Main Components of Overhead Lines
An overhead line may be used to transmit or distribute electric power. The successful operation of an overhead line main Components of Overhead Lines depends to a great extent upon the mechanical design of the line. While constructing an overhead line, it should be ensured that the mechanical strength of the line is such so as to provide against the most probable weather conditions.
In general, the main components of an overhead line are:
(i) Conductors which carry electric power from the sending end station to the receiving end station.
(ii) Supports which may be poles or towers and keep the conductors at a suitable level above the ground.
(iii) Insulators which are attach to supports and insulate the conductors from the ground.
(iv) Cross arms which provide support to the insulators.
(v) Miscellaneous items such as phase plates, danger plates, lightning arrestors, anti-climbing wires etc.
The continuity of operation in the overhead line depends upon the judicious choice of above components. Therefore, it is profitable to have a detail discussion on them.
The conductor is one of the important items as most of the capital outlay is invest for it. Therefore, the proper choice of material and size of the conductor is of considerable importance. The conductor material used for transmission and distribution of electric power should have the following properties :
(i) high electrical conductivity.
(ii) high tensile strength in order to withstand mechanical stresses.
(iii) low cost so that it can be used for long distances.
(iv) low specific gravity so that weight per unit volume is small.
All the above requirements are not find in a single material. Therefore, while selecting a conductor material for a particular case, a compromise is make between the cost and the required electrical and mechanical properties.
Commonly used conductor materials:
The most commonly use conductor materials for overhead lines are copper, aluminium, steel-cored aluminium, galvanised steel and cadmium copper.The choice of a particular material will depend upon the cost, the required electrical and mechanical properties and the local conditions.
All conductors used for overhead lines are preferably strand in order to increase the flexibility.
In stranded conductors, there is generally one central wire and round this, successive layers of wires containing 6, 12, 18, 24 ...... wires. Thus, if there are n layers, the total number of individual wires is 3n(n+1)+1. In the manufacture of stranded conductors, the consecutive layers of wires are twist or spirall in opposite directions so that Main Components of Overhead Lines layers are bind together.
Copper is an ideal material for overhead lines owing to its high electrical conductivity and greater tensile strength. It is always use in the hard drawn from as a stranded conductor.
Although hard drawing decreases the electrical conductivity slightly yet it increases the tensile strength considerably.
Copper has high current density i.e., the current carrying capacity of copper per unit of X-sectional area is quite large. This leads to two advantages. Firstly, smaller X-sectional area of the conductor is require and secondly, the area offer by the conductor to wind loads is reduce. Moreover, this metal is quite homogeneous, durable and has high scrap value.
There is hardly any doubt that copper is an ideal material for transmission and distribution of electric power. However, due to its higher cost and non-availability, it is rarely use for these purposes. Nowadays the trend is to use aluminium in place of copper.
Note: Solid wires are only used when the area of X-section is small. If solid wires are use for larger X-section and longer spans, continuous vibrations and swinging would produce mechanical fatigue and they would fracture at Main Components of Overhead Lines the points of support.
Aluminium is cheap and light as compared to copper but it has much smaller conductivity and tensile strength. The relative comparison of the two materials is brief below:
(i) The conductivity of aluminium is 60% that of copper. The smaller conductivity of aluminium means that for any particular transmission efficiency, the X-sectional area of conductor must be larger in aluminium than in copper. For the same resistance, the diameter of the aluminium conductor is about 1·26 times the diameter of copper conductor.
The increased X-section of aluminium exposes a greater surface to wind pressure and, therefore, supporting towers must be designed for greater transverse strength. This often requires the use of higher towers with the consequence of greater sag.
(ii) The specific gravity of aluminium:
(2·71 gm/cc) is lower than that of copper (8·9 gm/cc). Therefore, an aluminium conductor has almost one-half the weight of equivalent copper conductor. For this reason, the supporting structures for aluminium need not be make so strong as that of copper conductor.
(iii) Aluminium conductor being light, is liable to greater swings and hence larger cross-arms are require.
(iv) Due to lower tensile strength and higher coefficient of linear expansion of aluminium, the sag is greater in aluminium conductors.
Considering the combined properties of cost, conductivity, tensile strength, weight etc., aluminium has an edge over copper. Therefore, it is being widely use as a conductor material. It is particularly profitable to use aluminium for heavy-current transmission where the conductor size is large and its cost forms a major proportion of the Main Components of Overhead Lines total cost of the complete installation.
Steel cored aluminium:
Due to low tensile strength, aluminium conductors produce greater sag. This prohibits their use for larger spans and makes them unsuitable for long distance transmission. In order to increase the tensile strength, the aluminium conductor is reinforce with a core of galvanise steel wires. The composite conductor thus obtain is know as steel core aluminium and is abbreviate as A.C.S.R. (aluminium conductor steel reinforced).
Steel-cored aluminium conductor consists of the central core of galvanised steel wires surrounded by a number of aluminium strands. Usually, diameter of both steel and aluminium wires is the same. The X-section of the two metals are generally in the ratio of 1 : 6 but can be modified to 1:4 in order to get more tensile strength for the conductor. Fig. 8.1 shows steel cored aluminium conductor having one steel wire surround by six wires of aluminium. The result of this composite Main Components of Overhead Lines conductor is that steel core takes greater percentage of mechanical strength while aluminium strands carry the bulk of current. The steel cored aluminium conductors have the following advantages :
(i) The reinforcement with steel increases the tensile strength but at the same time keeps the composite conductor light. Therefore, steel core aluminium conductors will produce smaller sag and hence longer spans can be use.
(ii) Due to smaller sag with steel cored aluminium conductors, towers of smaller heights can be used.
Steel has very high tensile strength. Therefore, galvanise steel conductors can be use for extremely long spans or for short line sections expose to abnormally high stresses due to climatic conditions. They have been found very suitable in rural areas where cheapness is the main consideration. Due to poor conductivity and high resistance of steel, such conductors are not suitable for transmitting large power over a long distance. However, they can be use to advantage for transmitting a small power over a small distance where the size of the copper conductor Main Components of Overhead Lines desirable from economic considerations would be too small and thus unsuitable for use because of poor mechanical strength.
The conductor material now being employed in certain cases is copper alloy with cadmium. An addition of 1% or 2% cadmium to copper increases the tensile strength by about 50% and the conductivity is only reduce by 15% below that of pure copper. Therefore, cadmium copper conductor can be useful for exceptionally long spans. However, due to high cost of cadmium, such conductors will be economical only for lines of small X-section i.e., where the cost of conductor material is comparatively small compare with the cost of supports.
The supporting structures for overhead line conductors are various types of poles and towers called line supports. In general, the line supports should have the following properties:
(i) High mechanical strength to withstand the weight of conductors and wind loads etc.
(ii) Light in weight without the loss of mechanical strength.
(iii) Cheap in cost and economical to maintain.
(iv) Longer life.
(v) Easy accessibility of conductors for maintenance.
The line supports used for transmission and distribution of electric power are of various types including wooden poles, steel poles, R.C.C. poles and lattice steel towers. The choice of supporting structure for a particular case depends upon the line span, X-sectional area, line voltage, cost and local conditions.
These are make of seasoned wood (sal or chir) and are suitable for lines of moderate X-sectional area and of relatively shorter spans, say upto 50 metres. Such supports are cheap, easily available, provide insulating properties and, therefore, are widely use for distribution purposes in rural areas as an Main Components of Overhead Lines economical proposition. The wooden poles generally tend to rot below the ground level, causing foundation failure. In order to prevent this, the portion of the pole below the ground level is impregnate with preservative compounds like creosote oil. Double pole structures of the ‘A’ or ‘H’ type are often use (See Fig. 8.2) to obtain a higher transverse strength than could be economically provided using single poles.
The main objections to wooden supports are :
(i) tendency to rot below the ground level (ii) comparatively smaller life (20-25 years) (iii) cannot be use for voltages higher than 20 kV (iv) less mechanical strength and (v) require periodical inspection.
The steel poles are often use as a substitute for wooden poles. They possess greater mechanical strength, longer life and permit longer spans to be use. Such poles are generally used for distribution purposes in the cities. This type of supports need to be galvanised or painted in order to prolong its life. The steel poles are of three types viz., (i) rail poles (ii) tubular poles and (iii) rolled steel joints.
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. Moreover, they give good outlook, require Main Components of Overhead Lines little maintenance and have good insulating properties. Fig. 8.3 shows R.C.C. poles for single and double circuit. The holes in the poles facilitate the climbing of poles and at the same time reduce the weight of line supports.
The main difficulty with the use of these poles is the high cost of transport owing to their heavy weight. Therefore, such poles are often manufactured at the site in order to avoid heavy cost of transportation.
In practice, wooden, steel and reinforced concrete poles are used for distribution purposes at low voltages, say up to 11 kV. However, for long-distance transmission at higher voltage, steel towers are invariably employed. Steel towers have greater mechanical strength, longer life, can withstand most severe climatic conditions and permit the use of longer spans. The risk of interrupted serivce due to broken or punctured insulation is considerably reduced owing to longer spans. Tower footings are usually grounded by driving rods into the earth. This minimises the lightning troubles as each tower acts as a lightning conductor.
Fig. 8.4 (i) shows a single circuit tower. However, at a moderate additional cost, double-circuit tower can be provided as shown in Fig. 8.4 (ii). The double circuit has the advantage that it ensures continuity of supply. In case there is a breakdown of one circuit, the continuity of supply can be maintained by the other circuit.