Electrical Plus Notes

Sag Template  1) Sag template is used for allocating the position and height of the supports correctly on the profile.  2) The sag template is usually made up of transparent celluloid, perplex, or somtimes cardboard. 3) Sag template defines the limit of minimum clearance angle between the sag and the ground for safety purpose. Curves marked on the sag template are : 1 ) Hot template Curve or Hot Curve 2 ) Ground Clearance Curve 3 ) Support Foot or Tower Curve 4 ) Cold curve or Uplift Curve                                fig : 1 ( Note : Above image is enlarged in fig 2 )                                    fig : 2 Hot Curve :  The hot curve is obtained by plotting the sag at maximum temperature against span length. It shows where the supports must be located to maintain the prescribed ground clearance. Ground Clearance Curve : The clearance curve is below the hot curve. It is drawn parallel to the hot curve and at a vertical distance equal to the ground clearnc

CONTENT > Vibrations in Overhead Conductors > Types of Vibrations >  Precautions Vibrations in Overhead Conductors Vibration is one of the serious cause of conductor failure (i.e. having cracks , breaking of conductor , or flash over ) in overhead conductors of transmission lines. So, vibration should be kept within limits for safe operation of the transmission lines.  Types of Vibrations  Vibration in Power Lines can be categorized into three types caused under the influence of wind :  i) Resonant or Aeolian Vibration  ii) Galloping  iii) Simple swinging i) Aeolian Vibration  1) These vibrations are caused by aerodynamic forces generated as the wind blows across the conductor.       fig : Damage due to Aeolian vibration 2) The frequency of the vibration may receive a positive feedback from the conductor's natural vibration frequency.  3) This self- exciting vibration can cause cracks on the conductors due to fatigue and can cause damage particular

CONTENTS > Line Supports : > Properties for LIne Supports : > Types of Line Supports : > Advantages and Disadvantages of Line Supports : Line Supports Line support palys a major role in power transmission. Line support (like Poles and Towers) are used for supporting the overhead lines or wires. Desired properties for lIne Supports:   1) High mechanical strength to withstand the weight of conductors and wind load etc 2) Light in weight without the loss of mechanical strength. 3) Cheap in cost and economical to maintain. 4) longer life. 5) Easy accessibility of conductors for maintenance. Line Supports Types:   1) Wooden Poles 2) Steel Poles 3) R.C.C. Poles 4) Lattice steel towers   1) Wooden Poles :   These supports are cheap, easily available , provide insulating properties and widey used for distribution purpose in rural areas.  # Span upto 50 meters. # Made of seasoned wood (sal and chir)   Advantages  1) Wooden poles have lower elecrical conductiv

CONTENTS > Sag in Transmissison Line :  > Span between Supports : > Tension in Conductor :  > Formula for sag calculation : > Factors affecting sag : > Effect of ice loading on sag : > Effect of wind pressure on sag  : What is Sag in Transmission Line ? Sag in conductor : When the conductors are supported at the towers or poles, they are pulled and connected between supports but soon conductors will sag or dip under its own weight.   Maximum sag is essential in determining the ground clearance of the conductor. The sag should be minimum in order to reduce the conductor material and to avoid extra height of pole. Span :  The distance between adjacent supporting towers is called span.  Tension in conductor :  To avoid the mechanical faiilure of the conductor the tension in the conductor should be low. The tension in conductor depends upon conductor weight, effect of temperature, ice and wind loading.   So  sag and tension  in the conductor should be so

CONTENTS > Properties of Good Conductor > Types of conductors used in Overhead Transmision Lines A Conductor is one of the most important component of overhead lines. Proper selection of coductor is as important as selection of size and voltage .     Properties required for a good conductor : a) HIgh electrical conductivity b) High tensile strength  c) Lower cost  d) Corrosion resistive e) Light in weight  Types of Conductors used for Transmission Lines Following are four common types of overhead conductors that are used for overhead transimission and distribution to carry generated power from generating stations to the end users. 1) AAC : All Aluminium Conductor 2) AAAC : All Aluminium Alloy Conductor 3) ACSR : Aluminium Conductor Steel Reinforced 4) ACAR : Aluminium Conductor Alloy  Reinforced AAC :-  (a) All Aluminium Conductor is also called as Alumunuim Stranded Conductor.   (b) It has conductivity about 61% IACS( International Annealed Copper Stranded ).  (c)

Testing of Insulator : Electrical insulators are tested to avoid unwanted insulator failure, the electrical insulator must undergo the following tests:  1)   Flashover tests of insulator 2) Performance tests 3) Routine tests   Flashover test are mainly of three types :- (i) Power frequency dry Flashover Test of insulator :   Here insulator to be tested is mounted in a practical manner as it is used on poles  and Variable power frequeny voltage source are connected to the insulator. Applied voltage is gradually raised upto minimum flashover voltage. This voltage is maintained for one minute and observe that there should not be any flashover or puncture occurred. It should be capable to sustain at minimum specified voltage for one  minute. (ii) Power frequency Wet Flashover Test / Rain Test of Insulator  : Here Again the practical condition of insulator is  developed with applied variable power frequency voltage source. After that we create artificial raining condition by s

Causes of insulation Failure :   (a) Cracking of Insulator - Porcelain insulator mainly consists of three different materials  like main porcelain body, steel fitting arrrangement and cement   to fix the steel part with porcelain. The unequal expansion and contraction  of porcelain, steel and cement are the chief cause of cracking of insulator .   (b) Defective Insulation Material -  If the material used for insulator is defective then it gets easily punctured. (c) Porosity in the Insulation Materials -  When porcelain insulator is manufactured at low temperature then it will make insulator porous and it willl absorb moisture from air which reduces insulation. (d) Improper glazing of Insulator surface - If the surface of porcelain insulator is not properly glazed ,moisture can stick over it and cause insulation reduction. (f)   Mechanical Stresses on Insulator - If insulator has any weak portion due to manufacturing defect it may break from that weak portion due to  mechani

Types of Electrical insulator Insulator provides necessary insulation between line conductors or supports and prevent leakage current to flow from conductors to earth. Among several types of insulators the most comonly used insulators are Pin insulator, suspenson insulator , Strain type insulator , Shackle insulator , Stay or Guy insulator.    1) Pin type insulator :-  (a) Pin type insulator are used for supporting line conductors. It consists of single or multiple shells arranged in parallel and mounted on cross-arm of the pole or tower.    (b) One piece of insulators can be used upto 11kV, two piece upto 25KV  and 4 to 6 pieces upto 44 to 66 KV  (c) The designing of sheds or petticoats should be such that even when the outer surface is wet due to rain, the inner dry surface provides sufficient leakage resistance. (d) On the top of the insulator the conductor bound into a groove which is cemented on to a galvanized steel pin.   (e) Cost of pin typ