Electrical Transmission Tower: Types, Design & Parts

What is a Transmission Tower?

A transmission tower (also known as a power transmission tower, power tower, or electricity pylon) is a tall structure (usually a steel lattice tower) used to support an overhead power line. In electrical grids, they are used to carry high voltage transmission lines that transport bulk electric power from generating stations to electrical substations; utility poles are used to support lower-voltage sub-transmission and distribution lines that transport power from substations to electric customers.

Transmission towers must carry heavy conductors safely above the ground and withstand natural disasters. Therefore, designing these towers is a crucial engineering task, integrating civil, mechanical, and electrical engineering principles.

Transmission Tower Parts

A power transmission tower is essential for power transmission systems and consists of several parts:

1. The peak of the transmission tower
2. The cross arm of the transmission tower
3. The boom of transmission tower
4. Cage of transmission tower
5. Transmission Tower Body
6. Leg of transmission tower
7. Stub/Anchor Bolt and Baseplate assembly of the transmission tower.

These parts have been described below. Note that the construction of these towers is not a simple task, and there is a tower erection methodology behind building these high voltage transmission towers.

Peak of Transmission Tower

The portion above the top cross arm is called peak of transmission tower. Generally earth shield wire connected to the tip of this peak.

Cross Arm of Transmission Tower

The cross arms hold the transmission conductors. Their size depends on the transmission voltage, configuration, and stress distribution angle.

Cage of Transmission Tower

The portion between tower body and peak is known as cage of transmission tower. This portion of the tower holds the cross arms.

Transmission Tower Body

The tower body extends from the bottom cross arms to the ground and is vital for maintaining the ground clearance of the bottom conductor of the transmission line.

Transmission Tower Design

During design of transmission tower the following points to be considered in mind,

• The minimum ground clearance of the lowest conductor point above the ground level.
• The length of the insulator string.
• The minimum clearance to be maintained between conductors and between conductor and tower.
• The location of a ground wire with respect to outermost conductors.
• The midspan clearance required from considerations of the dynamic behavior of the conductor and lightning protection of the power line.

To determine the actual transmission tower height by considering the above points, we have divided the total height of the tower into four parts:

1. Minimum permissible ground clearance (H1)
2. Maximum sag of the overhead conductor (H2)
3. Vertical spacing between the top and bottom conductors (H3)
4. Vertical clearance between the ground wire and top conductor (H4)

Higher voltage transmission lines require greater ground clearance and vertical spacing. Therefore, high-voltage towers have higher ground clearance and larger spacing between conductors.

Types of Electrical Transmission Towers

According to different considerations, there are different types of transmission towers.
The transmission line goes as per available corridors. Due to the unavailability of the shortest distance straight corridor transmission line has to deviate from its straightway when obstruction comes. In the total length of a long transmission line, there may be several deviation points. According to the angle of deviation, there are four types of transmission tower–

1. A – type tower – angle of deviation 0^o to 2^o.
2. B – type tower – angle of deviation 2^o to 15^o.
3. C – type tower – angle of deviation 15^o to 30^o.
4. D – type tower – angle of deviation 30^o to 60^o.

As per the force applied by the conductor on the cross arms, the transmission towers can be categorized in another way-

1. Tangent suspension tower and it is generally A – type tower.
2. Angle tower or tension tower or sometime it is called section tower. All B, C and D types of transmission towers come under this category.

Apart from the above-customized type of tower, the tower is designed to meet special usages listed below:

These are called special type tower

1. River crossing tower
2. Railway/ Highway crossing tower
3. Transposition tower

Based on numbers of circuits carried by a transmission tower, it can be classisfied as-

1. Single circuit tower
2. Double circuit tower
3. Multi circuit tower.