Causes and Disadvantages of Low Power Factor

Causes and Disadvantages of Low Power Factor

Power factor is a very important concept in power system engineering. A low power factor penalizes consumers as well as the power utility companies (DISCOM). There are so many disadvantages and adverse effects of low power factors in the electrical power system network. In this article, we are going to discuss what are the disadvantages and adverse effects of low power factors in Electrical systems. Also, we will discuss the main causes of the poor power factor.

But before that, we have to understand the basic concepts of the Power factor and its importance in our electrical system. I have already discussed it in previous articles concepts and the importance of the power factor. So please follow my previous Article titled as: What is power factor and why is it important?

Ideally, the power factor should be unity (1)But in the Practical case, it is very difficult to achieve unity PF. There are so many causes of low power factors in electrical systems. So let’s discuss the effect and the Causes of low power factor and also the disadvantages of poor power factor.

What is the Low Power Factor

Power factor plays an important role in AC circuits since optimum utilization and consumption of power depends mainly on power factor. We all know that Power and current for single-phase and three-phase AC circuits are calculated as:
It is clear from both equations (1) and (2) that for a constant power and supply voltage, Current (I) is inversely proportional to CosФ i.e. power factor. So in other words, we can say that when the power factor increases, the current decreases and vice-versa. Hence in case of a low power factor, the load current will increase. This high load current causes the following disadvantages in the Electrical system.


Disadvantages of Low Power Factor

These are the main disadvantages of Low Power Factor in our electrical system.
  • Large kVA rating and size of Electrical equipment
  • Large conductor size and so higher cost of transmission line
  • High Transmission loss hence poor efficiency
  • Poor Voltage regulation
  • Penalties imposed by power utility companies (DISCOM)

Let’s discuss all these disadvantages in detail:

  1. Large kVA rating and size of Electrical equipment: We all know that most of Electrical Machineries (Transformers, Alternators, switchgear, etc.) are rated in kVA. However, it is clear from the below formula that the Power factor is inversely proportional to the kVA rating of electrical machines. i.e.  kVA = kW/ CosФ
    Therefore, for a low Power factor, a larger kVA rating of Machines. Hence large kVA rating of electrical equipment makes them costly and heavier in size.
  2. Effect on Transmission Lines (Larger conductor size and cost): At a low power factor, for transmitting a fixed amount of power at a constant voltage the conductor will have to carry more current. As the current carrying capacity of the conductor is directly proportional to the cross-sectional area of the conductor. So to transmit high current, greater conductor-size transmission lines are needed.
    For example, consider a single-phase AC motor with a load of 20kW on full load with a terminal voltage of 250V. Then At the unity power factor, the Full load current would be 20,000/250*1 = 80A. Whereas at a low power factor (say 0.8), the Full load current will be = 20000/250*0.8 = 100A.
    Here from the above example, it is clear that if a motor works at a poor power factor of 0.8 then it draws more line current than at unity power factor. Hence we required a greater conductor size at a low power factor. This increases the cost of the conductor as well as the transmission line.
  3. Large Copper Losses and Poor Efficiency: We know that copper losses become directly proportional to the square of the line current. Also, the line current becomes inversely proportional to the power factor of the circuit.
    Hence by combining these two relations, we concluded that copper losses will be inversely proportional to the square of power factor. So due to low power factor line current will be higher as well as copper losses will be higher. This results in poor efficiency of the power system network.
  4. Poor voltage regulation (High voltage drop): Since low power factor causes large line current to drown by the electrical equipment. So large current at a low lagging power factor causes a higher voltage drop in alternators, transformers, transmission lines, etc. This results in the decreased voltage available at the supply end of equipment and hence poor voltage regulation. So in order to keep receiving end voltage in the permissible limit, we need to install extra regulation equipment (Voltage regulator).
  5. A penalty from Electrical Power Supply Company: Electrical Power Supply Company imposes a penalty on the consumer in the electricity bill for maintaining a low power factor (below 0.9).

The above discussion leads to the conclusion that the Low power factor is an objectionable feature of an Electrical power system. We should always try to improve the power factor near to unity for efficient and economic operation of the electrical system. Now let’s discuss the main causes of low power factor in the electrical system.

Causes of Low Power Factor

The main cause for the low power factor is because of the highly inductive industrial load. When we say inductive industrial load, then of course we are talking about induction motors. Apart from induction motors, induction heating furnaces, and arc lamps are also the source of poor power factor.
In the case of inductive load, the current lag behind the voltage. Therefore power factor becomes a lagging nature. The following are the primary Causes of the Low power factor.

Inductive Load

  • 90% of the industrial load consists of induction motors (1-ϕ and 3-ϕ). Such machines draw magnetizing current to set up the magnetic field for its proper working and hence work at a low power factor
  • For induction motors, the power factor is usually extremely low (0.2 – 0.3) at light loading conditions and rises to  0.8 to 0.9 at full load.
  • The current drawn by inductive loads is lagging which results in a poor power factor.
  • Other inductive machines such as transformers, generators, arc lamps, electric heating furnaces, electric discharge lamps, etc. also work at low power factor.

Variations in power system loading

  • A modern power system is an interconnected power system. So according to the different sessions and times, the load on the power system is not always constant. It varies during the entire day. It is more during the morning and evening (Peak load) but less during the rest period of time.
  • When the system is loaded lightly, voltage increases which increases the magnetization current demand of the machines. This results in a Poor power factor.

Harmonic Current

  • The presence of harmonic current in the system also reduces the power factor of the system.
  • In some cases, due to improper wiring or electrical accidents which 3-phase power imbalance occurs. This results in a low power factor too.

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Hey, I'm Satish Gupta an Engineer by profession and blogger by passion. I am writer and founder of this blog, Here I publish contents related to Electrical and Electronics Engineering..

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