[Trade Journal]
Publication: Proceedings of the National Electric Light Association 43rd Convention
New York, NY, United States
p. 667-672, col. 1-2
Suspension Insulator Research
As this Sub-Committee was not appointed until a few weeks before the writing of this report, but little time was left for preparing any complete descriptions. This report gives a short résumé of the situation, particularly as it has existed in California and neighboring states. Additional tests to be conducted cannot now be described in accurate detail as the plans for such research are still in the making. It is certain, however, that there will be much useful data available for future committee reports, and that there will be a general development of the subject which should lead to important improvements in the design of insulators and to increased knowledge of their performance after installation. The Sub-Committee report follows:
The suspension insulator unit was used only to a very limited extent prior to the year 1908, when the first 100,000 volt transmission systems were placed in operation. At that time the engineering fraternity was not familiar with the requirements of materials for suspension units, and there were very few, if any, ceramists who understood what porcelain characteristics were necessary for these insulators.
During the early period competition in the insulator business was very keen and prices were naturally low, and this condition made it necessary for the insulator manufacturing companies to sell all of their production in order to escape severe financial losses. This price condition, together with the necessarily high requirements for mechanical strength, led the manufacturers to guard against over-firing, and they frequently produced under-fired units which were often slightly porous. After some years of service these units absorbed sufficient moisture to cause trouble. When the manufacturers were confronted with trouble from porous insulators they began over-firing, which still resulted in some porosity, but in the majority of cases the trouble was due to cracking on account of the inherent brittleness of porcelain.
The hardware and cement in the cap and stud type of suspension insulators have unquestionably been the cause of most cracking. This feature has been recognized for a number of years and the manufacturing companies have progressively at-. tempted to take care of the various stresses that cause deterioration. At first a felt or paper gasket was placed between the metal cap and porcelain body of the insulator. Initially these were left in place after the cement hardened, but in later years, because of ionization trouble, the paper washer was removed after the cement had set. The next step was placing an elastic washer between the porcelain insulator head and the metal cap before cementing; and a piece of similar material was used between the stud and insulator head, materials such as felt, paper and cork being used. In the later units, instead of the fibrous material spacer, a paraffine or asphaltum compound was used to prevent the space being filled with cement and to give an elastic medium between the metal and porcelain. In still later designs the cemented surfaces on both sides of the head were sanded; and with the most recent types the sanded surfaces and metal parts have been treated with elastic material in order to secure a yielding bond between the metal and porcelain.
The increased care used in the manufacture of insulators, together with more exact tests and im-proved methods of production, has evolved suspension units that are decidedly superior to those made six or seven years ago. There seem to be no definite tests with which to determine to what extent the methods employed are effective, nor which disclose the relative merits of the various practices followed by the several manufacturers. Time and actual service must be depended upon to tell the results, unless suitable methods are found for anticipating failure by means of reliable tests.
Some four years ago when the megger was coming into prominent use for the location of defective suspension units, it was thought that if a megger could be devised having a much wider range than the ordinary commercial instrument, a great deal might be learned about the porcelain structure, particularly in regard to porosity.
Professors Ryan and Clark developed a high duty megger, using 25,000 volts d. c. as an energy source, and an extremely sensitive galvanometer for measuring the small currents involved(1). This instrument constituted a megger capable of measuring, with a fair degree of accuracy, insulation resistance of ten million megohms. It was hoped then that such a device could readily measure the degree of porosity in the various types which were absorbing moisture and failing in service. This expectation, however, was not realized, but in making the test
other things of importance were brought to light, the most notable being that of the temperature coefficient of porcelain. It was found that a good porcelain unit was subjected to at least a ten to one change in insulation resistance caused by the daily temperature cycle, a modern ten inch unit varying from a resistance of one million to one hundred thousand megohms with a temperature change of from 20 deg. to 40 deg. cent.
There were numerous experiments for determining the porosity of porcelain. In one case the units were placed in cold water, the temperature of which was raised to the boiling point and then cooled before the units were removed. This was not successful, as it was found that a unit which had absorbed moisture in the field would improve under such treatment. Another test was the following: A unit was boiled and while still remaining in the liquid was subjected to a pressure of 1,000 pounds per square inch in an attempt to force moisture into the pore system of the insulator body. Samples of porous porcelain which had absorbed moisture were taken and sections of them were cut and carefully weighed in an effort to gauge the percentage of moisture. Many other ways were tried to determine the porosity of insulators failing in service, but only one method seemed at all effective and that was the Fuchsine dye test. This test consists of placing broken porcelain samples in steam vapor for two hours to remove the air by osmosis; then dropping them into a red dye liquid and subjecting the samples and liquid to a pressure of 1000 pounds while they cool to atmospheric temperature. The insulator samples, of course, remain submerged in the dye liquid throughout the cooling period. After the porcelain pieces have thoroughly cooled under pressure, for several hours, they are removed from the dye liquid and, after drying, are broken as nearly as possible at right angles to the original surfaces. If the porcelain is non-porous, the fracture should show no penetration of the red stain, while slightly porous porcelain will show streaks of red stain throughout.
The high voltage developments on the Pacific slope, together with the large percentage of failures of most suspension units, have caused anxiety regarding the future of long high voltage lines, such as those operating from 150 to 200 kv. With this in mind, a number of methods have been proposed to improve the voltage distribution throughout long strings of suspension units. It has been found from test that when more than five or six suspension units are placed in series, the addition of units in the string does not materially lessen the voltage duty of the unit next to the line conductor, where a constant voltage is applied to the string. Hence, for high voltage, it was feared that the electric duty imposed on the line insulator, together with the lowering of the arc-over voltage on the string, might limit the voltage that could be successfully used with suspension units.
The question of grading has long been considered and in Europe has been tried even on the lower voltage lines; but this has never been done in American practice. With a string of 15 standard 10 inch suspension discs it is possible to get as much as a 7 to 1 ratio in the voltage duty throughout the string. In other words, the unit next to the line conductor would be carrying seven times the voltage duty of a unit near the tower arm. Even if the units were designed so that they would properly withstand the voltage imposed upon them, the method would not be an efficient one, as the insulators will flash-over at an abnormally low value due to cascading. By the use of shields, or grading, this ratio in voltage duty between the units throughout the string can very readily be changed from a ratio of 7 to 1 to that of 2 to 1, which is a great improvement.
There is now under construction in the west, a 165,000 volt transmission line using ten 10-inch discs per string in suspension and one shield at the bottom of the string. This shield has a two-fold purpose—the reduction of the voltage duty of the first unit, and the elimination of all corona on the hardware. The voltage duty on the first unit is reduced over 20 per cent by the introduction of the shield, and added to this is the feature of producing a spreading field surrounding the insulator string which will tend to carry away any arc from the insulator string, should a flash-over occur.
Fig. 6 shows the string and shield in place. The voltage distribution on the various units throughout the string are shown by the curves, Fig. 7. Curve No. 1 shows the voltage duty on the units without shield. Curve No. 2 shows the voltage duty on the units with shield.
The table on page 670 covers tests on suspension type insulators conducted at Stanford University during the past four years. These insulators are of a variety of types and makes, some having seen service on the lines of western companies, while others were taken directly from factory shipments. It is interesting to note the high percentage of failures of the different types when the insulators are not subjected to either voltage or mechanical load, but simply weather conditions. These test results tend to emphasize several points which have been clearly established by tests and operating experience.
The majority of suspension insulator failures, especially among units manufactured during the last six years, have been due to temperature changes, voltage duty and loading in most cases having very little effect. From the table it will be noted that insulators 1-13A, LA, BL, and S, after some four years of yard treatment, following their previous service record, show a rate of failure of from 12% per cent to 15 per cent per annum for the four year period. This test checks with what has been found under service conditions with the same class of insulators on western transmission lines.
It is also interesting to note, at this point, that where insulators of the class mentioned above were placed in even temperature storage, the rate of depreciation was materially reduced and in some cases was negligible. For example: The 1-13A class insulators over a 750 day period were failing at a rate of 43.8 per cent under yard treatment, while the rate of failure under basement treatment was only 4.35 per cent. With the LA insulators the failure under yard treatment was 36.8 per cent, while under basement treatment the failure was 0 per cent. The BL class failed under the last period of yard treatment at 47.4 per cent, while under basement treatment the failure was only 9.52 per cent. The S lot had a failure under yard treatment of 29.4 per cent and a failure under basement treatment of 3.85 per cent.
From the table it will be noted that under the first period of basement treatment, the difference in number of failures between yard and basement treatment was not considerable, but after a continuation of two years it was quite pronounced. This basement treatment explains, to a large extent, the reason why insulators installed in stations have given good service, while the same kind of units installed on lines have been the source of a great amount of trouble.
It is also of interest to note the difference between LA and B insulators. Both were manufactured by the same company, one in 1912 and the other in 1916. One shows a 60 per cent depreciation in four years of yard treatment, and the other no depreciation. These facts are borne out by practice as well as test, as insulators made in 1914 of the B class show no depreciation after more than five years of line service, while insulators made one to two years prior to that time show a depreciation of from 6 per cent to 10 per cent per annum under service conditions.
The record of insulators marked G is gratifying, as these units were manufactured in the early stages of the suspension insulator art, 1907¬-1908, doing duty under severe line conditions on 100 kv. circuits up to 1914, when they were removed from the lines and shortly afterward sent to the laboratory for test. This insulator is of poor electrical design, having a low margin of safety between flash-over and puncture. The distribution of stresses is not well taken care of as with units of later design, as heavy streamers form under flash-over conditions, cracking the porcelain bodies by excessive heating. In spite of such defects, these insulators show only a 3.6 per cent depreciation during the four years of yard treatment. It must be remembered that these units are the old cap and stud type using cement. The surfaces are untreated, no gaskets or yielding material having been used in their assembly. This simply emphasizes the fact that a high grade of porcelain is more essential in the suspension insulator unit than good electrical design. It also indicates the fact that, with proper care, good life can be obtained from insulators of the cap and stud type.
It will be noted that type H insulators made in 1916 by the same manufacturer as the G group, show a much higher rate of depreciation. It is true that only a few samples were tested in this case, but access was had to the records of transmission lines where they are in use. These records tend to show the relatively greater merits of the units manufactured by the same company eight years previously. In other words, the early units were far superior to those made at the later date.
Another point clearly brought out by the tabulated results concerns insulators marked F. These units were not made of porcelain, but of a fused material which is moulded while in a liquid state and the hardware attached by cement in the usual manner, or an alloy is used. Regarding the difference between cement and alloy, the results would indicate no particular preference, but it is clearly shown that any material having a mortality of over 50 per cent in two years of yard treatment is inherently unfit for service, either because of improper design or other deficiency.
The results, taken as a whole, seem to indicate clearly why insulators next to the tower in suspension strings, and those used in dead end positions, have a higher rate of depreciation than other units. That is, these particular units pass through wider daily and seasonal temperature ranges than the other units, as the one in suspension next to the tower is a sunshade for the others. In stations, insulators are protected both from extreme temperature variations and from absorbing moisture, hence these insulators give better service, all of which is shown by laboratory and field results.
All tests to date seem to establish clearly that thermal fatigue is apparently the major factor in suspension insulator deterioration. Therefore, it seems logical that some method should be devised to develop a test whereby suspension units could be given, in a comparatively short interval of time, an aging effect equivalent to that of from 10 to 15 years' service.
Members of this Sub-Committee have been giving the matter of thermal fatigue testing considerable thought for several years. The insulator manufacturing companies have generously supplied the Sub-Committee with units, both with and without hardware, for making such tests. When it is seen that insulators of one type manufactured by one company in the early stages of the art are affording excellent service, and those of the same company manufactured some eight years later are giving poor service, with the reverse being true of another manufacturing concern, the advantages of carrying on such research work are obvious, and it is expected that these efforts will have the full approval of all member companies. Insulator manufacturers have some improvement to offer each year. In the past, the only way of determining whether or not these so-called improvements were effective was by actual use, but a test by time alone is of ten a very expensive and unsatisfactory experience. The Committee feels that it should continue these tests and investigations to predetermine, where possible, the effectiveness of alleged improvements. The temperature cycle tests seem to offer the means of ascertaining the ability of new units to withstand thermal fatigue; and research along these lines will undoubtedly produce impor¬tant information.
References:
Suspension Insulator Failures on the Lines of the Sierra and San Francisco Power Co., J. E. Woodbridge, Journal of Electricity, Power and Gas, March 6, 1915, p. 185. Investigation of Suspension Insulator Deterioration, J. E. Woodbridge A. I. E. E. Trans. 1916, Vol. XXXV, part II, p. 1467. Experiments on Porcelain Suspension Insulator Units, J. C. Clark, Trans. A. I. E. E., 1916, Vol. XXXV, Part II, p. 1453.
Ceramics in Relation to the Durability of Porcelain Suspension Insulators, H. J. Ryan, A. I. E. E., Trans. 1916, Vol. XXXV, Part II, p. 1437.
Constitution and Microstructure of Porcelain, A. A. Klein, Technologic Paper, Bureau of Standards, No. 80, 1916.
Porcelain as an Insulating Material from the Physical-Chemical Standpoint, Dr. Zoellner, Elektrotechnische Zeitschrift, Vol. 29, 1908, p. 1257, and Vol. 30, 1909, p. 95.
Deterioration of Porcelain Insulators in Service, J. A. Brundige, Trans. A. I. E. E., 1914, Vol. 33, p. 119
Discussion by L. C. Nicholson, Trans. A. I. E. E., 1913, Vol. 32, Part II, p. 1488
A. I. E. E. Trans. 1914, Vol. XXXIII, Part II, p. 1731.
Journal of Electricity, 1918, Vol. 40, No. 8, p. 395.
A. I. E. E. Trans. 1917, Vol. 36, pages 526 to 595, papers by A. 0. Austin, W. D. Peas-lee, J. A. Brundige; discussion by H. J. Ryan.
NOTE.—(1)Refer to "Experiments on Porcelain Suspen¬sion Insulator Units," Vol. 35, part 2, A.I.E.E. Trans. pages 1453-1466.
