Publication: Electrical Review - London
SOME NOTES ON HIGH-TENSION
INSULATORS FOR OVERHEAD TRANSMISSION LINES.
By C. J. GREENE.
(Continued from page 676.)
Insulator No. 1.—Test No. 1.—On running up the voltage, sparking began at 73,000 volts, but it was not until 84,000 volts was reached that the air between line and pin was broken down completely and a permanent arc established. This point is represented by circle No, 1 on the curve of arcing distances, p. 675. Immediately after the permanent arc was established, the insulator was punctured between line and pin, the puncture occurring at a point near the top of the insulator, and not in the neighbourhood of the transmission wire.
Insulator No. 2.—Two insulators of this type were tested, tests Nos. 2, 3 and 4 being carried out on one of them, and tests Nos. 5 and 6 being carried out on the other.
Test No. 2.—With the insulator in a clean and dry condition, sparking began at 80,000 volts, and a complete breakdown of the air took place at 90,000 volts, a permanent and vicious arc establishing itself between line and pin.
Test No. 3.—There being no means of carrying out a spray test, the insulator was wetted all over with clean fresh water and the pressure again applied. The first spark jumped across at 45,000 volts, and an arc almost established itself, but the heat thus generated caused the insulator to dry, and the arcing ceased. The pressure was then gradually increased, and with each slight increment of pressure an arc attempted to establish itself, but was extinguished by the further drying of the insulator. This phenomenon was repeated at every slight increase of pressure till at 90,000 volts, the insulator being then probably quite dry, a permanent arc was, established.
Test No. 4.—The same insulator was then taken, and the whole of its surface with the exception of that of the inside of the inner shed was coated with a mixture of coal dust and water, so as to represent the-condition of insulators working in the neighbourhood of collieries and railways., The first spark started again at about 50,000 volts, at which pressure the insulator began gradually to dry and clean itself, the discharge from the insulator throwing off the dirt. This process of drying and cleaning proceeded as the pressure was gradually increased up to 85,000 volts, at which pressure a permanent arc was set up. This process of the insulator cleaning itself is one of great interest and importance, but it is probable that it only takes place with sufficient rapidity to be of service at pressures much greater than are at present in use in this country. At low pressures it appears that the force of repulsion between the surface of the insulator and the similarly charged particles of dirt is not sufficient to cause these particles to be repelled. The reason why the permanent arc established itself at 85,000 volts instead of 90,000, is probably due to the fact that the insulator was not quite clean, and that the remaining particles of dirt acted as needle-points. Circle No. 2 on the curve of arcing distances represents the arcing voltage of the insulator dry, and circles Nos. 3 and 4 the voltage of the insulator wet and dirty.
Test No. 5.—Another insulator of exactly the same type was then taken and tested in a clean and dry condition. At 92,000 volts a permanent arc was set up between the transmission wire and the pin. Almost immediately afterwards the insulator appeared to puncture through the inner piece to the pin, the outer piece remaining sound.
Test No. 6.—To test whether this was so the pressure was again applied to the insulator, and on its reaching 70,000 volts the outer piece was also punctured, there now being a complete breakdown of the material in both inner and outer pieces. Circle No. 5 on the curve represents the-arcing voltage of test No. 5. There is, of course, no circle for test No. 6.
Insulator No. 3.—This insulator, it may be remarked, the standard insulator used on the Lancashire Electric Power Co's overhead 11,000-volt mains, so that the R.M.S. are between line and pin is, under normal conditions, about 6,500 volts, and it would appear from the tests that the insulator is capable of use with very much higher pressures.
Test No. 7.—The insulator was tested in a perfectly clean and dry condition. The first spark jumped over between line and pin at 94,000 volts, and a permanent arc established itself at 97,000 volts, whilst at 99,000 volts the insulator punctured right through from the line to the pin.
Test No. 8.—Another insulator of exactly the same type was tested under the same conditions. The first spark jumped over at 81,000 volts and a permanent arc was set up at 99,000 volts. We were unable to puncture the insulator at this pressure.
Test No. 9.—Another insulator of exactly the same type was tested under the same conditions. The first spark jumped over at 82,000 volts, and a permanent arc established itself at 101,000 volts. The insulator was not punctured at this voltage. Circles Nos. 7, 8 and 9 represent, the results, as plotted on the curve of arcing distances.
The exact positions of the punctures which took place are marked on the drawings of the insulators.
Direct Deductions from Tests.--It is, of course, impossible from the small number of insulators tested to attempt to formulate any hard and fast deductions; one would not be justified in doing so. It can only be stated that such tests as were carried out indicated certain results which it appears probable would be confirmed by carrying out a number of further tests under the same conditions, and the writer can only add that he has seen the results of quite independent tests carried out on the same type of insulator in another part of the country, which results agree almost in detail with those mentioned above. The writer, however, is not at liberty to publish these results.
Puncturing Voltage.—It appears, first, that an insulator does not by any means necessarily puncture in its thinnest place, nor does the puncture necessarily form the shortest path of breakdown between line and pin, the insulator puncturing at some point of local weakness which the electric pressure finds out. It is probably quite a mistake to make any single part of an insulator with a greater thickness than 1 inch. Masses of porcelain vitrified under great heat suffer from the same defects as large masses of iron which cool unequally. By unequal cooling, local internal stresses are set up in the mass of porcelain which manifest themselves at the first opportunity, since these stresses by trying to adjust themselves can only do so by fracture of the porcelain. This fracture may be caused by a blow, rough handling, exposure to heat or the rays of the sun, or by electric pressure. It is for this reason that insulators of any size should unquestionably be made in two or more pieces and then cemented together.
The fact that the position of breakdown cannot be fore-told also points to the absolute necessity of pressure testing every insulator before erection, and to the desirability of immersing the head of the insulator in water and filling the pin-hole when applying the test. By carrying out the test in this manner the weak points are subjected to a searching test. It is interesting to note that the average puncturing voltage of the porcelain tested is approximately 100,000 volts per inch.
From the table and curve mentioned above it is evident, and of course only to be expected, that all insulators of the same type and size will flash over at the same voltage when the conditions of test or working are similar, and it will be seen that this voltage---for the insulators tested—is in the neighbourhood of 10,000 volts per inch of air space. This ratio agrees almost exactly with the ratio obtained by the tests on the arcing distance between the rods. At the lower pressures the ratios agree exactly, but at the higher pressures it takes a greater pressure per inch of air space to cause a breakdown between line and pin than between rod and rod.
It will also be noted that in the cases of insulators Nos. 1 and 3, both of which have three sheds, the volts per inch of air-space are considerably greater than in the case of insulator No. 2, which only has two beds. The action of this third shed would therefore appear to be beneficial, the ratio being improved either on account of the greater creeping surface between line and pin or on account of some cooling action of the middle shed on the arc. The experiments carried out up to date are unfortunately insufficient to throw much light on this and cause any definite theory to he formed.