Home » BASIC FUNDA » SUPPORTS – POLE AND TOWER.

SUPPORTS:


Supports for overhead lines are mainly of two types (a) Pole and (b) Tower. Up to 33 kv in distribution system poles are used.
Properties of line supports are
(i) High mechanical strength for withstanding weight of conductor and wind pressure.
(ii) Must be economic for installation and maintenance.
(iii) Longer life.
(iv) Light in weight without sacrificing mechanical strength.
As per uses in different situation in line, poles or towers are various types
(a) Tangent pole or mid pole/tower: This type of poles or towers use in the mid section of the straight line. In this case both side of tower or pole maintain opposite tensile strength.
(b) Terminal or end pole/tower: The starting or ending pole or tower is known as terminal or end pole or tower. In this type tower or pole needs higher mechanical strength as line strings at only one end.
(c) Angle pole or Tower: This type of tower uses where line angled with certain degrees. High mechanical strength is also needed here as both side of tower or pole is not maintaining same tensile strength.
(d) Section tower or pole: The pole where line divides, known as section tower or pole. Tangent or angle, both types of tower may be used as section pole.

Poles: There are various types of pole are available, and uses as per requirement.

Wooden pole: Seasoned sal, chir or tick woods are used as poles in distribution system of shorter span (50m). Low tension line upto 11 kv are suitable for this type of pole. The main advantages are low cost, easy available, good insulator, low maintenance. The separate types of wood are use as per required tensile strength. Below ground level this types of poles are painted with creosote oil or alkatra to prevent corrosion.
The main drawbacks are short life span; decompose below ground level, less mechanical strength, periodical inspection.
These types of pole are not uses now a days.

Concrete pole: Now a days the most popular poles are concrete pole. There are two types of concrete poles are available (a) Reinforced cement concrete or R.C.C pole. (b) Pre-stressed cement concrete or P.C.C pole.
This type of concrete pole used up to 33 kv voltage level. It has high mechanical strength, low maintenance and greater life span. The main disadvantages are high cost than wooden pole, and transport cost is high as its heavy weight.
Tubular steel pole: These types of poles are often used with larger span than wooden pole. In developed localities where base area requirement is under consideration or immediate pole installation is required, this type of pole is preferable. Below ground level it colored with bitumen and galvanized rest of the part for prolonged life. It made by joint free steel tube or by welding sheet steel. Its high mechanical strength, light weight, long life is the advantages whereas it is costlier than wooden or concrete pole.
Rail pole: with high mechanical strength and weight, this type of pole is often used in 11 kv and 33 kv line. Below ground level this type of pole is painted by bitumen color and above ground painted by red oxide for improving life. The main disadvantages are its high cost and high transport cost due to heavy weight.

Pole related components:

Cross arm and Bracket: For holding insulators at pole, cross arm and bracket uses. It also keeps the conductors at safe distance. Cross arm are made by steel angle or wood. At low tension line, De-iron clamp and shackle insulator works as the substitute of cross arm.
When only one side crosses arm is used it known as side arm.
Bracing: To strengthen the cross arm, bracing is used which is made by steel or sometime wood is known as knee bracing.

Double pole structure: In case of 11kv and above, where angle pole is used with higher tension, sometimes double pole uses. The two poles are connected rigidly by bracing.
Stays and Struts: In case of terminal pole or angle pole, the tension at both side of tower are not same, for which the pole may bent. To support the pole, strays and struts are used. The stranded steel wire is used as stray rod. One G.I wire is used inside the stranded steel wire to safe men and animal. This is known as cattle guard.
Steel tower: In case of transmission line, where long distance has to be travelled with voltage above 33 KV, steel tower is used. It is much more costly than pole but even it is economical as it carries long span. It has greater mechanical strength and longer life. It minimizes the lightning trouble as each tower act as a lightning conductor. Normally steel towers are galvanized and sometimes painted.

Types of available towers are self supporting type, Guyed type, and chainnet type. Lattice type structure is the most common form of self supporting tower. On the other hand guys are used to secure more mechanical strength of the tower. In chainnet structure V shaped steel leg hinged by the strength of guys. In America and Canada guyed type tower are used whereas in India self supporting towers are available.
Self supporting towers are classified by (i) Shape of tower, (ii) tower base, (iii) number of circuit, (iv) angle of line deviation.

As per shape towers are (a) Barrel type, (b) Corset type, (c) portal type.

As per number of circuits, towers are single circuit and double circuit tower.

As per base these towers are two types (i) Narrow base type and (ii) Broad base type.
Narrow base type towers are preferable where span is lower whereas broad base type is used in case of larger span.

As per angle of line deviation, towers are (a) Suspension or tangent tower, (b) Angle or section tower.

Suspension towers are used for straight line up to deviation angle 2°. Maximum towers of transmission line are these types.
When the line deviation is greater than 2° or at section point Angle or section towers are used. These types of towers are heavier than suspension tower and it costly too.
As per Indian rule, tower types are i) ‘A’ type: Low base tangent tower.
ii) ‘B’ type: Angle tower with an angle 2 to 15°. It is also called light angle tower.
iii) ‘C’ type: Angle tower with an angle 15 to 30°. This is medium angle tower. The base is greater than ‘A’ and ‘B’ type.
iv) ‘D’ type: Angle or dead end tower with an angle 30 to 60° with broad base. It is heavy angle tower.

Tower related components:

Tower stub: For holding the tower properly, the foundation of the base of the tower concreted. Actually tower legs are joined with the concreted Iron stub. The stub are made by M.S angle and well deeply grounded with concrete.

Cross-arm of tower: For holding the insulator as well as conductor cross arm are used which is made by M.S angel.

Earthing: Every pole and tower is earthed by G.I rod in case of pole which is known as earth spike and by G.I pipe in case of tower. Normally earth is done around 2 to 4 meter from pole or tower.

Tower height: The height of tower depends upon the distance between the two supports. If the span is high, the height of tower should also be high.
Actually tower heights are calculated by
(i) Minimum ground clearance (h1),
(ii) Sag between conductors (h2),
(iii) Vertical spacing between conductor (h3),
(iv) Vertical spacing between ground wire and top conductor (h4)
So, the height of tower is H = h1+h2+h3+h4.

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