In this article we are going to discuss how power factor correction capacitors can be used, there fundamental principle, and 3 methods they can be deployed.
The low power factor is mainly due to the inductive load. In industry, inductive loads such as induction motors, lamps, etc. are mainly used for various purposes. These inductive loads have lagging current, which results in a low power factor. The low power factor has many drawbacks. Therefore, the improvement of the power factor is necessary to solve the problems associated with the deterioration of the power factor.
Principle of power factor improvement
The basic principle of power factor correction is to connect a leading current taker in parallel with the inductive load to neutralize the effects of lagging currents. The power factor correction capacitors are one such device. To better understand power factor optimization, let us consider an example.
The figure below shows a single-phase inductive load connected to a single-phase supply voltage V. This inductive load receives a lagging current with a power factor equal to cos Ø1.
When the power factor correction capacitors are connected to this load, the current Ic which drives the power supply voltage V will flow by 90°.
Thus the net line current I is the sum of the load phase current I and the capacitor current Ic.
The load reactive current component (IsinØ1) is partially neutralized by the main capacitor current Ic, so the resulting line current I lags the supply voltage by an angle of less than Ø1. This current I lags at an angle of Ø2 is shown in the figure. Since Ø2 < Ø1, cosØ1 < cosØ2. Therefore, the load power factor is corrected from the previous value cosØ1 to cosØ2.
Some important points:
The load reactive current component (IsinØ1) is partially neutralized by the main capacitor current Ic, so the net reactive current component of the line current is (IsinØ1 – Ic). However, the effective current component, IcosØ1, remains constant. So the total line current after power factor correction is given by:
I = IcosØ1 + j (IsinØ1 – Ic)
line current before power factor correction,
I = Icos Ø1 + jIsin Ø1
By comparing the above two currents, we can clearly see the line current drop due to power factor correction.
- The active current component is not affected by the power factor correction. This is because only the reactive current component is neutralized by the power factor correction scheme. This is also evident from the phase diagram. You can watch this:
- Icos Ø1 = I cos Ø1
- Since the active current component does not change, the active power remains the same if the supply voltage is constant.
- VI cos Ø1 = VI’ cos Ø1
- 3) Since the component of the reactive current is reduced from IsinØ1 to (IsinØ1 – Ic), this means that the reactive power demand of the equipment is also reduced.
- Therefore, reactive power = voltage x component of reactive current
- Reactive power demand before power factor correction= V sin Ø1
- Reactive power demand after power factor correction= V (IsinØ1 – Ic)=VIsin Ø1 – VIc= p.f lag kVAR before optimization – capacitor leads kVAR Therefore, the net reduction in reactive power= VIc
Power factor correction method
To improve the low power factor of equipment and improve the overall power factor of the plant or industry, power factor correction methods should be used.
Power factor correction can be performed using the following equipment:
Power Factor Correction Capacitors:
The power factor can be corrected by connecting the fixed power factor correction capacitors in parallel with the load which takes in the delayed reactive power. Since the capacitor is a generator of reactive power, the delayed reactive power demand of the equipment is provided locally by a stationary capacitor. Hence pf has been improved. This method is the same as described in Principles of Power Factor Correction. The advantages and disadvantages of the power factor correction method are shown below.
Features:
This method is very effective because the power factor correction capacitors does not consume real power.
Require little or no maintenance.
Installation is easy and requires less space.
Negatives:
Switching capacitor banks requires circuit breakers or special arrangements.
They are less than 10 years old.
Synchronous Capacitor:
A synchronous capacitor is an over-excited synchronous motor. When the synchronous motor is highly excited, it draws current in advance at a 90° angle from the supply voltage. In other words, you can say it works like a capacitor.
The power factor improvement is achieved by connecting a synchronous capacitor in parallel with the inductive load whose pf must be improved. For more information on power factor optimization for synchronous capacitors, see “What is power factor optimization in synchronous motors?”
Features:
Smooth power factor control can be achieved by varying the field excitation of a synchronous capacitor. For stationary capacitors, the power factor is controlled gradient rather than smooth. This means that running the capacitor bank will improve the power factor to some extent. If additional power factor correction is required, additional capacitors must be added to the bank. However, with synchronous capacitors, further improvement in power factor is achieved simply by changing the field excitation.
Thermal stability against short circuit current of the motor windings is high.
Negatives:
This method is less efficient compared to the capacitive method due to losses.
High maintenance cost.
The total cost of this method is higher compared to the fixed capacitor method up to 500 kVA.
Auxiliary equipment is required to start the synchronous capacitor. This is because synchronous motors do not start on their own.
Advanced Stage:
Phase advances are used for power factor correction in induction motors. Induction motors are known to operate at a delayed power factor. This is because the stator windings of an induction motor draw excitation current with a lag from the mains supply, which leads to the building of air gap flow. It is clear that if this excitation current can be supplied by other means, the electric current will be excused from supplying delayed reactive power to the stator windings.
Therefore, the power factor of the induction motor is improved.
Phase Advancer is mounted on the motor shaft of an induction motor and provides exciting ampere windings (MMF) to the rotor circuit at slip frequency. The amount of power factor correction depends on the amount of MMF provided by the phase presenter. By giving mmf greater than or equal to the required value, the induction motor can be run at a leading power factor.
