Power Factor Correction

Power factor correction (PFC) is designed to increase power factors and, thus, power quality. It lessens the burden on the electrical distribution system, improves energy efficiency, and lowers electricity costs. Furthermore, it reduces the likelihood of equipment instability and failure. Power factor adjustment is achieved by connecting capacitors that provide reactive energy in opposition to the energy absorbed by loads such as motors, which are located close to the load. This technique improves the power factor from the point where the reactive power source is attached, eliminating needless current flow in the network.

Table of Contents

• Basics of Power Factor
• What is Power Factor Correction?
• Power Factor Correction Formula
• Power Factor Correction Circuit
• Power Factor Correction using Capacitor Bank
• Power Factor Correction using Synchronous Condenser
• Phase Advancer
• The benefits of power factor correction
• Conclusion

Basics of Power Factor

Power quality is critical for efficient equipment performance, and power factor helps with this.  The power factor is a measure of how efficiently incoming power is used in an electrical installation. It is the ratio between active and apparent power when:

Active Power (P) = the power required for useful work such as giving light or pumping water, expressed in Watts or Kilowatts (kW).

Reactive Power (Q) is a measure of stored energy reflected to the source that produces no beneficial work, expressed in var or kilovar (kVAR).

Apparent power (S) is the vector sum of active and reactive power, given in volt-amperes or kilovolt-amperes (kVA).

What is Power Factor Correction?

Power factor correction is a technique for improving the power factor of alternating current (AC) circuits by lowering reactive power. These strategies increase circuit efficiency while decreasing the current taken by the load. Typically, capacitors and synchronous motors are employed in circuits to reduce inductive elements (and hence reactive power). These approaches are used to reduce seeming power rather than increasing genuine power.

In other words, it minimizes the phase difference between voltage and current. So it tries to keep the power factor close to unity. The power factor's most affordable value is from 0.9 to 0.95.

Power Factor Correction Formula

Assume a system has an inductive load; to correct the power factor, we have added extra equipment in the form of a capacitor.

Power factor correction

Image used courtesy of electrical4u 

The work of the capacitor here is to minimize the lagging effect of the inductive load. I_C  is the capacitor current that leads the voltage by 90°. Here, the angle between voltage V and I_R (resultant current after adding the capacitor) = ф₂ Angle between V and I_L (Load current before adding capacitor) = ф₁

Phasor Diagram
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ф₂  is less than ф₁, and thus, cosф₂ >cosф_{1 .}Thus, the value of power factor cosф₂ is increased. From the phasor diagram:

This formula for capacitance is used to improve the power factor.

Power Factor Correction Circuit

Power factor correction approaches mostly utilize capacitors or capacitor banks, as well as synchronous condensers. There are three techniques for correcting the power factor, depending on the equipment utilized.

1. Capacitor Bank
2. Synchronous Condenser
3. Phase Advancer

Power Factor Correction using Capacitor Bank

Capacitors or capacitor banks may have fixed or variable capacitance. They connect to an induction motor, a distribution panel, or the main power supply. The fixed-value capacitor is continuously connected to the system. A variable value capacitance alters the quantity of KVAR based on the system's requirements. The capacitor bank is connected to the load to rectify the power factor. If the load is three-phase, the capacitor bank can be connected using a star and delta configuration.

Star Connection
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Delta Connection
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Power Factor Correction using Synchronous Condenser

When a synchronous motor becomes overexcited, it absorbs the leading current and functions as a capacitor. The synchronous condenser is an overexcited synchronous motor that runs at no load. 

 Synchronous condenser   Image used courtesy of electrical4u 

When this type of equipment is linked in parallel to the supply, it draws a leading current. And boosts the system's power factor. When the load has a reactive component, it draws current from the system, which is a lag current. To neutralize the current, this device draws a leading current. 

Synchronous Condenser Phasor Diagram
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The current drawn by the load is IL before adding the synchronous condenser, and the power factor is фL. It takes current Im, and the power factor is фm after adding the synchronous condenser. Now the resultant current becomes I. Between the power factor angles фL and фm, фm is less than фL. So, according to the provided phasor diagram, cosфm is greater than cosфL.

Phase Advancer

Excitation current causes an induction motor to draw reactive current. If another source of excitation current is employed, the stator winding is isolated from it. And a motor's power factor can be enhanced. This layout can be accomplished by utilizing the phase advancer. The phase advancer is a simple AC exciter that is installed on the same shaft as the motor and connected to the motor's rotor circuit. It generates excitation current for the rotor circuit at slip frequency. If you supply more exciter current than is required, the induction motor might operate on the leading power factor.

The benefits of power factor correction

• Savings on electricity: Power factor correction (PFC) avoids reactive energy penalty, lowers kVA demand, and reduces power losses created by the installation's transformers and conductors.
• Increased available power: PFC technology installed on the low voltage side enhances the power available at the secondary of an MV/LV transformer. A high power factor optimizes an electrical installation by making better use of its components.
• Reduced installation size: Installing PFC equipment allows for a reduction in conductor cross-section because the compensated system absorbs less current for the same active power.

Conclusion

Power factor correction employs parallel-connected capacitors to counteract the effects of inductive devices and lessen the phase shift between voltage and current. Power factor correction is a technique that uses capacitors to minimize the reactive power component of an alternating current circuit, hence increasing efficiency and decreasing current. Power factor correction optimizes the phase angle between the supply voltage and current while keeping the real power consumption in watts constant, as we have seen that pure reactance consumes no real power. Adding additional impedance in the form of capacitive reactance in parallel with the coil above reduces and thus raises the power factor, lowering the circuit's rms current drawn from the supply.