Simulator 16 User Guide - PowerWorld

ATC Analysis Methods: Iterated Linear Step (IL)
The Single Linear Step is an extremely fast method for determining the ATC. However, because it only
uses present operating point information, controller changes are not taken into account. The
linearization assumes that all controllers are fixed. The Iterated Linear Step (IL) method provides an
alternative to the Single Linear Step that allows controller changes, but still performs its analysis in a
reasonable amount of time. The (IL) method operates as follows:
Perform Single Linear Step
Set the Stepsize = Minimum Transfer Limitation found in Single Linear Step which is greater than
the specified When iterating, Ignore Limiters below value
If [abs(Stepsize) = 100] then stop
o If the maximum 100 number of iterations is reached, a transfer limitation will be reported
highlighted in lime green with the Iteratively Found field set to OSCILLATING indicating that
the iterated process is oscillating. The Transfer Limit that is reported is the accumulated
transfer amount that has been ramped as of the last successful solution. The maximum
number of iterations is a value that is set internally. It is unlikely that this limit will ever be
reached because other mechanisms are in place to prevent oscillations.
o The iteration count is reset at the beginning of the iteration process.
Implement transfer by amount of Stepsize and resolve power flow
o The power flow is solved according to the options set with the Power Flow Solution Options.
o When calculating a transfer between injection groups, the Island-Based AGC Power Flow
Solution Options applicable to dispatch using an injection group are used when determining
which generators and loads will participate in the transfer. These options include whether or
not AGC status should be considered when determining which units will participate in the
ramping, whether or not to enforce generator MW limits, and whether or not to allow
negative loads. The power factor to use when adjusting load is also specified with these
options.
o To prevent oscillation of the solution, the Stepsize may be limited if the sign of the Stepsize
changes throughout the iterations. At each iteration, the direction of the Stepsize is checked
and the Stepsize is bounded based on the accumulated transfer at each iteration. The
accumulated transfer is the total amount that the transfer between the seller and buyer has
been ramped. The accumulated transfer is updated with the current Stepsize once it has
been determined that Stepsize can be achieved. At the start of the iterated process,
LowBound is initialized to a large negative number and HighBound is initialized to a large
positive number. At each iteration the following checks are performed:
If Stepsize > 0 then LowBound = (Accumulated Transfer)
Else If Stepsize < 0 then HighBound = (Accumulated Transfer)
If Stepsize >= (HighBound - Accumulated Transfer)*0.7
Then Stepsize = (HighBound – Accumulated Transfer)*0.7
Else If Stepsize