# What are various systems of power transmission?

## Various Systems of Power Transmission

For the transmission of electrical power from the generating stations to the substations for distribution, there are various types of power transmission systems are adopted. However, for the transmission of electric power, three-phase three wire transmission system is universally adopted.

The different possible systems for electric power transmission are discussed below.

## DC Transmission System

When the electrical power is transmitted using direct current or voltage, then the transmission system is called the DC transmission system. The transmission system can be further classified into following three types −

• DC Two-Wire System

• DC Two-Wire with Mid-Point Earthed

• DC Three-Wire System

## Single Phase AC System

When the electrical power is transmitted using alternating current or voltage and in this system any one of the three phases is used for the transmission of electric power. The single phase AC system is also classified into three types viz −

• Single-Phase Two-Wire System

• Single-Phase Two-Wire with Mid-Point Earthed System

• Single-Phase Three-Wire System

## Two Phase AC System

In a two-phase AC system, the electrical power is transmitted in the form of alternating quantities and any two of the three phase are used for transmitting the electric power. Depending upon the conductors used, the two phase AC system is of following two types −

• Two-Phase Three-Wire System

• Two-Phase Four-Wire System

## Three Phase AC System

When the electric power is transmitted by using the three phases or three line conductors, then the system is called the three phase AC system. There are two types of three phase AC system as −

• Three-Phase Three-Wire System

• Three-Phase Four-Wire System

## Comparison of Various Systems of Power Transmission

As we have seen above, there is a large number transmission systems of electrical power, but it is difficult to say which is the best system for transmitting electric power. As the cost of conductor material is one of the most important factor in a transmission system. Obviously, the best system of electric power transmission is the one for which volume of conductor material required is minimum. Thus, the volume of conductor material forms the basis of comparison among different transmission system for electric power.

Also, while comparing the amount of conductor material required in various systems, the proper comparison shall be made on the basis of same maximum stress on the dielectric, i.e., the comparison of systems has to be made on the basis of maximum voltage between conductors and maximum voltage between conductor and the earth.

The following table gives the ratio of conductor material in any system with respect to that in the corresponding two-wire DC system −

Transmission SystemSame Maximum Voltage to EarthSame Maximum Voltage between Conductors
DC Transmission System
Two-wire system                     1                     1
Two-wire with mid-point earthed                   0.25                      1
Three-wire system                 0.3125                   1.25
Single-Phase AC System
Two-wire system$$\mathrm{\frac{2}{cos^{2}\:\phi }}$$
$$\mathrm{\frac{2}{cos^{2}\:\phi }}$$
Two-wire with mid-point earthed$$\mathrm{\frac{0.5}{cos^{2}\:\phi }}$$
$$\mathrm{\frac{2}{cos^{2}\:\phi }}$$
Three-wire system$$\mathrm{\frac{0.625}{cos^{2}\:\phi}}$$$$\mathrm{\frac{2.5}{cos^{2}\:\phi }}$$
Two-Phase AC System
Two-phase three-wire system$$\mathrm{\frac{1.457}{cos^{2}\:\phi }}$$
$$\mathrm{\frac{2.914}{cos^{2}\:\phi }}$$
Two-phase four-wire system$$\mathrm{\frac{0.5}{cos^{2}\:\phi }}$$
$$\mathrm{\frac{2}{cos^{2}\:\phi }}$$
Three-Phase AC System
Three-phase three-wire system$$\mathrm{\frac{0.5}{cos^{2}\:\phi }}$$
$$\mathrm{\frac{1.5}{cos^{2}\:\phi }}$$
Three-phase four-wire system$$\mathrm{\frac{0.583}{cos^{2}\:\phi }}$$
$$\mathrm{\frac{1.75}{cos^{2}\:\phi }}$$

From the above comparison table, it is clear that −

• There is a great saving in conductor material if the DC system is adopted for the transmitting the electric power.

• In case of AC system, the three-phase AC system is most suitable system for transmitting the power due to saving in the conductor material. Also, this system is convenient and efficient than the other systems.