ATC Analysis Methods: Single Linear Step (SL)

 

The Single Linear Step approach is the most common ATC method. This method of ATC analysis uses only information about the present system state and sensitivities (mathematical derivatives) about the present system state. These sensitivities are embodied in the PTDF and LODF calculations.

Consider a transmission line with a limit of 10 MW, present loading of 5 MW and a PTDF of 10%. The estimated maximum transfer without causing an overload on the line is

 

Transfer Limitation = (Limit – Present Loading) / PTDF = (10 – 5) / 0.1 = 50 MW

 

When including contingency analysis, the OTDF (Outage Transfer Distribution Factor) and linearized estimates of post-contingency flows are used to determine the Transfer Limitation.

 

Transfer Limitation = (Limit – Post-Contingency Loading) / OTDF

 

If we find the Transfer Limitation for every transmission branch (and interface) during each contingency, then the ATC is equal to the smallest Transfer Limit value.

Contingency elements have an associated Status that determines under what conditions a particular action will be implemented. When ATC values are calculated using the single linear step method, contingency actions are not actually implemented. Because contingency actions are not actually implemented, TOPOLOGYCHECK and POSTCHECK actions cannot be applied after other actions have been performed and the power flow has been solved. By default, TOPOLOGYCHECK and POSTCHECK actions act as CHECK actions during the single linear step method. Whether or not model criteria are met for CHECK, TOPOLOGYCHECK, and POSTCHECK contingency actions will be determined at the zero transfer level rather than at the transfer level that determines the limiting transfer.

There Iterate on Action Status option that can be set with contingency options, will allow conditional actions to be treated more accurately as part of the linear step calculations. Instead of treating all TOPOLOGYCHECK and POSTCHECK as CHECK actions, these actions are processed after other actions have been linearly implemented. See the Contingency Iterated Linear Analysis topic for details on this process.

Note: Simulator also monitors the possibility that a transfer will reduce the flow on a line until the line reaches its limit for flow in the opposite direction.