[Trade Journal]
Publication: American Institute Of Electrical Engineers
New York, NY, United States
p. 438-440, col. 1-2
The Effect of Humidity on the Dry Flashover
Potential of Pin-Type Insulators
BY J. T. LITTLETON, JR. [1], Associate, A.I.E.E. and W.W. SHAVER [1], Non-member
Synopsis.—This paper shows that humidity affects considerably the flashover voltages of porcelain and Pyrex insulators. The flashover potential rises as the humidity is increased. The tests were made on pin-type insulators and on rods. The results show that this rising characteristic is a surface effect which varies with the absolute water content of the surrounding atmosphere.
INTRODUCTION
IT is probably generally recognized that the humidity or water content of the atmosphere has some effect on the flashover potential of insulators. The A. I. E. E. specifications for testing insulators[2] state that relative humidity measurements should be taken at the time of the test and recorded as part of the data. No correction factor for this condition has been worked out, however, and little seems to be known as to the quantitative effect of humidity changes. It seems to be a general belief that an increase in humidity leads to a decrease in flashover. Peek[3] has stated that humidity does not affect the flashover potential of sphere-gaps. Schwaiger[4] has given data on porcelain and glass rods in uniform tangential fields, showing a lowering of flashover potential with increase of humidity, and curves showing an increase in flashover potential on large insulators and constant flashover potential on small insulators with increase of humidity. Accordingly, it seemed worth while to determine the quantitative relationship between humidity and flashover potential on different types of insulators and insulators made of different materials, and, if possible, to work out a correction for this condition.
While the work on this problem has not been com pleted, inasmuch as no correction factor has been completed, it is believed that the measurements obtained are of sufficient value and interest to justify their presentation.
EXPERIMENTS
The electrical equipment consisted of a 150-kv-a., center-grounded, 60-cycle transformer with maximum voltage of 300 kv.. controlled by an induction regulator. Flashover voltages were measured in either of two ways; (1) by a voltmeter previously calibrated with a sphere-gap, or (2), directly by the sphere-gap in parallel with the insulator with suitable high resistances on each side of the gap to prevent errors due to oscillations from the insulator under test.
The insulators were mounted and the tests conducted as nearly as possible according to the A. I. E. E. specifications for testing insulators.[5] The tests were made in a wooden box 6 ft. square and 5 ft. high with a hinged cover, all lined with waterproof paper. These dimensions gave less than the standard clearance for the insulator mounting so that in this respect the tests were not made according to the standard specifications. This condition did not noticeably affect the results, as measurements made in the box agreed with those taken in the room for comparison. The iron pipe holding the insulator pin ran diagonally through the box and projected through the walls so that the one lead was connected to it on the outside. The other lead to the half-inch brass rod tied in the line groove was insulated from the chamber wall by a large bell shaped Pyrex insulator.
The humidity in the test chamber was controlled by the introduction of dry steam circulated throughout the box by a fan. Because of small leaks it was found necessary to introduce steam continuously in order to maintain a given humidity. Humidity measurements were made by means of a dry and wet bulb hygrometer suspended in the current of air from the fan, just inside a window through which the readings were taken. These results were converted to absolute humidity by means of the data given in the Psychrometric and Smithsonian tables.[6]
The experimental procedure was as follows; the insulator was mounted on the pin in the test chamber and a standard dry flashover test made with the natural humidity of the atmosphere. Steam was then introduced in the chamber and the humidity thereby increased and maintained at a constant value for 15 min. A second flashover test was then made and the cycle repeated. If it were desired to decrease the humidity after it had reached a point near saturation the cover of the box was raised and fresh air admitted. So far as possible, a given series of tests was made at a constant temperature, but on account of the steam used in the test, the temperature slowly increased, in an extreme case changing by as much as 4.5 deg. cent. However, temperature and pressure as well as relative humidity were recorded for each test and a correction applied to reduce the values to those for 25 deg. cent. and 760-mm. pressure.[7]
Measurements were made upon different types of Pyrex pin-type insulators (see Fig. 1) and upon porcelain pin-type insulators, (Fig. 2), varying from 16- to 44-kv. rating. Tests were also made on the effect of humidity on the flashover potentials between No. 6 copper wire electrodes wrapped around Pyrex rods of various diameters, and at various spacings. In addition, in order to determine whether or not the phenomenon was merely due to the humidity effect on the electrodes, measurements were made on the flashover potentials between a one-inch steel bar bent in the shape of a hook, and a loop of No. 8 copper wire, as well as between two loops of No. 6 copper wire.
RESULTS
Figs. 3 and 4 show the results of a series of tests on the different Pyrex insulators and porcelain insulators, respectively. In every case there was a marked increase in the flashover potential when the humidity was increased. These curves do not show the maximum variations that have been observed, as considerably greater variations are possible when the experiments are made at a higher temperature, the maximum possible water vapor content in that case being much greater. In general it was found possible to increase the flashover potential of both Pyrex insulators and porcelain insulators about 30 per cent over the value at a low humidity.
The results of some of the experiments on Pyrex rods are shown graphically in Fig. 5 and again, increased humidity caused increased flashover potential. The results shown were obtained with a rod 0.9 in. in diameter and with the spacings between the electrodes as indicated on the graphs.
Fig. 6 shows the results of the measurements on the flashover potentials between the loop of No. 8 copper wire and the one-inch steel rod bent in the form of a hook and between the two loops of No. 6 wire. In the first case, they were spaced so as to give a flashover potential about the same as that of a 44-kv. insulator; and in the case of the loops of No. 6 wire, the spacing was the same as when they were wrapped around a Pyrex rod. From the slopes of these curves, it is quite evident that the humidity effect on the flashover potential of an insulator and of a Pyrex rod cannot be attributed to any effect on the electrodes.
DISCUSSION AND CONCLUSIONS
As mentioned above, the curves show humidity values expressed in absolute units, since such units bring the measurements taken under different temperature and pressure conditions more nearly in agreement with each other than do those of relative humidity. Schwaiger7 shows data plotted on a relative humidity basis which also come out as a straight line relation if temperature and pressure are kept constant, but in order to correlate data made at different temperatures, or to work out a correction factor, it is necessary to use absolute humidities. In addition he shows that in the neighborhood of 100 per cent relative humidity the flashover potential falls very rapidly, this effect beginning at even 80 per cent humidity on certain insulators. This decrease in flashover potential was noted in our experiments at humidities above 90 per cent, but it is believed that this was due to condensation on the insulator surface, which could be avoided by having the insulator at the proper temperature. Schwaiger's results indicate no humidity effect on flashover potential with an insulator of approximately the same dry flashover potential as ;the Pyrex insulator No. 161 and the 16-kv. porcelain insulator. Our observations made on the latter insulators indicate a very definite effect and one which is also of considerable magnitude. Weicher8investi-gated the effect of humidity on the flashover potential between points and 2-cm. spheres so spaced as to give a flashover potential approximately the same as indicated in Fig. 6, and found that the flashover potential increased with increasing relative humidity. This does not agree with our observations on the flashover potentials between the two loops of copper wire and between the copper wire and the pin in which no humidity effect was observed.
The question as to whether the curves shown in Figs. 3 and 4, which were obtained under more or less artificial conditions, actually represent the phenomena when observed under standard conditions, is perhaps best answered by a reference to the point A on the curve in Fig. 3 for the 40-kv. Pyrex insulator. This point was obtained from the average dry flashover values of five 40-kv. insulators of the same type as used in this test. The tests on the five insulators were made in the standard way outside the test box under low, absolute humidity conditions some months previous to the time when the other results shown were obtained. When correction is made for temperature and pressure, however, this value shows good agreement with the other results obtained in the special test box. As a further proof of this, the points marked P on the curves represent measurements taken under natural humidity conditions at Purdue University Electrical Engineering Laboratory.
It is not proposed to advance at this time any theoretical explanation of the results obtained, as the data at hand are not sufficient to warrant the development of any very definite theory. The results given on the accompanying graphs show that the effect is a surface one which varies with the absolute water content of the surrounding atmosphere. Since this is true, it is not surprising that the results are similar for both Pyrex insulators and porcelain insulators, as in both cases the surface is of glass. It is perhaps worthy of note that the slopes for porcelain insulators are slightly steeper than those for Pyrex insulators having approximately the same flashover potential. This may be accounted for by the high chemical resistivity surface of Pyrex insulators.
Regardless of the cause of the phenomenon, experiments have shown that humidity has a marked effect on the dry flashover potentials of pin-type insulators. It is further apparent that the large change occasioned by humidity has a direct effect on the rating of an insulator. Extrapolation from the curve, together with temperature and pressure corrections, shows that a No. 441 Pyrex insulator at 760-mm. pressure and 35 deg. cent. temperature would flash over at 128 kv. with a 10 per cent relative humidity and at 193 kv. with a 90 per cent relative humidity.
Moreover, the apparent straight line relation between flashover potential and absolute humidity makes it possible to adopt a correction factor for the humidity effect on a given insulator, and to catalogue the dry flashover potential at a standard humidity as well as at
[not finished]