Unit2 Insulator

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INSULATOR


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The British Standard Specification givesparticulars of porcelain only, and requires that"the porcelain shall be ivory white, sound, free

from defects, and thoroughly vitrified so that theglaze is not depended upon for insulation".

This thorough verifications of the porcelain is of paramount importance, since the presence of pores or other air-spaces will lower the dielectricstrength.


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There are three types used in connectionwith overhead lines, viz. :

Pin-type. Suspension-type. Strain-type.


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For operating voltages :up to about 25,000 with ordinary designsof insulator a one-piece construction canbe adopted,up to about 45,000 volts a two-piece,

up to 66,000 volts a three-piece,and beyond this a four-piece insulator.


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Pin Insulators 22 kV / 33 kV


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Pin Insultors 11 kV


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Suspension Insulators the cost of a pin-type insulator increases very

rapidly as the working voltage is increased. For high voltages this type is therefore

uneconomical, and there is the further disadvantage that replacements are expensive.For these reasons, high-voltage lines areinsulated by means of suspension insulators inwhich, as their name indicates, the lineconductor is suspended below the point of support by means of the insulator or insulators.


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This ten-inch pattern (next slide) has been foundvery suitable for lines up to and including 33,000volts, where the conditions of mechanical

loading allow of its use. Since the ultimatemechanical strength is decided by the steellinks, and not by the porcelain, this pattern isvery strong and has the peculiar advantage that

the breaking of a porcelain disc will not allow theline to fall, or, in fact, interrupt the service, if astring of several units is used.


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ten-inch suspension insulator.


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For light low-voltage lines, say up to11,000 volts, the shackle insulator issuitable, but for higher voltages a string of suspension-type insulators is necessary.Where the tension is exceedingly high, asat long river spans, two, three, or even

four strings of insulators in parallel havebeen used.


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Advantages of using pin insulators

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Disadvantages of suspensioninsulators

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Testing of Insulators


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B.S. 137 groups the tests made oninsulators into three categories:

flash-over tests (a design test taken tothree insulators only to prove thecorrection of the design)

sample tests (to prove the quality of manufacture)routine tests (carried out on all insulators)


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50 per cent. dry impulse flash-over test.

The test is made on a clean insulator mountedas far as possible in the normal manner. Theimpulse generator delivers a positive 1/50

microsecond impulse wave of amplitude suchthat about half of the impulses applied causeflash-over of the insulator. The polarity is thenreversed, a negative 1/50 impulse being applied.

There must be at least 20 applications of theimpulse in each case, and the insulator must notbe damaged.


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Dry flash-over and dry one-minutetest.

A voltage of power frequency is applied to aclean insulator mounted as fas as possible in thenormal manner, and the voltage gradually

increased up to the specified value. This voltageis maintained for one minute. The voltage is thenincreased gradually until flash-over occurs. Theinsulator is then flashed-over at least four more

times, the voltage being raised gradually toreach flash-over in about 10 seconds.


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Wet flash-over and one-minute raintest.

The insulator is sprayed with water of resistance 9,000 - 11,000ohm-cm., drawn from a source of supply at a temperature within 10C of the ambient temperature in the neighborhood of the insulator under test, and directed at an angle of 45 degrees, the volume of water being equivalent to a precipitation of 0.12 in. per minute.

The insulator must withstand the test-voltage specified for oneminute. The insulator with 50 per cent. of the one-minute rain test-voltage applied to it is then sprayed for two minutes, the voltageraised to the one-minute test-voltage in approximately 10 secondsand maintained there for one minute.

The voltage is then gradually raised until flash-over occurs, and theinsulator is then flashed at least four more times, the time taken to

reach the voltage being in each case about 10 seconds.


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Sample Tests:

Temperature-cycle test. Mechanical test. Electro-mechanical test. Puncture test. Porosity test.


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Temperature-cycle test. In the temperature-cycle test, the insulator is

subjected three times to the followingtemperature cycle: immersed for T minutes in a

water-bath at not less than 70 C. higher thanthat of the main water, taken out, and immersedas quickly as possible in a main water bath andleft in this bath for T minutes. T = (15 + W/3)

where W = wt. of insulator in lbs. The insulator must withstand this series of tests withoutdamage to the porcelain or glaze.


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Mechanical test. Applied to pin insulators and line post insulators. The

test is a bending test, in which a load of three times thespecified maximum working load (twice for a postinsulator) is applied for one minute. There must be no

damage to the insulator, and in the case of the postinsulator the permanent set must be less than 1 per cent.In the case of the post insulator, the load is then raisedto three times, and there must be no damage to theinsulator or its pin (or pins). A permanent set-test is also

made on the pin-type. A load of twice the maximum,applied for one minute, must not produce a permanentset of more than 1 per cent.


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Electro-mechanical test. In this test, which is applied to suspension

or tension units only, the insulator ismechanically stressed to a tension of 2 1/2times the specified maximum workingload, this being maintained for one minute.Simultaneously, 75 per cent. of the dry

spark-over voltage is applied.


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Puncture test. In the case of pin or post insulators, the voltage is applied between

the pin and [the] lead foil bound over the top and side grooves. Inthe case of suspension units, between the metal fittings; theinsulator is completely immersed in insulating oil at roomtemperature, and the voltage raised as rapidly as is consistent withcorrect measurement. The insulator must not be punctured.

Alternatively, an impulse over-voltage test may be made in this casein air with the insulator arranged as for flash-over tests, a negativepolarity 1/50 microsecond wave is applied of twice the amplitude of the negative 50 per cent. impulse flash-over voltage, as determinedin test (a) above (i.e. a prospective voltage of twice the 50 per cent.flash-over voltage). Twenty such impulses shall then be applied.The procedure [is] to be repeated at prospective voltages of 2.5, 3,3.5 ... [etc] times the 50 per cent. flash-over voltage, and continueduntil the insulator is punctured or the limit of the generator reached.The insulator, to pass the test, must not be punctured by prospectivevoltages of three times, or less, the flash-over voltage.


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Porosity test. Pieces freshly broken from a complete

finished insulator to show no signs of impregnation (when further broken) after being immersed for 24 hours in a 0.5 per cent. alcohol solution of fuchsin under apressure of 2,000 lbs. per sq. in.


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Routine Tests:

Electrical routine test. Mechanical routine test.


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Electrical routine test. Pin insulators are inverted, and immersed in water

sufficiently deep to cover the attachment to the neckgroove; the spindle hole is also filled with water. The testis commenced at a low voltage, which is increased

rapidly until a flash-over occurs every few seconds. Thevoltage is maintained at this value for at least fiveminutes, or if failures occur, for five minutes after the lastpunctured piece has been removed. At the conclusion,the voltage is reduced to about one-third of the test-

voltage before switching off. With suspension units thereis, of course, no necessity for water immersion.


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Mechanical routine test. After assembly, every string insulator unit

is suspended in a horizontal or verticalposition, and a tensile load 20 per cent. inexcess of the maximum specified workingload applied for one minute.


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TABURAN BEZAUPAYA DALAM

PENEBAT RANGKAIANUNIT2/35

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Unit2 Insulator

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