Reactive Power

"Reactive power is a concept used by engineers to describe the background energy movement

in an Alternating Current (AC) system arising from the production of electric and magnetic

fields. These fields store energy which changes through each AC cycle. Devices which store

energy by virtue of a magnetic field produced by a flow of current are said to absorb reactive

power; those which store energy by virtue of electric fields are said to generate reactive

power."

"Most equipment connected to the electricity system will generate or absorb reactive power,

but not all can be used economically to control voltage. Principally synchronous generators

and specialised compensation equipment are used to set the voltage at particular points in the

system, which elsewhere is determined by the reactive power flows.

Synchronous Generators - Synchronous machines can be made to generate or absorb

reactive power depending upon the excitation (a form of generator control) applied. The

output of synchronous machines is continuously variable over the operating range and

automatic voltage regulators can be used to control the output so as to maintain a constant

system voltage."

http://www.nationalgrid.com/NR/rdonlyres/43892106-1CC7-4BEF-A434-7359F155092B/3543/Reactive_Introduction_oct01.pdf

Alternators are also called synchronous condenser due to the fact that it can generate or absorb reactive power by controlling the excitation voltage of the alternators. Over-Excited alternators can generate reactive powers or leading power factor.