Reactive Power

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Definition
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.


Sources of Reactive
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.
Synchronous Compensators - Certain smaller generators, once run up to speed and
synchronised to the system, can be declutched from their turbine and provide reactive power
without producing real power. This mode of operation is called Synchronous Compensation.
Capacitive and Inductive Compensators - These are devices that can be connected to the
system to adjust voltage levels. A capacitive compensator produces an electric field thereby
generating reactive power whilst an inductive compensator produces a magnetic field to
absorb reactive power. Compensation devices are available as either capacitive or inductive
alone or as a hybrid to provide both generation and absorption of reactive power.
Overhead Lines and Underground Cables - Overhead lines and underground cables, when
operating at the normal system voltage, both produce strong electric fields and so generate
reactive power. When current flows through a line or cable it produces a magnetic field which
absorbs reactive power.