CIVIL ENGG / PHYSICS HELP IN REPORT

Introduction

Transmission Line is considered as integrated system consisting of the supporting structure and functional system i.e. Electrical Systems and its components.

The geometry of the supporting structure is dependent on the electrical system which is based on the line voltage. The electrical system includes:

• Conductor subsystem consisting of conductor and its holding clamps.

• Ground wire subsystem consisting of ground wire and its holding clamps.

In design of tower for weight optimization, the basic parameters of the structure are constrained on the basis for electrical requirements and these include:

· Base width.

· Height of the tower.

· Outline of the tower

One of the important concern is the structure must be able to resist both vertical and lateral loadings. Although the forces due to wind or earthquake are dynamic in nature, it is assumed static forces to simplify the analysis and design process. Values of static wind loading equivalents and factors are provided in NESC loading (National Electrical Safety Code, 2002) and CP 3: Chapter V: Part 2:1972/BS 6399-2:1997- Code of practice for wind loads.

Specification and Properties of the Structure

A transmission line tower, like any other exposed structure, has a super structure suitably shaped, dimensioned and designed to sustain the external loads acting on the strung cables (conductors and ground wires) and the super structure itself. The superstructure has a trunk and a hamper (cage) to which cables are attached, either through insulators or directly.

A.S.C.E manual “Guidelines for Electrical Transmission Line Structural Loading” has distinguished the overall configuration of a transmission line structure on the basis of following requirements:

• Ground clearance requirements

• Electric air gap clearance requirements

• Electric and magnetic field limits

• Insulation requirements

• Structural loading

• Number of circuits

• Right of way requirements

Determination of height

The configuration of a transmission line tower is dependent on the following parameters:

· Minimum permissible ground clearance, h1

· Maximum sag, h2

· Vertical spacing between conductors, h3

Figure 3.1 Determination of Tower Height

· Vertical clearance between earthwire and top conductor, h4

CBIP in its “Transmission Line Manual” has summed up the total height of a transmission line tower as summation of the following:

h=h1 + h2 + h3 + h4

Ground Clearances

It is the minimum distance from the ground to the lowest point of the bottom conductor. It is fixed as per the requirement of electric air gap clearance and the electric and magnetic field limitations.

The minimum permissible ground clearance as per IE Rules, 1956, Rule 77(4) can be determined from the equation

CL = 5.182 + 0.305*K

Where K =

Spacing between Conductors (Phases)

1) Mecomb’s Formula

Spacing (cm) =

Where:

V = Voltage of System in kV

D = Diameter of Conductor in cm

S = Sag in cm

W = Weight of Conductor in kg/m

2) VDE Formula

Spacing (cm) =

Where:

V = Voltage of System in kV

S = Sag in cm

3) Still’s Formula

Spacing (cm) =

Where:

V = Voltage of System in kV

l = Average span length (m)

4) NESC Formula

Spacing (cm) =

Where:

V = Voltage of System in kV

S = Sag in cm

L = Length of Insulator String in cm

5) Swedish Formula

Spacing (cm) =

Where:

E = Line Voltage in kV

S = Sag in cm

L = Length of Insulator String in cm

Table 3.1 Recommended Ground Clearance for Voltage Levels

6) French Formula

Spacing (cm) =

Where:

E = Line Voltage in kV

S = Sag in cm

L = Length of Insulator String in cm

Length of suspension insulator string:

It is an important parameter in deciding the phase to minimum ground metal Clearance, which in turn decides the length of cross arms. It is a function of insulation level, power frequency voltage and service conditions (pollution, altitude, humidity).

Vertical clearance between ground wire and top conductor:

This vertical clearance is decided by the requirement of the peak clearance and the mid span clearance.

Peak clearance is dependent on the angle of shielding made by the ground wire to protect the power conductors against the direct lightning stroke and to conduct the lightning current to the nearest earthed point when contacted by a lightning stroke.

Mid span clearance is the spacing required at the null points between the ground wire and the conductor to safe guard the conductor from flashover during lightning.

Maximum Sag

Sag-tension calculation

The sag of the conductor is defined as the distance between the point of attachment of the cable to the insulator/ tower and the null point in the cable (earth wireand conductor). It is dependent on the size and type of conductor, climatic conditions (wind temp., snow) and span length.

The sag and tension of the conductor are subject to variations due to the changes in temperatures and loading. For spans of the order of 300 meters and less, the sag and tension calculation can be carried out by parabolic formula with sufficient degree of accuracy. For the case of very long spans, catenary formula gives more accurate results than parabolic.

The horizontal tension at the lowest point (H) shall normally be taken 60 N/mm2 for span less than 300 m and 100 N/mm2 for span greater than 300 m span. This tension will be treated as a pretension.

Table 3.2 Spacing Between Conductors

Table 3.2 Spacing Between Conductors (Cont’d)

Loads on Structure

Load on the structure are calculated in three directions: vertical, transverse, and longitudinal, the transverse load is perpendicular to the transmission line and the longitudinal act parallel to the line.

Vertical Loads (NESC Code)

The vertical load on supporting structures consists of the weight of the structure, loads during construction and maintenance plus the superimposed weight, including all wires, ice coated where specified. The vertical load is given by the following equations

Vertical load of wire Vw is given by the following equations:

Vw = weight of bare wire x vertical design span x OCF

Vertical design span is the distance between low points of adjacent spans

Longitudinal Loads

Longitudinal loads are unbalanced forces developed at the structure due to various conditions on the line such as broken wire loads, differential in adjacent span length in rugged terrain, inclined spans and non-uniform loading of adjacent spans.

Figurre 3.1 Sag and Tension Formulae

Transverse Loads

These are caused by wind pressure on wires and structure, and the transverse component of the line tension at angles.

Wind Load on Wires (NESC Code)

For wind on wires, the wind on the overhead ground wires and conductors are considered. The transverse load due to the wires is given by:

F = q x d x Horizontal Span x OCF

Where

F = Transverse Wind Load on Wire

q= Dynamic Pressure of the Wind

OCF= Overload Capacity factor

The horizontal span is the distance between midpoints of adjacent span

Wind Load on Structures (CP3: Chapter V: Part 2: Sept 1972)

The exposed areas of the structure members are also subjected to wind loads. The wind coefficient of the structure depends on the shapes of member sections, solidity ratio, angle of incidence of wind i.e. face wind or diagonal wind, and shielding. The transverse load due to wind is obtained by calculating the dynamic pressure due to wind speed, and multiplying with the effective area of the members modified by a coefficient as below:

Va = V x S1 x S2 X S3

Where V = Basic Wind Speed

S1 = Topography Factor

S2 = Factor of ground roughness, structure size and height above the ground

S3 = Factor of Statistical Concept

Dynamic Wind Pressure q = kVa2

Where K = 0.613

Va= Design Wind Speed

Hence Load due to Wind, F = CrqAe

Where

Cr= the Overall force Coefficient

q = Dynamic pressure of the wind

Ae = Effective area of face

Loading Criteria

The Loading Criteria for the transmission line as given by CBIP in “Transmission Line Manual” is as follows:

i. Reliability

ii. Security

iii. Safety

Reliability of a transmission system is the probability that the system would perform its function/ task under the designed load criteria for a specified period. Thus, this covers climatic loads such as wind loads and/or ice loads.

Security of a transmission system is the capacity of the system to protect itself from any major failure arising out of the failure of its components. Thus, this covers unbalanced longitudinal loads and torsional loads due to broken wires.

Safety of a transmission system is the ability of the system to provide protection against any injuries or loss of lives to human beings out of the failure of any of its components. Thus, this covers loads imposed on tower during the construction of transmission line and loads imposed on tower during the maintenance of transmission line.

Chapter Four

Analysis and Design

1

2

3

2 Analysis

The lattice tower has 12 panels basic wind speed of 47 m/sec with open terrain with well scattered obstructions having height generally 1.5 m to 10 m (this category includes normal country lines with very few obstacles) and return period of design loads of 50 years was considered. The basic wind speed is applicable at 10 m height above mean ground level and based on peak gust averaged over a short time interval of about 3 seconds corresponding to mean height above ground level in an open terrain.

1.

2.

3.

4.

4.1 Configuration of Tower

· Base Width: 8.5m X 8.5m (Square Base)

· Hamper (Cage) Width (L.C.A): 3.6m X 3.6m

· Topmost Hamper width (Ground Wire): 2m X 2m

· Total Tower Height: 50m

· Permissible Weight Span:

Normal Condition: Maximum: 525 mm; Minimum: 200 mm

Broken Wire Condition: Maximum: 315 mm; Minimum: 100 mm

· Design Span: 350 mm

· Wind Span = Normal Span: 400 mm

· Weight Span: 550 mm

· Concrete Level to Ground Level: 225 mm

4.2 Transmission Line Components

The following parameters for transmission line and its components are assumed:

· Transmission Line Voltage: 220 kV (A. / C.)

· Tower Type: Suspension Tower

· Tower Configuration: Vertical Conductor Configuration

· No. of Circuits: Double

· Angle of Line Deviation: 0 to 2 degrees

· Terrain Type Considered: Plain

· Terrain Category: 2 (Normal cross country lines with very few obstacles)

· Return Period: 50 yrs

· Wind Zone: 4

· Basic Wind Speed: 47 m/s

· Basic Wind Pressure: 71.45 kg/sqm

· Weight of Insulator Disk: 3.5kN

· Weight of Ground Wire Attachment: 2.00kN

· Cross Arm: Pointed

4.3 Conductor and Ground Wire Parameters

· Conductor Material: ACSR, (Aluminum Conductor Steel Reinforced)

· Maximum Temperature: 75°C (ACSR)

· Number of Ground Wires: Single

· G.W. Type: Earth wire – 7 / 3.66

· Shielding Angle: 30o

· Maximum Temperature: 53°C (7 / 3.66)

· Insulator Type: I String

· Number of Insulator Discs: 14

· Size of Insulator Disc: 255 * 145 mm (Skirt Diameter)

· Length of Insulator String: 2,340 mm

· Minimum Ground Clearance: 7,500 m Sag Error Considered: 160 mm

· Mid Span Clearance: 8,500 mm

· Minimum Height above G.L.: 50,000 mm

· Phase to Phase Clearance: Vertical Spacing between Conductors (Minimum): 7,500 mm.

· Horizontal Spacing between Conductors (Minimum): 8,500 mm

· Lightning Impulse Level (Air Clearance): 1700 mm

· Minimum Phase to Earth (Air Clearance): 1970 mm

· Minimum Thickness of Member: Leg Member, G.W. Peak and Lower Member of C.A.: 5 mm; Others: 4 mm

4.4 Sag and Tension

Sag Tension Calculation by using the Catenary formula for Span >300m computed in Excel Spreadsheet for both the conductor and ground wire. The results of the Sag and Tension Calculations are given in Table 4.1, the detailed calculations are shown in Appendix 1.

Table 4.1 Sag and Tension in Wires

	Conductors	G.Wire
Height of Crossarms, Z (m) (above Tower base)	28.2	36.2	44.2	50.0
Mean Wind Speed, VZ(m/s)	41.3	43.0	44.5	45.4
Dynamic Wind Pressure,Vd (N/m2)	1046.1	1136.0	1213.3	1263.7
Sag of Conductor, D (mm)	19.9	21.0	22.0	56.6
Tension, T (kN)	49.0	49.0	49.1	8.0

4.5 Loading

4.5.1 Wind Loading on Tower

The wind loads on the members of the tower were calculated according to BS CP3: Chapter V Part 2, 1972. The dynamic wind pressures calculated at the panel heights were applied to the panels to obtain equivalent load on the projected faces of the tower. The wind forces were applied to the leg members at the top and bottom ends of the panels. The forces due to the wind load are shown in Table 4.2. The detailed calculations and Panels are shown in Appendix 2.

4.5.2 Wind Loading on Conductor and Earth Wire

The wind loads on the members of conductors were calculated according to the NESC code, by applying dynamic wind pressure at the crossarms on the wind span and diameter of the wire. The wind forces were applied as transverse load at the tip of the crossarms. The forces due to the wind load on the wires are shown in Table 4.3. The detailed calculations are shown in Appendix 3.

4.5.3 Vertical Load

The vertical loads on the tower consists of the

· Self Weight of the tower members

This is computed automatically in the Software used for Analysis)

· The weight of the conductors, ground wire and insulators

This is computed by using the weight span of the wires

The longitudinal loads on the tower consist of forces due to broken wire conditions, and considered to be negligible under normal conditions

Table 4.2 Wind Force on Tower (Divided into 12 Panels)

Table 4.3 Loads due to Conductor and Ground Wire

The longitudinal and vertical loads due to the wires are shown in Table 4.3. The detailed calculation notes are shown in Appendix 3

4.6 Modeling of Tower

The transmission tower is modeled as a three dimensional space truss using STAAD Pro V8, the loading are then synchronized at the tip of peak and at the three tips of the cross arms. (Appendix 8)

4.7 Load and Load Conditions

The following Load Combinations were considered for the design of the Transmission tower: (Appendix 9)

· Normal Condition- SelfWeight of Structure, Vertical and Transverse Load of Wires

· Broken Wire Conditions

· Earth Wire-Self Weight, Vertical, Transverse and Longitudinal Load due to broken earth wire

· Top Right Conductor-Self Weight, Vertical, Transverse and Longitudinal Loads due to Broken Top right conductor

· Middle Right Conductor-Self Weight, Vertical, Transverse and Longitudinal Loads due to Broken middle right conductor

· Bottom Right Conductor-Self Weight, Vertical, Transverse and Longitudinal Loads due to Broken bottom right conductor

Figure 4.1a-d below depicts the loading conditions.

4.8 Preliminary Design of Tower

The tower is designed using the Limit States Design Approach of BS 5950:2000.

Preliminary member sizing is done to provide input data for the analysis of the tower, the result of the forces and stresses in the members during the first analysis is then used to redesign the tower. These steps are repeated until an optimum and conservative member section is attained

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Figure 4.1a Loading Tree for Normal Condition

Figure 4.1b Loading Tree for Broken Wire Condition (GW)

Figure 4.1c Loading Tree for Broken Wire Condition (TR)

Figure 4.1d Loading Tree for Broken Wire Condition (MR)

Figure 4.1b Loading Tree for Broken Wire Condition (BR)

Figure 4.2 2D Wire Frame Tower Model

Chapter Five

Results and Discussion

5

Results

5.1 Maximum Forces in Leg Members

The maximum forces in the Leg Members are as summarized as follows. (See Appendix 4 )

Table 5.1 Maximum compressive and Tensile Forces in Leg Members

5.2 Maximum Forces in the Cross Arms

The maximum forces in the Cross Arms from Analysis are summarized in the table below. (See Appendix 4)

Table 5.2 Maximum Forces in Cross Arms

5.3 Maximum Forces in the Horizontal Lattices

The maximum forces in the lattice members from Analysis are shown in the table below: (See Appendix 4)

Table 5.3 Maximum Forces in horizontal members