MODULE 2Note 1: Mechanical design of Overhead Lines

Overhead Line

• Overhead line is used to transmit or distribute electrical power

For long distance transmission and distribution overhead lines are always prefer over underground system because of economical and constructional reasons.

Main components of Overhead lines1) Conductor: Which carry electrical power from sending

end station to receiving end station2) Supports: This may be poles or towers and keep the

conductors at a suitable level above the ground.3) Insulators: Which are attached to the Supports and

insulate the conductor from the ground.4) Cross arm: Which provide support to the insulators.

5) Miscellaneous items: Such as phase plates, danger plates, lightning arrestors, anti-climbing wires etc.

1. Conductor Material

Conductor material Properties

• High electrical conductivity• High tensile strength• Low cost• Low specific gravity-weight per unit volume should be

small

1. Conductor Material

Commonly used materials are.

i. Copper,ii. Aluminium iii. steel cored aluminium,iv. galvanized steel v. cadmium copper.

All conductors used for overhead lines are preferably stranded in order to increase the flexibility

i. Copper:

• Copper have very good conductivity and greater tensile strength.

• It is always used in stranded form• Copper has high current density. Hence smaller cross section

of conductor is only required.• Copper is an ideal material for conducting electricity

Disadvantage: higher cost and non availability. So now a days aluminium is using for lengthy wires.

ii. Aluminium:• Conductivity of aluminum is 60% of copper hence greater

cross section of conductor is needed.• Specific gravity of aluminum is lesser than cu. Hence

weight is reduced, so lighter supporting structure is only required.• Aluminum conductor is light hence liable to greater

swing. So lengthy cross arm is required.• Sag is greater in aluminum. Because tensile strength is

less.• Aluminum is cheaper than cu.

iii. Steel Cored Aluminium.

• ACSR consist of central core of galvanized steel wires surrounded by a number of aluminium strands.

ACSR have following advantages.• Reinforcement increases the tensile strength. At the

same time composite conductor is still light. Hence it is well suited for long distance transmission.• Due to small sag tower for small height can be used.

iv. Galvanised Steel:

• Steel have very high tensile strength. • Hence galvanized steel is using as the conductor material

for extremely high span and for short line exposed for high stresses. • It is well suitable for rural areas where cheapness is the

primary consideration. • Because of high resistivity these type of conductors are

not using for high power transmission systems.

v. Cadmium Copper:

• Addition of cadmium with Cu. Increases the tensile strength of the conductor. Hence cadmium Cu. Can be useful for exceptionally long spans.

• Because of higher cost such material is only suitable for low cross section conductors.

2. Line Supporters

• Properties• High mechanical strength to withstand weight of

conductor and wind loads etc.• Light in weight without the loss of mechanical strength.• Cost should be less• Longer life• Easy accessibility of conductors for maintenance. Different types of Supportersi. wooden poles ii. steel polesiii. RCC poles iv. Steel towers

i. Wooden poles:

• These are made up of seasoned wood and are suitable for lines of moderate cross sectional area, and of shorter span.

• Such supporters are suitable for rural areas.

• It cannot be used for voltages higher than 20 kV

The steel poles are often used as a substitute for wooden poles.

• They possess greater mechanical strength, longer life and permit longer spans to be used.

• Such poles are generally used for distribution purpose in the cities.

ii. Steel poles.

• The reinforced concrete poles have become very popular as line supports in recent years. • They have greater mechanical strength, longer

life and permit longer spans than steel poles.• They give good outlook, require little

maintenance and good insulating properties. • Main disadvantage is high cost of transport due

to its heavy weight

iii. RCC poles

• For high voltage transmission for long distance steel towers are invariably using.

• Its have grater mechanical strength, good life, can withstand most severe climate conditions and permit the use of longer span.

• Tower footage are always earthed to reduce lightning impacts.

iv. Steel towers.

3. Insulators

• Properties of good insulators area. High mechanical strength.b. High electrical resistancec. High relative permittivity in order to have high dielectric

strength.d. The insulating material should be nonporous.e. High ratio of puncture strength to flash over

Different Types AreI. Pin Type insulatorsII. Suspension Type InsulatorsIII. Strain InsulatorsIV. Shackle Insulators

• Pin Type insulator is secured to the cross cross- arm on the pole

• There is a groove on the upper end of the insulator for housing the conductor.

• These are using for transmission and distribution up to a voltage of 33kV

Pin Type insulators

2. Suspension Type insulators• For high voltages instead of PIN type Suspension type

insulators are using• They consist of Number of porcelain disc connected in series

by metal link in the form of a string• The conductor is suspended at the bottom end of the string

while other end secured with the cross arm.

iii. Strain Insulators

• When there is a dead end of the line or there is a corner or sharp curve, the line is subjected to greater tension.• In order to relieve the line from excessive stress strain

insulators are used• Strain insulators consist of an assembly of suspension

insulators• If tension is too high Too or more Strings are used in parellel

iv. Shackle Insulators

• Shackle insulators are Directly fixed to the Pole with a bolt or to the cross arm.• It is using instead of strain insulators for low voltage

systems

4. Cross Arm• Which Provide support to the insulator• Made up of galvanised Steel or cast iron• Should have good mechanical strength

5. Miscellaneous • Danger Plate• Phase Plate• Lightning Arrestors• Anti- Climbing wires