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In the
context of electricity market operations, a primary concern is the ability
to transfer power across vast interconnected networks while meeting a
broad range of operating reliability constraints. A common scenario
consists of compensating load increases and/or generation outages in a
system area by raising the generation elsewhere. In order to ensure that
the grid does not get too close to its stability limits, it is therefore
important to evaluate the maximum transfer capability across the "links"
that interconnect the areas involved in such transactions.
A "link" identifies a group of
transmission lines that form a topological cut-set, i.e., their removal
splits the networkin two areas, one on
each side of the link. The maximum power that can be transferred across a
link is limited by thermal and stability constraints. In a sense, the
concept of "stability constrained link" is similar to the concept of
“congestion path”, with the difference that the former is concerned with
stability, rather than thermal, violations.
"Stability constrained
links" may appear in any multi-area power system where large MW blocks are
transferred between weakly interconnected areas. This is often the case in
longitudinal transmission networks that span distinct system areas with
significant load-generation unbalances.
The analysis of recent blackouts due to instability revealed that most of
them followed a similar pattern:
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Large MW blocks get
transferred from areas with inexpensively priced energy toward areas
where the load demand has increased due to an actual increase in load,
or perhaps because one or several local generating units are scheduled
for maintenance, or simply because the local generation is too expensive
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As a result, certain
links get loaded closer and closer to their stability limits
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At this moment, a
generation or transmission outage takes place. Typically, such incidents
evolve into cascading outages
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Since the link was already operating within
a small stability margin, the physical phenomena leading to blackout are
triggered and the wide-spread disturbance becomes unavoidable.
The detection of critical links is an
intrinsically difficult proposition. To begin with, the search of all the
possible links entails a graph topological procedure that may find
hundreds of thousands of links even for moderately sized networks. Once
the full set of links has been determined, a stability criterion is used
to compute the maximum transfer capability of each link. Then, the links
are ranked in the order of their stability margins.
This complex problem is solved quite
expeditiously by WeakLinks Finder™,
which is the computational engine of WeakLinks Professional™.
Given a solved load-flow case or a state estimate, the program:
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Detects all the "links",
or “cut-sets”,
within the transmission system
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Ranks them in the order
of their distance to the maximum transfer limit between the areas
situated on each side of the link
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Recommends a control strategy, i.e., raise
or lower the MW output of certain machines, which can help increase the
stability margin of the link.
The same array of
calculations is performed on user-defined links. The solution's
speed enables the fast evaluation of the power transfers between areas
known a priori to have stability limitations -- as often as needed,
for each transaction, off-line and in real-time.
These capabilities form a useful
complement to QuickStab® Professional, which is being used
off-line and in real-time in control centers in the United States, Europe,
Latin America and Asia. WeakLinks Finder™ identifies the stability
constrained links. This information is then passed to QuickStab®,
whose multi-area stability computational engine quickly determines how far
from instability are the areas separated by the link.
Together, these fast
and versatile applications stand out because they quickly detect and
quantify the risk of instability -- and allow utilities and system
operators to develop preventative and corrective strategies that can save
hundreds of millions of dollars in penalties and lost revenues caused by
blackouts.
For additional
information or to schedule a live demonstration please contact us via
e-mail
infoqs@eciqs.com, web
http://www.eciqs.com, telephone (212) 913-9154 or in writing to Energy
Consulting International, Inc., 405 Lexington Avenue, 26th
Floor, Chrysler Bldg, New York, NY 10174.
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