Advantages and disadvantages of Power factor improvement methods

Normally the power factor in large generating station with the whole load lies in between 0.8 to 0.9. However in some cases the power factor becomes lagging, so it is desirable to take necessary steps to improve the power factor in power systems. The following methods are the efficient methods to improve the power factor.

1) Static Capacitor method:

This is one of the power factor improvement methods as the power factor can be improved by connecting static capacitors in parallel with the inductive load. The static capacitors draw the leading current which minimizes or neutralizes the inductive lagging current in the power system. This improves the power factor in the power system. This is simple method of improving power factor. The connection of capacitors for single phase and three phase load is shown in below figures. For three phase, the capacitors can be connected in star or delta form.

Static Capacitor Method of power factor correction

Static Capacitor Method of power factor correction

Static Capacitor Method of power factor correction

Advantages:

  • Little maintenance is required as there is no rotating equipment.
  • Less cost because of static capacitors.
  • Easy connecting arrangement with less weight
  • Low losses in this arrangement.

Disadvantages:

  • Short life time (8 to 20 years)
  • Easily damaged for higher voltages compared to rated.
  • Once the capacitors are damaged, the repair of them is difficult.

Synchronous condenser method:

When the synchronous motor is over excited it takes the leading current which acts a capacitor. I.e. A synchronous motor with over excitation is called as synchronous condenser.  Same as static capacitors, if the synchronous condenser is connected in parallel with the power system loads, it neutralizes the lagging reactive component in the current and helps in improving power factor of the power system. The synchronous condenser method is explained with following example.

Synchronous Condenser Method of power factor correction

Synchronous Condenser Method of power factor correction

Synchronous Condenser Method of power factor correction

Fig shows the synchronous condenser connected in parallel with the inductive three phase load.  The synchronous condenser draws the current Im with the load current IL, The total current is the phasor sum of the Im and IL. As the Im is the leading to the supply voltage, then the phasor sum I is the improved power factor. Synchronous condensers usually used at major bulk supply sub stations for power factor improvement.

Advantages:

  • We can achieve step less power factor using synchronous condenser method by changing the magnitude of current with change in excitation.
  • Faults in synchronous condenser can be removed easily.
  • The motor windings offer higher thermal stability for higher short circuit currents.

Disadvantages:

  • Considerable losses in the motor.
  • Maintenance cost is high due to rotating component.
  • It produces noise.
  • At low power rating the cost is high compared to static capacitors method.
  • As the synchronous equipment is not self starting, so extra excitation equipment is needed for that purpose.

Phase advancer’s Method:

Phase advancers are used to improve the power factor for the induction motors. The low power factor of an induction motor is due to the fact that its stator winding draw more current for excitation which lags the supply voltage. So if the excitation ampere turns can be provided from some other AC source, then stator winding can be relived and the power factor can be improved. This job is effectively accomplished by the component called phase advancer.

The phase advancer circuit is mounted on the same rotor shaft and connected to rotor circuit for providing extra ampere turns at slip frequency for the motor during starting or less load and thus improves the power factor of the induction motor.

Advantages:

  • There will be considerable reduction in lagging KVAR because the phase advancer provider extra ampere turns at slip frequency of the motor.
  • This can be used with induction motor instead of synchronous motor in some conditions where it cannot be used.

Disadvantages:

  • It is not economical to use for the motors below 200Hp.
  • This can be used only for motors.
  • Maintenance is high because it is connected to rotating part.