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tower spotting

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Tower spotting

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tower spotting

1. 1. CHAPTER – 2 TOWER SPOTTING AND TOWER SCHEDULE 1.0 1.1 SAG TEMPLATE: A Sag Template is a very important tool with the help of which the position of towers on the Profile is decided so that they conform to the limitations of vertical and wind loads on any particular tower, and minimum clearances, as per I.E. Rules, required to be maintained between the line conductor to ground, telephone lines, buildings, streets, navigable canals, power lines, or any other object coming under or near the line. 1.2 A Sag Template is specific for the particular line voltage, the conductor used and the applicable design conditions. Therefore, the correct applicable Sag Template should be used. 1.3 A Sag Template consists of a set of parabolic curves drawn on a transparent celluloid or acrylic clear sheet duly cut in over the maximum conductor sag curve to allow the conductor curve to be drawn and the lowest points of the conductor sag to be marked on the profile when the profile is placed underneath it. 1.4 A typical calculation sheet for a sag template is enclosed at Appendix – A. 1.5 The set of curves in the sag template consists of: a) b) c) d) ‘Cold or Uplift Curve’ showing sag of conductor at minimum temperature (minus 2.5ºC) and still wind. ‘Hot or Maximum Sag Curve’ showing maximum sag of conductor at maximum temperature and still wind including sag tolerances allowed (normally 4%), if any, and under maximum ice condition wherever applicable. ‘Ground Clearance Curve’ which is drawn parallel to the ‘Hot or Maximum Sag Curve’ and at a distance equal to the specified minimum ground clearance for the relevant voltage. ‘Tower Footing Curve’ which is drawn parallel to the ‘Ground Clearance Curve’ and separated by a minimum distance equal to the maximum sag at the basic design span. 1.6 The Sag Template is plotted to the same scale as the profile, i.e., 1 cm = 20 M horizontal and 1 cm = 2 M vertical. It is generally plotted for spans up to 1000 metres. This is necessary for tower spotting when there are large variations in the ground levels along the line route. 1.7 A typical ‘Sag Template’ drawing is shown in Appendix – B. 2.0 2.1 TOWER SPOTTING: The Sag Template is applied to the profile by moving the same horizontally while always ensuring that the vertical axis is held vertical, i.e., in line with the vertical lines on the profile sheet. 2.2 The following clearances shall be provided between the lowest conductor of the line and the ground as per Rule 77 of the Indian Electricity Rules, 1956. Sl. No. Nominal System Voltage 1. 2. 3. 4. 132 kV 220 kV 400 kV 800 kV Minimum Ground Clearance (Metres) 6.10 7.00 8.84 12.40
2. 2. 50 Construction Manual for Transmission Lines 2.3 The left hand side of the tower footing curve is placed at the starting point of each section. Initially, the template is shifted to the right, ensuring at all times that the tower footing curve is touching the starting point, to a position where the ground clearance curve is just above the ground profile, i.e., the ground clearance curve should not touch or cross the ground line plotted on the profile. The second tower location is then marked at the point where the tower footing curve on the right hand side cuts the ground profile. 2.4 The second tower location is then used as the reference and the third tower location is marked in a similar manner as above. This is continued till the end of the section is reached. 2.5 It may be possible that a very short or very long span remains at the end of the section. In such cases, depending on the economics of the options, the span can be distributed evenly or other spans in the section can be increased (not normally exceeding the basic span) by using tower extensions wherever possible. 2.6 The ground clearance curve shall not only clear the route centre line profile but also the profile to the left or right of the centre line upto a distance equal to maximum cross arm spread on either side. 2.7 Besides normal ground clearance, the clearance between power conductor and objects like other power or telecommunication lines, houses, trolley wires, roads, railway tracks, canal embankments etc., is also to be checked. In these cases, the clearance of the conductor from these objects is to be maintained. 2.8 The requisite or extra clearance can be obtained either by reducing the span or providing extension to tower body depending on which alternative is most economical. Normally, 3 metre & 6 metre extensions are available for towers upto 220 kV. 220 kV Special Towers with 4.5 metre, 9 metre & 18 metre extensions designed for long spans are also available. 3 metre, 6 metre, 9 metre, 18 metre & 25 metre extensions are available for 400 kV towers. 2.9 The tower locations with extensions or towers with additional heights are marked on the ground profile at that point of the tower footing curve which is at a height equal to the tower extension or the difference in height with reference to the height of the bottom cross arm of the special tower (with extension, if provided) above the ground profile. This point above the ground profile is used as the reference / initial point for the tower footing curve when spotting the next tower location. 2.10 When a tower location with extension or a tower with additional height is to be marked on the ground profile, then a point is marked at that location on the ground profile which is at a height equal to the tower extension or the difference in height with reference to the height of the bottom cross arm of the special tower (with extension, if provided). This point above the ground profile is used as the reference point for the tower footing curve. 2.11 A figure showing the application of a sag template on the profile is given at Appendix – C. 2.12 In spans where towers are located at different ground levels, the lowest point of the conductor sag may be outside the span. This is termed as an “Uplift” condition. This indicates that the total weight of conductor is taken up by the tower at the higher ground level and the tower at the lower ground level is being pulled up by a force equal to the weight of conductor between the lower support and the lowest point of the conductor sag. If the upward pull of the uphill span becomes greater than the downward load of the next adjacent span, actual uplift will be caused and the conductor would tend to swing clear upwards of the tower. The suspension towers cannot be used under uplift conditions. This type of condition can be resolved by providing extensions to the suspension tower at the lower level or by using a B – type tower designed for uplift conditions.
3. 3. Tower Spotting and Tower Schedule 51 2.13 The intermediate spans in a section should preferably be as near as possible to the basic design span. In case any individual span becomes too short on account of undulations in ground profiles, one or more line supports of the section, wherever possible, may be extended by inserting standard body extensions to increase the span length. 2.14 While crossing over existing power lines, one of the towers of the crossing span of the new line is preferably located near the existing power line for taking advantage of the higher height of the conductors near the tower. This reduces the necessity of increasing the height of the towers of the new line for obtaining the requisite clearance. Double suspension / tension insulator strings, depending on the type of the towers, are to be used in the new line on such crossings. 2.15 While crossing below existing power lines of higher voltage, both the adequate ground clearance for the new line and the specified clearance of the new line from the existing power line shall be ensured. This can be achieved by using towers / structures of lesser height or by using sub station structures (if there is no right of way problem) in the new line. Alternatively, the height of the existing power line can be increased by providing tower extensions. Double suspension / tension insulator strings, depending on the type of the towers, are to be provided on the existing power line at such crossings. 2.16 The length of double suspension insulator strings is more than that of single suspension strings because of the yoke plates provided in them. Therefore, when double suspension insulator strings are used, additional ground clearance shall be provided when spotting towers so that the specified ground clearance is available after stringing. 2.17 The following clearances shall be provided between the lowest conductor of the line crossing over another line and the top most conductor / earth wire of the line crossing underneath as per Rule 87 of the Indian Electricity Rules, 1956. Minimum clearances in metres between lines crossing each other. Sl. No. 1. 2. 3. 4. 5. 6. Nominal System Voltage Low & Medium 11 – 66 kV 132 kV 220 kV 400 kV 800 kV 132 kV 3.05 3.05 3.05 4.58 5.49 7.94 220 kV 4.58 4.58 4.58 4.58 5.49 7.94 400 kV 5.49 5.49 5.49 5.49 5.49 7.94 800 kV 7.94 7.94 7.94 7.94 7.94 7.94 2.18 The minimum clearances required as per Rule 80 of the Indian Electricity Rules, 1956 shall be maintained, according to the voltage of the lower line, from the conductors of the line passing near a pole / tower or any supporting structure of the second line. 2.19 For crossing of a non – navigable river, the clearance of the bottom conductor of lines upto 220 kV shall be at least 3 metres above the highest flood level (HFL). The clearance of the bottom conductor of 400 kV lines in such a case shall be reckoned with respect to the highest flood level (HFL). 2.20 For crossing of navigable rivers, the clearance as approved by the concerned navigation authorities shall be maintained. 2.21 The crossing span of National Highways and major roads shall not normally exceed 250 metres. One of the towers of the crossing span can be located near the road in order to obtain