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Pharos University EE-272

Pharos University EE-272. Electrical Power Engineering 1 “Electrical Engineering Dep ” Prepared By: Dr. Sahar Abd El Moneim Moussa. Mechanical Design of OverHead Transmission Line. T.L Main components. An OHTL consists of main components : Conductors Insulators Support structures

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Pharos University EE-272

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  1. Pharos UniversityEE-272 Electrical Power Engineering 1 “Electrical Engineering Dep” Prepared By: Dr. SaharAbd El MoneimMoussa Dr. SaharAbd El MoneimMoussa

  2. Mechanical DesignofOverHeadTransmission Line Dr. Sahar Abd El Moneim Moussa

  3. T.L Main components • An OHTL consists of main components: • Conductors • Insulators • Support structures • Shield wires. Dr. Sahar Abd El Moneim Moussa

  4. 1- O.H.T.L. Conductors • The conductor material used for transmission and distribution should have the following properties: 1- High electrical conductivity 2- High tensile strength 3- Low cost so that it can be used for long distances 4- Low specific gravity so that the volume per unit length is small Dr. Sahar Abd El Moneim Moussa

  5. Overhead transmission lines are composed of aluminum conductors that arestranded in spiral fashion for flexibility. These are primarily classified as: • AAC: all-aluminum conductors • ACSR: aluminum conductors, steel reinforced • AAAC: all-aluminum alloy conductors Dr. Sahar Abd El Moneim Moussa

  6. Dr. Sahar Abd El Moneim Moussa

  7. Manufacturer’s Specifications: • Code. • Aluminum area. • Stranding Al/St. • Aluminum and steel diameter. • Outside conductor diameter. • Conductor area. • Approximate weight (kg/km), total, Al, St.. • Approximate ultimate strength. • Linear coefficient per C. • Final modulus of elasticity kg/cm2. • Resistance at 20 C Ohm/km • Geometric mean radius Dr. Sahar Abd El Moneim Moussa

  8. Dr. Sahar Abd El Moneim Moussa

  9. Line Sag & Line Span • LINE SAG: It is the difference in level between the points of supports and the lowest point on the conductor. • LINE SPAN: It is the horizontal distance between the two point of supports. Dr. Sahar Abd El Moneim Moussa

  10. B L h y-axis A d d2 s s d1 o H o V= w S x1 x2 x2 y L x-axis Supports at equal levels Supports at UNequal levels Dr. Sahar Abd El Moneim Moussa

  11. L y-axis d s s H o V= w S y Approximate Formulas: x-axis Directrix • Supports at EQUAL levels: Where: d: Sag of the conductor L: Span Length W: weight per unit length of the conductor : Length of the conductor Dr. Sahar Abd El Moneim Moussa

  12. B h A d2 d1 o x1 x2 x2 L • Supports at UNEQUAL levels: Therefore: x1 + x2 = L Dr. Sahar Abd El Moneim Moussa

  13. Effect of Ice: • The effect of ice covering the conductor is to increase its weight and the effective outside diameter of the conductor. • If d is the diameter of the conductor and t is the thickness of ice then the overall diameter of the conductor covered with ice is D = d+2t • The weight of ice per unit length (wi) equals the density of ice multiplied by the the volume of ice per unit length Where: i=950 kg/m3 Dr. Sahar Abd El Moneim Moussa

  14. Effect of Wind: • The wind pressure on the conductor is considered as a horizontal force on the projected area of the conductor. ww= Wind force per unit length wi= Wind pressure per unit area x projected area per unit length wi= Wind pressure x (d+2t) kg/m Dr. Sahar Abd El Moneim Moussa

  15. Finally the effect of both ICE & WIND: i- ii- iii- The sag, S represents the sag in a direction making an angle  with the vertical. iv- The vertical sag= S cos  ww  w+wi wt Dr. Sahar Abd El Moneim Moussa

  16. Example (a): A 132 kV transmission line has the following data; Wt. of the conductor= 680 kg/km Ultimate strength= 3100 kg Line span = 260 m Safety factor= 2 Calculate the height above ground at which the conductor has to be supported knowing the ground clearance required is 10 m Solution:(It will be solved in the lecture) Dr. Sahar Abd El Moneim Moussa

  17. Example(b): A transmission line has a span of 200 m. The conductor has a cross sectional area of 1.29cm2, weigh of 1170 kg/km and a breaking stress of 4218 kg/cm2. Calculate a sag for a safety of 5, allowing a wind pressure of 112kg per square meter of projected area, what is the vertical sag? Solution:(It will be solved in the lecture) Dr. Sahar Abd El Moneim Moussa

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