Angle towers

Angle towers, sometimes called semi-anchor towers, are used where the lines makes a horizontal angle

greater than two degrees (Figure 2.6). As they must resist a transverse load from the components of the line tension induced by this angle, in addition to the usual wind, ice and broken conductor loads, they are necessarily heavier than suspension towers. Unless restricted by site conditions, or influenced by conductor tensions, angle towers should be located so that the axis of the cross- arms bisects the angle formed by the conductors. Theoretically, different line angles require

different towers, but for economy there is a limiting number of different towers which should be used. This number is a function of all the factors which make the total erected cost of a tower line. However, experience has shown that the following angle towers are generally suitable for most of the lines :

1. Light angle - 2 to 150 _{line deviation}

2. Medium angle - 15 to 300 _{line deviation}

3. Heavy angle - 30 to 600 _{line deviation}

(and dead end)

While the angles of line deviation are for the normal span, the span may be increased up to an optimum limit by reducing the angle of line deviation and vice versa. IS:802 (Part I) - 1977 also recommends the above classification.

The loadings on a tower in the case of a 60 degree angle condition and dead-end condition are almost the same. As the number of locations at which 60 degree angle towers and dead-end towers are required are comparatively few, it is economical to design the heavy angle towers both for the 60 degree angle condition and dead-end condition,

whichever is more stringent for each individual structural member.

For each type of tower, the upper limit of the angle range is designed for the same basic span as the tangent tower, so that a decreased angle can be accommodated with an increased span or vice versa. It would be uneconomical to use 30 degree angle

towers in locations where angles higher than 2 degree and smaller than 30 degree are encountered. There are limitations to the use of 2 degree angle towers at higher angles with reduced spans and the use of 30 degree angle towers with smaller angles and increased spans. The introduction of a 15 degree tower would bring about sizable economics. Pilot suspension insulator string

- This shall be used if found necessary to restrict the jumper swings to design value at both middle and outer phases.

Unequal cross arms

- Another method to get over the difficulty of higher swing of Jumper is to have unequal cross arms.

2.3 400 kv single circuit towers

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The bundled conductors are kept in horizontal configuration with a minimum clearance of 11 mtrs. phase to phase.

The latticed parts are fully galvanised. Galvanised hexagonal round head bolts and nuts are used for fastening with necessary spring or plate washers.

Normally 4 types of single circuit towers are used as detailed below :-

a) "A" type towers :

These towers are used as tangent towers for straight run of the transmission line. These are called suspension or tangent towers. These towers can carry only vertical loads and are designed for carrying the weight of the conductor, insulators and other accessories. These towers are also designed for a deviation upto 2 degrees.

b)" B" type towers :

These towers can be used as sectionalising towers without angle and angle towers from 2 degrees up to 15 degrees deviation.

c) " C" type towers

These towers can be used for deviations ranging from 15 degrees up to 30 degrees. They are also being used as transposition towers without any angle.

d) "D" type towers :

These towers can be used as Dead End or anchor towers without any angle on the tower. Also these towers can be used for deviations ranging from 30

degree - 60 degree.

These towers are usually provided as terminal towers near gantry with slack span on one side or as anchoring tower before major river crossing, power line crossing, railway crossings etc.

Fig. 2.8 shows two types of tower configuration for 400 KV single circuit towers.

A section of 400 kv single circuit towers is shown in Fig.2.9.

2.4 400 KV Double circuit towers

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These towers are designed to carry two circuits consisting of 3 phases each, having bundled conductors. Here, the circuits are placed in a vertical configuration. A minimum phase to phase clearance of 8 mtrs. is maintained. A minimum clearance of 11 mtrs. is maintained from one circuit to another. Two earthwires are placed above each circuit in such a way to provide the required shielding angle.

Like single circuit towers, these towers are also galvanised, lattice steel type structures designed to carry the tension and weight of the conductor alongwith the insulators, earthwire and its accessories.

Normally these towers are identified as P (D/C suspension towers), Q, R & S (D/C tension towers) or as DA, DB, DC and DD respectively.

As in the single circuit towers, DA/P towers are used as suspension towers from O degrees-2 degrees deviations. DB/Q,DC/R and DD/S towers are used as tension towers with angle of deviation from 2 degrees-15 degrees, 15 degrees-30 degrees and 30 degrees - 60 degrees respectively.

DB towers are also used as sectionalising towers without angle.

DC tower is also used as transposition tower without any angle.

The Double Circuit towers are used while crossing reserved forest, major river crossings, narrow corridors near switchyards etc. so as to make provision for future transmission lines since the approval from various authorities can be obtained at one time (for example, from forest, aviation authorities etc.) and to minimise expenditure in laying foundations in rivers.

400 kv double circuit towers.

2.5 River-crossing tower

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The height and weight of the towers vary considerably depending on the span, minimum clearance above water, ice and wind loads, number of `unbroken' conductors, etc. Usually the governing specification requires that towers employed for crossing of navigable water ways be designed for heavy loading conditions and utilise larger minimum size members than the remainder of the line. In addition to these structural requirements, it is often necessary to limit the height of tall crossing towers because of the hazard they present to aircraft.

Fig.2.10 shows a view of 400 kv double circuit River crossing tower.

2.6 Railway crossing tower

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Angle or dead end towers (Type B,C or D) with suitable extensions and with double tension insulator strings are employed for railway crossing in conformity with the relevant specification of Railway Authorities.

2.7 High way crossing tower

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extension and with double tension strings are employed for high way crossing.angle towers are used for National High way crossing to make the crossing span as a single section so as to facilitate independent and prompt striginig.

2.8 Transposition tower

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2.8.1 Power transmission lines are transposed primarily