What are the different types of Transmission Line Supports?

Transmission Line Supports

The supporting structures used for overhead transmission line conductors, such as poles and towers, are called the transmission line supports.

Generally, the transmission line supports supposed to possess the following properties −

• The line supports should have high mechanical strength so that it can withstand the weight of conductors and wind loads, etc.

• It should be light in weight without the loss of its mechanical strength.

• It should have longer life.

• It should be cheap in cost and economical to maintain.

• Accessibility of line conductors for maintenance is to be easy.

There are various types of line supports available. The choice of supporting structure for a particular case depends upon the line span, area of cross-section, transmission line voltage, cost and local conditions, etc.

Types of Line Supports

The line supports used for transmission and distribution of electrical power are classified into following types −

• Wooden Poles

• Steel Poles

• R.C.C. Poles

• Lattice Steel Towers

Wooden Poles

The wooden pole supports are made up of seasoned wood such as sal or chir and are suitable for the transmission lines of moderate cross-sectional area and of relatively shorter spans up to 50 meters. The advantage of wooden supports are as follows −

• Cheap
• Easily available
• Provides insulating properties, etc.

Therefore, the wooden poles are widely used for distribution purposes in rural areas as an economical proposition. However, the wooden poles tend to rot below the ground level and hence, causing the foundation failure. Therefore, to prevent this, the portion of the pole below the ground level is impregnated with preservative compounds like creosote oil.

Also, the double pole structures of the ‘A’ or ‘H’ type are often used to obtain a higher transverse strength. The primary objections to the wooden pole supports are as follows −

• Tendency to rot below the ground level.
• Smaller life (about 20 to 25 years).
• Cannot be used for high voltages (> 20 kV).
• Have less mechanical strength.
• Require periodic inspection and maintenance.

Steel Poles

The steel poles are generally used for distribution purposes in the cities. These line supports possess greater mechanical strength, longer life and longer span as compared to the wooden poles. However, the steel poles need to be galvanised or painted for preventing from the corrosion.

The steel poles are of following types −

• Rail Poles
• Tubular Poles
• Rolled Steel Joints

R.C.C. Poles

The RCC (Reinforced Cement Concrete) poles are also used for the distribution purposes and have become very popular as line supports in recent years. The RRC poles have the following properties −

• Greater mechanical strength
• Longer life
• Longer line span than steel poles
• Good insulating properties
• Require little maintenance, etc.

The main problem associated with the use of RCC poles is the high transportation cost owing to their heavy weight. Thus, these poles are often manufactured at the site so that the heavy transportation cost can be avoided.

Lattice Steel Towers

The steel towers are employed for the transmission of electrical power to long distances at high voltage (> 11 kV). The main properties of the steel towers are as −

• Greater mechanical strength
• Longer life
• Can withstand most severe climatic conditions
• Can be used for longer line spans. This reduces the risk of interrupted service due to broken or punctured insulation considerably.

In practice, the tower footings are grounded by driving rods into the earth. This minimises the lightning troubles since each tower acts as a lightning arrestor.

The tower supports can be either single circuit or double circuit. The double circuit tower support has the advantage that it ensures continuity of supply, i.e. if there is breakdown of one circuit, the continuity of supply can be maintained by the other circuit.