Electrical machines work on the principle of conversion of electromagnetic energy.A part of input energy is consumed for creating and maintaining the magneticfield.This part of the input energy cannot be converted into active energy and is returned to the electrical network on removal of the magnetic field. This power is known as “reactive” power Q.This Reactive Power has one half of the Power in the positive area and the other half in the negative area. Unlike true power, reactive power is not useful power because it is stored in the circuit itself. This power is stored by
1)Inductors,
Because they expand and collapse their magnetic fields in an attempt to keep current constant,
2) Capacitors,
Because they charge and discharge in an attempt to keep voltage constant.
Reactive power (vars) is required to maintain the voltage to deliver active power (watts) through transmission lines. Motor loads and other loads require reactive power to convert the flow of electrons into useful work. When there is not enough reactive power, the voltage sags down and it is not possible to push the power demanded by loads through the lines.
Sources Of Reactive Power:
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 i.e. make free to rotate and produce phase difference, 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.
Power Triangle:
· Power dissipated by a load is referred to as true power. True power is symbolized by the letter P and is measured in the unit of Watts (W).
· Power merely absorbed and returned in load due to its reactive properties is referred to as reactive power. Reactive power is symbolized by the letter Q and is measured in the unit of Volt-Amps-Reactive (VAR).
· Total power in an AC circuit, both dissipated and absorbed/returned is referred to as apparent power. Apparent power is symbolized by the letter S and is measured in the unit of Volt-Amps (VA).