[Trade Journal]
Publication: Journal of the American Institute Of Electrical Engineers
New York, NY, United States
p. 445-448, col. 1-2
High-Tension Insulator Porcelain
W. D. A. PEASLEE
Electrical Engineer. Jeffery Dewitt Insulator Co.. Huntington, W. Va.
Porcelain used in the manufacture of high-tension insulators must meet certain requirements as to mechanical strength, ability to resist; sudden changes of temperature, porosity, homogeneity and temperature, coefficient of resistivity. A further suggestion as to the influences of the Piezo electric effect and the deterioration of seemingly perfect porcelain is presented with a brief discussion of the degree of progress made in the art to date.
INTRODUCTION
WHILE in the case of transmission line insulators it is true that the design of the insulators insofar as the utilization of the material is concerned is a matter of most efficient design of air insulators, that is, the boundary surface between the air and the supporting dielectric, the material of the insulator itself becomes of greater importance as the voltage of the line increases.
The shape design of the insulator will not be discussed here as that subject is reserved for a later communication. It is hoped that the following discussion of the factors affecting the material employed in the manufacture of high-tension insulators as a dielectric, may lead engineers to a more serious consideration of this vitally important question. The discussion is confined to porcelain as for many reasons this material has come to be accepted as the best for high-tension insulators.
FACTORS TO BE CONSIDERED IN THE SELECTION OF A PORCELAIN BODY AS A MATERIAL FOR HIGH-TENSION INSULATORS
Mechanical Strength: It is perfectly obvious that mechanical strength is one of the prime factors to be considered in an insulator material. One of the first duties of an insulator is to support the conductor, and it must do so under any conditions not severe enough to destroy some other element of the trans-mission line construction.
Insulators made today either in pin or suspension types, use porcelain either in compression or tension or both, and so it is of great importance to know the strength of porcelain under these conditions.
Porcelain being a brittle or rigid substance like cast iron, has no yield point as commonly known, and its first yield is complete rupture. There seems to be no indication that the stressing of porcelain to a point close to its ultimate strength injures it either mechanically or electrically. Indeed, the accumulated evidence of a large number of tests covering combined electrical and mechanical tests, fatigue tests and high-frequency tests indicate that such stressing has no effect whatever upon these properties. It is probably quite safe to say that properly vitrified porcelain must be stressed beyond its ultimate strength to rupture it even under repeated strains. Tension and compression tests both confirm this statement.
It is, therefore, immaterial whether the porcelain be used for compression or tension, provided that maximum momentary stress does not reach the ultimate strength of porcelain. For engineering reasons an ample safety factor must always be provided for. Ordinary porcelain, made up of the usual three ingredients,—clay, flint and feldspar— has been found by several investigators to have strengths reaching as a maximum 40,000 lb. per sq. in. for compression and 1500 lb. per sq. in. for tension. Naturally different proportions of these ingredients produce a porcelain, or to use a ceramic term, "Body," (which covers all bodies made up of the above or similar ingredients and vitrified) of somewhat different mechanical characteristics, but these figures cover rather the upper limit of strength for a body having the required characteristic as to dielectric qualities and ability to withstand sudden temperature changes.
The wide difference between these values has led many engineers to distrust porcelain used under tension, but provided the stresses are kept within the proper limits, from an engineering safety factor point of view, there is no more valid ground for this attitude than there is to condemn cast iron whenever used in tension.
Under the stimulus of the demand by transmission engineers for better insulation a great deal of work has been done on porcelain mixtures or bodies, and certain types are now available, whose strength runs up to 65,000 lb. per sq. in. in compression and 12,500 lb. per sq. in. in tension. This, indeed, is not the ultimate limit, as indications point to the commercial production of bodies with even greater strength which will at the same time retain the other necessary requirements.
In making tension and compression tests on porcelain it is very necessary that the application of the stress be made in such a manner as to place the porcelain either in pure compression or tension as desired. The compression tests are made on small blocks and the pressure. applied through lead or blotting paper disks. The tension tests are made on test pieces consisting of a short, straight section of accurately determined area between two conical end pieces. These conical end pieces are gripped in a specially designed multiple part clutch faced with soft lead or blotting paper sheets; and remarkably consistent results are obtained in this manner.
Ability to Resist Sudden. Changes in. Temperature. A great many insulator failures are traceable directly to the inability of certain porcelain bodies to resist sudden changes in temperature. The first rays of sunlight on a frosty morning have often been the signal for insulator failures directly attributable to this weakness.
Also a body sensitive to this change is more difficult to manufacture reliably as it will develop internal strains which, added to the applied service strains, will produce rupture of the porcelain at very low applied loads. The existence of these strains has been shown by polarized light under microscopic examination.
Any mixture or body that in the shape and size employed will not, when completely equipped with hardware, withstand an indefinite number of alternate immersions in boiling and freezing water, should never be employed in the manufacture of high-tension insulators. This test should be insisted upon by purchasing engineers. Such bodies are made today, and some are made that will stand even greater ranges. At least two are known which can be heated red hot and thrown into a bucket of water without cracking.
Porosity. Porous porcelain is responsible to a large degree for the unsatisfactory condition of the insulator situation of today. One of the greatest insulator manufacturers has recently stated in a published article that non-porous porcelain cannot be produced.
This statement is challenged as the writer's investigations, both in this country and in the laboratories of France, indicate that it can be produced and under manufacturing conditions. Porcelain can be, and is, produced today that is in an engineering sense, absolutely non-porous. This statement is made after a great deal of careful research and on the strength of many porosity tests both by the impregnation method and the method used in ceramic analysis with a psycho-meter and a sample crushed to a 100-mesh fineness. The impregnation test has been carried in our laboratories to very high pressures, and is so penetrating that, under microscopic examination of the penetration of the colorant material, it has been observed in the cleavage cracks of microscopic quartz crystals.
Furthermore, a body having the slightest porosity, as indicated by the psychometer test, shows a decided penetration under the impregnation test. Porosity is of two kinds, discussed in ceramic language as open pore and closed pore types. In the open pore type the pores or voids are connected by capillary passages, while in the second, or closed pore type, the voids or pores are isolated. Though the percentage of porosity may be the same in the two cases, it is obvious that the general character of porosity and its effect on insulators is different in the two cases. In general, the first form is a product of under-firing and the second of over-firing, though many instances have been found wherein over-firing produces the first type of porosity. There are probably many ceramists even today, who will dispute the statement that over-firing will produce porosity, but it can be very readily demonstrated by proper tests. In this connection the curve of Fig. 1 shows the behavior of one mixture or body and indicates very well the effect of the temperature of firing on the porosity of the body. The porosity developed on over-firing on this body was of the closed pore type and on under-firing of the open pore type. In securing the data for making this curve the samples were fired to given temperatures and cooled, and the porosity and tensile strength values were taken at room temperature. In other words the points on the curve represent maximum temperature of firing of each sample and not the temperature of the test. The porosity determinations were made by the psychometer method, the impregnation method being in any case merely qualitative.
In practise the effect of open pore type of porosity is too well-known to discuss here. The development of megger and buzz stick tests are ample evidence of the degree to which this factor has entered into the troubles encountered in the insulator field. Tests to determine porosity in units at the factory are needed, and the man who develops a 'method whereby we can detect porous insulators at the factory without destroying them, will be a true benefactor of the transmission engineers and porcelain manufacturers.
In the production of insulators a method has been developed that is very valuable in production control testing. A solution of fuchsine dye in wood alcohol is used and unglazed pieces placed in it under pressure. The slightest degree of porosity of the open pore type is indicated by a deep penetration of the dye into the body of the test piece. Indeed, as mentioned before, it is so penetrating that the microscope has shown it forced into the cleavage cracks of minute quartz crystals. If test pieces of the same shape and volume as the insulators being fired are properly distributed in the kiln the fuchsine test on these pieces will furnish a very reliable indication of the condition of neighboring pieces as to porosity.
Some very interesting developments have recently been brought out by the use of very high pressures on the solubility of porcelain in water under certain conditions, and it is probable that considerable light will soon be thrown upon certain types of insulator depreciation as a result of these developments.
It has been found possible to produce insulators of non-porous porcelain within the ordinary limits of quantity manufacture and by means of this method of control to prevent the porous insulators, a few of which are unavoidable in commercial manufacture, from going to the customers. Closed pore porosity is commonly indicated by a swelling of the insulators, and can be watched closely by gauges applied to the finished product.
Temperature Resistivity Coefficient. The temperature resistivity coefficient of porcelain is large and negative. The curves of Fig. 2 give an idea of this characteristic and of the improvement that has been made in it. The curve-marked "Conventional Porcelain" in Fig. 2 is the standard mixture of clay, flint and feldspar, and the curve marked "New Body" is one of the recent developments, the formula for which naturally cannot be disclosed. In this curve the points on the curve represent measured resistance attained at the temperatures noted by the abscissas of the curve. For instance, a certain sample of conventional porcelain will have a resistance of two megohms at slightly over 700 degrees fahr. (371 deg. cent.) whereas the "New Body" in an exactly similar piece has a resistance of two megohms at 1160 degrees fahr. (627 deg. cent). These curves will give some idea of the progress that has been made in this respect. The importance of this feature has been discussed by various writers, and it must be stated here that this characteristic may be improved but will always be negative even in pure quartz. It has an important bearing on the mechanism of insulator failures under transient voltages. This has been discussed by the writer in previous papers before the institute. (1)
Piezo Electric Effect. One of the most baffling difficulties in the insulator situation is the deterioration of seemingly perfect units after a time, and the acquiring by non-porous porcelain of a certain porosity. Some recent investigations indicate that this may be intimately connected with Piezo electric qualities of quartz crystals. Under the microscope it will be seen that porcelain as ordinarily made, consists of rather large particles of quartz in a vitreous magma. It is well-known that if certain crystals such as quartz are subjected to a pressure on two diametrically opposite faces which are parallel with the major axes, a potential difference is set up on the faces perpendicular to those upon which the pressure is applied, which varies directly as the pressure.
The converse of this is also true and if a difference of potential is applied to two opposite faces parallel to the major axes the crystal is subjected to a squeezing action and a change in dimensions of the crystal results. These changes in dimensions if strongly resisted by the surrounding magma will set up enormous local stresses between the magma and the crystals.
Now what happens when this porphyritic mass is placed in an alternating electro-static field? According to the theory of probabilities a large number of these quartz crystals are so arranged that their major axes are not parallel to the field of force. This alternating field applying potentials as described will set up in these crystals a change in mechanical dimensions 120 times per second in the case of a 60-cycle system. This will result in a vibratory movement of these crystals. This vibratory action may be detrimental in two ways,—first a rupture of the crystal itself, along cleavage planes, and second a rupture between the crystals and the surrounding magma.
A concentrated leakage current results through the spaces between the magma and the crystals when the crystal is at its greatest deformation and the potential differences are at maximum. It will also tend towards making the porcelain porous in an entirely different manner (that is, by cracks and fracture voids) from the types before mentioned and the vibratory action of the crystals may aid in the introduction of moisture into the body. This decreases its value as a dielectric and as mentioned before the solvent action of such moisture may have an accelerating effect upon the deterioration of the porcelain.
Regardless of the degree to which this effect contributes to the deterioration of the porcelain when in service on high-tension lines the body to be sought is one wherein the quartz is dissolved as completely as possible in the feldspathic magma.
Great progress has been made in this direction and in a later communication some data, which is now being prepared, may be given that it is hoped will be of some help in the solution of this ever present problem.
CONCLUSION
The insulator problem at present is one whose solution is to be sought in the ceramic field. Aside from more rational shape design on the part of electrical engineers, the improvement must come in the development of porcelain bodies which meet, to the greatest degree possible, the above requirements. Great progress has been made in this respect and the statement that we may very soon see the commercial production in insulator form of such bodies is now amply warranted.
To be presented at the Annual. Convention of the A. I. E. E., June 29, 1920.
(1) "Insulator Failures Under Transient Voltages," W. D. A. Peaslee, A. I. E. E. TRANSACTIONS, page 1237, Vol. 35; "Insu¬lator Situation ....," W. D. A. Peaslee, A. I. E. E. TRANS¬ACTIONS, page 401, Vol. 36.
