Mechanical Design of Transmission Lines

General Considerations Electrical Considerations for T.L. Design: Low voltage drop Minimum power loss for high efficiency of power transmission. The line should have sufficient current carrying capacity so that the power can be transmitted without excessive voltage drop or overheating.

Conductivity of Conductor: R = ρ.L/A , or R = L/Ϭ. A Where: L: Conductor length. A: Conductor cross sectional area. ρ: resistivity Ϭ: Conductivity (Ϭ= 1/ρ)

PL= 3 I2 .R The conductor conductivity must be very high to reduce Conductor resistance R and hence reduce losses PL= 3 I2 .R

Mechanical Considerations for T.L. Design: The conductors and line supports should have sufficient mechanical strength: to withstand conductor weight, Conductor Tension and weather conditions (wind, ice). The Spans between the towers can be long. Sag will be small. Reducing the number and height of towers and the number of insulators.

Heat expansion coefficient must be very small. Rt = R0. (1 + α0 .t) αt = α0/(1+ α0.t) α t is the heat expansion coefficient at t.

TYPES OF CONDUCTORS MATERIALS

1- All Aluminum Conductors (AAC) lowest cost – low mechanical strength Used for small span

2- Aluminum Conductor Steel Reinforced (ACSR) 1- Steel strands 2- Aluminum strands ACSR (26/7)

Advantages of ACSR High mechanical strength can be utilized by using spans of larger lengths. A reduction in the number of supports also include reduction in insulators and the risk of lines outage due to flash over or faults is reduced. losses are reduced due to larger diameter of conductor. High current carrying capacity.

3- All Aluminum Alloy Conductor (AAAC) وهى عبارة عن سبيكة من الألمونيوم والماغنسيوم والسيليكون المعالجة حراريا - له متانة عالية (أكبر نسبة متانة مع الوزن) – وبالتالى يمكن زيادة المسافة بين الابراج مما يقلل من التكلفة - له مقاومة أقل وبالتالى يقلل من الفقد فى القدرة - غير معرض تآكل الجلفنة كما فى النوع السابق

4-Aluminum Conductor Alloy Reinforced (ACAR) وفيه استبدل القلب المكون من أسلاك الصلب بقلب من أسلاك من سبيكة من الالمونيوم

Types of Supports Wooden Poles Reinforced Concrete Poles Steel Poles Lattice Structure Steel Towers

Wooden Poles

Reinforced Concrete Poles

Steel Poles

Lattice Structure Steel Towers الأكثر استخداما فى الجهد العالى: الأعلى نسبة متانة/ الوزن الأطول عمرا سهولة التركيب والتجميع تتحمل قوى ميكانيكية عالية يعيبها: - وجوب دهانها من وقت لآخر - تحتاج اساسات خرسانية – تكاليف نقلها عالية

Types of Towers 1- Suspension Tower 2- Tension Tower 3- Angle Tower 4- End Tower

1- Suspension Tower

2- Tension Tower

3- Angle Tower

4- End Tower This type of towers exists in the beginning and at the end of the line which exposed to tension in one side.

SAG AND TENSION CALCULATIONS

Sag of Transmission Lines Sag of T.L depends on: Conductor weight. Span length, Tension in the conductor, T Weather conditions (wind , ice). Temperature.

Minimum Clearance between the ground and the conductor kV C (m) 0.4 5.5 11 5.5 33 6.0 66 6.2 132 6.2 220 7.0 400 8.4

Conductor Spacing Spacing = (S )0.5 + V/150 Where: S: Sag in meters. V: Line voltage in kV.