my learnings on transmission tower

R.SARAVANAN, PGET, L&T UAE 1

TRANSMISSION TOWER

R.Saravanan, PGET, L&T, UAE


Power in UAE..?

Production capacity – 18.74 GW. (lack in peak seasonal times)

Lack of natural gas

Gulf Cooperation Council – UAE, Kuwait, Qatar, Bahrain, Saudi Arabia & Oman

GCC began region-wide power grid – demand

UAE has no spare power capacity

Phase 3 of GCC grid to southern system of UAE

In Dec’2009 $20 billion contract to Korean Electric Power – 4 nuclear reactors

1st reactor may 2017 – each reactor 1400 MW


Electric power transmission..? The bulk transfer of electrical energy, from generating power

plants to substations Power is usually transmitted through overhead power lines Underground power transmission has a significantly higher cost

and greater operational limitations - urban & sensitive areas

Overhead Power lines..?An electric power transmission line suspended by

towers It is the lowest-cost method of transmission for large

quantities of electric energy (most of insulation by air)The bare wire conductors on the line are generally

made of aluminum


Transmission tower..?

• Tall structure usually a Steel lattice tower, used to support an overhead power line

• Electricity pylon – UK & parts of Europe• Ironman – Australia• Hydro tower in parts of Canada


TOWERGEOMENTRY

TOWERTYPES

ELECTRICALCLEARANCES

DESIGN PARAMETER

LOAD CALCULATION

ANALYSIS & DESIGN FOUNDATION STUB SETTING


TOWER GEOMENTRY

ANATOMY

BRACINGS

EXTENSIONS


Tower Anatomy

Peak - supports G.WCage - b/w peak & tower bodyCross Arm - Support

Conductor/G.WBoom – supports power

conductors (horizontal)Tower body – main portion,

connects cage/boom to foundation/(leg/body )extensions


Bracings Provided for interconnecting the legs To afford desired slenderness ratio for economical tower design Framing angle b/w bracings & main leg members shall not be <

15 degree Patterns area) Single web systemb) Double web or warren systemc) Pratt systemd) Portal systeme) Diamond Bracing systemf) Multiple Bracing System


1.Struts are designed in compression & Diagonals in tension2.NARROW BASE 3.66Kv single circuit

1.Tension diagonal give eff.support to compression one @ pt of connections2.Used in both large and small towers

1.Shear carried by diagonal member(t)2.Large deflection under heavy loads3.Unequal shears at top of four stubs for design


1.1half of Horizontal member in T & another C2.Advantageous to use it in BOTTOM panel3.Extensions & Heavy river crossing

1. Similar to waran system2.Horizontal member carry no primary loads designed as redundant supports

1.Increse in strenght reducing member sizes2.Increase in No.of bolts, fabrication & erection cost,3.Overal reduction in Wt & cost of steel


Tower ExtensionBody ExtensionLeg Extension

Body Extension

Used to Increase the height of tower to obtain the reqd min Ground clearance & over

road crossings, river crossings, ground obstacles

Body extensions upto 7.5m height in steps 2.5m can be used & thus form a part of

standard tower

Extensions having greater heights (25m) the suitability is checked by reducing span

length and angle of deviation. Practice in tower industry is also to specify negative body

extension (portion of tower body is truncated)


Leg Extension

Tower Leg extensions are required when the tower was spotted in the undulated

surface / Hilly terrain.

While spotting the tower locations in hilly areas requires more benching or revetment

or both are involved , but suitable hill side (leg extensions) can be used to minimize

benching or revetment or both.

Two types of Leg extension :

i) Universal leg extension

ii) Individual leg extension

Types of Tower

5) No. of Circuits• Single Circuit• Double Circuit• Multi-Circuit

6) Deviation Angle.• Ranges from 0 to 90 deg.

1) Type of Insulator• Suspension• Tension/Dead end• Transposition

2) Type of Support• Self Supporting• Guyed

3) Shape at the base• Square• Rectangle

4) kV Rating.• Ranges from 33 to 1200 kV• HVDC

R.SARAVANAN, PGET, L&T UAEEDRC-TL Design


Vertical Configuration Horizontal Configuration


Tension TowerSuspension Tower


Guy Towers


Conductor Configuration

R.SARAVANAN, PGET, L&T UAE 2166 kv 132 kv 220 kv 400 kv


66 kv 132 kv 220 kv 400 kv


Tower Nomenclature

Sr. No. Nomenclature Deviation Remark

1 A/DA/S/SLC/T0/TDL/QA/SA/V 0-20 Suspension Tower

2 B/DB/AT/DLB/TD2/QB/X 0-300 •Used Small angle tower.• Used as a Section Tower

3 C/DC/BAT/DLC/TD3/QC/CZ 30-60• Used as Medium Angle Tower•Used as a Transposition

4 D/DD/BAT/DE/TD6/TDT/QD/DE 60-900/Dead End

•Used as a large angle Tower•Used as a Dead End Tower

R.SARAVANAN, PGET, L&T UAE

Height of Tower Structure

24

4321 hhhhH

Height of tower is determine by-

h1=Minimum permissible ground clearance

h2=Maximum sag

h3=Vertical spacing between conductors

h4=Vertical clearance between earth wire

and top conductor


ELECTRICAL CLEARANCES

Sr. No

Type of Clearance 132 kV 220 kV 400 kV 765 kV

1 Ground Clearance 6.1 m 7.0 m 8.84 m 15.5 m

2 Live Metal Clearance in mm Swing

132 / 220 400 / 765

•Suspension insulator 15 15 1530 1980 3050 4400 (25°)

30 30 1370 1830 1860 1300 (55°)

45 - 1220 1675 -

60 1070 - -

•Tension Insulator 0 0 1530 2130 3050

•Jumper 10 20 1530 2130 3050 4400

20 40 1070 1675 1860 1300

30 - 1070 - - -

3 Mid Span Clearance (m) 6.1 8.5 9.0 12.4

4 Shielding Angle (Deg) 30 30 20 20

5 Phase to Phase Clearance Vertical 3.9 m 4.9 m

Horizontal 6.8 m 8.4 m


Right of Way :

Sr. No Type of Clearance 132 kV 220 kV 400 kV 765 kV

1 ROW width 27 m 35 m 52 m 85 m


DESIGN PARAMETERS

Transmission Voltage

Number Of Circuits

Climatic Conditions

Environmental and Ecological

Consideration

Conductor

Earth Wire

Insulators

Span


Economic Voltage of Transmission of Power

E = Transmission voltage (KV) (L-L). L = Distance of transmission line in KM KVA=Power to be transferred1506.15.5

KVALE

Standard Voltage - 66,110,132, 220, 400 KV


ConductorAluminum is used it has about half the weight of copper for the same resistance, as well as being cheaperTypes:AAC : All Aluminium conductors.AAAC : All Aluminium Alloy conductorsACSR : Aluminium conductors, Steel-ReinforcedACAR : Aluminium conductor, Alloy-Reinforced

Bundle conductorBundle conductors are used to reduce corona loses & audible noiseIt consists of several conductors cables connected by non-conducting spacersIt is used to increase the amount of current that may be carried in lineAs a disadvantage, the bundle conductors have higher wind loadingSpacers must resist the forces due to wind, and magnetic forces during a short-circuit

spacers


Earth Wire

Earth wire provided above the phase conductor across the line and

grounded at every tower.

It shield the line conductor from direct strokes

Reduces voltage stress across the insulating strings during lightning strokes

Galvanized steel earth wires are used

Aerial marker balls (>600mm dia) (Red, Orange, White)

Shield angle

25°-30° up to 220 KV

20° for 400 KV and above


Insulators Insulator are required to support the line

conductor and provide clearance from

ground and structure.

Insulator material-

High grade Electrical Porcelain

Toughened Glass

Fiber Glass

Type of Insulator-

Disc Type

Strut Type

Long Rod Insulator


Insulator Strings Disc insulator are joint by their ball

pins and socket in their caps to form

string.

No of insulator disc is decided by

system voltage, switching and lighting

over voltage amplitude and pollution

level.

Insulator string can be used either

suspension or tension.

Two suspension string in parallel

used at railways, road and river

crossing as statutory requirement.

Swing of suspension string due to

wind has to be taken into consider.

single string

Double string


Design Span lengths

1.Basic SpanMost economic spanLine is designed over level groundThe requisite ground clearance is obtained at maximum specified temperature


2.Ruling SpanAssumed design span that will produce, between dead endsIt is used to calculate the horizontal component of tension (which is applied to all spans b/w anchor pts)Tower spotting on the profile is done by means of sag template, (which is based on ruling span)

Ruling span = √ ( L1^3 + L2^3 +….+L6^3 / L1 + L2 + … + L6)

3.Average Span

Mean span length between dead endsIt is assumed that the conductor is freely suspended such that each individual span reacts to change in tension as a single average span

Average span = (L1+ L2+...+L6) /6


4.Wind Span 5.Weight Span

Horizontal distance between the lowest point of conductor, on the two spans adjacent to the towerThe lowest point is defined as point at which the tangent to sag curveIt is used in design of cross-arms

Half the sum of the two spans, adjacent to supportIt is assumed that the conductor is freely suspended such that each individual span reacts to change in tension as a single average span

Wind span = 0.5(L1 + L2)

Weight span = a1 + a2


Determination of Base Width

The base width(at the concrete level) is the distance between the centre of gravity at one corner leg and the centre of gravity of the adjacent corner leg.

A particular base width which gives the minimum total cost of the tower and foundations.

The ratio of base width to total tower height for most towers is generally about one-fifth to one-tenth.

38

Ryle Formula


Determination of Weight of tower

Rough approximationFrom knowledge of the positions of conductors & ground wire above ground level & overturning momentsRyle gives empirical formula in term of its height & maximum overturning moment at base

132 kv – 1.7 metric tones220 kv – 2.5 metric tones400 kv – 7.7 metric tones765 kv – 14 metric tones

Approximate values


LOADINGSLoads are applied in all three directions namely Transverse ( FX ), Vertical ( FY) and Longitudinal (FZ) direction.• Transverse loads consists of –

Wind on Conductor Wind on Insulator Component of Wire Tension in Transverse Direction

(Deviation Load) Wind on Tower Body

• Vertical Load consists of – Weight of Wire Weight of Insulator Weight of Line man & Tools Self Weight of Tower

• Longitudinal Load Consist of – Component of Unbalanced pull of the wire in the

longitudinal direction.


Loads on TowerNormal Condition

Broken Wire Condition


•Loads are calculated as per the guide lines furnished in specification/standard.•Standards for Calculation of Loads

IS – 802 – 1977 IS – 802 – 1995 DIN – VDE 0210 ASCE Manual IEC – 826

• The loads are calculated for following Conditions. Reliability / Working condition Security / Broken wire condition Safety / Erection & maintenance Condition


ANALYSIS & DESIGN

• Analysis is carried out by finite element software

STAAD

• Required FOS is provided in input file to find out ultimate force

• The critical compression and tension in each member group is found out

• Members and Connections are designed for these forces.

• Iterations are carried out for the optimum usage of tower.


FOUNDATIONIt costs 10-30 % of overall cost of towerIt is the last step in designing process but precedes the constructionOverload factors assumed in designs are 2.2 under Normal condition & 1.65 under broken-wire conditions

Data's for foundation design


0.5 to 2m dia Shaft depth 3 to 15m Skin friction between

ground & shaft resists uplift

Used in usa, acceptance for wide use in India

Uplift loads are resisted by undistrube material

Develop uplift load of 2 to 3times that of an iidentical footing without undercut

Non-cohesive soil For non-cohesive soils

such as uncemented sand or gravel

Provide pad footing without undercut

Usually followed in INDIA at present


Adopted in firm cohesive soils

Undercut on the pads Experience shows that

this type of footing develop resistance to uplift 2 to 3 times that given footing without undercut

Hybrid design Large uplift force are

to be resisted SBC is low

Augered footing with more than one bulb is used to increase the uplift capacity

35m long under reamed to 2.5 times dia of shaft

Clayey black cotton soils & medium dense sandy soils


In usa ,canada Steel corroded,

periodic excavation & maintanence

Medium dry sand, clay or sandy caly soils (no special precautions necessary)

The steel is treated with one coat of bituminous paint & top coat of asphalt

Suitable in areas with rock out crop

Based on uplift, the anchor be single bar or group of bars welded to tower leg

Vertical bars below stub angle form cage for footing

Grouted to a depth of about 50 times dia into the rock

Special circumstances River crossing towers

& towers on embankments

The raft at bottom makes the foundation substantially rigid to minimize differential settlement


Raft foundationPyramid chimney type foundation


Important steps in tower erection

The stubs are set with the help of stub setting templates

Excavated pits are lean concreted to correct level

Stubs are placed on lean concrete pad

Alignment is carried by four plumb bobs hung from centre of the horizontal bracing

If any pit over excavated by mistake, the extra depth should be filled by concreting

After the stub is set, the heel distance of four faces of the tower and two diagonals should be checked

Stub-setting

my learnings on transmission tower

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