Transmisiion line design concept

TRANSMISSION TOWER
R.Saravanan, PGET, L&T, UAE

R.SARAVANAN, PGET, L&T UAE 1

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 R.SARAVANAN, PGET, L&T UAE 5

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


TOWER GEOMENTRY

EXTENSIONS

BRACINGS

ANATOMY


Tower Anatomy
 Peak - supports G.W
 Cage - b/w peak & tower
body
 Cross Arm - Support
Conductor/G.W
 Boom – 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 are
a) Single web system
b) Double web or warren system
c) Pratt system
d) Portal system
e) Diamond Bracing system
f) Multiple Bracing System


1.Struts are designed 1.Tension diagonal 1.Shear carried by
in compression & give eff.support to diagonal member(t)
Diagonals in tension compression one @ 2.Large deflection
2.NARROW BASE pt of connections under heavy loads
2.Used in both large 3.Unequal shears at
3.66Kv single circuit
and small towers top of four stubs for
design


1.1half of Horizontal 1. Similar to waran 1.Increse in strenght
member in T & system reducing member
another C 2.Horizontal member sizes
2.Advantageous to carry no primary 2.Increase in No.of
use it in BOTTOM loads designed as bolts, fabrication &
panel redundant supports erection cost,
3.Extensions & 3.Overal reduction in
Heavy river crossing Wt & cost of steel


Body Extension
Tower Extension Leg 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
1) Type of Insulator 5) No. of Circuits
• Suspension • Single Circuit
• Tension/Dead end • Double Circuit
• Transposition • Multi-Circuit
2) Type of Support 6) Deviation Angle.
• Self Supporting • Ranges from 0 to 90 deg.
• Guyed
3) Shape at the base
• Square
• Rectangle
4) kV Rating.
• Ranges from 33 to 1200
kV
• HVDC

EDRC-TL Design R.SARAVANAN, PGET, L&T UAE

Vertical Configuration Horizontal Configuration


Suspension Tower Tension Tower

Guy Towers


Conductor Configuration


66 kv 132 kv 220 kv 400 kv

Tower Nomenclature
Sr.
Nomenclature Deviation Remark
No.

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

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

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

60- •Used as a large angle Tower
4 D/DD/BAT/DE/TD6/TDT/QD/DE 900/Dead •Used as a Dead End
End Tower


Height of Tower Structure
Height of tower is determine by-

H h1 h2 h3 h4

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.
Type of Clearance 132 kV 220 kV 400 kV 765 kV
No
1 Ground Clearance 6.1 m 7.0 m 8.84 m 15.5 m
2 Live Metal Clearance in mm Swing
132 / 400 /
220 765
•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.
Type of Clearance 132 kV 220 kV 400 kV 765 kV
No
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

L KVA E = Transmission voltage (KV) (L-L).
E 5 .5 L = Distance of transmission line in KM
1 .6 150 KVA=Power to be transferred


Aluminum is used
it has about half the weight of copper for the
Conductor
same resistance, as well as being cheaper
Types:
AAC : All Aluminium conductors.
AAAC : All Aluminium Alloy conductors
ACSR : Aluminium conductors, Steel-Reinforced
ACAR : Aluminium conductor, Alloy-Reinforced

Bundle conductors are used to reduce corona
loses & audible noise
It consists of several conductors cables
connected by non-conducting spacers
It is used to increase the amount of current
that may be carried in line
As a disadvantage, the bundle conductors
have higher wind loading
Spacers 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


single string
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
Double string
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.


Design Span lengths

1.Basic Span
Most economic span
Line is designed over level ground
The requisite ground clearance is obtained
at maximum specified temperature


2.Ruling Span 3.Average Span
Assumed design span that will produce,
between dead ends Mean span length between dead ends
It is used to calculate the horizontal It is assumed that the conductor is
component of tension (which is applied to all freely suspended such that each
spans b/w anchor pts) individual span reacts to change in
Tower spotting on the profile is done by tension as a single average span
means of sag template, (which is based on
ruling span) Average span = (L1+ L2+...+L6) /6

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


4.Wind Span 5.Weight Span
Half the sum of the two spans, Horizontal distance between the
adjacent to support lowest point of conductor, on the two
It is assumed that the conductor is spans adjacent to the tower
freely suspended such that each The lowest point is defined as point
individual span reacts to change in at which the tangent to sag curve
tension as a single average span It is used in design of cross-arms

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.
Ryle
Formula

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

38

Determination of Weight of
tower
Rough approximation
From knowledge of the positions of conductors & ground wire above ground level
& overturning moments
Ryle gives empirical formula in term of its height & maximum overturning moment
at base

Approximate values

132 kv – 1.7 metric tones
765 kv – 14 metric tones


LOADINGS
Loads 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 Tower
Normal 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.


Data's for foundation design

FOUNDATION
It costs 10-30 % of overall cost of
tower
It is the last step in designing process
but precedes the construction
Overload factors assumed in designs
are 2.2 under Normal condition & 1.65
under broken-wire conditions


 0.5 to 2m dia
 Uplift loads are  Non-cohesive soil
 Shaft depth 3 to 15m
resisted by undistrube  For non-cohesive soils
 Skin friction between
material such as uncemented
ground & shaft resists
 Develop uplift load of sand or gravel
uplift
2 to 3times that of an  Provide pad footing
 Used in usa,
iidentical footing without undercut
acceptance for wide
without undercut  Usually followed in
use in India
INDIA at present


 Adopted in firm  Hybrid design  Augered footing with
cohesive soils  Large uplift force are more than one bulb is
 Undercut on the pads to be resisted used to increase the
 Experience shows that  SBC is low uplift capacity
this type of footing  35m long under
develop resistance to reamed to 2.5 times
uplift 2 to 3 times that dia of shaft
given footing without  Clayey black cotton
undercut soils & medium dense
sandy soils


 In usa ,canada  Suitable in areas with  Special circumstances
 Steel corroded, rock out crop  River crossing towers
periodic excavation &  Based on uplift, the & towers on
maintanence anchor be single bar embankments
 Medium dry sand, clay or group of bars  The raft at bottom
or sandy caly soils (no welded to tower leg makes the foundation
special precautions  Vertical bars below substantially rigid to
necessary) stub angle form cage minimize differential
 The steel is treated for footing settlement
with one coat of  Grouted to a depth of
bituminous paint & about 50 times dia
top coat of asphalt into the rock

Pyramid chimney type foundation
Raft foundation

Stub-setting
 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

Transmisiion line design concept

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