Slack Bus Control

Currently there is a limitation in Simulator associated with putting the area that contains the system slack bus on AGC control, such as participation factor, economic dispatch, or optimal power flow.

A problem arises because it is mathematically impossible to uniquely specify the interchange for every area in an electrical island. One area always has to be available to "pick up the slack". For example in a two area system area A cannot export 100 MW to area B, while area B simultaneously exports 50 MW to A.

A remedy is to never place the slack area on AGC control. While this solves the problem, it prohibits economic dispatch, OPF, or participation factor control of the slack area. In this case, it is best to first move the island slack bus to a different area.

We have not yet developed a solution that works in all circumstances. Simulator does solve correctly as long as all the other areas are on AGC control and can actually be controlled. However, if, for example, an area runs out of generation so that it can not control its ACE, then the extra has to be picked up by the slack area. Simulator has problems with this, particularly when the slack area is on participation factor control.

Users are generally advised to avoid putting the slack area on AGC control. However, Simulator will work correctly if an entire electrical island is in a single area or super area, placed on AGC control.