Generation Dispatch and Transfer Limit InteractionTransaction
Scheduling
Most FCITC computations are based
on the assumption that generation participation factors for sending
and receiving subsystems are user defined and do not change. In
this subsection, the abbreviation FPF FCITC is used to denote
FCITC analysis computed using Fixed Participation Factors (FPF).
This is the simplest method of FCITC analysis. However, under
many conditions generator participation factors are unknown or
the user may have an option to select participation factors and
generators to be used in a transfer.
The use of fixed participation factors
raises the following questions and considerations:
How will study transfer FCITC change if a user specifies different
participation factors?
How will study transfer FCITC change if a user specifies different
generators to participate?
Which are the most limiting contingencies/monitored elements
that limit study transfer with respect to all possible combinations
of participating generators and participation factors? Did the
FCITC analysis actually find the most limiting contingency?
What are the possible FCITC variations due to different dispatch
patterns? The answer to this question may have an impact on
the determination of the transmission reliability margin (TRM).
Which are the generators in the importing systems that have
the greatest positive impact on FCITC? These generators can
be defined as the must-run units for a selected study transfer.
Generators in the importing system that must run with respect
to every import scenario can be defined as the must-run units
for the control area.
All of the above questions are quite
difficult to answer using traditionally available tools. Multiple
FPF FCITC runs could help answer some of these questions, but
cannot provide a complete solution considering all the possible
dispatch variations.
FPF FCITC calculations do not check
generator limits and do allow study transfer specification of
any transfer level. It is the user’s responsibility to check
that all generators will be within these limits for the selected
transfer level. This approach simplifies subsystem definitions
and data maintenance. It is, for example, common to specify only
a few representative buses as generation sources and sinks. However,
this approach may result in violation of generator capacity limits
and associated incorrect FCITC limits.
Within Generation Sensitivity Analysis
(GSA), PSS™MUST always observes generator limits. For this
reason, if an FPF FCITC analysis determines limits that do not
appear in the GSA calculation, the FPF FCITC calculations will
result in artificial limit violations caused by generation dispatch
violating generator limits.
PSS™MUST has a GSA function
designed to answer the above questions. Within this subsection,
we will use the notion of the study system to denote the subsystem
for which the user has requested to find an FCITC sensitivity
due to variations in participation factors. The GSA function finds
the dispatches in the study system (importing or exporting) that
minimize study transfer FCITC (i.e., the worst generation dispatch).
The GSA function can determine:
Generators in the importing/exporting systems whose output
adjustments (decrease or increase, respectively) would result
in the lowest import levels.
ALL contingencies and constraints that may be limiting with
respect to all possible combinations of generator adjustments
in the importing/exporting systems.
Transfer security margins. If the worst dispatch pattern cannot
create violations by adjusting user-specified generators, then
the specified transfer level is secure with respect to any dispatch
in the study system.
In most cases, the GSA function
determines several of the worst dispatches for different monitored
elements/contingency pairs. As a result, the worst dispatch analysis
may find more limiting contingencies/constraints than the FPF
FCITC analysis. GSA analysis can also point out some local problems
that are not related to the study transfer FCITC. Several effective
ways to use GSA are to:
Include all generators in the importing or exporting system.
Examine the contingencies/monitored element pairs to identify
local problems and must-run generators.
Remove the local must-run generator from the importing subsystem
list, customize monitored elements using the EXCLUDE/CHANGE
file, and repeat analysis.
Currently, PSS™MUST assumes
that the opposing system is adjusted according to fixed participation
factors. Therefore, for a specific transfer level, reduction/increase
in the dispatch of a generator in the study subsystem is made
up by fixed participation factor adjustments for the opposing
system.
It is important to understand the
difference between the worst generation dispatch method and the
optimal power flow (OPF) based approaches method. These two
methods have opposite objectives. When OPF is used for transfer
studies, it is used mostly to maximize transfer capability.
In contrast, the GSA function finds the worst dispatch pattern
that creates (yes, creates!) violations, e.g., minimizes
transfer limits. If the GSA function cannot create violations,
then this proves that the specified transfer level is secure.
Therefore, OPF and GSA functions find the upper and lower boundary
values for transfer levels between selected subsystems.
