For transmission of power at High voltage Transmission lines needed to be laid. For laying Transmission lines Transmission towers are most widely used in world. Transmission towers are used for following purposes:-
1. To separate high voltage conductors from surroundings and from each other.
2. To keep conductors at sufficient height for safe limits.
An overhead transmission line usually consists of three conductors or bundles of conductors containing the three phases of the power system. The conductors are usually aluminum cable steel reinforced (ACSR), which are steel core (for strength) and aluminum wires (having low resistance) wrapped around the core.
Tower designing is most important part as towers has to sustain natural calamities. There are following parts in Transmission towers:-
1. Tower top
2. Cross arm
4. Insulator string
5. Fork (K-Frame)/ Cage of Transmission tower
6. Tower body & Leg of Transmission tower
7. Tower base
8. Vibration damper
Every part details will be discussed below:-
1. Tower top:- Tower top is used to carry earth shield wire. This is connected at tip of tower also known as earth wire
2. Cross arm:-
Cross arm is used to hold insulators. Insulators are used to carry transmission conductors. Dimensions of cross arm depends upon level of voltage to be transmitted.
Beam is the portion between cross arms. This is used to hold cross arms.
4. Insulator strings:-
Insulator string is used to carry transmission lines. There no. very depending upon the level of Transmission voltage to be transmitted. As the voltage level increase no. of insulators increases in a string.
5. Cage of Transmission tower(Fork K-Frame):-
Cage is the main structure of Transmission towers which provides support to whole body of Transmission tower.
6. Tower body and leg of Transmission tower:-
They are base of transmission tower. During design of Transmission tower minimum ground clearance of the lowest conductor point above the ground level is depends upon Tower body and leg of transmission tower. Higher the voltage to be transmitted higher will be the ground clearance required than higher will be the width between legs of Transmission towers.
7. Tower base:-
Base of tower is the main holding of whole Transmission tower.
8. Vibration Damper:-
As clear from its name these are used for damping out vibrations due to wind in transmission towers. If dampers are not used than conductors get fatigue from where it is hanged. There re 2 types of vibration dampers (i) VORTX/ Stock bridge Type (ii) Spiral vibration damper
Vortex/ Stock bridge type vibration dampers are most widely used.
Transmission tower height is crucial point for any Transmission tower design. Transmission towers height to be designed depending upon:-
(a) Minimum ground clearance higher the level of voltage to be transmitted higher will be the ground clearance.
(b) Maximum sag of conductor. Sag is necessary evil in transmission lines, you can’t lay transmission conductors without sag.
(c) Vertical gap between top and bottom conductors.
(d) Gap between ground wire and top conductor.
A typical transmission tower with horizontal configuration height is 100 feet. These types of towers are designed to bear the vertical load of the conductor weight and horizontal loads from wind against the towers and the conductors.
There are following types of transmission towers:-
a) Suspension/Tangent towers <2°
b) Small angle towers 2 ° -15 °
c) Medium angle towers 15 ° -30 °
d) Large Angle towers 30 ° -60 °
e) Dead end towers >60 °
f) Transposition towers(4Xarms)
g) Special towers – JC, MC, NB, NBMC
h) Pole mounted termination towers
i) Mono poles