Three Phase System|Advantages Of Three Phase System

Three Phase System: In an electrical power system, there are two types of systems, namely: single-phase and poly-phase systems.In a three-phase system, three conductors can carry three alternating electrical quantities at the same frequency.

A single-phase system consists of two wires, where the current flows through one wire and returns through another wire, when it is energized by two terminals.

Generally, in most of the households and small industries where the required capacity of a motor is not greater than 5 horsepower, single-phase systems are used.

But nowadays, a three-phase system, which is a type of poly-phase system, is used to generate, transmit and distribute electrical energy.

In a three-phase system, three conductors can carry three alternating electrical quantities at the same frequency.

The electrical quantities in these three conductors reach the same peak amplitude at different
instances, as shown in Figure given below.

If any one of the alternating quantities is taken as a reference, the other two are delayed by one-third and two-thirds of a cycle of the alternating quantity i.e., 120° apart.

Therefore, three-phase power systems can be viewed as the combination of three separate single-phase systems, with 120° phase difference.

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Contents

A THREE-PHASE POWER SYSTEM

The colour codes of the wires used in a three-phase system vary from country to country.

In India, Red (R), Yellow (Y) and Blue (B) are the colour codes used in three-phase systems.

The two different configurations by which the three wires in a three-phase system are connected are: star (Y) connection and delta (D) connection.

The different types of three-phase power systems are:

(i) three-phase, three-wire system and
(ii) three-phase, four-wire system.

The fourth wire in a three-phase four-wire system is the neutral wire, represented in black colour.

It is known that the three-phase power system can be used as source and load.

As a source, a three-phase power system can be used as either three or four-wire star connection or three-wire delta connection.

Similarly, as a load, depending upon the application, the type of connection and
configuration of a three-phase power system varies.

Advantages of a Three-phase Power System

The advantages of a three-phase system over a single-phase system are:

The power-to-weight ratio of a three-phase system is high when compared to a single-phase system i.e., for the same electrical power, the size of energy source required in a three-phase system is less when compared to a single-phase system.
For transmission of electrical power, the requirement of conductor material in a three-phase system is less, when compared to a single-phase system.
Instantaneous power is always constant in a three-phase system, when compared to pulsating power in a single-phase system.
A three-phase system will have better power factor and efficiency, when compared to a single-phase system.
For a given size of the system, higher output power can be obtained in a three-phase system, when compared to a single-phase system.
A three-phase system is efficient, reliable, economical and has better regulation when compared to a single-phase system.
When a fault occurs in a single line, the other two lines can be used to transmit the power to the load.
Motors that operate on a three-phase supply do not require any starting devices like a capacitor to run it.
Torque produced in the motor using a three-phase supply is uniform, when compared to pulsating
torque produced by using a single-phase supply.
A three-phase system can be used to supply the electrical energy for a single-phase load.
A three-phase supply can be rectified into DC supply with very low ripple factor, when compared to a single-phase supply.
Parallel operation is easy in a three-phase system, when compared to a single-phase system.

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