POWER FACTOR AND ITS IMPROVEMENT

POWER FACTOR AND ITS IMPROVEMENT

INTRODUCTION

Power factor is a ratio of true power over apparent power. Power factor is denoted with cosɸ. Power factor also known as cosine of angle between voltage and current in A.C. System.

In an inductive circuit, current lags behind the voltage by some angle. Since the current lags behind the voltage, this power factor is known as lagging power factor. In case of capacitive circuit, current leads the voltage by some angle then power factor is known as leading power factor.

power-triangle
Power Triangle

In power triangle ΔOAB:

OA = Apparent Power

OB = Active or True Power

AB = Reactive Power

Power Factor,

DISADVANTAGES OF LOW POWER FACTOR

In single phase AC circuit, Power is given by,

In three phase AC circuit, Power is given by,

It is clear from the above equation that the load current is inversely proportional to power factor at constant voltage and current. Lower the power factor, higher is load current and vice versa. The higher load current at lower power factor causes the following disadvantages:

1. Greater Conductor Size

At low power factor, the amount of load current increases to deliver the same amount of power. To carrying the large amount of current, the conductor size is increased. This increases the cost of transmission and distribution.

2. Larger kVA rating of equipments

The electrical equipment like alternators and transformers are rated in kVA. We know that:

The kVA rating is inversely proportional to power factor. This means that with decrease in power factor, the kVA rating of equipment is increased which increases the capital cost.

3. Increase in copper loss

We know that the current is inversely proportional to power factor. With decrease in power factor, the current increases which further increase the I2R losses. This also results in poor efficiency of power system.

4. Poor Voltage Regulation

Low lagging power factor results in large voltage drops in transformers, alternators, transmission and distribution lines. This leads to poor voltage regulation.

Due to above disadvantages, The efforts are made to improve the power factor of the equipments operating at poor power factor.

 

POWER FACTOR IMPROVEMENT

Usually the value of power factor in power system lies between 0.8 to 0.9 lagging. However, sometimes its value falls below 0.8. In such cases, the various methods are used to improve the power factor. The power factor of the power system can be improved by using:

  • Static Capacitors
  • Synchronous Condensers
  • Phase Advancers

1. Static Capacitor

The power factor can be improved by connecting capacitors in parallel with the load which is operating at lagging power factor. The capacitors draws leading current which neutralize the lagging reactive component of original load current, Thus improves the overall power factor. For the three phase balance load, the capacitors can be connected in star configuration or in delta configuration. Mostly, The capacitors are connected in delta because in that case the size of each capacitor reduces and become one third to that of the capacitors which are connected in star. Static Capacitors are mostly employed in factories for power factor improvement.

static-capacitor-bank
Static Capacitor Arrangement

Advantages:

  1. Static capacitors are light in weight and can be easily installed.
  2. Static capacitors has no moving parts.Thus, they require almost no maintenance.
  3. Static capacitors have small losses, hence no energy consumption.

Disadvantages:

  1. Static capacitors have smaller life.
  2. They damaged quickly when the voltage exceeds the rated value.
  3. The damaged capacitors cannot be repaired. 
 

2. Synchronous Condensers

The synchronous condensers are basically an over excited synchronous motor which is working at no load. The over excited synchronous motors takes leading current and behaves like a capacitor. To improve the power factor, the over excited synchronous motor is connected in parallel with the load which operating at lagging power factor. The leading component of current taken by synchronous motor neutralizes the lagging component of the original load current and thus, The overall power factor is improved.

synchronous-condenser
Synchronous Condenser Circuit

The synchronous condensers are used for the load above 500kVA. Below this load, Static capacitors are used to improve the power factor because they are economical. Synchronous condensers are also used to improve the power factor of transmission lines. For this, synchronous condensers are installed at the receiving end stations. The main disadvantage of synchronous condensers is that they sometimes fall out of synchronism.

synchronous-condenser-phasor-diagram
Phasor Diagram

For an example, Consider a 3-phase inductive load taking current I at lagging power factor cosɸ1. A 3-phase synchronous condenser is connected in parallel with the load which draws current Is at leading power factor. The load current I lags behind the voltage by an angle of ɸ1 before the synchronous condenser is connected in the circuit. When the synchronous condenser connected in parallel with the load it takes Is, which leads the voltage by an angle ɸs. The resultant current I’ is the vector sum of I and Is and its angle of lag is ɸ2. It is clear that ɸ2 is less than ɸ1 and cosɸ2 is greater than cosɸ1. Thus, the power factor of the load is improved from cosɸ1 to cosɸ2.

 

LOCATION OF POWER FACTOR IMPROVEMENT EQUIPMENT

The power factor improvement equipments are installed near the location of apparatus which is responsible for low power factor. Static capacitors are connected at the terminals of induction motors, induction furnaces, arc lamps etc. In case of transmission line, Static capacitors are installed at the receiving end of transmission line i.e. receiving sub stations.

Connecting the static capacitors at the terminals of motor and switching the capacitors with the motor load is a very effective method for improving the power factor. In this type of installation, No extra switches or protective devices are required. The corrective capacitance is supplied only when the motor is operating. The synchronous condensers which are installed at the receiving end in case of transmission line are, relieve the transmission line, transformers and the alternators installed at generating station from excessive current due to poor power factor.

 

ADVANTAGES OF IMPROVED POWER FACTOR

We know that, The current is inversely proportional to power factor. That means when the power factor increases, the current decreases. Thus , all the disadvantages of low power factor which mostly occur due to high load current is reduced with improved power factor. The improved power factor has following advantages:

  1. Copper loss are reduced in power system due to reduction in current.
  2. Reduction in size of conductor to deliver the power.
  3. Voltage level at the load end is increased because of less voltage drop and better regulation.
  4. For the same kW output power, The smaller kVA rating is required.

Leave a Reply