Power factor places an important role in AC power system, since the total power depends on this factor.
Total power and current for single phase and three phase is shown in below
For single phase P = VLIL Cos Φ
IL= P / VL Cos Φ
IL œ 1 / Cos Φ
P = √3VLIL Cos Φ
IL = P / √3 VL Cos Φ
It is clear by the above equations, the load current in single phase and three phases is inversely proportional to power factor in power system. I.e. low power factor in a power system leads to higher load current which is a major disadvantage of low power factor.
Disadvantages of Low Power factor:
Poor Power factor or power factor less than unity causes 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.
Causes of Low Power factor:
The Above discussion id cleared that the low power factor is undesirable in power system network. The following are the causes of low power factor in power system.
- Most of the AC motors are induction type whether it is single phase or three phase which have low power factor. Generally induction motors have lesser power factor during light loads (0.2 to 0.3) and increase it to 0.8 or 0.9 at full load. So Operating the induction motors at lesser load reduces the overall power factor of the system.The motor selection should be proper at it should be always operated at maximum rated power to improve the power factor of power system.
- The loads like Arc lams, electric discharge lams and an industrial heating furnace operates at lesser power factor.
- We know that the load on the power systems varies from high to low as per demand. During low load periods due to high supply voltage increases the magnetization current which causes the low power factor of the power system.