What is Transmission Tower Erection Methodology ?

What is Transmission Tower Erection Methodology ?

Transmission Tower Definition

A transmission tower is a tall structure used to support overhead power lines, ensuring the safe and efficient transmission of electricity over long distances.

Build-Up Method

This method is most commonly used for the erection of 6.6 kV, 132 kV, 220 kV, and 400 kV transmission line towers due to the following advantages :

• Tower materials can be supplied to the site in a knocked down conditions which facilitates easier and cheaper transportation.

• It does not require any heavy machinery such as cranes etc.

• Tower erection activity can be done in any kind of terrain and mostly throughout the year.

• Availability of workmen at cheap rates.

This method consists of erecting the towers, member by member. The tower members are kept on the ground serially according to the erection sequence to avoid search or time loss. The erection progresses from the bottom upwards.

The four main corner leg members of the first section of the tower are erected and secured first. Sometimes, multiple leg sections of each corner are bolted together on the ground before being erected.

The cross braces of the first section which are already assembled on the ground are raised one by one as a unit and bolted to the already erected corner leg angles. First section of the lower thus built and horizontal struts (belt members) if any, are bolted in position. For assembling the second section of the tower, two gin poles are placed one each on the top of diagonally opposite corner legs.

These two poles are used to raise parts of the second section. The leg members and braces of this section are hoisted and assembled. The gin poles are then shifted to the top of the second section to raise parts of the third section. The gin poles are moved up as the tower grows.

This process is continued until the complete tower is erected. Cross-arm members are assembled on the ground and raised up and fixed to the main body of the tower. For heavier towers, a small boom is rigged on one of the tower legs for hoisting purposes. The members/sections are hoisted either manually or by winch machines operated from the ground.

For smaller base towers/vertical configuration towers one gin pole is used instead of two gin poles. In order to maintain speed and efficiency, a small assembly party goes ahead of the main erection gang and its purpose is to sort out the tower members, keeping the member’s incorrect position on the ground and assembling the panels on the ground which can be erected as a complete unit.

Section Method

In the section method, major sections of the tower are assembled on the ground and the same are erected as units. Either a mobile crane or a gin pole is used. The gin pole used is approximately 10 m long and is held in place by means of guys by the side of the tower to be erected.

The two opposite sides of the tower section of the tower are assembled on the ground. Each assembled side is then lifted clear of the ground with the gin or derrick and is lowered into position on bolts to stubs or anchor bolts.

One side is h held in place with props while the other side is being erected. The two opposite sides are then laced together with cross members and diagonals; and the assembled section is lined up, made square to the line. After completing the first section, the gin pole is set on the top of the first section. The gin rests on a strut of the tower immediately below the leg joint. The gin pole then has to be properly guyed into position.

The first face of the second section is raised. To raise the second face of this section it is necessary to slide the foot of the gin on the strut of the opposite face of the tower. After the two opposite faces are raised, the lacing on the other two sides is bolted up. The last lift raises the top of the towers.

After the tower, the top is placed and all side lacing has been bolted up all the guyed are thrown off except one which is used to lower the gin pole. Sometimes whole one face of the tower is assembled on the ground, hoisted, and supported in position. The opposite face is similarly assembled and hoisted and then the bracing angles connecting these two faces are fitted.

Ground Assembly Method

This method consists of assembling the tower on the ground and erecting it as a complete unit. The complete tower is assembled horizontally on even ground, aligned with the direction of the line for fitting cross arms. On sloping ground, careful packing of the lower side is needed before assembly starts.

After the assembly is complete the tower is picked up from the ground with the help of a crane and carried to its location, and set on its foundation. For this method of erection, a level piece of ground close to footing is chosen from the tower assembly.

This method is not useful when the towers are large and heavy and the foundations are located in arable land where building and erecting complete towers would cause damage to large areas or in hilly terrain where the assembly of the complete tower on the sloping ground may not be possible and it may be difficult to get the crane into position to raise the complete tower.

In India, this method is not generally adopted because of the prohibitive cost of mobile crane, and the non-availability of good approach roads to tower locations.

Helicopter Method

In the helicopter method, the transmission tower is erected in section. For example, the bottom section is first lifted on to the stubs and then the upper section is lifted and bolted to the first section and the process is repeated till the complete tower is erected.

Sometimes, a fully assembled tower is raised using a helicopter. Helicopters lift these towers from the fabrication yards and transport them to the line locations. The helicopter hovers over the location while the tower is securely anchored.

The ground crewmen connect and tighten the tower guys. As soon as the guy wires are adequately tensioned the helicopter disengages and files to the marshaling yard. This method is adopted where the approach is very difficult or to speed up the construction of the transmission line.