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

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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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

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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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

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Insulators

PropertiesHigh mechanical

StrengthHigh electrical

ResistanceHigh relative permittivity

non-porous, free from impurities and cracks

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Insulators: Pin type insulators

Used for TD upto 33 kV

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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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

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Insulators: Shackle insulators

Used only in Distribution line

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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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

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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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

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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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

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Factors Affecting Corona

Atmosphere

Conductor size

Spacing between conductors

Line voltage

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Important Terms

Critical disruptive voltage

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Important Terms

Visual critical voltage

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Important Terms

Power loss due to corona

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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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

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Methods of Reducing Corona Effect

By increasing conductor size

By increasing conductor spacing

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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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

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Calculation of Sag

When supports are at equal levels

T

wl

T

lwSSag

82

)2/(,

22

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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Calculation of Sag

When supports are at unequal levels

04/08/2023Md. Forhad Zaman, Lecturer, Dept. of EEE, RUET

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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