mechanical design of overhead lines
TRANSCRIPT
04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET
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Mechanical Design of Overhead Lines
INTRODUCTIONAdvantages of Overhead lines with comparison of Underground Cables
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Main Components of Overhead Lines
Conductors
Supports
Insulators
Cross arms
Miscellaneous items
•Copper•Aluminum•Steel-cored aluminum•Galvanized steel •Cadmium copper
04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET
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Overhead Lines: Wooden poles
• 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
04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET
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Overhead Lines: RCC poles
High cost of transport owing to their heavy Weight
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Overhead Lines: Steel Towers
•Long distance transmission at higher
voltage
•Greater mechanical strength
•Longer life
•Can withstand most severe climatic
conditions
•Permit the use of longer spans
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Insulators
PropertiesHigh mechanical
StrengthHigh electrical
ResistanceHigh relative permittivity
non-porous, free from impurities and cracks
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Insulators: Pin type insulators
Used for TD upto 33 kV
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Insulators: Suspension type insulators
Used for TD greater than 33 kV
AdvantagesCheaper
Can be repaired
FlexibilityProtect
from lightning
04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET
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Insulators: Strain insulators
Used in dead end of the line
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Insulators: Shackle insulators
Used only in Distribution line
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Potential Distribution over Suspension Insulator String
Mutual Capacitance/
Self Capacitance
Shunt Capacitance
Voltage does not distribute itself uniformly
The disc nearest to the conductor has maximum voltage
D.C. voltage distributes uniformly
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String Efficiency
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
04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET
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String Efficiency: Mathematical expression
•Self capacitance of each disc
is C
•Shunt capacitance C1
•Where C1 = KC
33 V
VefficiencyString
Inequality in voltage distribution increases with the increase of number of discs.Shorter string has more efficiency than the larger one.
The greater the value of K (= C1/C), the more non-uniform is the
potential.
Nearest to the conductor has maximum voltage
04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET
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Methods of Improving String Efficiency
By using longer cross-arms
By grading the insulators
By using a guard ring
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Important Points
The maximum voltage appears across the disc nearest to the conductor (i.e., line conductor).
The voltage across the string is equal to phase voltage i.e.,
Voltage across string=Voltage between line and earth = Phase Voltage
Line Voltage= √3 × Voltage across string
Assignment(Submit before the next class)
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
04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET
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Corona
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
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Factors Affecting Corona
Atmosphere
Conductor size
Spacing between conductors
Line voltage
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Important Terms
Critical disruptive voltage
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Important Terms
Visual critical voltage
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Important Terms
Power loss due to corona
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Advantages and Disadvantages of Corona
Advantages
Virtual diameter of the conductor is increased
Reduces the effects of transients produced by surges
Disadvantages
Lost of Energy
Ozone is produced by corona
Non-sinusoidal voltage drop and hence inductive interference with neighboring communication lines
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Methods of Reducing Corona Effect
By increasing conductor size
By increasing conductor spacing
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Sag in Overhead Lines
The difference in level between points of supports and the lowest point on the conductor is called sag
Conductor sag and Tension
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Calculation of Sag
When supports are at equal levels
T
wl
T
lwSSag
82
)2/(,
22
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Calculation of Sag
When supports are at unequal levels
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Effect of wind and ice loading
i
w
ww
w
tan T
lwSSag t
2,
2
cosSSagverticalThe
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