Mechanical Design of Overhead Lines 1
INTRODUCTION Advantages of Overhead lines with with comparison of Underground Cables
Main Components of Overhead Lines 2
Conductors Supports • Copper • Aluminum Insulators • Steel-cored aluminum • Galvanized steel arms • Cadmium 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 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.
String efficiency =
V
3 ×V 3
Methods of Improving String Efficienc Efficiency y 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 voltage i.e., (Submit before class) Voltage across string=Voltage betweenthe linenext and earth = Phase Voltage
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 corona..
THEORY OF CORONA FORMATION
Factors Affecting Corona 17
Atmosphere Conductor size Spacing between conductors Line voltage
Important Terms 18
Critical disruptive voltage
Important Terms 19
Visual critical voltage
Important Terms 20
Power loss due to corona
Advantages and Disadvantages Disadvantages of 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 supports and the lowest point on the conductor is called sag
Conductor sag and Tension
Calculation of Sag 24
When supports are at equal levels
Sag , S =
w(l / 2) 2T
2
2
=
wl
8T
Calculation of Sag 25
When supports 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
tan θ =
ww w + wi
2
Sag , S =
wt l
2T
The vertical Sag = S cos θ