Electrical Transmission Tower: Types, Design & Parts

What is a Transmission Tower?

A transmission tower, power tower or electricity pylon supports the conductors, shield wires, insulators and fittings of an overhead line. Steel lattice towers are common, but tubular steel, concrete, wood and composite structures can also carry transmission lines. The line may connect generating plants, electrical substations or other grid nodes. Utilities select towers or poles by loading, corridor, construction and maintenance needs rather than voltage alone.

The structure must keep conductors within required clearances while resisting conductor tension, dead weight, wind, ice and construction loads. Broken-wire, maintenance and site-specific extreme-event load cases may also govern. Electrical-line design, structural analysis and geotechnical design must work together.

Transmission Tower Parts

A lattice tower in a power transmission system can use the parts named in the frozen list. Terminology and component layout vary between utilities and structure families.

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.

The following descriptions refer to the illustrated lattice-tower form. Its engineered tower erection methodology must control temporary stability, lifting loads, member sequence, worker safety and quality checks.

Peak of Transmission Tower

On this tower form, the peak is the portion above the highest conductor crossarm. It supports one or more shield-wire attachment points at the location set by the lightning-shielding design.

Cross Arm of Transmission Tower

Crossarms support insulator assemblies and position the phase conductors. Their geometry and strength depend on electrical clearances, conductor arrangement, insulator movement and the design load cases.

Cage of Transmission Tower

Some lattice-tower drawings call the upper portion that carries the crossarms the cage. The term is not universal. Its bracing transfers crossarm loads into the main tower body.

Transmission Tower Body

The main body transfers upper-structure loads into the legs and foundations. Its height contributes to ground clearance for the lowest conductor of the transmission line, but sag, terrain, insulator movement and structure geometry also affect that clearance.

Transmission Tower Design

The frozen list covers several electrical geometry inputs. A complete tower design also needs route survey data, climatic loads, reliability targets, material rules, foundation conditions, construction loads and maintenance access.

• 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.

The following four-part height breakdown is a teaching aid for a level span. Final structure spotting uses the surveyed profile, governing sag cases, clearance rules, insulator swing and adjacent-span geometry.

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)

Required electrical clearances generally increase with system voltage and insulation level. Tower height does not follow voltage alone: span length, conductor sag, terrain, crossing requirements, phase arrangement and structure type can produce different geometry at the same voltage.

Types of Electrical Transmission Towers

Transmission structures can be classified by line angle, structural duty, material, circuit count or special crossing need.
A transmission line route changes direction to follow its approved corridor and avoid constraints. Angle structures resist the transverse component of conductor tension at those deviations. The frozen A-to-D ranges below are one project convention, not universal 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.

Structural duty provides a more general classification:

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.

A route may also need a purpose-designed structure for a major crossing, phase transposition or unusual equipment arrangement.

The frozen list gives common examples of these special-purpose structures.

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

A structure can also be classified by the number of three-phase circuits it carries:

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