Ancillary services (electric power)
"those services necessary to support the transmission of electric power from seller to purchaser given the obligations of control areas and transmitting utilities within those control areas to maintain reliable operations of the interconnected transmission system."
and identifies six different kinds of ancillary services:
- scheduling and dispatch
- reactive power and voltage control
- loss compensation
- load following
- system protection
- energy imbalance
Scheduling and Dispatch
Usually performed by the Independent System Operator or Transmission System Operator, both are services dedicated to the commitment and coordination of the generation and transmission units in order to maintain the reliability of the power grid.
Scheduling refers to before-the-fact actions (like scheduling a generator to produce a certain amount of power the next week), while dispatch refers to the real-time control of the available resources.
Reactive power and voltage control
Reactive power can be used to compensate the voltage drops, but must be provided closer to the loads than real power needs (this is because reactive power tend to travel badly through the grid). Notice that voltage can be controlled also using transformer taps and voltage regulators.
- U.S. Federal Energy Regulatory Commission 1995, Promoting Wholesale Competition Through Open Access Non-discriminatory Transmission Services by Public Utilities, Docket RM95-8-000, Washington, DC, March 29.
- E. Hirst and B. Kirby, “Ancillary Services,” Oak Ridge National Laboratory, Technical Report ORNL/CON 310, February 1996. http://www.ornl.gov/sci/btc/apps/Restructuring/Ancillary_Services.pdf