TRANSMISSION LINE SUMBUL.H.ABIDI ELECTRICAL DEPARTMENT S.P.C.E.
COMPONENTS OF OVER HEADLINE:- Conductor Main comp- Earth Onents of Line Wire. Over head support line Line Insulator
The line supports are poles and the chief requirements for such supports are :i) They must be mechanically strong with factor of safety of 2.5 to 3 .ii ) They must be light in weight without the loss of strength .iii ) They must have least number of parts .iv ) They must be cheap .v) Their maintenance cost should be minimum .vi ) They must be easily accessible for point and erection of line conductors .vii ) They must have longer life .viii ) They must be of pleasing shape
The different types of poles which can be used as line supports are : a. Wooden poles . b. Steel tubular poles c. Reinforced concrete poles . d. Steel towers . Fig.(1):Single phase single-circuit
Spacing between the conductors The most suitable spacing between the conductors can be arrived at by mathematical calculations. It can only be obtained by empirical formulae which have been obtained from practical considerations.Fig: Three-phase single circuit horizontal disposition of conductor and steel towers
Generally the following formulae are used for obtaining spacing between the conductors : VSpacing= S 150 Where, S=sag in meters V=voltage in KV
Transmission Line Span Supporting Tower Tension TowerTension Tower Insulator Sag
Wooden Poles•Cheapest•Use for lines where span is short and tension is low.•Pole has limitation of height and diameter.•Wood has natural insulating property and lesser flashovertakes place due to lightning.•For greater strength double pole structure of ‘A’ or ‘H’ areused.Concrete Poles•Concrete poles are reinforced to give greater strength.•It has longer life span than that of wood•The maintenance cost is less.•They are heavy and are likely to get damageduringloading,unloading,transportation and erection due totheir brittle nature.•Nowadays pre-stressed concrete supports aremanufactured in pieces.
Steel poles•Tubular steel poles or girder steel masts is favored for low and mediumdistribution voltages.•Longer span is possible.•Poles need to be galvanized or painted periodically to prevent corrosion.•Maintenance is expensive.Steel Towers•Lines of 66 KV are supported on towers.•They fabricated from painted or galvanized angle section which aretransported separately and erection done on site.•Long life and high degree of reliability•They can withstand severe weather conditions.•The height of the towers depend on operating voltage and span length.•The leg of the tower are set on concrete foundation
Types of steel towers1. Tangent towers• They are used for straight runs.• Stress is due to weight of line,wind and ice load.• Extra forces due to break in the line on one side.• Base is either square or rectangle.• Suspension type insulators used.2. Deviation tower• They are used where transmission line changes direction.• They have broader base , stronger members and are costlier• Strain insulators are used.They are classifiedi. Small angle towers(2o-15ochange in direction)ii. Medium angle towers(15o-30ochange in direction)iii. Large angle towers(30o-60ochange in direction)
Transmission line (Steel tower)
Transmission line (Steel tower)
TYPES OF INSULATORS USED IN OVERHEAD SYSTEM.• Pin type insulator.• Suspension type insulator.• Strain type insulator.• Shackle type insulator.
PIN INSULATORSThe pin type insulator is secured to the cross-arm on the pole. There isa groove on the upper end of the insulator for housing the conductor.The conductor passes through this groove and is bound by theannealed wire of the same material as the conductor.Pin type insulators are used for transmission and distribution of electricpower at voltages up to 33 kV. Beyond operating voltage of 33 kV, thepin type insulators become too bulky and hence uneconomical.
Causes of failures •Insulators are required to withstand both mechanical and electrical stresses. •The electrical stress is primarily due to line voltage and may cause the breakdown of the insulator. • The electrical breakdown of the insulator can occur either by flash over or puncture. •In flashover, an arc occurs between the line conductor and insulator pin (i.e., earth) and the discharge jumps across the air gaps, following shortest distance. Figure shows the arcing distance (i.e. a + b + c) for the insulator. •In case of flash-over, the insulator will continue to act in its proper capacity unless extreme heat produced by the arc destroys the insulator. • In case of puncture, the discharge occurs from conductor to pin through the body of the insulator. When such breakdown is involved, the insulator is permanently destroyed due to excessive heat. In practice, sufficient thickness of porcelain is provided in the insulator to avoid puncture by the line voltage. •The ratio of puncture strength to flashover voltage is known as safety factor .
Ball and socket type suspension insulator Iron Cap Locking Key Ball Socket Insulators Head CompressionExpansion Layer Loading Imbedded Sand Cement Insulating Glass or PorcelainSkirt Petticoats Steel Pin Ball Corrosion Sleeve for DC Insulators
Number of Insulators per StringLine Voltage 69 kV 4–6 115 kV 7–9 138 kV 8–10 230 kV 12 345 kV 18 500 kV 24 765 kV 30–35
ADVANTAGES•Each unit is designed for operating voltage of about 11KV .•The string is free to swing in any direction.•It is designed for longer span and higher mechanical loading.•There is decreased liability to lightning disturbance if thestring is suspended from a metallic structure, which work aslightning shield.DISADVANTAGES•Spacing between conductor is increased•Height of tower increases•Cost is high.
Strain or Tension Insulator•Strain or tension insulators are designed for handling mechanical stresses at anglepositions where there is a change in the direction of the line at termination of line.•Suspension insulator are arranged in a horizontal position. They are used to handlemechanical stresses and take the pressure off a conductor at the end of atransmission line, at a sharp corner or curve or over long river crossings.•Strain insulators are typically used for higher voltage transmissions.Shackle Type Insulators•Shackle type insulators, similar to strain type insulators, are used on sharp curves,end poles and in section poles.•However, unlike strain insulators, shackle insulators are designed to support lowervoltages. These insulators are single, round porcelain parts that are mountedhorizontally or vertically.
V-StringsA single string of insulator follows the conductor and sways like a pendulum in astrong side wind.V-strings are used to prevent conductor movement at towers. They are used for highvoltage transmission system.INSULATOR MATERIAL1. Porcelain with glazed surface2. Toughened glass3. Polymer Insulator Lattice Guyed tower