What is power factor

Index

•  Power  Factor
• Types of power factor

               1. Lagging power factor

                2. Leading power factor

• Causes of low power Factor
• Disadvantages of low power factor
• Power Factor Improvement or Correction

              1. Using Static Capacitor

              2. Using Synchronous Condenser

              3.Phase Advanser
  

What is power factor?

      The ratio of the active power (P), measured in KW and the apparent power (S), measured in KVA is defined as the power factor.
      Also,the power factor is used to represent the fraction of total energy that is used for doing useful work and the fraction of energy is stored in the form of magnetic energy in the inductor and capacitor of the circuit, and its values lies between 0 and 1.

     The most economical value of power factor lies between 0.9 and 0.95. The power factor of the circuit can be lagging ,leading and unity.

                                                      

1.      The ratio between actual power and apparent power is called power factor

Actual or real power = KW

Apparent power = KVA

Power factor = KW/KVA

2.      The cosine of angle between current and voltage is called power factor

3.      The ratio between resistance and impedance is called power factor

Types of Power Factor:

1. Lagging power factor

2. Leading power factor 

1. Lagging power factor. 

  Lagging power factor means ,"If  the current (I) flowing in the circuit leads voltage (E) in the circuit, then the power factor is said to be lagging ".

                       The pictorial representation of the lagging power factor, based on the phase angle between the voltage (E)  and current (I). 

     

2. Leading power factor 

 Leading power factor means ,"If the voltage (E) in the circuit leads the current (I) flowing in the circuit, then the power factor is said to be leading ".

               The pictorial representation of the leading power factor, based on the phase angle between the voltage (E)  and current (I). 

      

Causes of low power factor.

The different causes of low power factor are given below :

•  Single-phase and three-phase induction motors, which are normally used in AC circuits, where the current lags the voltage by 90`, as it is purely inductive in nature.
•  Other inductive equipment, which cause low power factors are :transformers,generators, arc lamp,electric furnaces and so on.
•  Load variation in the power system i.e., if the power system is loaded lightly, the power factor becomes low, as the current drawn by the equipment increases due to increases in voltage.
•  Existence of harmonic current reduces the power factor.
•  Imbalance in the power system due to improper wiring or electrical accident.

    Disadvantages of low power factor .

• Current drawn by the circuit will be high.
• Copper loss in the equipment will be high and hence, the efficiency of the equipment decreases.
• Equipment gets overheaded due to the copper loss, which in turn increases the stress on the insulation of the cable and makes it weak.
• As the current drawn by the circuit increases, the size of the conductor has to be increased to carry such current, which in turn increases the cost.
• Increases the KVA rating of the machine, which increases the size of the equipment, thereby increasing the cost.
• Voltage drop in the equipment will be increased, thereby affecting the voltage regulation of the equipment.
• Leads to decrease in the active power, which results in uneconomic operation of the equipment.

Power factor  Improvement or Correction :

   If the power factor of the system is low, improvement or correction of power factor is necessary. Adjusting the power factor of the system closer to unity using some equipment is known as power-factor improvement or correction.Power factor improvement or correction reduces the apparent power consumed by the load and hence, the current drawn by the load will decreases. In other words, power factor correction or improvement is the injuction of reactive power into the circuit to neutralize the effect of lagging current. Static capacitors or synchronous condensers or phase advancers are used to improve or correct the power factor of the system.

Using Static Capacitors :

    Static capacitors, whose ratings vary from 15 kVAR to 10000 kVAR, are used as devices to improve or correct power factor of the system. In a three- phase system, the capacitors arranged in star or delta connection arc used to improve the power factor.

Advantages of Static Capacitors:

1. Simplest method of power factor improvement or correction.
2. Maintenance required in the system is less.
3. Easier installation and less weight.

Disadvantages of Static Capacitors:

1. Life span of capacitor is short i.e.,8-10 years.
2. Gets easily damaged due to over voltages.
3. Once the capacitor is damaged, it is difficult to replace.
4. Production of switching surges and harmonics happen due to constant switching.

                                                                                                                                                                             Applications of Static Capacitors:

                           Capacitors in the range of a few hundred kVAR are used in industrial distribution circuits; capacitors of 500-3000 KVAR rating are used in small distribution substations and those with larger ratings are used in large substations.

Using Synchronous Condensers:

            The working of a synchronous motor can be varied, based on the excitation given to its windings. The excitation to the winding is classified as over-excitation, under-excitation and normal-excitation. A synchronous motor in over-excitation and no load condition draws leading current and starts to act like a capacitor and is known as a synchronous condenser. Varying the field excitation can control the amount of kVAR supplied to the load using synchronous condensers.

Advantages of Synchronous Condensers:

1. Life span of the equipment is longer i.e.,almost 25 years.
2. Power factor control is fexible and reliable.
3. Harmonics have no effects on synchronous condensers , as ther is no switiching mechanism.

Disadvantages  of Synchronous Condensers:

1. Losses in the synchronous condensers are high.
2. They are expensive and have a high maintenance cost.
3. The noise generated by them pollutes the environment.
4. Auxilliary device is required , as a synchronous condenser is not self- starting.
5. Use synchronous condenser is uneconomical for equipmnt below 500 kVA.

      Applications of Synchronous Condensers:

                       Synchronous condensers are used in large factories, industries and large substations to improve the power factor and voltage regulation.

Using Phase Advancers:

        In a induction motor, the power factor is low, as the stator winding draws lagging current from the supply. Phase advancer, which can be used only in induction motors to improve the power factor, ia a simple AC exciter. It is mounted on the shaft of the induction motor and is connected to the rotor circuit. Phase advancer supplies exciting ampere-turns to the rotor circuit of induction motor at slip frequency, which improves the power factor of the induction motor.