Brief technical description

Many electrical devices have inductive loads. These all require both active electrical power and reactive electrical power. The active electrical power is converted into useful mechanical power, while the reactive electrical power is used to maintain the device’s magnetic fields. This reactive electrical power is transferred periodically in bot h directions between t he generator and the load (at the same frequency as the supply). Capacitor banks and buried cables also take reactive energy.

Vector addition of the real (active) electrical power and the reactive electrical power gives the apparent power. Power generation utilities and network operators must make this apparent power a vailable and transmit it. This means that generators, transformers, power lines, switchgear, et c. must be sized for greater power ratings than if the oad only drew active electrical power.

Power supply utilities (both on-site and off-site) are faced with extra expenditure for equipment and additional power losses. External suppliers, therefore, make additional charges for reactive power if this exceeds a certain threshold. Usually, a certain target power factor of cost of between 1.0 and 0.9 (lagging) is s pecified, at which point the reactive energy requirement is significantly reduced.

(Electrical) power factor = Real power/Apparent power

For example, using the power triangle, if:

• real power = 100 kW and apparent power = 142 kVAr
• then the power factor = 100/142 = 0.70.

This indicates that only 70 % of the current provided by the electrical utility is being used to produce useful work.

If the power factor is corrected, for example by installing a capacitor at the load, this totally or partially eliminates the reactive power draw at the power supply company. Power factor correction is at its most effective when it is physically near to the load and uses state-of-the-art technology.

The power factor can change over time so needs to be checked periodically (depending on site and usage, and these checks can be anything from 3 to 10 years apart), as the type of equipment and the supplies listed (above) change over time. Also, as capacitors used to correct the power factor deteriorate with time, these also require periodic testing (most easily carried out by checking if the capacitors are getting warm in operation).

Other measures to take is:

• to minimise operation of idling or lightly loaded motors
• to avoid operation of equipment above its rated voltage
• to replace standard motors as they burn out with energy efficient motors
• even with energy efficient motors, however, the power factor is significantly affected by variations in load. A motor must be operated near of a high power factor design