Power Factor Correction

Power Factor correction

In this article, we are going to discuss the Power Factor Correction Methods. Here we will cover all the concepts related to Power factor correction such as What is power factor correction? Why Power factor correction? All methods of power factor correction and its advantages and disadvantages, Power factor correction formula in detail. But before that, we have to clearly understand the concepts of power factor and causes and disadvantages of low power factorI have already written an article on this topic. So please follow these articles through the given link.

What is Power Factor Correction?

Power factor correction is a technique of improving the power factor of Electrical system near to unity (1) by using extra electrical equipment which can absorb or supply reactive power to the circuit. Actually, the concept behind the power factor correction is Reactive power compensation technique, which can be done by using a capacitor bank and synchronous condenser in parallel of the circuit.

Power factor correction will not affect the amount of true or real power of the circuit, whereas it will reduce the apparent power and reactive power in the circuit. It also reduces the total line current drawn by load and hence the burden of an electrical system.

The main source of low power factor is Inductive loads in the circuit, which normally operates at very low power factor (0.2 - 0.3) at light loading conditions and rises to approx 0.8 at full load. Since low power factor has negative effects in the power system. So the techniques used to improve the power factor of the overall electrical circuit are known as Power factor correction.

Why Power Factor Correction?

To understand the concepts let us consider an example of a single-phase induction motor. It draws a total line current of 10 Amps at a power factor of 0.75. Then the useful current will be 10x0.75 = 7.5 A. Also the useful power to the motor is 230 × 7.5 = 1.725kW. But the total input power of the motor that has to be supplied is 230 × 10 = 2.3 kVA.
Without power factor correction, to achieve the required output of 1.725 kW (7.5 A) we have to supply a power of 2.3 kVA (10 A) to the motor. Also current of 10 A is flowing in the circuit but only 7.5 A of that current is producing useful output. Hence burden on system and also utility bill increases due to poor power factor.
Why power factor correction
There are so many benefits of power factor correction such as:
  1. Reduction in Electricity bill
  2. Reduction in copper losses of transformers and distribution equipment which increases the efficiency of the system.
  3. Reduction in voltage drop which increases voltage regulation.
  4. It reduces the line current which decreases the burden on cables and also increases the life of the equipment.

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 pure inductor, 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

The various methods of Power-factor improvement are as follows:
  1. By using a capacitor bank
  2. By using synchronous condenser
  3. By using phase advancer

1. Power factor correction by using a capacitor bank

In this method, A bank of capacitors is connected in parallel across the load. Since the capacitor takes leading reactive power, Overall reactive power taken from the source decreases. Hence the system power factor improves. A control system usually provided to monitor the power factor of the system and switches the capacitor ON and OFF as per requirement.
Here Star and Delta connected capacitor bank is shown in the figure.

power factor correction by capacitor bank

Both the connections improve the system power factor. But generally, delta connected capacitor bank is preferred for power factor improvement. Because in delta connected system the capacitor value required per phase becomes one-third times (small in delta) of star-connected system.
Advantages and Disadvantages of this method are as mentioned below.

Advantages

  • Low losses.
  • Low maintenance.
  •  Lightweight.
  • No foundation required. Hence Easy to install.

Disadvantages

  • Capacitors get easily damaged due to overvoltage
  • Short life (8-10 years)
  • Once damaged,  the repair is costly and uneconomical.
  • Due to constant switching, switching surges and harmonics may be produced.

2. Power factor correction by Synchronous condensers

When a 3-phase synchronous motor runs in overexcited mode, it draws 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 its excitation. When under excited, it operates at a lagging power factor and therefore absorbs reactive power from the bus. When overexcited it works at leading power factor and generates reactive power therefore, behaves as a capacitor.

A static capacitor bank provides PF control in discrete steps whereas a synchronous condenser furnish continuous control of power factor correction.
Advantages and Disadvantages of this method are as mentioned below.

Advantages:

  • 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:

  • 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 capacitor bank offers fast response.
  • A synchronous condenser can be installed only at one place whereas 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.

 3. Power factor correction by Phase Advancer

  • It can be used only for Induction Motors.
  • Since we know that stator winding of induction machine draws lagging current from the main 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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