The Basics of Power Factor in Electrical Distribution Systems

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In AC circuits, power is utilized in two basic forms: Active or Useful power measured in KW and Reactive power measured in KWAR. Before we get to understanding the concept of power factor, let us define the terms measured above:

Active Power
This is the useful working Power (also called Actual Power or Real Power). It is the power that actually powers the equipment and performs useful work. This power is measured in KW.

Reactive Power
It is the power that magnetic equipment (transformer, motor and relay) needs to produce the magnetizing flux.

Apparent Power
It is the vectorial summation of KVAR and KW. It is measured in KVA. So vectorially,
KVA = KW + KVAR

In summary, Reactive power is required to set up the magnetic field while the Active power produces the useful work (shaft horsepower in the case of electric motors). Total Power is the vector sum of the two and typically represents what we pay for in electricity bills. Having understood the basic concepts above, we now define power factor.

Power Factor (P.F.) is the ratio of working power to apparent power. This is defined mathematically as:



Thus, for a given KVA:
  • The lower the ratio of KW to KVA, the lower the power factor 
  • The higher the ratio of KW to KVA, the larger the power factor.
Power factor has a maximum value of 1.
The Power Triangle illustrates this relationship between KW, KVA, KVAR, and Power Factor:



 
From this power triangle, we can deduce that a system with good power factor will possess the following characteristics:
  • KVAR would be very small (approaching zero)
  • KW and KVA would be almost equal
  • The angle (formed between KW and KVA) would approach zero 
  •  Cosine would then approach one
  • Power Factor would approach one  
In order to have an efficient system, we want power factor to be as close to 1.0 as possible. In practice however, our electrical distribution sometimes has a power factor much less than 1.0.  What then causes low power factor in the electrical distribution system?

Causes of Low Power Factor
The cause of low power factor in the electrical distribution system especially in an industrial complex is mainly Inductive Loads. Most plant loads are Inductive and require a magnetic field to operate. The magnetic field is necessary, but produces no useful work. Typical inductive loads found in a plant are:
  • Transformers
  •  Induction motors
  • Induction generators (wind mill generators)
  • High intensity discharge (HID) lighting

These inductive loads constitute a major portion of the power consumed in industrial complexes. Reactive power (KVAR) required by inductive loads increases the amount of apparent power (KVA) in the electrical distribution system. This increase in reactive and apparent power results in a low power factor.