Causes and Disadvantages of Low Power Factor


    Introduction

    Power factor is a very important concept in power system engineering. A low power factor penalizes consumer as well as the utility company both. There are so many disadvantages and adverse effects of low power factor in the electrical system.
    In this article, we are going to discuss what are the disadvantages and effects of low power factor in Electrical system? Also, we will discuss the causes of low power factor.
    But before that, we have to understand the basic concepts of Power factor and its importance in the electrical system. We have already written an article about the concepts and importance of 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 Practical case, it is very difficult to achieve unity PF. There are so many causes of low power factor in electrical systems. Here in this article, we will discuss the effect and disadvantages of low power factor, and the Causes of low power factor.

    Low Power Factor


    Power factor plays an important role in AC circuit since the power consumption depends on this factor. We all know that Power and current for single phase and three phase AC circuits are calculated as:

    It is clear from both the equations (1) and (2) that for a constant power and supply voltage, Current (I) is inversely proportional to CosФ i.e power factor. In other words, we can say that when power factor increase, current decreases and vice-versa.
    So in case of low power factor, the load current will increase. This high load current causes the following disadvantages in Electrical system.

    Disadvantages of Low Power Factor

    Following are the major Disadvantages of Low Power Factor in our electrical system.

    1.    Large kVA rating and size of Electrical equipment: 

         We all know that most of the Electrical Machineries (Transformers, Alternators, Switchgears, etc) are rated in kVA. But, it is clear from the below formula that Power factor is inversely proportional to the kVA rating of electrical machines.
    i.e.  kVA = kW/ CosФ
    Therefore, for low Power factor, larger will be kVA rating of Machines. Large kVA rating of electrical equipment makes them costly and heavier in size.

    2.  Effect on Transmission lines (Greater conductor size and cost): 

         At low power factor, to transmit 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. To transmit high current, greater conductor size transmission lines are needed.
    For example, consider a single phase AC motor with load 20kW on full load with terminal voltage 250V. Then At unity power factor, Full load current would be= 20,000/250*1 = 80A. Whereas at low power factor (say 0.8), 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 lower power factor of 0.8 then it draws more line current than at unity power factor. Hence we required greater conductor size at low power factor. This increases the cost of the conductor.

    3.  Large Copper Losses and poor Efficiency: 

         We know that copper losses become directly proportional to the square of line current. Also, the line current becomes inversely proportional to power factor of the circuit. Hence by combining these 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 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 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 to the consumer in electricity bill for maintaining low power factor (below 0.9).
    The above discussion leads to the conclusion that Low power factor is an objectionable feature of Electrical power system. We should always try to improve 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 causes for the low power factor are because of the inductive load. In the case of inductive load, the current lag behind the voltage. Therefore power factor becomes lagging nature.
    Following are the main Causes of Low power factor.

    1. 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 are lagging which results in 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.

    2. Variations in power system loading

    • A modern power system is the interconnected power system. So according to the different session and time, 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 increase the magnetization current demand of the machines. This results in Poor power factor.

    3. 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 in which 3- phase power imbalance occurs. This results in low power factor too.
    Must Read our next Article: Power Factor Correction

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