What is Power Factor Correction?
Power factor correction is a technique of improving the power factor of an Electrical system near unity (1) by using extra electrical equipment that can absorb or supply reactive power to the circuit. Actually, the concept behind the power factor correction is the Reactive power compensation technique, which can be done by using a capacitor bank and synchronous condenser in parallel to the circuit.
Why Power Factor Correction?
There are so many benefits of power factor correction such as:
- Reduction in Electricity bill
- Reduction in copper losses of transformers and distribution equipment which increases the overall efficiency of the system.
- Reduction in voltage drop in the circuit which increases voltage regulation.
- It reduces the line current which decreases the burden on cables and also increases the life of the equipment.
- Penalties imposed by power utility companies (DISCOM) for poor power factor can be avoided.
Importance of power factor improvement
The improvement of power factor is very important for both consumers and generating stations as discussed below:
- For consumers: A consumer has to pay electricity charges for his maximum demand in kVA plus the units consumed. If the consumer improves the power factor, then there is a reduction in his maximum kVA demand and consequently, there will be annual savings due to maximum demand charges. Although power factor improvement involves extra annual expenditure on account of p.f. correction equipment, yet improvement of p.f. to a proper value results in the net annual saving for the consumer.
- For generating stations: A generating station is as much concerned with power factor improvement as the consumer. The generators in a power station are rated in kVA but the useful output depends upon kW output. As station output is kW = kVA × cos φ, therefore, number of units supplied by it depends upon the power factor. The greater the power factor of the generating station, the higher is the kWh it delivers to the system. This leads to the conclusion that the improved power factor increases the earning capacity of the power station.
Power Factor Correction Formula
As we discussed in my previous Article, A low power factor is mainly due to lagging current drown by the inductive loads. So before moving to the methods of power factor correction, please note the following points.
- For the pure inductors, the current lags behind the voltage by 90 degrees.
- For a pure capacitor, current leads the voltage by 90 degrees.
- So it is simply that if we use a capacitor in parallel to draw the leading current we can cancel the effect of lagging inductive current. Hence we can improve the power factor of the circuit.
Power Factor Correction methods
- By using a capacitor bank
- By using a synchronous condenser
- By using phase advancer
Power factor correction by using a capacitor bank
Both connections improve the system power factor. But generally, a delta-connected capacitor bank is preferred for power factor improvement. Because in delta delta-connected system the capacitor value required per phase becomes one-third times (small in delta) of a star-connected system.
Advantages of Capacitor bank
- Low losses.
- Low maintenance.
- No foundation is required. Hence Easy to install.
Disadvantages of Capacitor bank
- Capacitors get easily damaged due to over-voltage
- Short life (8-10 years)
- Once damaged, the repair is costly and uneconomical.
- Due to constant switching, switching surges and harmonics may be produced.
Power factor correction by Synchronous condensers
When a 3-phase synchronous motor runs in overexcited mode, it draws a leading current and behaves as a capacitor. Hence synchronous condenser is nothing but an overexcited synchronous motor running without mechanical load (at no load).
The most attractive feature of the synchronous condenser is that it gives dynamic power factor correction over a wide range of excitation. When under-excited, it operates at a lagging power factor and therefore absorbs reactive power from the bus. When overexcited it works at the leading power factor and generates reactive power therefore, it behaves as a capacitor. A static capacitor bank provides PF control in discrete steps whereas a synchronous condenser furnishes continuous control of power factor correction.
Advantages and Disadvantages of Synchronous condenser are mentioned below:
Advantages of synchronous condenser
- Reliable and long life span (almost 25 years)
- Flexible and continuous control of power factor.
- Not affected by harmonics.
- No switching required hence free from switching surges.
Disadvantages of synchronous condenser
- It has more losses as compared to a capacitor bank
- High maintenance cost
- It produces Noise pollution
- It has slow response due to the large time constant of its field circuit whereas the capacitor bank offers a fast response.
- A synchronous condenser can be installed only at one place whereas the capacitor bank can be distributed at many places hence due to this the capacitor bank is more effective in controlling reactive power flow and voltage profile.
- Uneconomical for below 500kVA.
Power factor correction by Phase Advancer
- It can be used only for Induction Motors.
- Since we know that stator winding of The induction machine draws lagging current from the mains supply.
- Hence, to improve the power factor of Induction motor, we have to supply this lagging current from an alternative source. Therefore this alternative source is the phase advancer.
- A phase advancer is basically an AC exciter, which is mounted on the same shaft of the main motor and connected in the rotor circuit. It supplies exciting ampere turns to the rotor circuit of induction motor at a slip frequency. So this way we can improves the power factor of the induction machine.
- Another attractive feature of phase advancer is that if we supply more amp-turns than needed, the motor will operate in an overexcited state (at leading PF).
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