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.Power flows, both actual and potential, must be carefully controlled for a power system to operate within acceptable voltage limits. Reactive power flows can give rise to substantial voltage changes across the system, which means that it is necessary to maintain reactive power balances between sources of generation and points of demand on a 'zonal basis'. Unlike system frequency, which is consistent throughout an interconnected system, voltages experienced at points across the system form a "voltage profile" which is uniquely related to local generation and demand at that instant, and is also affected by the prevailing system network arrangements. National Grid is obliged to secure the transmission network to closely defined voltage and stability criteria. This is predominantly achieved through circuit arrangements, transformers and shunt or static compensation.
SOURCES OF 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.1) 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.
2) 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.
3) 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.
4) 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. A lightly loaded overhead line is a net generator of reactive power whilst a heavily loaded line is a net absorber of reactive power. In the case of cables designed for use at 275 or 400kV the reactive power generated by the electric field is always greater than the reactive power absorbed by the magnetic field and so cables are always net generators of reactive power.
5) TRANSFORMERS:
Transformers produce magnetic fields and therefore absorb reactive power. The heavier the current loading the higher the absorption.
6) CONSUMER LOADS:
Some loads such as motors produce a magnetic field and therefore absorb reactive power but other customer loads, such as fluorescent lighting, generate reactive power. In addition reactive power may be generated or absorbed by the lines and cables of distribution systems.