OVERHEAD TRANSMISSION AND DISTRIBUTION LINE DESIGN GUIDELINES OVERVIEW
What are the guidelines to be considered in designing overhead electrical lines?
What are the guidelines to be considered in designing overhead electrical lines?
In order to minimise the risk of failure of an overhead line it is necessary to ensure that each component of an overhead line has been designed to meet all the electrical and mechanical loads likely to be experienced in service as far as reasonably practical. In order to achieve this, every line and every structure in that line could be individually designed to meet the project requirements. This would be extremely time consuming and is probably only justified for high value transmission lines. Another approach is to utilise a range of standard structures with pre-designed electrical and mechanical capabilities and apply them to a particular project.
Selection of Insulators
One of the most important and yet one of the most vulnerable links in transmission and distribution is insulators. Porcelain and toughened glass are the materials principally used for supporting conductors on overhead lines, and although these materials are relatively brittle and inelastic, they have proven service experience and are still widely used. The design of synthetic type insulators has improved both electrically and mechanically in recent times and they are being used in urban areas to minimise radio interference and in areas where gunshot or stone throwing is a problem. Insulator damage may occur due to such widely varying causes as lighting (puncture), power arcs, stone throwing, corrosion, gunshot and pollution.
One of the most important and yet one of the most vulnerable links in transmission and distribution is insulators. Porcelain and toughened glass are the materials principally used for supporting conductors on overhead lines, and although these materials are relatively brittle and inelastic, they have proven service experience and are still widely used. The design of synthetic type insulators has improved both electrically and mechanically in recent times and they are being used in urban areas to minimise radio interference and in areas where gunshot or stone throwing is a problem. Insulator damage may occur due to such widely varying causes as lighting (puncture), power arcs, stone throwing, corrosion, gunshot and pollution.
Conductors
Economically, conductors represent between 20 to 40% of the total cost of a line; consequently their selection is of prime importance. In earlier days of electrical power transmission, copper was mainly used as the material of overhead line conductors, however with the expansion of electricity networks, several factors, such as price, weight, availability and conductivity, have virtually compelled Overhead Line Design Engineers to concentrate on aluminium based conductors, eg.
AAC = All Aluminium Conductor
ACRS = All Aluminium Conductor Steel Reinforcement
AAAC = All Aluminium Alloy Conductor
Economically, conductors represent between 20 to 40% of the total cost of a line; consequently their selection is of prime importance. In earlier days of electrical power transmission, copper was mainly used as the material of overhead line conductors, however with the expansion of electricity networks, several factors, such as price, weight, availability and conductivity, have virtually compelled Overhead Line Design Engineers to concentrate on aluminium based conductors, eg.
AAC = All Aluminium Conductor
ACRS = All Aluminium Conductor Steel Reinforcement
AAAC = All Aluminium Alloy Conductor
Conductor Sag Tension Theory
A parabola is the shape of a cable that supports a uniform horizontal load. An example of a parabola is the cable of a suspension bridge that supports the deck below. Whereas a catenary is the shape that is formed by a hanging cable whose weight is a constant per unit of arc length. The word catenary comes from the Latin word catena, meaning chain. Provided that the sag is less than 9% of the span length, there is less than 1% difference in their shapes. So for most practical distribution applications the parabola will suffice and is the assumption generally used for distribution design. The mathematical formulae, which are derived for the parabola, are much simpler than the catenary formulae.
A parabola is the shape of a cable that supports a uniform horizontal load. An example of a parabola is the cable of a suspension bridge that supports the deck below. Whereas a catenary is the shape that is formed by a hanging cable whose weight is a constant per unit of arc length. The word catenary comes from the Latin word catena, meaning chain. Provided that the sag is less than 9% of the span length, there is less than 1% difference in their shapes. So for most practical distribution applications the parabola will suffice and is the assumption generally used for distribution design. The mathematical formulae, which are derived for the parabola, are much simpler than the catenary formulae.
Crossarms
Crossarms may be either wood or steel construction but the general design procedure is the same. Wood crossarms do however have significant benefits with regard to electrical performance associated with lightning outage performance. The mechanical loads to which crossarms may be subjected should take into account the conditions likely to be experiences in service so as to minimise the probability of failure, as far as reasonably practicable, these mechanical loads should be determined in accordance with ESAA C(b)1.
Crossarms may be either wood or steel construction but the general design procedure is the same. Wood crossarms do however have significant benefits with regard to electrical performance associated with lightning outage performance. The mechanical loads to which crossarms may be subjected should take into account the conditions likely to be experiences in service so as to minimise the probability of failure, as far as reasonably practicable, these mechanical loads should be determined in accordance with ESAA C(b)1.
Poles
Pole structures, and particularly single member pole supports are used to carry both high and low voltage conductors. Good pole supports, properly chosen with regards to local conditions and requirements, are a decisive factor in ensuring high continuity of service, long life of equipment and low maintenance costs.
Pole structures, and particularly single member pole supports are used to carry both high and low voltage conductors. Good pole supports, properly chosen with regards to local conditions and requirements, are a decisive factor in ensuring high continuity of service, long life of equipment and low maintenance costs.
Pole Foundations
The design of support foundations is rather more difficult than the design of other overhead line components, as the properties of soil are not as definite as those for other materials such as steel, aluminium, copper etc., and consequently for design purposes soil properties are selected within very widely varying limits.
The design of support foundations is rather more difficult than the design of other overhead line components, as the properties of soil are not as definite as those for other materials such as steel, aluminium, copper etc., and consequently for design purposes soil properties are selected within very widely varying limits.
Ground Stays
It is necessary to stay overhead line supports at locations where the loads exceed the capacity of the pole/foundations so that the stay wire, rod, bed log/screw anchor etc. take the pull due to the conductors. Too much attention cannot be directed to the design, making off and setting of stays, as the future safety of the line, particularly under adverse weather conditions, depends equally as much on being correctly stayed as it does on the proper erection of the conductors. In the economic design of stays it is essential to match the strength of the component parts, ie. The stay wire, rod, bedlog etc.
It is necessary to stay overhead line supports at locations where the loads exceed the capacity of the pole/foundations so that the stay wire, rod, bed log/screw anchor etc. take the pull due to the conductors. Too much attention cannot be directed to the design, making off and setting of stays, as the future safety of the line, particularly under adverse weather conditions, depends equally as much on being correctly stayed as it does on the proper erection of the conductors. In the economic design of stays it is essential to match the strength of the component parts, ie. The stay wire, rod, bedlog etc.
In over head transmission, Electrical power transmission, which can be defined as transport of generator-produced electric energy to loads- is conducted through different types of electrical cables.
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