air clearance design of transmission line

Air Clearance Design

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Chapter- 4 AIR CLEARANCE DESIGN

4.1 INTRODUCTION:

 Air clearance calculations calculations are required to provide adequate adequate clearance of a conductor from the ground and other conductors under all loading conditions, conditions, so as to ensure the reliability of the system and safe operation. This is achi achiev eved ed by fast fasten enin ingg the the cond conduc ucto tors rs to the the supp suppor orts ts with with the the he help lp of insulators keep in the worst conditions of operation in mind (i.e. swinging the conductor due to wind pressure up to ! ° ". The minimum ground clearance depends on the locations and the weather conditions. #ndian electricity act lays down a clearance of $% feet (!.$& m" to be provided for ''k and for every additional ''k or apart of it additional $ feet ().')& m" clearance is to be provided. This can be appro*imated as+ 4.2 BASIC DEFINITIONS (TERMINOLOGY

#n transmission lines planning and design, the following terms are commonly used+ $.

asic or normal span

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uling or equivalent span

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Average span

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/ind span

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/eight span

4.2.1 Ba!"# $r %$r&a' %$r&a' !pa%

The normal span is the most economical span for which the line is designed over level ground, so that the requisite ground clearance is obtained at the ma*imum specified temperature. 4.2.2 R'"%) $r e*"+a'e%t e*"+a'e%t !pa%

The ruling span is the assumed design span that will produce, between dead ends, the best average tension throughout a line of varying span lengths with

Air Clearance Design



%$changes in temperature and loading. #t is the weighted average of the varying span lengths. The average span is the mean span length between dead ends. #t is assumed that the conductor is freely suspended such that each individual span reacts to changes in tension as a single average span. All sag and tension calculations are carried out for the average span, on this assumption. Two basic hypotheses must be considered for e*treme loading+ $. 1ow temperature at a given wind speed -. 2igh wind at a given temperature  Apart from this other hypothesis shall also be considered+ $. 2igh temperature no wind for ground clearance -. 3ean temperature for coldest month for Aeolian vibration 4ormally longer ruling span shall be considered during design stage. 4.2., Dea E% Spa%

 A dead end span is one in which the conductor is dead5ended at both ends. Dead end spans should preferably be avoided, but in certain cases field conditions dictate their use. #t is frequently convenient to cross a river, a valley with a single long span. /hen such a span occurs, it is desirable to dead5ended it at both ends to avoid using a ruling span that would be too long for the rest of the line. 4.2.4 "% Spa%

The wind span is that on which the wind is assumed to act transversely on the conductors and is taken as half the sum of two spans, ad6acent to the pylon. This span will be used to calculate the wind load of conductor on tower. 4.2./ e")ht !pa%

The weight span is the hori7ontal distance between the lowest points of the conductors, on the two spans ad6acent to the tower. The lowest point is defined as the point at which the tangent to the sag curves. The weight span is used to calculate the dead load of conductor on tower. Typical figure for wind span and weight span is given below.

Air Clearance Design

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4., S0IELDING ANGLE:

The necessary protective angle between a line through ground wire and outer most conductor and vertical through of 9verhead :round /ire (92:/" has been much discussed sub6ect. ;*perience shows that the protective angle is about ')8 for proper shielding of phase conductor against lightening strokes.
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 Fig 4.1 Cross arm and String of a tower  Air Clearance Design

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Air Clearance Design

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1et,  a @ 3inimum air clearance between phase conductor and tower structure.  -a@ 1ength of cross arms.  b@ /idth of upper potion of tower.  c@ Distance of phase conductor from centre of the tower  l @ 1ength of insulator string. 2$@2eight of lowest conductor from the ground. 2-@ 2eight of middle conductor from the ground. 2'@2eight of upper conductor from the ground. 2t@ 2eight of tower. = @
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Air Clearance Design

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• D"!ta%#e $8 pha!e #$%#t$r 8r$& the #e%tre $8 t$er # C @ (mbB-" c @ -a  (bB-" cm c @ (- -$>.!%&"  ('->.'0 B -" cm c @ 0)'.&'0 cm # 5 9.,;,9 &

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 310.51 + 219.571   1     cm 1 −         2 * 219.57   3    2

y @ !%> cm = 5 /.76 &

• * @y B ' cm @!%> B ' @ $>' cm

Air Clearance Design

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Air Clearance Design

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