Simulator 16 User Guide - PowerWorld

entered to determine the highest transfer level at which the contingency can be solved. This loop
attempts to determine a more accurate transfer level at which the contingency fails to solve
rather than simply reporting the transfer level at which the last power flow was successful
regardless of the current step size. To start the iterative process, the current step size is reduced
by half. The ramping is performed and the contingency is solved. The step size will remain the
same as long as the contingency can be solved. If the contingency fails to solve, the step size will
be reduced by half again. The ramping will continue until the contingency no longer solves, the
accumulated transfer during the process meets the original step size, or the step size becomes
smaller than the step size tolerance.
An additional check is done during the iterative process on the failed contingency. When the
contingency does solve, the flow on the monitored element is checked to determine if it exceeds
its limit. If it is overloaded, the step size will be reduced by half until the monitored element is no
longer over its limit. While this check is being done on the monitored element limit, the ramping
continues out to the specified amount and the contingency is solved.
During the entire iterative process on a failed contingency solution, the step size is limited to be
between zero and the step size which was ramped when the contingency failed to solve for the
first time.
Tolerances in ATC Tool
When using one of the iterated methods, tolerances must be set correctly with the ATC tool or
injection changes will end up being picked up by the system slack instead of the seller and buyer or
no injection changes will be made at all. When studying a transfer between injection groups, the
Transfer Tolerance will dictate what the MVA Convergence Tolerance, set with the Power Flow
Solution Options on the Simulator Options Dialog, and the Island-Based AGC Tolerance can be.
Before the analysis starts, the MVA Convergence Tolerance will be checked to make sure that it is
less than 0.1*(Transfer Tolerance). If not, the MVA Convergence Tolerance will be set to
0.1*(Transfer Tolerance). The Island-Based AGC Tolerance will also be checked to make sure that it
is less than 0.5*(Transfer Tolerance). If not, the Island-Based AGC Tolerance will be set to
0.5*(Transfer Tolerance). The MVA Convergence Tolerance will then be checked to make sure that it
is less than 0.2*(Island-Based AGC Tolerance). If not, the MVA Convergence Tolerance will be set to
0.2*(Island-Based AGC Tolerance). This will order the tolerances such that (MVA Convergence
Tolerance) < (Island-Based AGC Tolerance) < (Transfer Tolerance). The original tolerances will be
restored when the initial state stored with the ATC tool is restored.
When studying a transfer between areas or super areas, there are two AGC tolerances to deal with.
These are both checked to make sure that they are less than 0.5*(Transfer Tolerance) and are set
to that value if they are not. Both of these tolerances should also be greater than the MVA
convergence tolerance. The smaller of the two tolerances in multiplied by 0.2 and the
MVA convergence tolerance must be less than this value. If not, the MVA convergence tolerance is
set to 0.2*(smaller of the two area/super area AGC tolerances). All tolerances will be restored to
their original values when the initial state stored with the ATC tool is restored.
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