Mechanical Design of Overhead Lines
- The line should have sufficient current carrying capacity so that the required transfer can takes place without excessive voltage drop or overheating.
- The line losses should be small and the insulation of the line should be adequate to cope with the system voltage.
- The tension in the conductor should be well below the breaking load and reasonable factor of safety should be used.
- Adequate clearance between the lowest point on the line and ground must be maintained.
- The supports for an overhead line must be capable of carrying the load due to the conductors and insulators (including the ice and wind loads on the conductors) together with the wind load on the support itself.
- The supports generally used are wooden poles, RCC poles, steel tubular poles and steel towers.
- The design of a support depends to a growth extent on whether the support is rigid or has a certain amount of flexibility in the direction of the line.
- Poles made of chemically treated wood are used for distribution lines especially in areas where ample supplies of good quality wood are available.
- For low voltage lines only one pole is used but for 33 kV lines two poles in A or H formation are used.
- Poles made of reinforced cement concrete are stronger but more costly than wood poles.
- They are widely used for distribution line upto 33 kV in urban areas
- Lines of 66 kV and above are invariably supported on steel towers.
- They are fabricated from painted or galvanized angle sections which can be transported separately and the erection done on site.
- Steel towers have the advantage of a very long life can a high degree of reliability.
- Can withstand very severe weather conditions.
- The height of the tower depends on the line voltage and span length.
- The forces which have to be taken into account in the tower design include vertical loads of conductor, insulators, fittings and tower itself, wind pressure on conductor and wind pressure on tower itself.
- There must be adequ ate sp acing betwe en conductors so that they do not come within sparking distance of each other even while swinging due to wind.
Where S = Sag in metres V = Line voltage in kV
Supports at Same Level
- The line is assumed to be flexible and sags below the level AB due to its weight.
- The exact shape of the line is that of a catenary.
Except for lines with very long span and large sag.
Let l = Length of span, i.e., horizontal distance between supports, in metres
S = Sag at mid span, in metres
T = Conductor tension (Assumed constant over the whole span) in Newtons w = Conductor weight, N/m
Effect of Ice and Wind
- In addition to its own weight, a transmission line conductor is also subject to wind pressure.
- A coating of ice may also be formed on the conductor of the lines in hilly areas during severe winter season. d = Diameter of conductor, metres t = Radial thickness of ice, metres
- The overall diameter of ice covered D = d + 2t.
- Volume of ice per metre length of conductor
- The weight of ice is approximate 8920 N/m3.
- Weight of ice per metre length of conductor wi = 2.8 × 104 t(d + t)N/m
- The wind pressure is assumed to act horizontally on the projected area of the ice covered conductor.
- For a wind pressure of a Newton per sq.m of projected area, wind load Fw
Fw = pD N/m
- The total force Ft acting on the conductor per metre length
- The force Ft lies in the new plane of conductor and is inclined to the vertical at an Ðg which is given by
- If T is the limiting tension and F t is the total per metre on the conductor under worst conditions then the sag in the new plane
The vertical sag is S cos g.
- Total Length of Conductor
Supports at Different Levels
- Figure shows such a section suspended between two supports B and C which are at different levels.
The curve BOCA is the complete parabola with A and B at the same level.
- Let l be the actual span (horizontal distance between B and C), lc be the span of the complete parabola.
- The above theory is valid even when the two supports B and C fall on the same side of origin O
i.e., span is less than
Factors Affecting Sag
- Weig ht of C o nductor : Sa g is directly proportional to weight per unit length of conductor.
- Span : A longer span causes more sag.
- Sag is proportional to square of span.
- C ond uct or Tens io n : Sag is inversely proportional to conductor tension.
- An inrease in conductor tension causes more stresses in the conductor and more load on insulators and towers.
- Ground Clearance : To maintain minimum clearance, it ma be necessary to increase the height of the towers if higher value of sag is desired (so as to keep conductor tension within safe limit).
- A sag template is a convenient way of locating the towers in the field.
- The tower footing line indicates the actual position of tower on ground.