Disadvantages of Low Power Factor


Disadvantages of Low Power Factor

The disadvantages of low power factors are three. The first is that transmission lines and other power circuit elements are usually more reactive than resistive.

  • Reactive components of current produce larger voltage drops than resistive components, and add to the total IZ = (I(R + LX)) drop, therefore, the system-voltage regulation suffers more and additional voltage- regulating equipment may be required for satisfactory operation of the equipment using power. T
  • he second disadvantage is the inefficient utilization of the transmission equipment since more current flow per unit of real power transmitted is necessary due to the reactive power also carried in the power lines.
  • If the current necessary to satisfy reactive power could be reduced, more useful power could be transmitted through the present system. The third disadvantage is the cost of the increased power loss in transmission lines.
  • The increased power loss is due to the unnecessary reactive power which is in the system. The reactive power losses vary as the square of the reactive current or as the inverse of the power factor squared.

low power factor

Power in a Three Phase AC Circuit = P = √3 V x I CosФ
And Current in a Three Phase AC Circuits = I = P / (3 V x CosФ)
I ∝1 /CosФ….… (1)
Power in a Single Phase AC Circuits = P = V x I CosФ
And Current in a Three phase AC Circuits = I = P / (V x CosФ)
I ∝ 1/CosФ……… (2)
It is clear from both equations (1) an (2) that Current “I” is inversely proportional to CosФ i.e. Power Factor. In other words, When Power Factor increases, Current Decreases, and when Power Factor decreases, Current Increases.
Now, In case of Low Power Factor, Current will be increased, and this high current will cause to the following disadvantages.

1) Effect on Transmission lines(Greater conductor size):

At low power factor, to transmit or distribute the power at a constant voltage requires more current. To transmit high current, higher conductor size transmission lines are needed. For example, In the case of single phase AC motor with load 20Kw on full load with terminal voltage 250V.

At unity power factor full load current would be 20,000/250 = 50A. But at less power factor = 20000/250*0.8 = 100A. Now all the transmission lines and conductor and motor conductors should be rated to 100A instead of 80A. This increases the cost of transmission lines.

2) Effect on Generators, Transformers:

It is known that the electrical machinery (Generators, Transformers) etc are rated in KVA rather than kW

Where KVA = KW/ power factor

I.e The KVA is inversely proportional to KVA for a given KW. For lower power factor the electrical machinery should be having higher KVA rating to drive constant KW load. For example to drive a 20KW load with a unity power factor requires an electrical machine with 20KVA.If the power factor is lesser then the electrical machine higher than 20KVA is required to drive the load. Then the size and cost of the electrical machine becomes expensive.

3) high losses in power system

Due to the high current for low power factor, the copper losses increases in the transmission conductors and switch gear machinery.

4) High Voltage drops (poor voltage regulation):

The large current at low lagging power factor causes greater voltage drops in alternators, transformers and transmission lines. This results in decrease in voltage at the driving end and enables the use of extra equipment to counter act the voltage drop like voltage stabilizers. This increase the cost of power supply system.

The low power factor reduces the handling capacity of the system. I.e. the reactive competent in the current prevents the full use of machinery in power system.


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