# Comparison of Conductor Material in Underground Electric System

## Underground Electric Transmission System

The system of electric power transmission uses underground cables for transmitting the electric power from the generating station to the consumers is known as underground electric transmission system.

Underground cables have low visibility and are not affected by bad weather. However, the cost of these cables is comparatively high and their laying process is also time consuming.

## Types of Underground Transmission System

Based on the type of supply (i.e. AC or DC) and the number of conductors used, the underground electric transmission system can be classified into the following types −

• Underground DC System

• Two-Wire DC System
• Two-Wire DC System with Mid-Point Earthed
• Three-Wire DC System
• Underground 1-Phase AC System

• 1-Phase Two-Wire AC System
• 1-Phase Two-Wire with Mid-Point Earthed
• 1-Phase Three-Wire AC System
• Underground 2-Phase AC System

• 2-Phase Three-Wire AC System
• 2-Phase Four-Wire AC System
• Underground 3-Phase AC System

• 3-Phase Three-Wire AC System
• 3-Phase Four-Wire AC System

## Comparison of Conductor Material in Underground Systems of Transmission

In underground system, if the electric power is transmitted using the multi-core belted type cables, then the chief electric stress on the insulation is between conductors. Therefore, under such situations, the comparison of conductor material is made on the basis of maximum voltage between the conductors.

For comparing the conductor material needed in the underground electric power transmission system, the following assumptions are to be made −

• The same amount of electric power (say P watts) is transmitted by each system.

• The maximum voltage between conductors (say Vm) is the same in each system.

• The length (say l meters) over which the electric power is transmitted remains the same in each case.

• The line losses (say W watts) are the same in case of each system.

The table given below shows the ratio of conductor material required in any underground system with respect to the underground 2-wire DC system. Here, cos𝜙 is the power factor of the load in AC System.

SystemVolume of Conductor Material (Same maximum voltage between conductors)
DC System
(i) 2-wire system1
(ii) 2-wire with mid-point earthed1
(iii) 3-wire system1.25
Single-Phase AC System
(i) 1-phase 2-wire system$\mathrm{\frac{2}{cos^{2}\, \phi }}$
(ii) 1-phase 2-wire with mid-point earthed$\mathrm{\frac{2}{cos^{2}\, \phi }}$
(iii) 1-phase 3-wire system$\mathrm{\frac{2.5}{cos^{2}\, \phi }}$
Two-Phase AC System
(i) 2-phase 3-wire system$\mathrm{\frac{2.914}{cos^{2}\, \phi }}$
(ii) 2-phase 4-wire system$\mathrm{\frac{2}{cos^{2}\, \phi }}$
Three-Phase AC System
(i) 3-phase 3-wire system$\mathrm{\frac{1.5}{cos^{2}\, \phi }}$
(ii) 3-phase 4-wire system$\mathrm{\frac{1.75}{cos^{2}\, \phi }}$