Tower spotting - Mechanical Design of Lines and Grounding

Tower spotting

From the field book entries, the route plan with enroute details and level profile shall be plotted and prepared to scale of 1:2000 horizontal & 1:200 vertical on 1.0, 10 mm squared paper as per approved procedure. Reference levels at every 20 metres along the profile are also to be indicated on the profile besides, reduced levels at undulations. Areas along the profile, which, in the view of the contractor, are not suitable for tower spotting, shall also be clearly marked on the profile plots. If the difference in levels be too high, the chart may be broken up according to requirement. A 10mm overlap shall be shown on each following sheet. The chart shall progress from left to right. Sheet shall be 594 mm wide in accordance with the IS. For ‘as built’ profile these shall be in A1 size.

With the help of approved sag template and tower spotting data, tower locations shall be marked on the profiles. While locating the towers on the profile sheet, the following shall be borne in mind.

a) SPAN

The number of consecutive spans between the section points shall not exceed 15. A section point shall comprise of tension point as under as applicable: -

For D/C line ‘DB’ type or ‘DC’ type or ‘DD’ type tower as applicable

For S/C line ‘B’ type or ‘C’ type or ‘D’ type tower as applicable

b) EXTENSION

An individual span shall be as near to the normal design span as possible. In case an individual span becomes too short with normal supports on account of undulations in ground profile, one or both the supports of the span may be extended by inserting standard body extension designed for the purpose according to technical specification.

c) LOADING

There shall not be any upward force on Suspension towers under normal working conditions and the Suspension towers shall support at least the minimum weight span as provided in the designs. In case uplift is unavoidable, it shall be examined if the same can be overcome by adding standard body extensions to the towers failing which tension towers designed for the purpose shall be employed at such positions.

d) RAILWAY CROSSINGS

All the Railway crossings coming enroute the transmission line have already been identified by the HVPN. At the time of detailed survey, the Railway crossings shall be finalized as per the regulation laid down by the Railway Authorities. The following are the important features of the prevailing regulations (revised in 1987):

i)  The crossing shall be supported on DD/D type tension towers only (as the case may be) on either side and Double tension insulator string shall be used on both the towers on the side of the crossing.

ii)  The crossing shall normally be at right angle to the Railway track.

iii) The minimum distance of the crossing tower shall be at least equal to the height of the tower plus 6 metres away measured from the center of the nearest Railway track.

iv)  No crossing shall be located over a booster transformer, traction switching station, traction sub-station or a track cabin location in an electrified area.

v)  Minimum ground clearance above Rail level of the lowest portion of any conductor under condition of maximum sag shall be maintained as per latest Railway regulations amended from time to time.

vi) The approval for crossing Railway track shall be obtained by the HVPN from the Railway Authorities, however six copies of profile and plan, tower and foundation design and drawings, required for the approval from the Railway Authorities shall be supplied by the contractor to the HVPN.

e) ROAD CROSSING

At all important road crossings, the towers shall be fitted with normal suspension or tension insulator strings depending on type of tower, but the ground clearance at the highest point of the roads under maximum temperature and still air shall be such that even with the conductor broken in adjacent span, ground clearance of the conductor from the road surfaces will not be less than 6.100 meters . At all National Highways, tension towers with double insulator strings on crossing side shall be used.

f) RIVER CROSSING

In case of major river crossing, towers shall be of suspension type and the anchor towers on either side of the main river crossing shall be ‘DD/D’ type tower. For navigable river, clearance required by navigation authority shall be provided. For non-navigable river, clearance shall be reckoned with respect to highest flood level (HFL).

g) POWER LINE CROSSINGS

Where the line is to cross over another line of the same voltage or lower voltage, towers with suitable extensions shall be used. Where the line is to cross under the power lines, gantries shall be used. Provisions to prevent the possibility of its coming into the contact with other overhead lines shall be made in accordance with Indian Electricity Rules,1956. In order to reduce the height of the crossing towers, it may be advantageous to remove the ground wire of the line to be crossed (if this is possible and permitted by the owner of the line to be crossed).All the works related to the above proposal shall be deemed to be included in the scope of the contractor except if modifications are required to line below, in which case, the conditions to be agreed upon. Suitable Extension for towers over 11kV line crossing shall be used, where requisite electrical as per I.E. rules is not available.

h) TELECOMMUNICATION LINE CROSSING

The angle of crossing shall be as near to 90o as possible. However, deviation to the extent of 30o may be permitted under exceptionally difficult situations. When the angle of crossing has to be below 60o, the matter will be referred to the authority incharge of the telecommunication system. On a request from the contractor, the permission of the telecommunication authority may be obtained by the HVPN. Also, in the crossing span, power line support will be as near the telecommunication line as possible, to obtain increased vertical clearance between the wires.

i) DETAILS ENROUTE

All topographical details, permanent features, such as trees, building etc. 13.5 m on either side of the alignment for 132kV line shall be detailed on the profile plan.