Main Consideration in the Mechanical Design of Transmission Lines

Main Consideration in the Mechanical Design of Transmission Lines

Main Consideration in the Mechanical Design of Transmission Lines

Overhead transmission lines are supported between two towers. In that case overhead transmission lines have to overcome stress and strain due to weather effects.

Transmission lines are need to be designed such tension does not increases beyond allowable limit.

Major factors affecting design of transmission lines are:

1. Conductor parameters
2. Sag-tension relation
3. Effect of changes in temperature and weather parameters
4. Allowable clearances from the ground.

(1)Loading effect on transmission conductors

Loading effect is mainly due to:

1. Conductor weight
2. Ice weight deposited on transmission conductor
3. Wind load acting horizontally on the conductor.

For designing of transmission line loadings are divided as

1. Heavily loading
2. Medium loading
3. Lightly loading

Different Types Of Transmission Line Loading

(2) Span, Sag and Tension

Span is the distance between towers which depend on the voltage and the conductor size of the transmission line. These transmission lines are sag or dip down due to their own weight.

So there is a need to have minimum clearance between the lowest point of the conductor and the ground. This clearance is adjusted by pulling of conductors resulting into tension on conductor.

When span is larger, sag will be larger.

In order to have minimum ground clearance, it is a need to increase the tower height which increases the cost.

Span selection is depends on the conductor size and the mechanical loading. Most commonly used spans for voltage levels are given in Table.                          

Span and Minimum Ground Clearance at Different Voltage Level

(3) Ground Clearances

It is a need to have minimum allowable clearance between lowest point of the conductor and ground in order to safe ground life and the property. Ground clearances depend on voltage level of the transmission line.

Minimum clearance distance is taken as 6m for low voltage and it increases with voltage magnitude. Above table shows the most commonly used minimum ground clearances.

(4) Earthing of EHV transmission line

Earthing is used to divert lighting and fault current to the ground without affecting system stability.

Earthing conductor can be rod, wire, etc.

For earthing copper-clad steel wires are used most commonly.

Minimum dimension of earthing wires are given below.

                       

Material                                        Dimension

Copper wire                                 35 mm²

Copper clad steel wire               35 mm²

Steel wire                                     50 mm²

Copper plate                               1 × 0.5 mm²

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