Telegraph and power transmission insulators Insulator (electricity)

1 telegraph , power transmission insulators

1.1 material
1.2 design
1.3 types of insulators
1.4 suspension insulators
1.5 history

telegraph , power transmission insulators

power lines ceramic insulators in california, usa

overhead conductors high-voltage electric power transmission bare, , insulated surrounding air. conductors lower voltages in distribution may have insulation bare well. insulating supports called insulators required @ points supported utility poles or transmission towers. insulators required wire enters buildings or electrical devices, such transformers or circuit breakers, insulate wire case. these hollow insulators conductor inside them called bushings.

10 kv ceramic insulator, showing sheds


material

insulators used high-voltage power transmission made glass, porcelain or composite polymer materials. porcelain insulators made clay, quartz or alumina , feldspar, , covered smooth glaze shed water. insulators made porcelain rich in alumina used high mechanical strength criterion. porcelain has dielectric strength of 4–10 kv/mm. glass has higher dielectric strength, attracts condensation , thick irregular shapes needed insulators difficult cast without internal strains. insulator manufacturers stopped making glass insulators in late 1960s, switching ceramic materials.

recently, electric utilities have begun converting polymer composite materials types of insulators. these typically composed of central rod made of fibre reinforced plastic , outer weathershed made of silicone rubber or ethylene propylene diene monomer rubber (epdm). composite insulators less costly, lighter in weight, , have excellent hydrophobic capability. combination makes them ideal service in polluted areas. however, these materials not yet have long-term proven service life of glass , porcelain.

design

high voltage ceramic bushing during manufacture, before glazing.

the electrical breakdown of insulator due excessive voltage can occur in 1 of 2 ways:

a puncture arc breakdown , conduction of material of insulator, causing electric arc through interior of insulator. heat resulting arc damages insulator irreparably. puncture voltage voltage across insulator (when installed in normal manner) causes puncture arc.
a flashover arc breakdown , conduction of air around or along surface of insulator, causing arc along outside of insulator. insulators designed withstand flashover without damage. flashover voltage voltage causes flash-over arc.

most high voltage insulators designed lower flashover voltage puncture voltage, flash on before puncture, avoid damage.

dirt, pollution, salt, , particularly water on surface of high voltage insulator can create conductive path across it, causing leakage currents , flashovers. flashover voltage can reduced more 50% when insulator wet. high voltage insulators outdoor use shaped maximise length of leakage path along surface 1 end other, called creepage length, minimise these leakage currents. accomplish surface moulded series of corrugations or concentric disc shapes. these include 1 or more sheds; downward facing cup-shaped surfaces act umbrellas ensure part of surface leakage path under cup stays dry in wet weather. minimum creepage distances 20–25 mm/kv, must increased in high pollution or airborne sea-salt areas.

suspension insulator string (the vertical string of discs) on 275 kv suspension pylon.



suspended glass disc insulator unit used in suspension insulator strings high voltage transmission lines


types of insulators

these common classes of insulator:

pin type insulator - name suggests, pin type insulator mounted on pin on cross-arm on pole. there groove on upper end of insulator. conductor passes through groove , tied insulator annealed wire of same material conductor. pin type insulators used transmission , distribution of communications, , electric power @ voltages 33 kv. insulators made operating voltages between 33kv , 69kv tend bulky , have become uneconomical in recent years.
post insulator - type of insulator in 1930s more compact traditional pin-type insulators , has rapidly replaced many pin-type insulators on lines 69kv , in configurations, can made operation @ 115kv.
suspension insulator - voltages greater 33 kv, usual practice use suspension type insulators, consisting of number of glass or porcelain discs connected in series metal links in form of string. conductor suspended @ bottom end of string while top end secured cross-arm of tower. number of disc units used depends on voltage.
strain insulator - dead end or anchor pole or tower used straight section of line ends, or angles off in direction. these poles must withstand lateral (horizontal) tension of long straight section of wire. in order support lateral load, strain insulators used. low voltage lines (less 11 kv), shackle insulators used strain insulators. however, high voltage transmission lines, strings of cap-and-pin (suspension) insulators used, attached crossarm in horizontal direction. when tension load in lines exceedingly high, such @ long river spans, 2 or more strings used in parallel.
shackle insulator - in days, shackle insulators used strain insulators. nowaday, used low voltage distribution lines. such insulators can used either in horizontal position or in vertical position. can directly fixed pole bolt or cross arm.
bushing - enables 1 or several conductors pass through partition such wall or tank, , insulates conductors it.
line post insulator
station post insulator
cut-out

suspension insulators

pin-type insulators unsuitable voltages greater 69 kv line-to-line. higher transmission voltages use suspension insulator strings, can made practical transmission voltage adding insulator elements string.

higher voltage transmission lines use modular suspension insulator designs. wires suspended string of identical disc-shaped insulators attach each other metal clevis pin or ball , socket links. advantage of design insulator strings different breakdown voltages, use different line voltages, can constructed using different numbers of basic units. also, if 1 of insulator units in string breaks, can replaced without discarding entire string.

each unit constructed of ceramic or glass disc metal cap , pin cemented opposite sides. in order make defective units obvious, glass units designed overvoltage causes puncture arc through glass instead of flashover. glass heat-treated shatters, making damaged unit visible. mechanical strength of unit unchanged, insulator string stays together.

standard suspension disc insulator units 25 centimetres (9.8 in) in diameter , 15 cm (6 in) long, can support load of 80-120 kn (18-27 klbf), have dry flashover voltage of 72 kv, , rated @ operating voltage of 10-12 kv. however, flashover voltage of string less sum of component discs, because electric field not distributed evenly across string strongest @ disc nearest conductor, flashes on first. metal grading rings added around disc @ high voltage end, reduce electric field across disc , improve flashover voltage.

in high voltage lines insulator may surrounded corona rings. these typically consist of toruses of aluminium (most commonly) or copper tubing attached line. designed reduce electric field @ point insulator attached line, prevent corona discharge, results in power losses.

a recent photo of open wire telegraph pole route porcelain insulators. quidenham, norfolk, united kingdom.


history

the first electrical systems make use of insulators telegraph lines; direct attachment of wires wooden poles found give poor results, during damp weather.

the first glass insulators used in large quantities had unthreaded pinhole. these pieces of glass positioned on tapered wooden pin, vertically extending upwards pole s crossarm (commonly 2 insulators pole , maybe 1 on top of pole itself). natural contraction , expansion of wires tied these threadless insulators resulted in insulators unseating pins, requiring manual reseating.

amongst first produce ceramic insulators companies in united kingdom, stiff , doulton using stoneware mid-1840s, joseph bourne (later renamed denby) producing them around 1860 , bullers 1868. utility patent number 48,906 granted louis a. cauvet on 25 july 1865 process produce insulators threaded pinhole: pin-type insulators still have threaded pinholes.

the invention of suspension-type insulators made high-voltage power transmission possible. transmission line voltages reached , passed 60,000 volts, insulators required become large , heavy, insulators made safety margin of 88,000 volts being practical limit manufacturing , installation. suspension insulators, on other hand, can connected strings long required line s voltage.

a large variety of telephone, telegraph , power insulators have been made; people collect them, both historic interest , aesthetic quality of many insulator designs , finishes. 1 collectors organisation national insulator association, has on 9,000 members.