Correction of Power Factor With Capacitors

Description

Power Factor is the phase relationship of current and voltage in AC electrical distribution systems. Under ideal conditions, current and voltage are "in phase" and the power factor is "1.0." If inductive loads such as motors are present, power factors less than 1.0 (typically .80 to .90 or lower) can occur.

Low power factor, electrically speaking, causes more current to flow in power distribution lines in order to deliver a given number of kilowatts to an electrical load. The effects are:

  • Power distribution systems in the building, or between buildings, can be overloaded by excess current

  • The excess current produces more losses within the distribution systems, and larger voltage drops result

  • Costs can be incurred if the electric utility charges a penalty for low power factor

Generating and power distribution systems owned by an electric utility have their capacity measured in KVA (kilovolt amps).

With unity power factor (1.0), it would take 2,000 KVA of generating and distribution network capacity to deliver 2,000 kW. If the power factor dropped to .85, however, 2,353 KVA of capacity would be needed. Thus, we see that low power factor has an effect on generating and distribution capacity.

Furthermore, ordinary electric meters found in residential and small commercial buildings do not register power factor or KVA. The net result on the electric utility is:

  • Low power factors overload generating and distribution networks with excess KVA

  • The excess KVA load may require additional generating capacity

  • The increased load increases utility distribution losses and reduces the voltage available to customers

If you own a large building, consider correcting poor power factor for any or all of these reasons:

  • To reduce power factor "penalty" charges from the electric utility

  • To restore the current capacity of overloaded conductors within the building or building complex

  • To reduce conductor losses in the building

  • To reduce voltage drop within the building

The most common power factor correction device is the capacitor. It improves the power factor because the effects of capacitance are exactly opposite those of inductance.

The var or KVAR rating of a capacitor shows how much reactive power the capacitor will supply. Since this kind of reactive power is caused by inductance, each kilovar of capacitance decreases the net reactive power demand by the same amount. A 15 KVAR capacitor, for example, will cancel out 15 KVA of inductive reactive power.

Capacitors can be installed at several points in the electrical system and will improve the power factor between the point of application and the power source. However, the power factor and the increased current draw between the load and the capacitor will remain unchanged. Capacitors are usually added at each piece of offending equipment, ahead of groups of motors (ahead of motor control centers or distribution panels) or at main services.

The application of capacitors depends on where the capacitors are to be connected, type of mounting, enclosure, voltage, etc. An electrical contractor or electrical or mechanical engineer can assist you in determining the best route for your power factor correction.