1

C O N T E N T S

Foreword Preface

1. Introduction

2. Chapter 2 - Design Parameters

2.1 Span of River Crossing 2.2 Climatic Conditions

2.2.1 Wind 2.2.1.1 Basic Wind Speed 2.2.1.2 Reference Wind Speed (VR) 2.2.1.3 Design Wind Speed, Vd 2.2.1.4 Terrain Roughness Co-Efficient (k2) 2.2.1.5 Design Wind Pressure (Pd) 2.2.1.6 Wind Loads

2.2.1.6.1 Wind Load on Tower 2.2.1.6.2 Wind Load on Conductor and

Groundwire

2.2.2 Temperature 2.2.3 Isokeraunic Level

2.2.3.1 Lightning Consideration for Tower Design

2.2.4 Seismic Intensity 2.2.5 Ice Formation

3. Chapter 3 - Clearances

3.1 Live Line Metal Clearances 3.2 Minimum Clearances above Ground and River 3.3 Clearance Between Conductor and Groundwire

3.3.1 Clearance between Conductor and Groundwire at Tower Shield Angle

3.3.2 Clearance between Conductor & Groundwire at Mid Span-Mid Span Separation

3.4 Spacing between Conductors/Phase to Phase Clearance

4. Chapter 4 Transmission Line Components

4.1 Normal Practices 4.2 Power Conductor 4.3 Insulator Strings 4.4 Earthwire

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5. Chapter 5 Span of River Crossing Sag Tension Calculation

5.1 River Crossing Span/Design Span 5.2 Anchor Span 5.3 Wind Span 5.4 Weight Span 5.5 Ruling Span/Equivalent Span 5.6 Sag Tension Calculation

6. Chapter 6 Philosophy of Design

6.1 Return Period 6.2 Type of Tower 6.3 Wind on Tower 6.4 Deviation Angle

7. Chapter 7 Requirements of Broken Wire for River Crossing Tower (Suspension Type)

7.1 Single Circuit Tower 7.2 Double and Multiple Circuit Tower 7.3 Requirement of Brekage of V Insulator String

8. Chapter 8 - Loadings

8.1 Requirement of Loads 8.2 Nature of Loads

8.2.1 Transverse Loads (T) 8.2.2 Vertical Loads (V) 8.2.3 Longitudinal Loads (L)

8.3 Loading Criteria 8.4 Angle of Incidence of Wind 8.5 Transverse Loads (TR) Reliability Condition

8.5.1 Wind Load on Conductor/Groundwire 8.5.2 Wind Load on Insulator String 8.5.3 Wind Load on Tower 8.5.4 Transverse Load frm Mechanical Tension of Conductor

and Groundwire Due to Line Deviation 8.5.5 Total Transverse Load (TR) under Reliability Condition

8.6 Transverse Load (TS) Security Condition 8.7 Transverse Load During Construction and Maintenance

Safety Condition 8.7.1 Normal Condition 8.7.2 Brokenwire Condition

8.8 Vertical Loads (VR) Reliability Condition 8.9 Vertical Loads (VS) Security Condition 8.10 Vertical Loads During Construction and Maintenance (VM)

Safety Condition

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8.11 Longitudinal Loads (LR) Reliability Condition 8.12 Longitudinal Load (LS) Security Condition 8.13 Longitudinal Loads during Construction and Maintenance (LM)

Safety Condition 8.13.1 Normal Condition 8.13.2 Brokenwire Condition

8.14 Loading Combination under Reliability, Security and Safety Conditions 8.14.1 Reliability Condition (Normal Condition) 8.14.2 Security Condition (Brokenwire Condition) 8.14.3 Safety Condition (Construction and Maintenance)

8.14.3.1 Normal Condition 8.14.3.2 Broeknwire Codntion

9. Chapter 9 Special Considerations

9.1 Truncation Facilities 9.2 Maximum Height of Tower 9.3 Use of Special Conductor 9.4 Steel Quality 9.5 Minimum Thickness of Angle Section used 9.6 Bolts and Nuts 9.7 Slenderness Ratio 9.8 Redundant Member 9.9 Aviation Requirements

9.9.1 Painting of Towers 9.9.2 Span Markers 9.9.3 Night Aviation Signals

9.10 Platforms and Ladders 9.11 Maintenance Fixtures 9.12 Foundations 9.13 Life Expectancy

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MANUAL FOR DESIGN OF TOWERS FOR LONG SPAN RIVER CROSSING

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1.0 INTRODUCTION

India is a vast country and full of major and minor rivers. While laying transmission lines across various states, it is sometimes necessary to cross rivers. Normally rivers are crossed by providing tower on either bank of the river. Wherever river crossing span is more than a kilometer, towers in the mid-stream of the river with special pile or well foundations are provided. Therefore, in the river crossing stretches tall towers are used to minimize the number of costly foundations.

Design of tall tower is complex because of the fact that tall tower encounters more wind forces. Therefore, selecting correct parameters for developing river crossing tower design is of utmost importance. In this manual, guidelines for selecting parameters and developing river crossing tower design are presented.

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CHAPTER 2 DESIGN PARAMETERS

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2.1 SPAN OF RIVER CROSSING

River Crossing Span is the distance between two River Crossing Towers or two tall towers placed on the banks for crossing the river. Normally this span ranges between 600 to 1000 metres. Height of River Crossing Tower is finalized depending on crossing span. Deciding crossing span is an important aspect of designing river crossing tower.

2.2 CLIMATIC CONDITIONS

The reliability of a transmission system is dependent on the accuracy of the parameters related to climatic conditions considered for design. The design of tower will vary with variation in climatic conditions. The following are the main climatic and other parameters which should be considered in developing design of transmission line towers.

1. Wind 2. Temperature 3. Isokeraunic Level 4. Seismic Intensity 5. Ice formation

2.2.1 WIND

2.2.1.1 Basic Wind Speed

The wind speeds have been worked out for 50-year return period based on the updated wind data of 43 dyne pressure tube (DPA) anemograph stations and study of other related works available on the subject since 1964. The basic wind speed data have been published by Bureau of Indian Standards in IS:875-1988 in active co-operation with Indian Meteorological Department as shown in figure-1. This map represents basic Wind Speed based on peak gust velocity averaged over a short time interval of about 3 seconds and corresponds to 10 m height above mean ground level in terrain category-2 for 50-year return period.

Based on the wind speed, the entire map has been divided into six wind zones with max wind speed of 55m/sec and min wind speed of 33 m/sec. Basic wind speeds for six wind zones are given in Table-1.

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TABLE-1

WIND ZONE BASIC WIND SPEED (METRE/SEC.)

1 33 2 39 3 44 4 47 5 50 6 55

2.2.2 REFERENCE WIND SPEED (VR)

It is the extreme value of wind speed over an averaging period of 10- minute duration and is to be calculated from basic wind speed 'Vb' by the following relationship:-

VR = Vb/Ko

Where Ko is a factor to convert 3 second peak gust speed into average speed of wind during 10 minutes period at a level of 10 meters above ground. Ko is to be taken as 1.375.

2.2.3 DESIGN WIND SPEED,Vd

Reference wind speed obtained in Clause 2.2.2 shall be modified to include the following effects to get the design wind speed.

Vd = VR x K1 x K2.

Where,

K1 = Risk co-efficient (Reliability level-3 i.e 500-year return period to be considered) (See Table - 2)

K2 =Terrain Roughness Co-efficient. (Terrain Category-1 for large stretch of water to be considered) (See TAble-3)

TABLE 2

RISK CO-EFFICIENT K1 FOR RELIABILITY LEVEL 3 AND VARIOUS WIND ZONES.

Reliability Level 3 Co-Efficient K1 For Wind Zones (500 years return period) Wind zone: 1 2 3 4 5 6 Risk Co-efficient K1 1.17 1.22 1.25 1.27 1.28 1.30

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2.2.4 TERRAIN ROUGHNESS CO-EFFICIENT (K2)

Terrain Roughness Co-efficient (K2) value to be considered is 1.08 under category-1 applicable for coastal areas, large stretches of water and deserts etc.

2.2.5 DESIGN WIND PRESSURE (Pd)

The design wind pressure on towers, conductors and insulators shall be obtained by the following relationship:-

Pd = 0.6 Vd2

Where Pd = Design wind pressure in N/m2 and

Vd = Design wind speed in m/sec.

Design wind pressures Pd for Reliability level 3 and Terrain Category 1 applicable for tall towers have been worked out for various wind zones and are given in Table-3.

TABLE 3

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RELIABILITY TERRAIN CATEGORY DESIGN WIND PRESSURE Pd in LEVEL N/m2 FOR WIND ZONES _____________________________________________________________

1 2 3 4 5 6

3 1

(500 Yrs. (Large stretch of 552 838 1120 1320 1520 1890 return water, coastal period) area, desert etc. _____________________________________________________________

2.2.6 WIND LOADS

(A) WIND LOAD ON TOWER

In order to determine wind load on tower, the tower is divided into different panels having height 'h'. These panels should normally be taken between the intersections of the legs and bracings. For a lattice tower, the wind load FWT in newtons, for wind normal to face of tower, on a panel of height 'h' applied at the centre of gravity of the panel is

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Fwt = Pd x Cdt x Ae X GT

Pd = Design Wind pressure, in N/m2

Cdt = Drag Co-efficient pertaining to wind blowing against any face of the tower. Values of Cdt for different Solidity Ratios are given in Table-4.

Ae = Total net surface area of the legs and bracings of the Panel projected normal to wind in square meters (The projections of the bracing elements of the adjacent faces and of the plan and hip bracings may be neglected while determining the projected surface of a face).

GT = Gust Response Factor, specific to the terrain category 1 and dependent upon on the height above ground. Values of GT are

given in Table-5.

TABLE - 4

DRAG CO-EFFICIENT, Cdt FOR TOWERS

SOLIDITY RATIO* DRAG CO-EFFICIENT, Cdt upto 0.05 3.6 0.1 3.4 0.2 2.9 0.3 2.5 0.4 2.2 0.5 and above 2.0

Intermediate values may be linearly interpolated

* Solidity Ratio is equal to the effective area (projected area of all individual elements) of a frame normal to the wind direction divided by the area enclosed by the boundary of the frame normal to the wind direction.

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TABLE 5

GUST RESPONSE FACTORS FOR TOWERS (GT) AND FOR INSULATORS(GI)

HEIGHT ABOVE GROUND VALUES OF GT AND GI FOR IN METRES TERRAIN CATEGORY -1

UPTO 10 1.70 20 1.85 30 1.96 40 2.07 50 2.13 60 2.20 70 2.26 80 2.31 90 2.37 100 2.42 110 2.49 120 2.55 130 2.60 140 2.67

150 2.74 160 2.80 170 2.85 180 2.92

Intermediate values may be linearly interpolated.

B. WIND LOAD ON CONDUCTOR AND GROUNDWIRE

The load due to wind on each conductor and groundwire, Fwc in Newtons applied at supporting point normal to the transverse face shall be determined by the following expression:-

Fwc = Pd x L x d x Gc x Cdc Where: Pd = Design wind pressure in N/m2 L = Wind span, being sum of half the spans on either side of

supporting point in meters. d = Diameter of Conductor/groundwire in metres. Gc = Gust Response factor which takes into account the turbulance of the wind and the dynamic response of the conductor. Values

of Gc are given in Table-6 for average height of conductor above the ground.

Cdc = Drag Co-efficient which is 1.0 for conductor and 1.2 for groundwire.

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The average height of conductor/groundwire shall be taken upto clamping point on tower less two third the conductor/groundwire sag at minimum temperature and no wind.

The total effect of wind on bundle conductor shall be taken equal to the sum of the wind load on all sub-conductors without accounting for possible masking effect of one sub-conductor over the other.

TABLE - 6

VALUES OF GUST RESPONSE FACTOR Gc FOR CONDUCTOR/ GROUNDWIRE ______________________________________________________________

TERRAIN HEIGHT VALUES OF Gc CATEGORY ABOVE GROUND (SPAN IN METRES) (metres) 400 500 600 700 800 and above _____________________________________________________________________________________________

1 10 1.60 1.56 1.53 1.50 1.47 20 1.83 1.79 1.75 1.70 1.66

40 2.00 1.95 1.90 1.85 1.80 60 2.12 2.07 2.02 1.96 1.90 80 2.20 2.15 2.13 2.06 1.97 100 2.25 2.20 2.15 2.08 2.02 120 2.27 2.22 2.18 2.11 2.05 130 - - - - 2.08 140 - - - - 2.10 ______________________________________________________________

C) WIND LOAD ON INSULATORS

Wind load on insulator string 'FWi' shall be determined for the length of string from the attachment point with tower to the centre of the conductor.

Fwi = 1.2 x Pd x Ai x Gi

Pd = Design Wind Pressure in N/m2

Ai = 50 per cent of the area of insulator string projected on a plane parallel to the longitudinal axis of the string (1/2xdiameter x length)

Gi = Gust Response Factor, specific to the terrain category depends on the height above ground.Values of Gi are given in

Table-5.

In case of multiple insulator strings no masking effect shall be considered.

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2.3 TEMPERATURE

To evolve design of towers, three values of temperatures i.e maximum, minimum and everyday are very important. Tower height as well as sag and tension calculations of conductor and earthwire vary with the change in these three temperatures.

The temperature range varies for different seasonal conditions. The absolute ambient maximum and minimum temperatures which may be encountered at different locations in country are given on the Map of India in Fig.2 and Fig.3 respectively. The temperatures given in these maps are the air temperatures in shade. The maximum conductor temperature may be obtained after allowing for increase in temperature due to solar radiation and heating effect of current over the absolute ambient maximum temperatures given in Fig.2. After considering several factors such as annealing effect of metal, excess power to be transmitted in emergency during summer, etc, the following design temperatures have been fixed for designing transmission lines:-

Permissible Conductor/Earthwire Temperatures:

a) Maximum permissible Temperature of ACSR Conductor = 75 deg.c (85 C may be considered for higher thermal rating of line)

b) Maximum permissible Temperature of AACSR Conductor = 85 deg.c c) Maximum permissible Temperature of Earthwire = 53 deg.c d) Minimum Temperature = Between -5 deg.c to +10 deg.c depending

on the location of transmission line. Generally 0C is adopted.

e) Everyday Temperature = 32 deg.c

For colder regions where minimum temperature is below minus 5deg.C, everyday temperature may be considered 15 deg.C or as decided by the utility. Maximum Temperature may also be suitably decided by the utility.

2.4 ISOKERAUNIC LEVEL

2.4.1 LIGHTNING CONSIDERATION FOR TOWER DESIGN

As the overhead transmission lines pass through open country, they are subjected to the effects of lightning. The faults initiated by lightning can be of following types:

a) Back Flashover: When lightning strikes on tower or on the earthwire near the tower which raises the tower potential to a level resulting in discharge across the insulator string.

b) Mid-span Flashover: When lightning strikes on earthwire raising local potential of the earthwire such that a break down in the air gap between earthwire and phase conductor may take place. This is protected by keeping mid-span separation between conductor and earthwire more than at tower.

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Shielding failure : When lightning strikes on the phase conductor directly resulting in a flashover across the insulator string.

The above type of faults can be minimized by suitably choosing the shielding angle and keeping the tower footing resistance at the minimum and not more than 10 0hms.

2.5 SEISMIC INTENSITY

The transmission line tower is pin-jointed structure, flexible and free to vibrate and maximum wind pressure is the chief criterion for design. Occurrence of earthquake and maximum wind condition is unlikely to take place simultaneously and, further, seismic stresses are considerably diminished by the flexibility and the vibration of the structure. This is also in line with recommendation given in Cl.No.3.2(b) of IS:1893-1984. Seismic loads for tower design are therefore not considered. However, in regions where earthquakes are experienced, the earthquake forces may be considered in tower foundation design in accordance with IS:1893-1984.

2.6 ICE FORMATION:

Design of towers constructed in snow and ice incident areas will be dealt with in a separate document.

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CHAPTER 3 CLEARANCES

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3.1 LIVE LINE - METAL CLEARANCES TABLE 7

Conductor metal clearances adopted in the country for transmission lines of 66KV and above are given as under:

System Voltage (kv)

Double suspension insulator string

V-Insulator String

Swing from Vertical (Degree)

Minimum Clearance (mm)

Minimum clearance (mm)

66AC

Nil 15 30 45 60

915 915 760 610 610

NOT USED

132AC Nil 15 30 45 60

1530 1530 1370 1220 1070

NOT USED

220AC

Nil 15 30 45

2130 1980 1830 1675 - -

NOT USED

400AC Nil 22 44

3050 3050 1860

3050

800AC Wind Zone 33m/sec 39m/sec.

Nil 22 45

5600 4400 1300

5100(from bottom of cross-arm) 5600 (from side of tower)

800AC 44m/sec. 47m/sec.

Nil 27 55

5600 4400 1300

5100(from bottom of cross-arm) 5600 (from side of tower)

800AC 50m/sec. 55m/sec.

Nil 30 60

5600 4400 1300

5100(from bottom of cross-arm) 5600 (from side of tower)

500HV DC

-

-

3750

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3.2 MINIMUM CLEARANCES ABOVE GROUND AND RIVER

TABLE 8

System voltage (kv)

66 AC

132 AC

220 AC

400 AC

800 AC

500 HVDC

Minimum Ground Clearance (mm)

5500 6100 7000 8840 15000 12500

Minimum Clearance Above River or lake or water stretches Non Navigable river (from HFL) (mm)

3650 4300 5100 6400 13200 10650

Minimum Clearance Above Navigable River (From HFL) (mm)

19000 19220 20100 21900 25900 21900

3.3 CLEARANCE BETWEEN CONDUCTOR AND GROUNDWIRE

3.3.1 CLEARANCE BETWEEN CONDUCTOR AND GROUNDWIRE AT TOWER SHIELD ANGLE

The height and location of groundwire shall be such that line joining groundwire and outermost conductor shall make an angle with vertical equal to shield angle. The angle of shield is affected by height of tower. For tall river crossing towers lower angle of shield is provided than for normal towers in the line

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SHIELD ANGLE

TABLE-9

Nominal voltage (KV)

66 AC

132 AC

220 AC

400 AC

800 AC

500 HVDC

Angle of Shield for Normal tower

30 30 30 20 20 10

Angle of Shield for Tall River x-ing Tower

20 20 20 10 10 10

3.3.2 CLEARANCE BETWEEN CONDUCTOR & GROUNDWIRE AT MID SPAN (MID SPAN SEPARATION)

The minimum mid-span clearances between conductor and ground wire in tall/river crossing towers will be as given in Table-10.

TABLE-10

Nominal voltage (KV)

66 AC

132 AC

220 AC

400 AC

800 AC

500 HVDC

Minimum Mid-Span Clearance (mm)

3000 6100 8500 9000 12000 9000

The clearance between the conductor and groundwires at mid-span is kept more than the clearance at tower to avoid flash-over from groundwire to conductor when hit by lightning stroke. The usual practice in this regard is to maintain groundwire sag at 90 percent of conductor sag at minimum temperature in no-wind condition for ensuring that the sag of groundwire does not exceed that of conductor under any climatic condition. This arrangement also improves angle of shield in the mid-span.

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3.4 SPACING BETWEEN CONDUCTORS/PHASE TO PHASE CLEARANCE

The spacings between conductors/phases for long spans tower are recommended to be calculated from the following formulae:-

Vertical Clearances (m)

= 0.75 (F75+Ik) +V/150

Horizontal Clearances

= 0.62 (F75+Ik ) + V/150

Where; F75 = Sag in metres at 75 C and no Wind Condition for ACSR & at 85C & no wind for AACSR Conductor .

Ik = Length of Insulator string in metres V = Line Voltage in KV

Various formulae used in different countries for calculation of phase to phase spacing are given in Annexure-I.

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CHAPTER 4 TRANSMISSION LINE COMPONENTS

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4.1 NORMAL PRACTICES

Normal practices adopted in respect of conductor, insulator and earthwire are given in paras 4.2, 4.3 and 4.4. However each case is to be examined specifically for adequacy and optimality of design.

4.2 POWER CONDUCTOR

Conductors normally used in the lines for various voltage levels are also used for river crossing stretches. However, for exceptionally long span crossings ALUMINIUM ALLOY CONDUCTOR STEEL REINFORCED (AACSR) is used to reduce the tower height by minimising the sag. Various ACSR conductors and their equivalent AACSR conductors used for different voltage lines are given below:-

TABLE-11

Sl. No.

Description ACSR AACSR

A Voltage System Code Name No.of conductor /phase Stranding/wire diameter

Total sectional area Overall diameter Approx.weight Calculated DC resistance at 20C Min. UTS Modulus of elasticity Co-efficient of linear exp. Maximum allowable temp.

66kV ACSR DOG One 6/4.72mmAL +7/1.57mm ST. 118.5mm2 14.15mm 394 kg//km 0.281 Ohm/km 32.41 kN 76 GN/m2 19.80x10-6/ C 95C

66kV AACSR DOG One 6/4.72mm Al. Alloy +7/1.57mm ST 118.5mm2 14.15mm 394 kg/km. 0.314300/km 47.15 kN 76 GN/ m2 19.8x10 6/C 95C

B Voltage Level Code Name No.of conductors/phase Stranding /wire diameter

Total sectional area Overall diameter Approx. weight Calculated DC resistance at 20C Minimum UTS Modulus of elasticity Co-efficient of linear exp Maximum allowable temp.

132kV/110kV ACSR ACSR PANTHER One 30/3mmAL+7/3mm St.

261.5mm2 21.00mm 974 kg/km 0.14 Ohm/km 89.07kN 80 GN/m2 17.80x10-6/deg.C 95C

132kV110kv AACSR PANTHER One 30/3mm Al. Alloy +7/3mm st. 261.5mm2 21.00mm 974 kg/km 0.156700/km 118.19 80 GN/m2 17.8x10 6/C 95C

C Voltage Level Code Name

220kV ACSRZEBRA

220kV AACSR ZEBRA

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Sl. No.

Description ACSR AACSR

No.of conductor /phase Stranding/wire diameter

Total sectional area Overall diameter Approx.weight Calculated DC resistance at 20C Min. UTS Modulus of elasticity Co-efficient of linear exp. Max.allowable temp.

One 54/3.18mmAL+7/3.18mmSt. 484.5mm2 28.62mm 1621 kg/km 0.06868 Ohm/km. 130.32 kN 69 GN/m2 19.30x10-6/deg.C. 95C

One 54/3.18Al Alloy+7/3.18 St 484.5 mm2 28.62mm 1621 kg/km 0.0775000/km 182.41 kN 69 GN/m2 19.3x10-6/C 95C

D Voltage Level Code Name

No.of conductor /phase

Stranding/wire diameter

Total sectional area Overall diameter Approx.weight Calculated DC resistance at 20C Min. UTS Modulus of elasticity Co-efficient of linear exp. Max.allowable temp.

400kV ACSR MOOSE

Two (Twin Bundle)

54/3.53mmAL+7/3.53mm St. 597mm2 31.77mm 2004 kg/km 0.05552 Ohm/km. 161.2 kN 69 GN/m2 19.30x10-6/deg.C 95C

400kV AACSR `MOOSE

Two(Twin Bundle)

54/3.53 AL Alloy 7/3.53 597mm2 31.77 mm 2004 kg/km. 0.62312Ohm/ km 224.71 kN 69 GN/m2 19.3x10-6/ C 95C

E. Voltage Level

Code Name No.of conductors/phase

Stranding /wire diameter

Total sectional area Overall diameter Approx.weight Calculated DC resistance at 20C Minimum UTS Modulus of elasticity Co-efficient of linear exp. Maximum allowable temp.

800kVAC/500kVHVDC

ACSRBERSIMIS QUADRUPLE BUNDLE

42/4.57AL+7/2.54 ST

725mm2 35.05mm 2181kg/km 0.04189 Ohm/km 154kN 62 GN/m2 21.50x10-6/deg.C 95C

800kVAC/500kVHVDC AACSRBERSIMIS QUADRUPLE BUNDLE 42/4.57ALLOY+7/ 2.54 ST 724.4mm2 35.04 mm 2180 kg/km. 0.04189 Ohm/km 232.53kN 62 GN/m2 21.30x10-6/C 95C

4.3 INSULATOR STRINGS

Following types of insulator strings are generally used for River/Long span crossing stretches for various voltage lines. However each case is to be examined.

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TABLE - 12

VOLTAGE (KV)

TYPE OF STRING

SIZE OF DISC(DIA X SPACING (MM)

NO. OF DISCS

ELECTRO MECHANICAL STRENGTH OF DISC (kN)

MECHANICAL STRENGTH OF STRING (kN)

REMARKS

66 DOUBLE I 255 X 145 2 X 6 45 2 X 45

132 DOUBLE I 255 X 145 2 X 10 45 2 X 45

220 DOUBLE I 255 X 145 2 X 15 70 2 X 70

400 DOUBLE I 255/280 X 145 2 X 24 120 2 X 120

400 DOUBLE V 255/280 X 145 4 X 24 90 2 X 90 FOR WIND ZONE 1,2,3 AND 4

400

DOUBLE V

255/280 X 145

4 X 24

120

2X120

FOR WIND ZONE 5 & 6

500 HVDC DOUBLE V

280 X 170 4 X 35 210 2 X 210

800 DOUBLE V

320 X170 4 X 38 210 2 X 210

4.4 EARTHWIRE

Normal and special earthwire used in the line for various voltages is adopted for River crossing stretches also. However for longer spans, higher strength earthwires are sometimes used to minimise the sag. Earthwires used for various voltage lines are given below.

TABLE NO.-13

SL. NO.

DESCRIPTION NORMAL EARTHWIRES SPECIAL EARTHWIRES

A SYSTEM VOLTAGE 400KV/500KV/800kV 400KV/500KV/800kV Material of Earthwire Galvanised Steel AACSR No. Of Continuous EW Two Two Stranding/Wire diameter 7/3.66mm 16/2.86 Al-Alloy

19/2.48 Steel Total Sectional Area 73.65 mm2 194.6 mm2 Overall Diameter 10.98mm 18.12mm Approximate Weight 583 kg/km 1005 kg/km Resistance at 20 deg. C 2.5 Ohms/km 0.3211 Ohm/km. Minimum UTS 68.4 kN 143.2 kN Modulus of Elasticity 102 GN/m2 130.5 GN/m2 Co-efficient of linear expansion 11.5 x 10-6/deg.c 14.5 x 10-6/ C

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SL. NO.

DESCRIPTION NORMAL EARTHWIRES SPECIAL EARTHWIRES

Max. Allowable Temp 53 C 53C B SYSTEM VOLTAGE 220kV, 132kV, 66kV To be selected if

required based on specific parameters

Material of Earthwire Galvanised Steel No. . of Earthwire One Stranding/Wire diameter 7/3.15MM Total Sectional Area 54.55 mm2 Overall Diameter 9.45mm Approximate Weight 428 kg/km Calculated D.C Resistance at 20C 3.375 ohms/km Minimum UTS 5710 kgs Modulus of Elasticity 190 GN/m2 Co-efficient of linear Expansion 11.5 x 10-6/deg.c Max. Allowable Temp 53 C 53C

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CHAPTER 5 SPAN OF RIVER CROSSING SAG TENSION CALCULATION

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5.1 RIVER CROSSING SPAN/DESIGN SPAN

Crossing span is the distance between two River Crossing towers or two tall towers placed on banks for crossing the river. Height of the River crossing tower or tall tower is fixed depending on crossing span. Crossing span is an important parameter for designing river crossing or tall tower.

5.2 ANCHOR SPAN

The distance between River crossing tower and Anchor tower on either bank of the river is Anchor span. In some cases Anchor span is kept little more than normal span to avoid uplift on Anchor tower consequent to tall river crossing tower.

5.3 WIND SPAN

The wind span is the sum of the two half spans adjacent to the River crossing tower under consideration.

5.4 WEIGHT SPAN

The weight span is the horizontal distance between the lowest point of the conductors on the two adjacent spans.

5.5 RULING SPAN/EQUIVALENT SPAN

The ruling/equivalent span is calculated from the following formula between Anchor tower to Anchor tower on either side of the river.

(L13+ L23 + L33) Ruling Span = ____________________ ( L1 +L2 + L3)

Where L1 = Span between Anchor tower and River crossing tower on one side of river.

L2 = River Crossing span L3 = Span between River crossing tower and Anchor tower on

other side of the river.

5.5 SAG TENSION CALCULATION

The Sag and Tension calculation for River Crossing stretches are very important. Sag and Tension calculations are carried out for

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ruling/equivalent span following catenary equation which decides the tension that will prevail in the conductors between anchor tower to anchor tower. Heights of River Crossing and Anchor towers are finalised based on the sag at crossing span and Anchor span calculated with the tension of equivalent/ruling span at maximum temperature no wind condition. The factor of safety for conductor at starting condition should not be less than 5.0 at no wind condition and everyday temperature. Formula used for Sag and Tension calculation using catenary equation are given in Annexure-II.

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CHAPTER 6 PHILOSOPHY OF DESIGN

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6.1 RETURN PERIOD

River crossing or Tall Towers are very important structures. Any failure in these type of structures requires long restoration time and results in huge revenue loss due to disruption in transmission of power. The practice everywhere in the world is to design this type of structure with highest reliability. Indian Standard IS:802 (Part-1/Sec-I):1995, recommends 500-years return period for design of tall River Crossing towers.

6.2 TYPE OF TOWER

River crossing tower should normally be a suspension tower. This is required for imposition of minimum load on tall tower and at the same time for convenience of stringing operation. River crossing tower should always be provided with double suspension strings of insulators of suitable strength in I or V configuration.

6.3 WIND ON TOWER

River crossing towers are also to be designed for diagonal wind acting on tower conductors/earthwire and insulators. This is to be considered in addition to the wind load acting perpendicular to tower, conductor and earthwire. The methodology for calculating wind on tower is given in Clause No.2.2.6 (A).

6.4 DEVIATION ANGLE

Normally suspension towers with zero degree line deviation are used for crossing rivers. This is because of distinct advantage during stringing at tall towers. However deviation angle upto 5 deg may be provided on River Crossing towers to take care of any eventualities and to provide flexibilities during spotting of the towers.

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CHAPTER 7 REQUIREMENT OF BROKENWIRE FOR RIVER

CROSSING TOWER (SUSPENSION TYPE)

_____________________________________________________________

7.1 SINGLE CIRCUIT TOWER

Any one phase or groundwire broken, whichever is more stringent for a particular member.

7.2 DOUBLE AND MULTIPLE CIRCUIT TOWER

Any one phase or groundwire broken whichever is more stringent for a particular member

7.3 BROKEN LIMB CONDITION FOR V INSULATOR STRING

For V insulator strings one limb broken condition to be considered. In such a case the transverse and vertical loads shall be transferred to outer limb attachment point.

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CHAPTER 8 LOADING

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8.1 REQUIREMENT OF LOADS

a). Climatic loads related to reliability requirement b). Failure containment loads related to security requirements

c). Construction and maintenance loads related to safety requirement.

8.2 NATURE OF LOADS

8.2.1 TRANSVERSE LOADS (T) :

a) Wind Load on tower structure, conductor, groundwire and insulator strings.

b) Components of mechanical tensions of conductor and groundwire.

8.2.2 VERTICAL LOADS (V)

a) Loads due to weight of each conductor and groundwire based on appropriate weight span, weight of insulator strings and fittings.

b) Self weight of structure c) Loads during construction and maintenance

8.2.3 LONGITUDINAL LOADS (L)

Unbalanced horizontal loads in longitudinal direction due to mechanical tension of conductor and/or groundwire during broken wire condition. The mechanical tension of conductor/groundwire is the tension corresponding to 100 per cent design wind pressure at everyday temperature or 36 per cent design wind at minimum temperature after accounting for drag co-efficient and gust response factor.

8.3 LOADING CRITERIA

Loads imposed due to action of wind are calculated under the following criteria for river crossing tower and anchor tower:

CRITERION-I - Everyday temperature and full design wind pressure.

CRITERION-II - Minimum temperature and 36% of design wind pressure

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8.4 ANGLE OF INCIDENCE OF WIND

8.4.1 The wind direction on tower and conductor shall be considered as follows:

(i) Angle of incidence of wind 90 to the tower and 90 to the conductor, earthwire and insulators

(ii) Angle of incidence of wind 45 to the tower and 45 to the conductor, earthwire and insulators

(iii) Angle of incidence of wind 0 to the tower ( longitudinal direction )

Tower members will be designed for whichever case is more stringent for a particular member.

8.5 TRANSVERSE LOADS (TR) - RELIABILITY CONDITION

8.5.1 WIND LOAD ON CONDUCTOR/GROUNDWIRE

The load due to wind on each conductor and groundwire normal to the line applied at supporting point shall be determined by the following expression:

Fwc = Pd x L x d x Gc x Cdc Fwc = Wind load in Newtons Pd = Design Wind pressure (Table - 3) L = Wind span being sum of half the span on either side of

supporting point, in metres. d = Diameter of cables in metres Cdc = Drag Co-efficient, taken as 1.0 for conductor and 1.2 for

groundwire. Gc = Gust response factor, taking into account the turbulance of

the wind and the dynamic response of the Conductor. Values of Gc are given in Table 6 for the terrain category 1 and average height of conductor/ground wire above the ground.

The average height of conductor/groundwire shall be taken upto clamping point of top conductor/groundwire on tower less two third the sag at maximum temperature and no wind.

8.5.2 WIND LOAD ON INSULATOR STRING

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Wind load on insulator strings shall be determined from the attachment point to the centre line of the conductor in the direction of wind as follows:

Fwi = Pd x Ai x Gi X Cdt

Where:

Fwi = Wind load on insulator string in Newtons (N) Pd = Design wind pressure in Newton per sq.m (N/m2) Table 3

Ai = 50 per cent of the area of insulator string projected on a plane which is parallel to the longitudinal axis of string in m2

Gi = Gust response factor, peculiar to the ground roughness and depends on the height of Insulator attachment point above ground, values of Gi for the terrain category 1 are given in Table 5.

Cdt = Drag Co-efficient, to be taken as 1.2

In case of multiple string including V string no masking effect shall be considered The total effect of wind on multiple string set shall be taken equal to sum of the wind load on the individual insulator strings.

8.5.3 WIND LOAD ON TOWER

The wind load on tower is to be calculated considering the following:

The wind direction on tower and conductor shall be considered as follows:

(i) Angle of incidence of wind 90 to the tower and 90 to the conductor

(ii) Angle of incidence of wind 45 to the tower and 45 to the conductor

(iii) Angle of incidence of wind 0 to the tower ( longitudinal direction )

In order to determine the wind load on tower, the tower is divided into different panels. These panels should normally be taken between connecting points of legs and bracings. For square and rectangular lattice towers, the wind load for wind normal to the longitudinal and transverse faces of the tower, on a panel of height h applied at the centre of gravity of the panel is:-

Fwt = Pd (1+0.2Sin22)(ST1Cd T1 Cos2 +ST2 CdT2 Sin2) GT

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Where,

Fwt = Wind load on tower panel in Newtons (N) Pd = Design wind pressure in Newton/sq.m (N/m2)

ST1 = total surface area projected normally on the members of face 1 of the panel with height h.

ST2 = total surface area projected normally on face 2 of the\ members of face 2 of the same panel.

FACE-1

0

FACE-2

The projection of the bracing elements of the adjacent faces and of the plan and hip bracings members may be neglected while determining the projected surface area of a face.

CdT1 & CdT2 = Drag co-efficient relevant to face 1 & 2 for wind perpendicular to each face values of CdT1 & CdT2 for different solidity ratios are given in Table - 4. Solidity ratio is equal to the effective area (projected area of all the individual elements)of a frame normal to the wind direction divided by the area enclosed by the boundary of the frame normal to the wind direction.

GT = GUST RESPONSE FACTOR, peculiar to the ground roughness depending on height of CG of panel above ground level. Values of GT are given in Table -5.

8.5.4 TRANSVERSE LOAD FROM MECHANICAL TENSION OF CONDUCTOR AND GROUNDWIRE DUE TO LINE DEVIATION

This load acts on the tower as component of mechanical Tension of Conductor and groundwire

Fwd = 2 x T x Sin Where 2 Fwd = Load in Newtons T = Maximum tension of conductor or groundwire in newtons at

everyday temperature and 100% full wind pressure or at minimum temperature and 36% of full wind pressure whichever is more stringent.

= Angle of Deviation.

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8.5.5 TOTAL TRANSVERSE LOAD (TR) UNDER RELIABILITY CONDITION

TR = Wind loads on conductor, Groundwire, Insulators and tower and Line deviation loads. (Clauses 8.5.1, 8.5.2,8.5.3 and 8.5.4)

= Fwc + Fwi + Fwt + Fwd

Fwc, Fwd are to be applied on all conductor and groundwire points. Fwi is to be applied on insulator attachment point. Fwt wind on tower is to be applied at groundwire peak, cross-arm levels and below bottom x-arm level at suitable panel points as decided by the designer.

8.6 TRANSVERSE LOAD (TS) - SECURITY CONDITION

8.6.1 Transverse loads due to wind action on tower structure, conductor, groundwires and insulators shall be computed as per Clauses 8.5.1, 8.5.2 & 8.5.3 Sixty (60)% wind span shall be considered for brokenwire and 100% for intact span.

8.6.2 Transverse loads due to line deviation shall be the component of 100% mechanical tension of conductor and groundwire as defined in Clause no. 8.5.4. For broken conductor tension is nil.

Fwd = T Sin for intact conductor as per Clause 8.5.4. 2 8.7 TRANSVERSE LOAD (TM) DURING CONSTRUCTION AND

MAINTENANCE - SAFETY CONDITION

8.7.1 NORMAL CONDITION

8.7.1.1 Transverse load due to wind action on tower structure, conductors and groundwires and insulators shall be taken as nil. Transverse loads due to mechanical tension of conductors and groundwires at everyday temperature and no wind on account of line deviation shall be taken.

8.7.1.2 Transverse load due to mechanical tension of conductor or groundwire at everyday temperature and no wind on account of line deviation shall be considered as follows:

TM = 2 x T1 x Sin/2 Where

TM = Load in Newtons T1 = Tension in Newtons of conductor/groundwire at everyday temperature and nil wind.

= Angle of deviation of line.

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8.7.2 BROKENWIRE CONDITION

8.7.2.1 Transverse loads due to wind on tower structure, conductor, groundwire and insulators shall be taken as nil.

8.7.2.2 Transverse load due to mechanical tension of conductor or groundwire at everyday temperature and no wind on account of line deviation shall be considered as follows;

TM = T1 x Sin 2

TM = Load in Newtons T1 = 100% of tension in Newtons of conductor and groundwire at

everyday temperature and no wind condition.

= Angle of deviation.

8.8 VERTICAL LOADS (VR) - RELIABILITY CONDITION

8.8.1 Loads due to weight of each conductor and groundwire based on appropriate weight span, weight of insulator strings and accessories.

8.8.2 Self weight of tower structure upto point/level under consideration.

Where minimum vertical loads are stringent for any particular member, the weight of conductor/groundwire corresponding to minimum design weight span plus weight of insulators string and accessories shall be taken.

8.9 VERTICAL LOADS (VS) - SECURITY CONDITION

8.9.1 Loads due to weight of each conductor or groundwire based on appropriate weight span, weight of insulator strings and accessories for the broken wire condition. Load due to weight of broken conductor/groundwire shall be taken as 60% of weight span. For intact wire the vertical loads shall be considered as given in Clause 8.2.2 (a) and (b).

8.9.2 Self weight of structure upto tower panel under consideration.

8.10 VERTICAL LOADS DURING CONSTRUCTION AND MAINTENANCE (VM) - SAFETY CONDITION

8.10.1 Same as clause no. 8.8.1 multiplied by over load factor 2.0

8.10.2 Same as Clause 8.8.2

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8.10.3 Load of 1500 N shall be considered acting at each cross-arm tip as a provision for weight of line man with tools.

8.10.4 Load of 5000 N at cross-arm tip to be considered for cross-arm design upto 220kV and 10,000 N for 400kV and higher voltages on one conductor attachment at a time.

8.10.5 Peak should be designed for lifting of cross-arm. Normally cross-arm is lifted with the help of a pully fitted in peak. Therefore effectively two times the weight of cross-arm shall be acting on the peak. Therefore peak is to be designed for following dead weight/vertical loads independent of any other loads.

TABLE-14

TYPE OF CONDUCTOR BUNDLE

VERTICAL LOAD (N) FOR PEAK DESIGN

1. Single Conductor 5,000 2. Twin bundle Conductor

10,000

3. Multiple bundle Conductor

20,000

8.10.7 All bracings and redundant members of the tower which are horizontal or inclined upto 15 degree from horizontal shall be designed to withstand an ultimate vertical load of 1500N acting at Centre mid point, independent of all other loads.

8.11 LONGITUDINAL LOADS (LR)-RELIABILITY CONDITION

8.11.1 Longitudinal loads shall be taken as nil.

8.12 LONGITUDINAL LOAD (LS) - SECURITY CONDITION.

8.12.1 Horizontal load in longitudinal direction due to mechanical tension of conductors and groundwires shall be taken for loading criteria specified in Clause No.8.2.3 for broken wire(s). Longitudinal loads at broken phases shall be considered as 100% of mechanical tension of conductor(s)/groundwire. For intact wires these loads shall be considered as nil.

8.13 LONGITUDINAL LOADS DURING CONSTRUCTION AND MAINTENANCE (LM) - SAFETY CONDITION

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8.13.1 NORMAL CONDITION

This load shall be taken as nil

8.13.2 BROKENWIRE CONDITION

8.13.2.1 Longitudinal loads during construction simulating brokenwire condition as given in Clause 8.13.2.2 will be based on stringing of one earthwire or one complete phase at one time.

8.13.2.2 Longitudinal loads during stringing on River crossing tower fitted with suspension string should normally be imposed during pushing of the running block through the sheave. It will apply only on one complete phase of sub-conductors or one earthwire. It will be taken as 10,000 N per sub-conductor or 5000N on one earthwire. Over load factor of 2.0 will be applied on these loads

8.14 LOADING COMBINATION UNDER RELIABILITY,SECURITY AND SAFETY CONDITIONS

8.14.1 RELIABILITY CONDITION (NORMAL CONDITION)

8.14.1.1 Transverse loads as per clause 8.5

8.14.1.2 Vertical loads as per Clause 8.8

8.14.1.3 Longitudinal loads as per clause 8.11

8.14.2 SECURITY CONDITION (BROKENWIRE CONDITION)

8.14.2.1 Transverse loads as per clause 8.6

8.14.2.2 Vertical loads as per clause 8.9

8.14.2.3 Longitudinal loads as per Clause 8.12

8.14.3 SAFETY CONDITION (CONSTRUCTION AND MAINTENANCE)

8.14.3.1 NORMAL CONDITION

1. Transverse load as per 8.7.1 2. Vertical load as per clause 8.10 3. Longitudinal load as per clause 8.13.1

8.15 BROKENWIRE CONDITION

1. Transverse load as per Clause No.8.7.2 2. Vertical load as per Clause No.8.9 3. Longitudinal load as per Clause No.8.13.2

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CHAPTER 9 SPECIAL CONSIDERATIONS

_____________________________________________________________

9.1 TRUNCATION FACILITIES

Normally river crossing tower design once designed, is used for various other river crossings subsequently also with suitable truncation provided they fall in same wind zone. Therefore, panel heights in the bottom of the tower shall be suitably sized, so that after truncation of each panel, matching desired spans are achieved. For example, if the towers is designed for 1000m span, by truncating last panel it should be suitable for about 900m span. Similarly, by further truncation; spans of about 800m, 700m etc., should be achieved. Provision should be kept accordingly while designing the tower.

9.2 MAXIMUM HEIGHT OF TOWER

Height of tower depends on the span and ground profile. Where natural contours permit, a combination of normal suspension and angle/dead-end towers with extensions if required can be used for crossing the rivers. It is not advisable to install very tall towers, since tall structure is likely to encounter higher wind forces and is susceptible to failure. Normally spans upto maximum 1000m are crossed for various voltage levels with tower height of around 130 to 145m. In isolated cases spans more than 1000m are also crossed where tower heights are sometimes more than 170 metres.

9.3 USE OF SPECIAL CONDUCTOR

For suitable reduction in tower height, special conductors of higher strength are used in the river crossing stretches. Aluminium alloy conductor steel reinforced (AACSR) are popular in this regard. Using of such conductor not only reduces tower height but also saves in tower weight.

9.4 STEEL QUALITY

Normally, members of tall river crossing tower are loaded heavily. This is due to extraordinary height of towers, conductor and groundwire above ground level encountering heavy wind. To have desired capacity in tower members to withstand heavy loads, combination of mild and high tensile steel are normally used. HT steel to be used shall have yield stress not more than that of steel equivalent to IS:8500-490B/BSEN-10025-S355 JR.

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9.5 MINIMUM THICKNESS OF ANGLE SECTION USED

River crossing towers are subjected to heavy loads. Therefore angle section less than 5mm thickness should not be used in the tower to achieve adequate rigidity in the structure. The minimum thickness of angle section shall be not less than following.

i) Leg members, main members of crossarm and peak = 6mm ii) Other members including bracings and redundants = 5mm

9.6 BOLTS AND NUTS

Normally two sizes of bolts (diameters) are used in the tower. One size is used for lightly loaded members and the higher size bolt is used for heavily loaded member especially legs. This is required to minimize the number of bolts in heavily loaded members.

To avoid any error during tower erection in respect of use of lower size bolt, it is preferable to have distinguishable gap between the two sizes of bolts used. For example, if one size of bolt is 16mm dia, the second size should be 24mm dia bolt.

9.7 SLENDERNESS RATIO

Slenderness ratio for members shall be computed in accordance with IS-802 (Part-1/Sec-2)-1992. Slenderness ratio (kl/r) for compression and tension members shall not exceed the following values:-

a) Leg members including corner members of earthwire peak and lower corner members = 120 of the cross arms in compression. b) For other members carrying computed stresses = 200 c) For redundant members. = 200 d) For members under tensile stress only = 375

9.8 REDUNDANT MEMBER

All the redundant members should be designed for slenderness ratio of not more than 200. Redundant members connected to leg member should be designed for actual axial load or for 2.5% load carried by the leg member whichever is more stringent. Adequate number of bolts shall be provided in the redundant members accordingly.

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9.9 AVIATION REQUIREMENT

9.9.1 PAINTING OF TOWERS

9.9.1.1 The full length of the tower shall be painted over the galvanised surface in contrasting bands of orange or red and white. The bands shall be horizontal. The width of the colour band shall be as per relevant aviation regulations, prevalent at the time of execution of the project.

9.9.1.2 SURFACE PREPARATION

The etching of galvanised surface of erected tower members with suitable etching or wash primer is to be carried out as per IS:1477 to ensure the adhesion of paint coating. After etching of galvanised surface of tower, a coat of zinc primer is to be applied.

9.9.1.3 PAINTING REQUIRMENTS

Two coats of alternate bands of international orange and white paint as explained above and conforming to amendment No.1 July 94 of IS-5613 (Part 3/Sec-1)-1989 or Annex-14 , volume 1 of ICAO Rules are to be applied. The painting of towers shall generally conform to relevant provisions in IS-1477 (Parts-I and II). The paints to be used for painting shall be in accordance with IS-2074.

9.9.2 SPAN MARKERS

In the river crossing span, sphere type span markers of 600mm diameter shall be mounted on the earthwire. The sphere shall be half orange and half white conforming to amendment No.1 July 1994 to IS-5613 (Part 3/Sec-1-1989 or ICAO rules,Annex-14,Volume-1. The markers shall be suspended from earthwires at interval of 36 metres. The design of the markers and their fixing arrangement shall be such that they withstand the wind pressure and do not induce undue vibratory strain in earthwire.

9.9.3 NIGHT AVIATION SIGNALS

Night aviation lamps are also provided on the river crossing towers following ICAO rules Annex-14 , volume-1. Lamps of required intensity are to be provided on the tower at various levels as per Civil Aviation Rules and conforming to amendment No.1 July 94 of IS-5613 (Part 3/Sec-1)-1989.

9.10 PLATFORMS AND LADDERS

River crossing and tall tower shall be provided with step bolts of not less than 16mm diameter and 175mm length, spaced not more

36

than 450mm apart from 3.5 meter above ground level upto 30m height. Thereafter in continuation of the step bolts, ladders alongwith protection rings shall be provided upto top of the tower. Suitable platform using 6mm thick chequered plate alongwith suitable railings shall be provided. Platform is to be provided at each cross-arm level, and peak level from ladder end to tip of cross-arm and peak. Rest platforms are also to be provided at an interval of every 20m height. The platform shall be fixed to the tower by using countersunk bolts.

9.11 MAINTENANCE FIXTURES

Suitable maintenance fixtures are to be provided on the tower cross-arms and peak. Platforms are to be provided in the cross arm following the centre line of cross-arm spreading from tower body to cross-arm tips. The platform width should not be less than 750mm. This platform shall work as bird guard also protecting insulators from bird spitting. Rest platforms are to be provided for climbing every 20m height from 30 m onwards from ground level.

9.12 FOUNDATIONS

Foundations for river crossing tall towers are very important. Foundation designs should be developed based on soil investigations carried out at the river crossing locations. Since the foundation loads are of high magnitude, open foundation can be adopted only at those locations where soil parameters are good. In case of weak soil, pile and well foundations are recommended.

Setting of stubs for river crossings tower is difficult due to its large base width. Common practice is to use anchor bolts which can be easily installed over the pile, well or normal foundations. Wherever the foundations are constructed midstream and there is substantial height difference between normal river level and top of foundation, suitable steps made of reinforcement are to be provided on the foundation for climbing of man with T&P for maintenance purposes.

9.13 LIFE EXPECTANCY

Normally transmission lines are constructed for 30 years life span. But it is seen, most of the transmission lines are in service in the country even after 50 years. Considering this aspect, most of the transmission lines are expected to work for more than 50 yrs. River crossing towers are designed with higher reliability factor and expect higher life span in comparison to normal towers in the line.

-.-.-.-.-.

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ANNEXURE-I CALCULATION OF PHASE SPACING ON LONG SPAN TRANSMISSION LINE: FORMULAE ADAPTED BY DIFFERENT COUNTRIES

Germany

0.75 f +40 + 1k + V m (Vertical) 150

0.62 f +40 + 1k + V m (Horizontal) 150

Austria

0.9 f +40 + 1k + V m (Horizontal) 150

Belgium

0.75 f + 1k + V m (Vertical) 150

0.62 f + 1k + V m (Horizontal) 150

France

0.9 f + 1k + V m 150

U.S.A.

0.75V + 3.26 f inch

Poland

0.75 f +40 + 1k + V m (Vertical) 150

0.65 f +40 + 1k + V m (Horizontal) 150

Sweden

6.5 f + 0.7 V cm

Czechosiovakia

25 + V + 7 f cm

Cananda

300+3(La- LR) + 83 x f+15 + 10 (V VR)mm In which f = Max. Sag f+40 = Sag at 40C 1k = Length of Insulator String (assumed as 4 m) V = Voltage in kV La = Actual span in m limited to 450 m LR = Reference span in m (50 m) VR = Reference Voltage in kV (5 kV) F15 = Sag at 15C

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ANNEXURE-II

CALCULATION OF SAG INCLUDING CATENARY EFFECT

A. INPUT DATA Unit 1. Catenary span = L m 2. Tension at Maximum tremperature = T1 Kg 3 Height difference = H m 4. Weight of conductor = W Kg/M 5. Span for river crossing section = L m 6 RL of Location (High Tower) = RH m 7 Height of bottom X-arm (High Tower) = BCH 8 Sag error = SE m 9 Length of Suspension Insulator = DS m 10 Length of Hanger = HD m 11 RL of HFL = HFL m 12 Minimum ground clearance = MGCL m

B. CALCULATIONS:

Null point from high tower (L1) = L + T1 x H 2 W x L

= L1 M

L2 = 2 * L1 = 2 x L1

= L2 M

Parabolic sag for L2 =( L2 )2 x W x L x L ( L ) 8 x T1

= S1 M

Length of cable for half span =L3 = L1 + 2 x S1 x S1

3 x L1

= L3 M

Tangential tension =T2 = L3 x W = T2

Resultant tension at Max. temp.=T3 = sqrt( T1 xT1 - T2xT2

= T3 Kg

Catenary sag = S2 = T3 x ( cosh ( W x L1 ) - W T3

= S2 M

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RL of null point = RLNP = RH+ BCH - S2 - SE - DS

= RLNP

Available Clearance = ACL = RLNP - HFL

= ACL

ACL > MGCL

40