cases yields a calculated basis for evaluating the economics of the transmission enhancement. It is important to note
that the impact of a transmission enhancement cannot be assessed using measures of current congestion, such as congestion rent, that reflect only
base case conditions. The simulation techniques used for this congestion study did not incorporate voltage stability limits, which require modeling
using a full alternating current optimal power flow solution for each study interval; this adds significant complexity to the modeling process.
Transient stability, another important physical constraint, may not be possible to model under unconstrained network dispatch.
U.S. Department of Energy / National Electric Transmission Congestion Study / 2006 9