Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) PDF Download

Mechanical Design of Overhead Lines

INTRODUCTION

  • 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.

LINE SUPPORTS

  • 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.

Wooden Poles 

  • 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.

RCC Poles 

  • 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

Steel Towers 

  • 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.

Spacing 

  • 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.

 Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

 Where S = Sag in metres V = Line voltage in kV

SAG

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

 Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

  • Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

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

 Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

  • 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

Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

  •  The force Ft lies in the new plane of conductor and is inclined to the vertical at an Ðg which is given by

 Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) 

  •  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

 Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

The vertical sag is S cos g. 

  • Total Length of Conductor

 Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

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.

 Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) 

  •  Let l be the actual span (horizontal distance between B and C), lc be the span of the complete parabola.

 Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) 

  •  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  Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) 

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).

SAG TEMPLATE

  • 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.
The document Mechanical Design of Overhead Lines | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Electrical Engineering SSC JE (Technical).
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FAQs on Mechanical Design of Overhead Lines - Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

1. What are the key factors to consider in the mechanical design of overhead lines?
Ans. The key factors to consider in the mechanical design of overhead lines include the type and size of conductors, the tension and sag of the lines, the choice of supporting structures, the environmental conditions, and the required clearances from surrounding objects.
2. How does conductor size affect the mechanical design of overhead lines?
Ans. The conductor size affects the mechanical design of overhead lines as it determines the current-carrying capacity and the mechanical strength of the lines. Larger conductors can carry more current but may require stronger supporting structures to withstand the increased weight and wind loading.
3. What is the significance of tension and sag in the mechanical design of overhead lines?
Ans. Tension and sag are critical considerations in the mechanical design of overhead lines. Tension ensures that the conductors remain properly stretched and prevent excessive sag, which could lead to clearance issues or interference with surrounding objects. Proper tensioning also helps minimize the risk of conductor galloping or oscillating under wind conditions.
4. How do environmental conditions affect the mechanical design of overhead lines?
Ans. Environmental conditions such as wind speed, ice loading, and temperature variations significantly impact the mechanical design of overhead lines. Strong winds can exert considerable forces on the conductors and supporting structures, requiring appropriate design measures to ensure structural integrity. Similarly, ice loading during winter can increase the weight on the conductors, necessitating stronger supports. Temperature variations may cause thermal expansion and contraction, which need to be accounted for in the line design.
5. What are the clearance requirements for overhead lines in relation to surrounding objects?
Ans. Overhead lines must maintain specific clearances from surrounding objects to ensure safety and prevent electrical faults. These clearances depend on factors such as line voltage, conductor type, and the nature of nearby objects (e.g., buildings, roads). Adhering to these clearance requirements avoids potential hazards, such as short circuits, accidental contact, or damage to the lines.
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