The nature of reactive power

All inductive (i.e. electromagnetic) machines and devices that operate on AC systems convert electrical energy from the power system generators into mechanical work and heat. This energy is measured by kWh meters, and is referred to as “active” energy.

In order to perform this conversion, magnetic fields have to be established in the machines. The magnetic field is created by the circulation of current in coils, which are mainly inductive. The current in these coils is therefore lagging by 90° relative to the voltage, and represent the reactive current absorbed by the machine.

It should be noted that while reactive current does not draw power from the system, it does cause power losses in transmission and distribution systems by heating the conductors.

In practical power systems, load currents are invariably inductive, and impedances of transmission and distribution systems predominantly inductive as well. The combination of inductive current passing through an inductive reactance produces the worst possible conditions of voltage drop (i.e. in direct phase opposition to the system voltage).

For these two reasons (transmission power losses and voltage drop), the Network Operators work for reducing the amount of reactive (inductive) current as much as possible.

Fig. L4: An electric motor requires active power P and reactive power Q from the power system

Reactive power of capacitors

The current flowing through capacitors is leading the voltage by 90°. The corresponding current vector is then in opposition to the current vector of inductive loads. This why capacitors are commonly used in the electrical systems, in order to compensate the reactive power absorbed by inductive loads such as motors.

Inductive-reactive power is conventionally positive (absorbed by an inductive load), while capacitive-reactive power is negative (supplied by a capacitive load).

As reactive-inductive loads and line reactance are responsible for voltage drops, reactive-capacitive currents have the reverse effect on voltage levels and produce voltage-rises in power systems.

Equipment and appliances requiring reactive energy

All AC equipment and appliances that include electromagnetic devices, or depend on magnetically coupled windings, require some degree of reactive current to create magnetic flux.

The most common items in this class are transformers, reactors, motors and discharge lamps with magnetic ballasts (see Fig. L5).

The proportion of reactive power (kvar) with respect to active power (kW) when a piece of equipment is fully loaded varies according to the item concerned being:

• 65-75% for asynchronous motors (corresponding to a Power Factor 0.8 – 0.85)
• 5-10% for transformers (corresponding to a Power Factor close to 0.995)

Fig. L5: Power consuming items that also require reactive energy