The United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

  Federal Energy Regulatory Commission (FERC)

Federal Energy Regulatory Commission

The Federal Energy Regulatory Commission is the United States federal agency with jurisdiction over interstate electricity sales, wholesale electric rates, hydroelectric licensing, natural gas pricing, and oil pipeline rates...

 defines the ancillary services as:

"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, 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

Tap (transformer)

A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. This means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output...

 and voltage regulator

Voltage regulator

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components...

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