758Analysis and Design of Multi Circuit Transmission Line Tower PDF

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International Journal of Emerging Technology in Computer Science & Electronics (IJETCSE)

ISSN: 0976-1353 Volume 13 Issue 1 –MARCH 2015.

56

Analysis and Design of Multi Circuit Transmission

Line Tower

Sakthivel. T#1

and Sanjeevi. R*2

# * (Civil Engineering): dept. Easwari Eng. College, Ramapuram, Chennai – 600 089, India

[email protected]

Abstract— Due to rapid urbanization and space constraints

narrow base towers provide an alternative solution to

conventional broad based towers. A narrow base 200 KV multi

circuit tower is analysed using STADD.PRO. The choice of tower

is made based on the available right of way. As per IS 5613 (Part

2/Sec 2) the required right of way for a 220 KV broad base toweris 35 m. But when adequate right of way is not available the base

width of the tower is constrained. There is no standard of

specification or limit up to which the tower acts as broad base or

narrow base tower. The tower is considered as narrow base when

its width is constrained. The tower is in Mumbai. It is a four

circuit double peak tower and consists of 2 earth wires, 12 cross

arms (6 box cross arms on one side and 6 triangular cross arms

on the other side of the tower). It is a dead end tower with

deviation of 0º - 15º. Since it is a dead end tower could also be an

angle tower. The narrow base tower is also an angle tower with

deviation of 30º-60º. The design wind pressure for the tower is 757

N/m². The maximum sag of the conductor is 9.626 m. The

maximum sag of the conductor occurs at maximum temperature

and nil wind. The transverse loads, vertical loads and thelongitudinal loads on the towers are calculated for reliability

condition, security condition, and safety condition. The electrical

clearance diagram for the tower is drawn using AUTOCAD. The

height of the tower is 56.335 m, and its width at plinth level at a

body extension of 6 meter is 7.343 m. The entire tower is

separated into 9 blocks. The tower is separated into panels

consisting of A-Pattern, X-Pattern, bracings, and leg members.

The body wind is calculated based on the boundary area and the

projected area, and it acts at 10 wind points in the longitudinal

face of the tower. The tower is designed based on IS 802 (Part 1/

Sec2).

Index Terms—Transmission Line Tower; Narrow Base

Tower; Angle Tower; Sag and tension; Body Wind Calculation

I. INTRODUCTION

Electric power plays an important role in the life of the

community and development of various sectors of economy. In

India, high priority is given to power development programs.

Transmission line (TL) structures support the phase conductors

and shield wires of a transmission line. Due to rapidurbanization and space constraints, narrow base structures

provide solution as an alternative approach to conventional

broad based towers. The tower is designed as angle tower and

also dead end tower.

Fig. 1 Dead End Tower

TABLE I PROPERTIES OF CONDUCTOR

Material All Aluminum AlloyConductor (AAAC)

Units

Standing

(Aluminium/Steel) 61/3.31

Diameter 29.79 mm

Cross Sectional Area 5.25 cm²

Ultimate Tensile

Strength 14603 kg

Unit Weight 1.448 kg/m

Modulus of Elasticity 550800 kgf/cm²

Coefficient of LinearExpansion 2.30E-05 /°C

Wind Span 350 m

W/A 0.2758 kg/m/cm²

TABLE II PROPERTIES OF GROUND WIRE

Material Units

Standing

(Aluminium/Steel) 7/3.15

Diameter 9.45 mm

Cross Sectional Area 0.5455 cm²

Ultimate Tensile

Strength 5710 kg





57

Unit Weight 0.43 kg/m

Modulus of Elasticity 1933000 kgf/cm²

Coefficient of LinearExpansion 0.0000115 /°C

Wind Span 350 m

W/A 0.788267644 kg/m/cm²

TABLE III PROPERTIES OF INSULATOR

Single Tension Insulator Details Units

Number of Insulator (Single

Tension Insulator) 2

Length of Insulator 3.416 m

Diameter of Insulator 0.305 m

Weight of Insulator (Maximum) 150 Kg

Weight of Insulator (Minimum) 150 Kg

Number of Pilot Insulator

Required 1

Length of Pilot Insulator 2.500 m

Diameter of Pilot Insulator 0.280 m

Weight of Pilot Insulator

(Maximum) 150

Kg

Weight of Pilot Insulator(Minimum) 150 Kg

Coefficient for Insulator Wind

Pressure 0.50

Factor for Unbalanced Tension 0.0

II. DETERMINATION OF WIND PRESURE

The wind intensity is lower at the ground level and the air flowis turbulent because of friction with the rough surfaces of theground. The basic wind speed map of India is applicable at 10m height above mean ground level for the six wind zones ofthe country. The basic wind speed Vb is 44 m/s. themeteorological wind speed VR is 32 m/s. Vd = VR x K1 x K2.Pd = 0.6 Vd². Pd = design wind pressure in N/m2. The designwind pressure is Pd = 757 N/m².

III. SAG AND TENSION

The sag is calculated for various temperature and wind

pressure conditions. It could be calculated by catenary method

or by parabolic method. The parabolic method of sag and

tension calculation is followed in IS: 5613 (Part 2/Sec 1). The

sag and tension are determined for the following temperature

and wind pressure combinations,

• Everyday temperature and nil wind

• Everyday temperature and full wind

• Maximum temperature and nil wind

• Minimum temperature and 36 % of full wind

• Minimum temperature and nil wind

The maximum temperature of all aluminum alloy conductor is

85º (Clause 10.2.4, IS: 802 (Part 1/Sec 1). The maximum

temperature of the ground wire is 53º (Clause 10.2.4, IS: 802

(Part 1/Sec 1).

The maximum sag of the conductor occurs at maximumtemperature and nil wind

TABLE IV SAG AND TENSION OF CONDUCTOR

Tem

p º C

Wind

Pressure

(kg/m2)

Tension

(Kg)

Factor of

Safety

(Required)

Factor

of

Safety

(Actual)

Vertical

Sag

(m)

Tension

%

0 0 4030.11 1.4285 3.62 5.50 27.60

32 0 3138.37 4.5454 4.65 7.06 21.49

32 175.22 7564.48 1.4285 1.93 2.93 51.8085 0 2303.50 1.4285 6.34 9.626 15.77

0 63.08 5198.40 1.4285 2.81 4.27 35.60

IV. LOAD CONDITIONS

The tower is designed as dead end and angle tower. The loads

acting on the tower are considered for the following loading

conditions, for angle tower (30º-60º) and for dead end tower

(0º-15º).

• 30º deviation reliability condition

• 30º deviation security condition – Loads for intact

spans

•

30º deviation security condition – Loads for brokenspans

• 30º deviation safety normal condition

• 30º deviation safety broken condition – Loads for

intact spans

• 30º deviation safety broken condition – Loads forbroken spans

• 30º deviation anticascading condition



spans

• 60º deviation security condition – Loads for broken

spans


• 60º deviation safety broken condition – Loads for

intact spans

• 60º deviation safety broken condition – Loads forbroken spans

• 60º deviation anticascading condition



spans




spans• 15º deviation safety normal condition

V. ELECTRICAL CLEARANCES

The design of transmission line towers are classified into

structural design and electrical design. The electrical clearance

forms the electrical design of transmission line tower. As per

clause 13.1 of IS 5613 (Part 2/Sec 1) a minimum ground

clearance of 7 meter is provided. As per clause 7.3.2 the

vertical spacing between conductors is a minimum of 4.9





58

meter. As per clause 13.2 the mid span clearance between earth

wire and power conductor is 8.5 m.

Fig. 2 Electrical Clearance Diagram

VI. STAAD.PRO MODELLING OF NARROW BASE TOWER

The tower is modelled using Staad.Pro. During the modelling

phase the tower is separated into various blocks for the ease of

analysis and design. The blocks comprise of bracings of A-

Pattern, X-Pattern and V-Pattern. A sample panel with A-Pattern is shown in the following figure.

Fig. 3(a) Panel of Tower Fig. 3(b) Tower Model

Fig. 4 Sample Loading Tree

VII. GENERATION OF LOADING TREE

The lengths of the members are identified from the towermodel. The broken wire conditions are identified. Either earth

wire or one conductor could be broken in a phase or two





59

conductors and an earth wire can be broken as per clause 16 of

IS 802 (Part 1/Sec 1). The sample loading tree is shown in the

following figure.

VIII. BODY WIND CALCULATION OF TOWER

Trial sections are assigned to the generated model.

The projected area and the boundary area of the tower arecalculated. Then the body wind is determined on 10 wind

points in the tower. Then the wind load is obtained. The body

wind is applied in the longitudinal face of the tower.

TABLE V BODY WIND ON TOWER

Wind

Point

(kg/m2)

Body

Wind

(Kg)

10 560

9 2961

8 1994

7 1996

6 2086

5 2270

4 2543

3 3076

2 2716

1 969

IX. LOAD CASES AND COMBINATIONS

The load combinations and combination load cases

are assigned and the tower is analysed. Then the forces are

obtained and grouping of forces is done.

Fig. 5 Load Case Details

X. PRIMARY DESIGN OF TOWER

The tower is designed based on IS 802(Part 1/Sec 2). Trial

sections are assigned.

As per clause 6.3 of IS 802 (Part 1/Sec 2) the limiting

slenderness ratio is 120 for leg members, ground wire peak

member, and lower members of cross arms in compression;

and for other members except redundant members the limiting

slenderness ratio is 200, and for redundant members the

limiting slenderness ratio is 250. The total weight of the tower

without redundant member is 51667.84 Kg.Star angle and quadruple plate angle sections are used for legmembers. Double angle sections are used for bracings

subjected to more forces. Single angle sections are used for

belt members.

The factor of safety provided for leg members is 1.01-1.1. The

factor of safety provided for bracings and belt members vary

from 1.2-1.3. The factor of safety for the cross arm bottommembers is 1.3-1.4.

The sections used for leg members are 150x150x20 QL,

150x150x16 QL, 200x200x25 star angle, 200x200x20 star

angle, 200x200x16 star angle, 150x150x20 star angle, and

150x150x12 star angle.

The various members used for bracings are 150x150x10 DL,130x130x10 DL, 120x120x10 L, 100x100x10 L, and

100x100x8 L.

The various members used for belt and plan-x-bracing are

100x100x10 L, 100x100x8 L, 80x80x6 L, 70x70x6 L.

High tensile steel of fy=350 N/mm² is used for leg members,

top cross arm and bottom cross arm members. Mild steel of

fy=250 N/mm² is used for belt, bracings.

The leg members are split into maximum of 3 rvv. The belt

members are split into maximum of 3 rvv. Very few bracings

are split to 4 rvv in order to maintain the slenderness ratio and

produce effective members.

XI. DESIGN OF TOWER

In order to validate the design, the tower is analyzed and

designed using STADD.PRO. The total weight of the tower

obtained using STADD.PRO is 48784.76 Kg. Since the result

shows that the tower weight could be reduced further. Hence,

the body wind is calculated. Then the body wind is applied on

the tower and the maximum compression and tension forces

acting on the tower are obtained. The weight of the tower

obtained by optimizing the angle section is 48608 Kg.

XII. INTERPRETATION OF RESULT

The maximum displacement is experienced by the dead endtower for 0º condition. Since the dead end tower will be

located in the substation it may not be a narrow base tower,

however if the due to space constraints if the dead end tower is

supposed to be a narrow base tower then the dead end tower’s

safety should be given more importance than the economy.





60

Fig. 6 Displacement of the Tower for Critical Load Combination

XIII. GRAPHICAL REPRESENTATION OF MOMENT, FORCE AND

DISPLACEMENT

The graph of moment, force, and displacement is shown in the

following figure.

0.70

0.70

0.70

0.70

1.40

1.40

1.40

1.40

2 4 66.03

1 50

1.22

-0.688

2.51

Mz(kNm)

Fig. 7 Moment of Beam 1 (Leg Member)

50

50

50

50

100

100

100

100

150

150

150

150

2 4 66.03

1 5

62.6

-104

Fy(kg)

Fig. 8 Vertical Force along Y axis for Beam 1 (Leg Member)

10000

10000

10000

10000

20000

20000

20000

20000

2 4 66.03

1 5

15187 13575

Fx(kg)

Fig. 9 Horizontal Force along X axis for Beam 1 (Leg Member)

XIV. ADVANTAGES OF NARROW BASE TOWER

Even though the narrow base tower might not be completely

economical as a conventional tower, it is the only best solution

for urban cities. And with adaption of emerging trends and

technologies in engineering, narrow base towers could be an

alternative to conventional broad base towers, when adequate

right of way is not available.

XV. COCLUSION

The weight of the tower is optimized by using sections of

various sizes. The reduced weight is obtained by using single

angle sections for belt members. Since the scope of the project

is not obtaining the least weight of the tower with adequatesafety, the width of the tower is not increased further, to

validate the economy of the tower. Further the redundant

members are not designed. Considerable change in the total

weight of the tower would be obtained after the redundant

members are designed. The reduction in weight is achieved by

using angle sections of thickness varying from 5-6 mm, but not

less than 5 mm.

ACKNOWLEDGMENT

We would like to thank the management and staff of Easwari

Engineering College for their guidance and support.

References

[1] Alam M. J. and Santhakumar, A. R, “Reliability analysis and fullscale testing of Transmission Tower”, Journal of Structural Engineering ©

ASCE., Vol. 122, No. 3, pp. 338-344, (1996).

[2] Albermani F., Chan R. W. K., and Kitipornchai S, ‘Failure analysisof transmission towers’, ‘Engineering Failure Analysis’, © ELSEVIER., Vol.

16, pp. 1922-1928, (2008).

[2] CBIP – Central Board of Irrigation and Power manual[3] IS: 802 (Part1/Sec1):1995 – Use of structural steel in overhead

transmission line towers – Code of practice. Part 1 – Materials, Loads and

Permissible Stresses. Section 1 – Materials and Loads.[4] IS: 802 (Part1/Sec2):1992 – Use of structural steel in overhead

transmission line towers – Code of practice. Part 1 – Materials, Loads andPermissible Stresses. Section 2 – Permissible Stresses.

[4] IS: 5613 (Part 2/Sec 1): 1985 – Code of Practice for Design,

Installation and Maintenance of Overhead Power Lines. Part 2 – Lines above11 KV and up to and including 220 KV. Section 1 – Design.

[5] IS: 5613 (Part 2/Sec 2): 1985 – Code of Practice for Design,

Installation and Maintenance of Overhead Power Lines. Part 2 – Lines above11 KV and up to and including 220 KV. Section 2 – Installation and

Maintenance.

[6] Knight G. M. S and Santhakumar A. R, “Joint effects on behaviourof Transmission Towers,” Journal of Structural Engineering © ASCE., Vol.

119, No. 3, pp. 698-712, (1993).

Sakthivel. T, Post Graduate student, Structural Engineering

Department, Easwari Engineering College





61

. Sanjeevi. R, Assistant Professor, Structural EngineeringDepartment, Easwari Engineering College

758Analysis and Design of Multi Circuit Transmission Line Tower PDF

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