Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

Electrical Engineering SSC JE (Technical)

Electrical Engineering (EE) : Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

The document Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev is a part of the Electrical Engineering (EE) Course Electrical Engineering SSC JE (Technical).
All you need of Electrical Engineering (EE) at this link: Electrical Engineering (EE)

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.

 Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRevChapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

 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

 Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

  • Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

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

 Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

  • 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

Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

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

 Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev 

  •  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

 Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

The vertical sag is S cos g. 

  • Total Length of Conductor

 Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

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.

 Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev 

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

 Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev 

  •  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  Chapter 6 Mechanical Design of Overhead Lines - Notes, Power System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev 

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.


Offer running on EduRev: Apply code STAYHOME200 to get INR 200 off on our premium plan EduRev Infinity!

Related Searches

Exam

,

Chapter 6 Mechanical Design of Overhead Lines - Notes

,

Sample Paper

,

ppt

,

Previous Year Questions with Solutions

,

Summary

,

pdf

,

Important questions

,

Semester Notes

,

shortcuts and tricks

,

Chapter 6 Mechanical Design of Overhead Lines - Notes

,

study material

,

video lectures

,

MCQs

,

mock tests for examination

,

Viva Questions

,

Electrical Engineering Electrical Engineering (EE) Notes | EduRev

,

Power System

,

Extra Questions

,

Electrical Engineering Electrical Engineering (EE) Notes | EduRev

,

Electrical Engineering Electrical Engineering (EE) Notes | EduRev

,

past year papers

,

Power System

,

Chapter 6 Mechanical Design of Overhead Lines - Notes

,

Power System

,

Objective type Questions

,

Free

,

practice quizzes

;