EE35T - Overhead Line Design and Transmission Line Construction

The fundamental purpose of a Transmission or Distribution Line is to carry the active power from one point to another.

A Transmission line should possess the following characteristics:

Components of a High Voltage Transmission Line

1. Conductors

There are several different types of conductors that are used to transmit power and these include:

(i) ACSR - Aluminum Conductor Steel Reinforced. This is the most popular conductor that is used because of its high strength and relatively low cost. It comprises aluminum strands bound around a steel core. The most common are 6/1, 26/7, 54/7.

(ii) ACSR/AW - ACSR Conductor with Aluminum clad steel reinforced core. This is very useful in corrosive environments.

(iii) ACSR/SD - ACSR Conductor that is self damping. It is more expensive than regular ACSR, and comprises two trapezoidal layers of conductor around a steel core.  The strands are made of #6201 Aluminum, and the structure makes them self damping against Aeolian Vibration. They can be strung at very high tensions.

(iv) ACAR - Aluminum Conductor Alloy Reinforced. This comprises strands of #1350 Aluminum around a core made of #6201 Aluminum. It is lighter than ACSR, but more expensive and just as strong. It is used in corrosive environments.

(v) AAC-1350 - Aluminum Conductor made of #1350 Strands. It is used in construction that requires good conductivity and short spans.

(vi) AAAC-6201 - Conductor composed of #6201 Aluminum Alloy. It is stronger than ACSR, and lighter, but more expensive. It is used for long spans in corrosive environments.

Some factors to be considered when selecting the transmission line conductors include:

Finally, the size of the conductor has to be considered. Again, several factors are used in determining the size of the conductor to be used.

Voltage Drop Considerations: The conductor meets the minimum size requirement but transmits the power with an acceptable loss. It is often expressed as a maximum voltage drop of 5%. The total series impedance is equal to the maximum allowable voltage drop divided by the maximum load current.

Thus: 

Thermal Capacity: The conductor should be able to carry the maximum long term load current without overheating. The Conductor is assumed to withstand a temperature of 75 degrees celsius without a decrease in strength. Above this temperature, the strength decreases.

Economic Considerations: The conductor is rarely sized to meet the minimum requirements. The total cost per kilometer or mile must be taken into account as too the present worth of energy losses associated with the conductor. There must also be some compensation for load growth.
 

2. Insulators

There are two types of insulators: Suspension Type and Pin Type. The function of the insulator is to support and anchor the insulator. Additionally, they also insulate the conductor from ground and tend to be made of either glass or porcelain and in some cases, ceramic.
 

3. Support Structures

These serve the purpose of keeping the conductors at a safe height from ground as well as at an adequate distance from each other. The construction of the support is dependent on the cost. The cost takes into account the design and the materials as well as transportation and labour. Galvanised steel self supporting towers as well as wooden H-frame and K-frame are commonly used. The erection of structures is an important part of transmission line construction. The method chosen is dependent on:

Additionally, there are several factors that need to be considered when choosing the method of construction. These include: The above factors are determined by whether there is the choice to use maximum equipment and minimum labour or minimum equipment and maximum labour.

Location of Poles and Structures: Poles and Structures have to be located in observance of the right of way (See Definitions below). The initial step when locating the poles is to establish a plan-profile drawing. These drawings show a topographical contour map of the terrain along the right of way, and a sideview profile of the line, showing elevations and towers. The plan profile drawing acts as a worksheet as to what needs to be done, in dealing with the problems that are posed. They are used to complete the work with respect to structure spotting.

Structure spotting is a process that determines the height, location and type of consecutive structures on the plan profile drawing. Structure spotting should closely conform to the design criteria established for the line. The following steps should be taken when spotting structures:

Towers have to be buried at a certain depth to ensure that they do not collapse. The depth may be from 6 feet up to the height of the tower.

Two types of towers are used:
1. Towers used for straight runs
2. Towers used when bends have to be made in the path of the line (Deviation Towers)

In putting down deviation towers, guyed wires and guyed blocks have to be used to balance the tensile forces on the tower. When two forces act on a tower (which is usually the tension of the line), a resultant force is produced. A guyed wire is used to counteract this resultant force so as to prevent the tower from collapsing. The guyed block is also used, and this is the buried block to which the guyed wire is connected. The block is usually buried at an angle to negate the resultant force on the line. The above description is figuratively shown below.


Figure 1. This is the diagrammatic representation of the use of the deviation tower with the Guyed Wire
 


Figure 2. This is the diagrammatic representation of the use of the Guyed Block







Some Basic Definitions to be familiar with

Right Of Way:

This is the legally granted free space that may be leased or purchased when constructing overhead lines. The right of way has to take into account the environmental and aesthetic value of the area through which the line passes. In locating towers and stringing the lines, the electricity commission has to determine the route of the line. Once this is established, then it is necessary to determine the right of way. In some cases, the right of way cannot be obtained, and as a result, alternate routes, in which the right of way can be obtained must be devised. Right of Way must be clear of trees, or any obstructions which may cause the line to fault, or touch, or even result in the tower collapsing.

Sag:

Sag is defined as the vertical distance between the point where the line is joined to the tower and the lowest point on the line.


Figure 3. Diagram showing the definition of sag.

The sag is as a result of the tensioning of the line and must not be too low otherwise the safety clearances may not be met. Also, the sag had to be such that it caters for ice loading in the winter of temperate climates. If the sag is large, and the line becomes heavily loaded, then the sag will further increase and breach the safety clearances. Similarly, if the sag is low, then when the line contracts in the winter, a low sag will indicate a high tension, and as a result of this contraction, the line may snap. Sag is inversely proportional to the tension of the line, and is given by the formula below.

For high tensions, the sag should be small.
For low tensions, the sag should be high.

Clearances must also be observed when stringing a line. The normal clearances for overhead lines are shown in the table below.
 

Voltage Level
Clearance to Ground
less than 66kV
20 feet (6.1m)
66kV to 110kV
21feet (6.4m)
110kV to 165kV
22feet (6.7m)
greater than 165kV
23feet (7.0m)

Span and Ruling Span:

Span is the horizontal distance between two towers. The Ruling Span is defined as the assumed uniform span that most closely resembles the variety of spans that are in any particula section of the line. The ruling span is used to calculate sag and clearances on the plan profile drawing, and it is necessary in structure spotting. When stringing the line, the general rule is that the spans in the line should not be more than twice the ruling span, or less than half of the ruling span. The approximate relationship for the ruling span is given by the formula below.