Types of Overhead Line Supports
We can divide overhead line supports in two categories. They are poles and towers. The function of overhead line support is to keep the overhead conductors at a certain safe height from the ground. Up to 33kv line we normally use pole type supports. Beyond 33kv that is 66 KV and above we use tower type supports. According to the position of the supports in an overhead line, supports are differently named. The support from which the overhead line starts is called the terminal or dead-end support. The terminal or dead-end support only gets tension from one side because the conductors of the other side are pension free.
Where the orientation of overhead line changes, an angle pole or angle tower is used. In that case because of the diversion of the line the tensions from two sides of the tower or pole are exactly not opposite to each other. Therefore there will be a resultant force acting on the power pole.
The support located in between two angle points of the overhead line is called tangent pole or tower. In the case of a transmission line, this tower is also known as the suspension tower. Since the tension is applied to that tower from two opposite directions, there will be a very small or no resultant tension acting on the tower.
The overhead transmission towers which externalize the transmission line in many sections are called section towers. A section tower may be an angle tower or a tangent tower.
Overhead Line Support Poles
The poles used to support an overhead line are generally made of wood, concrete or steel. Whether we use wooden poles, concrete poles or steel poles depends on the location, cost, maintenance cost, and importance of the overhead line. The overhead line may be supported on the different types of supporting pole arrangements. These may be single-pole, double-pole, three-poles, or four-pole arrangement. The poles used as tangent poles are normally single-pole structures. For terminal points of the line, high angle points of the line and for supporting service transformers the double-pole type structure is normally used. For fixing a service transformer or similar type of equipment on the double-pole structure cross bracing arrangement is used. The double-pole structure is generally called DP in short. Sometimes, due to the constructional view, it is also called H-pole or A-pole. In addition to that, for some specific reasons, such as for tapping the overhead line or where the diversion angle is more than 60 degrees, the three-pole or four-pole structures are used.
Although for domestic low tension supply overhead line only single poles are used for all the purposes and positions.
Previously, for 400/230V LT line and even for 11KV HT line, the wooden poles were used. Wooden poles were also used for 33 KV lines. The cost of a wooden pole is quite cheap. Also, the expenditure for the foundation work is very low. Proper wood treatments make a wooden pole quite durable and to last intact for a long time. This is the reason previously wooden poles were the suitable choice for supporting an overhead line. The sal wood the most suitable means of making a wooden pole. The density of the sal wood is 815 kg per cubic meter. Not only the sal wood, but there are also many other timbers used for the purpose. Presently, for the preservation of forests, the use of wooden poles has become obsolete.
The wooden poles used for the purpose should be straight enough. Also, the wood should not be rotten. There should not be any cracks or holes in the wood used for wooden poles. Sometimes it becomes difficult to find out a long straight wood. In those cases, two smaller poles can be connected together to form a long straight pole.
Wood Treatment for Wooden Poles
The wood must be seasoned properly before using it as a supporting-pole of an overhead line. Seasoning means making the wood properly dry before using it. The dryness prevents the attack of fungus. The fungus makes a wood rotten quickly. Termites also damage the strength of the wood. Moisture and temperature also accelerate the deterioration of the quality of wood. The effect of moisture is maximum below and just above the ground level. So, proper chemical treatment is required to prevent the attack of termites.
Alcatraz mixed with oil or copper chrome arsenic is used for the treatment of wooden pole. The treatment with copper chrome arsenic is called the Ascew treatment of wood. In this process of treatment, the wooden poles are kept in a closed vessel filled with copper chrome arsenic. The pressure inside the container is maintained up to 100 kg per square meter for at least one hour. Due to the high pressure, the chemical is properly inserted inside the wood.
The top of the must be properly curved cut to avoid any accumulation of rainwater on the pole top. The necessary grooves are also to be provided on the upper side of the pole to fit cross arms.
For fixing cross-arms necessary holes to be made in the grooves. The standard diameter of the holes is generally from 17 to 20 millimeters. It is mandatory to make the grooves and holes before putting the poles for chemical treatment.
Concrete Poles for Overhead Electric Lines
The concrete poles used for supporting overhead electric lines are of two types. They are RCC type and PCC type. Presently, PCC type electric poles are widely used for supporting 11KV and 400/230V electric lines. Sometimes, PCC type electric poles are also used for 33kv lines. The cost of a concrete pole is more than the cost of a wooden pole of the same length. But it is cheaper than a rail pole of the same size. The longevity of a concrete pole is much more than that of a wooden pole. Moreover, it is almost maintenance-free. The strength of a concrete pole is weaker than a rail pole but much more than that of a wooden pole.
Disadvantages of using Concrete Poles
1. The weight of a concrete pole is much higher than that of a similar size wooden pole.
2. Because of the weight, the transportation of the pole to the working spot is quite difficult.
3. The workforce required during the installation of a concrete pole is higher enough compared to that of a wooden pole.
4. Careless handling can break the poles during installation.
Structure of Concrete Poles
Concrete poles are made by casting cement. To increase the tensile strength of the poles, iron wires are used inside the concrete. For earthing, one 25 mm x 3 mm copper strip is used inside the concrete. One axial hole is provided otherwise for or passing the earth wire through it. For fitting and fixing of cross arms and other accessories, a concrete pole is provided with a necessary number of 20 mm diameter holes.
The cross-section of a PCC pole is intentionally made rectangular. The tensile strength of the pole is more along the wider side of the cross-section. The wider side of the cross-section is always kept along the bisection of the angle between the conductors of two sides of the pole.
Categories of Concrete Poles
Depending on the tensile strength or tension load carrying capacity, concrete poles are divided into 11 categories.
|Pole Class||Length||Length Inserted in the Ground||Maximum Tensile Strength|
|Class 1||16.5 to 17 m||2.4 m||3000 Kg|
|Class 2||16.5 to 17 m||2.4 m||2300 Kg|
|Class 3||16.5 to 17 m||2.4 m||1800 Kg|
|Class 4||16.5 to 17 m||2.4 m||1400 Kg|
|Class 5||14.5 to 16 m||2.3 m||1100 Kg|
|Class 6||11.5 to 12 m||2.0 m||1000 Kg|
|Class 7||11.5 to 12 m||2.0 m||800 Kg|
|Class 8||11.5 to 12 m||2.0 m||700 Kg|
|Class 9||9.5 to 11 m||1.8 m||450 Kg|
|Class 10||8 to 9 m||1.5 m||300 Kg|
|Class 11||6 to 7.5 m||1.2 m||200 Kg|
Steel Tubular Pole for Overhead Line
The tensile strength of a steel tubular pole is much more than that of a concrete and wooden pole. With proper and regular maintenance, we can make the lifespan of a steel tubular pole much longer than that of a concrete and wooden pole. The cost of a steel tubular pole is much higher than that of a concrete and wooden pole. These poles are widely used for 400/230 volt LT and 11 KV medium voltage lines. Steel poles are also used for 33kv lines. There are two types of steel poles available in the market for the purpose. One is the stepped steel pole and another is the swaged steel pole. The first one is called ISPT and the second one is called ISWP. Both of these poles are jointless steel tubes. Sometimes, steel tubular poles can be made by welding steel sheets.
The athletic look of a steel tubular pole is much better than wooden and concrete poles. The diameter of the steel tubular poles used for HT lines is higher than the diameter of tubular poles used for LT lines. The portion of the pole inserted inside the ground must be coated with bitumen on both inside and outside of the tube to protect against corrosion and rust. The portion of the pole above the ground level should be coated with red oxide (only on the outer surface of the tube). Every Steel tubular pole must be provided with a 14 mm dia hole at a height of 300 m from the ground level to provide earthing connection. Also, each pole is provided with a pole-cap on the top to prevent the entry of rainwater inside the tube.
Rail Poles for Supporting Overhead Lines
Rail poles are the strongest poles used in an electrical overhead line. But the cost of the rail pole is much higher than other types of poles used for supporting overhead electrical lines. In addition to that, the weight of a rail pole is significantly high. Therefore the handling of a rail pole is a very difficult task. Because of its heaviness, the cost of installation of rail poles is also higher than that of other poles.
Types of Rail Poles
Rail poles are normally not used for 400/230V LT lines. Rail poles are often used for 11KV and 33KV lines. The rail poles used for overhead lines are of four standard sizes.
1. 30 kg per meter,
2. 37 kg per meter,
3. 45 kg per meter,
4. 52 kg per meter.
The third type rail poles are generally used for 11KV lines and forth type rail poles are generally used for 33KV lines. The standard length of a rail pole varies from 9 m to 13 m.
Installation of Rail Poles as Overhead Supporting Structures
Before using the rail poles as overhead supporting poles, they need to be coated with red oxide. It is also recommended to coat the rail poles with bitumen up to a certain height from the ground level. It increases the longevity of the pole. The cross-section of a rail pole is shown below.
The tensile strength of a rail pole is more along the y-axis than the x-axis. For using a rail pole as a tangent pole, the x-axis of the cross-section is always kept along the overhead line. For a terminal pole, the y-axis of the cross-section is kept along the overhead line. For an angel pole, the y-axis of the cross-section is kept along the bisection of the angle between the conductors of two sides of the pole.