Mechanical Design Of Overhead Lines 2x2x3p

Mechanical Design of Overhead Lines 1

INTRODUCTION Advantages of Overhead lines with comparison of Underground Cables

Main Components of Overhead Lines 2

Conductors • • • • •

s Copper Aluminum Insulators Steel-cored aluminum Galvanized steel arms Cium Cross copper Miscellaneous items

Overhead Lines: Wooden poles 3

• Tendency to rot below the ground level • Comparatively smaller life (20-25 years) • Cannot be used for voltages higher than 20 kv • Less mechanical strength and • Require periodical inspection

Overhead Lines: RCC poles 4

High cost of transport owing to their heavy Weight

Overhead Lines: Steel Towers 5

• Long distance transmission at higher voltage • Greater mechanical strength • Longer life • Can withstand most severe climatic conditions • Permit the use of longer spans

Insulators 6

Properties High mechanical Strength High electrical Resistance High relative permittivity non-porous, free from impurities and cracks

Insulators: Pin type insulators 7

Used for TD upto 33 kV

Insulators: Suspension type insulators 8

Advantages Cheaper Used for TD greater than 33 kV

Can be repaired Flexibility

Protect from lightning

Insulators: Strain insulators 9

Used in dead end of the line

Insulators: Shackle insulators 10

Used only in Distribution line

Potential Distribution over Suspension Insulator String 11

Shunt Capacitance

Voltage does not distribute itself uniformly

Mutual Capacitance/ Self The Capacitance

disc nearest to the conductor has maximum voltage D.C. voltage distributes uniformly

String Efficiency 12

The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency.

The greater the string efficiency, the more uniform is the voltage distribution

String Efficiency: Mathematical expression 13

• Self capacitance each disc is C Nearest to theof conductor has maximum voltage

• Shunt capacitance C1

• Where The C1 greater = KC the value of K (= C1/C), the more non-uniform is the potential.

Inequality in voltage distribution increases with the increase of number of discs. Shorter string has more efficiency than the larger one.

V String efficiency = 3 ×V3

Methods of Improving String Efficiency 14

By using longer cross-arms By grading the insulators By using a guard ring

Important Points 15

The maximum voltage appears across the disc

nearest to the conductor (i.e., line conductor). The voltage across the string is equal to phase

Assignment (Submit before class) Voltage across string=Voltage betweenthe linenext and earth = Phase Voltage voltage i.e.,

Example 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 8.10, 8.11, 8.12

Line Voltage= √3 × Voltage across string

Corona 16

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona. THEORY OF CORONA FORMATION

Factors Affecting Corona 17

Atmosphere Conductor size Spacing between conductors Line voltage

Important 18

Critical disruptive voltage

Important 19

Visual critical voltage

Important 20

Power loss due to corona

Advantages and Disadvantages of Corona 21

Advantages Disadvantages Virtual diameter of the Lost conductor of Energy is increased Reduces the effects of transients produced by surges Ozone is produced by corona Non-sinusoidal voltage drop and hence inductive interference with neighboring communication lines

Methods of Reducing Corona Effect 22

By increasing conductor size By increasing conductor spacing

Sag in Overhead Lines 23

The difference in level between points of s and the lowest point on the conductor is called sag

Conductor sag and Tension

Calculation of Sag 24 When s are at equal levels

2

2

w(l / 2) wl Sag , S = = 2T 8T

Calculation of Sag 25 When s are at unequal levels

Effect of wind and ice loading 26

Assignment (Submit before the next class) Example 8.17, 8.17, 8.19, 8.20, 8.21 , 8.22, 8.23, 8.24, 8.25, 8.26, 8.26

ww tan θ = w + wi

2

wt l Sag , S = 2T

The vertical Sag = S cos θ