What is transmission line Supports ?
The function of line support is obviously to support the conductors. Line support must be capable of carrying the load due to insulators and conductors including the ice and wind loads on the conductors along with the wind load on the support itself. The line supports are of various types including wood, steel and reinforced concrete poles, and steel towers either of the rigid or flexible type.
A distinction is drawn between straight poles which lie in direct line of transmission and normally only support the conductors and the special poles which may carry some load due to conductor tension. These latter supports may be angle towers, terminal towers, towers at tee-off parts, anchor towers, or towers for some special purpose such as for crossing the rivers. The dimensions of these line supports are governed by ISI regulations. In case of telegraph or railway line crossing special requirements are to be met with.
The main requirements of the line supports are:
1. High mechanical strength to withstand the weight of conductors and wind load etc.
2. Light in weight without the loss of mechanical strength.
3. Cheaper in cost.
4. Low maintenance cost.
5. Longer life.
6. Good looking
7. Easy accessibility for painting and erection of line conductors.
The choice of line supports for a particular situation depends upon the line span, cross-sectional area, line voltage, cost and local conditions.
The design of an overhead line support depends upon the fact whether the support is rigid or has a certain amount of flexibility in the direction of the line. Wooden poles and some special types of steel structures are of latter type and only the transverse wind pressure occurring upon the conductors and upon the support itself is usually considered in their design. The longitudinal pull of the conductors is normally balanced on either side of the support but in the event of breaking of one or more conductors on one side, there will be an unbalanced load which may be far in excess than transverse wind pressure. With flexible supports this unbalanced load is quickly absorbed by an increase in sag in undamaged span on account of bending of the supports on each side of the wrecked span towards the adjoining spans. After three or four spans the longitudinal pull becomes negligible. A certain general rigidity in the longitudinal direction is provided for in practice by using rigid anchoring towers at an interval of 1.5 km or so. These anchoring towers are designed to withstand the breaking of one wire in three on one side as well as the transverse load.
In case of rigid supports such as the lattice-steel broad-base structures equal strength is usually provided in both the longitudinal and transverse directions and every tower is designed to withstand the unbalanced load on account of breaking of one wire in three on the same side. In addition, anchor towers are often provided, in which the support is capable of withstanding the failure of two conductors out of three, or even of all conductors on one side.
1. Wooden Poles.
These supports are cheapest, easily available, provide insulating properties and therefore, are extensively used for the distribution purposes specially in rural electrification keeping the cost low. Their use is usually limited to low pressures (up to 22 kV) and for short spans (up to 60 metres). In districts having a plentiful supply of suitable timbre and where the cost of transporting steel towers is high single and 'H' poles have been used for overhead lines operating at voltages up to 130 kV and average span lengths of 150 metres.
Sal or chir wooden poles up to 11 metre length with minimum circumference of 38 cm at the top and 66 cm at the bottom are used. These poles must be straight, strong with gradual taper and free from large knots. The poles should be properly seasoned to prevent rapid decay due to opening of cracks. To prevent decay owing to snow and rain, the wooden poles are protected by an aluminium, zinc or cement cap at the top. The portion of the pole i.e., 2 metres from bottom which is buried in the ground must be treated with creosite oil or any preservative compound.
The wooden poles well impregnated with creosite oil or any preservative have life from 25 to 30 years. Wooden poles are very elastic and lines employing wooden supports are often designed throughout for the transverse load. Longitudinal strength at terminals and for anchor support is provided by means of guys. Double pole structures of A or H types are often employed for obtaining a higher transverse strength than that could be economically provided by means of single poles.
Single-member poles, shown in above Fig. , are ordinary poles and are used in all positions where there is no undue stress or tension and where no transformer or switchgear are to be mounted on them.
"A" poles shown in Fig. are used mainly where bends in the lines cause strain and single poles are not suitable.
“A” poles consist of two member poles spaced at the base and joined at the top, held together by cross-bars in the form of the letter A.
"H" poles shown in Fig. comprising two single poles strapped together by steel or wooden cross-pieces are used mainly where transformers and switchgear are to be mounted on them.
Four member poles, shown in Fig., comprise two 'H' units in the form of a square joined by cross-bars.
They are used where extra-heavy transformers and switchgear are required, usually at the junction of number of circuits.
The height of a wooden pole depends upon clearance above the ground surface and secondly, the number of cross-arms and other equipment to be attached. Normally, the height of wooden pole is 10 to 12 m.
The main drawbacks of wooden supports are : tendency to rot below the ground level, comparatively smaller life, less mechanical strength and requirement of periodical inspection. Wooden poles are covered under IS: 876-1961.
2. Steel Poles.
The steel poles are of three types (i) tubular poles (ii) rail poles and (iii) rolled steel joists. The tubular poles are of round cross section, the rail poles are of the shape of track used for railways and rolled steel joists are of I cross section.
Such poles possess greater mechanical strength and permit use of longer spans (50-80 m) but cost is higher. Their life is longer than that of wooden poles and life is increased by regular painting. These poles are set in concrete muffs at the foundation inorder to protect them from chemical action.
The average life of steel poles is more than 40 years. The advantages of tubular poles are that these are lighter in weight and easy to install though initial cost is little more as compared to wooden poles. It does not require special equipment for its erection. The tubular poles are covered under IS: 2713-1969. Tubular poles in height of 9 to 11 m are generally used for distribution purposes in cities to give good appearance. Steel rail poles in height of 11 m and 13 m are used for transmission purposes at 11 kV and 33 kV respectively.
3. RCC Poles.
Poles made of reinforced cement concrete (RCC), usually called the concrete poles, are extensively used for low voltage and high voltage distribution lines up to 33 kV. Their construction should conform to the standard specification for RCC work, but in no case the dimensions shall be less 25 cm x 25 cm at the bottom and 13 cm x 13 cm at the top. RCC poles must conform to IS: 785-1964.
These poles are of two types in shape. One type is of square Cross section from bottom to top. The other type has rectangular bottom and square top with rectangular holes in it to facilitate may be manufactured at site itself to avoid heavy cost of transportation. However, prestressed concrete poles, called the PCC poles, are less bulky and lighter than RCC poles.
PCC poles are extensively used on 11 kV and lt lines. PCC poles must conform to IS: 1678-1960.
4. Lattice Steel Towers.
Though there is no hard and fast rule but wooden poles are generally used for distribution purposes in rural area, the steel tubular poles and concrete poles are usually used for distribution in urban area to give good appearance and steel rails or narrow-base, lattice-steel towers are used for transmission at 11 kV and 33 kV and broad-base lattice-steel towers are used for transmission purposes at 66 kV and above. The broad-base, lattice-steel towers are mechanically stronger and have got longer life.
Due to their robust construction long spans (300 m or above) can be used and are much useful for crossing fields, valleys, railway lines, river etc. Even though these are two to four times costlier than wooden poles, yet for tall supports and longer spans these prove more economical. Furthermore their reliability is of a high degree. The substantial construction of the towers renders them capable of withstanding the most severe climatic conditions, and immune from destruction by forest fires. The risk of service interruptions, due to broken or punctured insulators, is considerably reduced owing to use of large spans. Lightning troubles are also minimized as each tower is a lightning conductor, whereas on wooden pole lines shattered poles and wrecked line sections are not infrequent.
Steel towers are fabricated from painted or galvanized angle sections which can be transported separately and the erection done on site. At a moderate cost these can be designed for double circuit giving a further insurance against discontinuity of supply. In case of breakdown to one circuit it is possible to carry out repairs while maintaining the continuity of supply on the other circuit. The height of the tower depends on the line voltage and length of span. The legs of the towers are set in special concrete foundations.
The forces to be taken into account in the design of a tower are vertical loads of conductors, insulators, fittings and tower itself, wind pressure on conductors and wind pressure on tower itself.
Steel towers can be broadly classified as tangent towers and deviation towers. Tangent towers can be used for straight runs of the line and up to 2° line deviation from the straight run. The line is straight or along the tangent to the line route. In such towers the stress is because of the weight of the conductors, ice and wind loads. In addition extra forces due to break in the line on one side of the tower is also to be considered in the design of towers. The base of such a steel tower may be square or rectangular. Insulators used with such towers are suspension types.
For deviations exceeding 2º, special angle towers, sometimes called the deviation towers, are used. They are used where the transmission line changes direction. Such towers have broader base and stronger members as they are to withstand the resultant force due to change in direction in addition to the forces to which the tangent towers are subjected. Insulators used with such towers are of strain type. The cost of deviation tower is comparatively larger than that of a tangent tower hence it is designed to withstand heavy loading as compared to standard or tangent tower.
Deviation towers are further classified as small and towers (for 20 to 15º change in direction), medium angles towers (for 15° to 30° change in direction) and large angle towers (for 30° to 60° change in direction and dead end).
For protection against corrosion the steel towers are periodically painted or galvanised. The life of steel towers can be made almost indefinitely large by a reasonable amount of attention to their maintenance.
Advantage of single circuit and double circuit design
Single Circuit Design
1. Its structure is lighter in weight and requires less strong foundation because it is subjected to low wind pressure on conductors and structure itself.
2. It needs much lower support for equal conductor clearance to earth but it requires more way leave for same number of circuits.
3. Two earth wires are required for single circuit as these cannot be disposed at the top.
4. Danger of flashover is most unlikely and repairs can be carried out without danger to workmen from other circuits.
5. Reliability regarding continuity of supply is less.
6. It is more expensive for two circuits than the double circuit design.
7. Greater spacing of conductors is required resulting in greater inductive reactance.
8. The phase performance along the line is unbalanced as The central conductor passes at the top of the support, which is an obvious drawback.
Double Circuit Design
1. Its structure is heavier in weight and of more height. It requires relatively stronger foundation. It is subjected to more wind pressure.
2. It needs taller structure but less way leave for equal number of circuits.
3. Only one earth wire is required for two circuits and more protection against lightning is had due to its disposition at the top.
4. There is always danger from the other live circuit.
5. Reliability regarding continuity of supply is more.
6. It is most economical and cheaper.
7. Lesser spacing of conductors is required, hence the inductive reactance is less.
8. It gives better approach to the triangular arrangement, hence the phase performance will be more balanced.