Overhead power lines are used to transmit and distribute electric power. The successful operation of overhead transmission lines depends on the mechanical design of lines. The main components of overhead transmission line are towers, conductors, earth wire, stock Bridge damper, jumper wire, spacer, Aerial Marker Balls, span and sag. Let discuss these components of overhead transmission line in detail.
The main supporting unit of the overhead transmission line is tower. Transmission tower carries the heavy transmission conductors (higher than 66KV ) at a suitable high level from the ground surface. Tower is made up of steel.
Transmission Line Tower Parts :
Transmission tower consists of three main parts. Lower part is the tower body, middle part is the cage while upper part is called the peak of transmission tower. The sides of transmission tower contains cross arms where insulators ↗ are suspended. Overhead conductors are supported by insulators and they are not directly connected to the tower. Moreover, cross arms keeps the conductors at a safe distance from the tower structure. Hence touching the tower doesn’t cause any damage. One thing to be ensure that towers should have a capability to resist all kind of forces like conductor weights, unbalanced conductors and natural calamities (i.e. wind pressure).
ACSR (Aluminum conductor steel reinforced) also known as aluminum bare conductors are generally used for overhead transmission lines. These conductors carry electric power from generating stations to distribution substations. The outer part of conductor consists of multiple wires (strands) of aluminum while the center of conductor is made up of steel material. Steel provides mechanical strength to the conductor while aluminum has a good conductivity so it conducts maximum power. Stranding is done to make the conductor stronger and more flexible. For low voltages, overall aluminum is used.
Earth wire is the topmost wire in overhead power lines. It is also known as shielding wire and is installed at the top of tower above the phase conductors. It is connected directly to the tower with no insulators attached. Shield wire protects the other conductors from lightening by diverting the lightening stroke currents to the ground. It is kept above the phase conductors (shielding angle is about 30) in such a way that the lightning over voltages cannot directly hit the other wires or phase conductors. Earth wire in transmission system is made up of optical ground wire (OPGW). During normal condition, earth wire doesn’t carry any current.
Distance between the two adjacent towers is called span. Span is dependent on the conductor size, voltage level of power line and environmental conditions like (wind, ice loading etc.). During transmission, one thing to be noted that the conductor is not too much stretched between the adjacent towers in order to save conductor material. If the stress in the conductor reach beyond certain value, there is a possibility that the conductor (transmission line) may break due to excessive tension. Hence there is a maximum horizontal spacing limit at which the towers or poles are placed so that they don’t fall on the ground during high wind and ice loading.
Sag in Transmission Line:
In order to keep sufficient tension in the overhead conductors, they are not fully stretched and allowed to have dip or sag under their own weight. We can define sag as:
“Vertical difference in level between the point of support and the lowest point of conductor is called sag”.
Sag plays an important role in the mechanical designs of overhead lines. If sag is increased, more height of supporting towers is required which increases the cost. Similarly, if sag is kept minimum in order to save conductor material, the conductor can be subjected to extra tension. In that case, there is a possibility of mechanical failure of transmission lines. Hence the value of sag should be kept in such a way that the tension in the conductor is in the safe limits and the cost of transmission line also doesn’t go so high.
Stock Bridge Damper :
Due to high pressure wind or high voltages running along the overhead conductors, transmission lines vibrate and oscillate due to which there are chances that strands of overhead conductors become damage or break. Moreover, if the magnitude of vibration is high for long period of time, the hardware of disc insulators may also break as well as cross arms of the tower might also bend. In order to avoid such situations, stock Bridge damper is installed in overhead transmission lines.
Transmission line vibration dampers is used to control wind induced vibrations. It consists of clamps, damper weights and damper cable. Damper weights is connected to overhead conductors through clamp which is made up of aluminum alloy. When conductors vibrates, the clamp also oscillates and flexible damper cable causes the damper weights placed at its ends to vibrate. By oscillating ups and down, damper weights controls the vibrations of overhead conductors.
Jumper Wire :
Line conductors in a transmission line are not connected to each other. Consider the below diagram in which the conductor is connected to the insulator at one side of the tower. At the other side of tower, another conductor is used. Hence there is no electrical connection between the power conductors in a transmission line.
The electrical connection between these conductors is provided by a loop joining the two sides of pole which is termed as jumper. In other words, we can say that jumper is a short length conductor wire used in a transmission line in order to maintain the continuity of electrical power. Moreover, jumper wire is also used as a switch. In case of any fault, it is not necessary to disconnect the whole power line. A jumper wire in that particular area is disconnected and a faulty section can be easily cleared.
Line Spacer in Transmission Line :
Due to high voltages in transmission lines, multiple conductors are used. Spacers are placed in between the multiple conductors in order to prevent them from interacting with each other. They also provide support to these conductors. In the below fig, line spacer is installed in the transmission line having four conductors.
Aerial Marker Balls on Power Lines :
The big orange, yellow red or white balls hanging on the transmission and distribution power lines are known as Aerial Visibility Marker Balls. They are generally found near mountains, valleys and around airports. Actually when you are flying in in these particular areas, overhead conductors are almost invisible. The marker balls are placed on the power lines for making the line conductors visible to the low flying helicopters and aircrafts.
These are the main components of overhead transmission line.