Testing of Insulators

[Trade Journal]

Publication: The Journal of Electricity, Power and Gas

San Francisco, CA, United States
vol. 13, no. 2, p. 143-145, col. 1-2


Four Institute "Introductories" on Transmission.

 

IN these columns for January* there appeared an announcement of the appointment of a committee, known as the Committee on High Tension Transmission, by the American Institute of Electrical Engineers, for the purpose in general of collecting data respecting present practices and successful methods in electric power transmission at high voltage; "in a word," continues the announcement, "the endeavor will be to make this committee the clearing house for experience and ideas bearing on high tension transmission."

The first efforts of this committee are now in evidence in the papers which appear below, and each of which is submitted as an "Introduction to a Discussion" upon the particular topic indicated in its title, and upon which subject Institute members are requested to express their opinions either personally or by letter, before the regular meeting of the Institute to be held in New York City on March 27th, next. In contributing to a discussion, it is requested that the matter under discussion be taken up under the several heads, and in the manner made use of in the "Introduction," and that following the treatment of these heads, there be introduced any other matter which the "Contributor" may deem advisable. When a member takes part by mail in more than one of the discussions taking place at the same meeting, it is requested that he embody his several "Contributions" in separate letters.

The scope of these "Introductions," the prominence of their authors, and the fulness of their experiences, together with the thoroughness which is always characteristic of Institute discussions, give abundant evidence of the growing importance of the Transmission Committee. That it will become most potent in the influence it will exert upon transmission engineering is not to be doubted.

Although not embodied in Mr. Blackwell's paper, the accompanying illustrations of high tension insulators under test at the Oakland substation of the Standard Electric Company of California, are here presented for the first time as a fitting contribution to the discussion. The photographs from which these half tones were made were taken under break-down tests, wherein the potential used at the instant of photographing ranged between 120,000 and 130,000 volts at sixty cycles. The insulators and pins used in the test were identical with those specified in Figure 1 of Mr. Chesney's paper, which follows, and the testing set was of the standard Stanley type, which has been described in these columns heretofore. **—THE EDITOR.

 

 

THE TESTING OF INSULATORS. **

 

By F. O. BLACKWELL.

 

ELECTRIC power transmission cannot be successful unless it is able to deliver uninterrupted power. Continuous operation, so far as the transmission line is concerned, depends largely upon the effectiveness of the insulator which is employed. Insulators must, therefore, be obtained which will not fail in service, and this can only be assured by the thorough testing of each one that goes on the electric lines.

The potential that can be employed safely for the transmission of power is now limited by the pressure the insulators will bear.

Transformers that are reliable and not excessive in cost can be built for twice the voltage that any line yet constructed will withstand.

As the distance over which power can be transmitted with a fixed cost of conductor varies with the potential, the length of transmission lines is to a great extent limited by the insulator.

The design of new and improved types of insulators is, therefore, most important, and these can only be developed by experiment with adequate testing facilities. In order to ascertain the value of such insulators, no method of testing can equal a practical trial under conditions of actual service. Placing new insulators on power transmission lines in commercial operation is impracticable in most cases and should only be permitted after they have successfully withstood tests to demonstrate their ability to stand operating conditions. These tests should duplicate as nearly as possible the electrical and mechanical strains set up in the insulators under the very severe conditions that would ever be met with on a transmission line.

There are certain facts which must be considered if correct deductions are to be made from insulator tests. For instance, we cannot test each insulator with a given number of volts continuously as it would be in service. As is well known, all insulating materials are most apt to break down on long applied electric stress. The prepared cloth wrappings used on the windings of electrical machinery will stand instantaneously two or three times the potential that they will carry continuously. Glass and porcelain are not affected by time to the same extent as organic materials, but we know that both are punctured by long continued applications of lower pressures than they have withstood in test.

The shape of the potential wave also has a pronounced effect in breaking down insulation. A wave may be either flat-topped or peaked, so that the maximum instantaneous potential is much less or greater than that of a sine wave of the same square root of the mean square potential. We might have for the same potential as read by the voltmeter, maximum instantaneous potentials which differ as much as two to one.

In air, the maximum point of the wave determines the distance which the current will jump. Different generators or even the same generator under different conditions of load will show widely varying arcing distances for the same potential.

Insulating materials being more affected by time than air, show in their ability to resist puncture that the average potential of the wave is more important than the maximum.

It is not safe to assume the potential either by the voltmeter or air gap as the true potential for determining the insulating value, as it is somewhere between the two. Moisture in the atmosphere also affects the arcing distance. In steam, a given potential will jump twice as far and in a fog 25 per cent, farther than under ordinary conditions. Of course, if the altitude is high and the air more rarified, the arc will also jump a greater distance. I would like to call attention to the characteristics of the apparatus required for testing insulators.

The alternators generally used for long distance transmission plants give very nearly a sine wave and therefore the testing generator should be one which will give a sine wave under all conditions. It is not sufficient to do so at full potential and no load, as tests are made with all degrees of excitation and with both leading and lagging currents.

The armature reaction should be as small as possible, which means that the generator should be much larger than would ordinarily be thought necessary. It is also desirable to have a high reluctance in the magnetic circuit to secure stability when running with weak fields and permit of control with a reasonable amount of field resistance.

There should be but one transformer used to step up to the highest potential required and its reactance should be as low as possible. A number of transformers in series is particularly bad, as it gives poor regulation and leads to great uncertainty as to the actual potential to which an insulator is being subjected.

I have known testing sets with transformers in series and a generator of poor regulation to vary widely in the relation of the generator volts and the length of the spark gap due to change of wave form with different magnetic saturations of the apparatus and different numbers of insulators and consequently various capacities on the testing circuit. The only certain way to determine the real potential is to have a step-down instrument transformer on the high potential circuit.

Assuming that insulators are to be passed upon for a specific transmission plant, they should first be inspected to see that they are free from cracks, bubbles or pits that will impair their strength or in which moisture can lodge. If of porcelain, the glaze should cover all the outer surfaces. The glaze is of no insulating value in itself, but dirt sticks to unglazed surfaces. Experience has shown that porcelain insulators which are not absolutely non-absorbent are worthless. The best porcelain shows a polished fracture like glass. If there is any doubt about the quality of the porcelain in this respect, it should be broken into small pieces, kept in a hot dry place for some time, weighed and immersed in water for a day. When taken out of the water the weight should be the same as at first. A puncture test should be made by setting the insulator in a cup of salt water, filling the pin hole also with water and slowly increasing the potential between the top and bottom until the desired test potential is reached or the insulator either punctures or arcs over the surface.

 

Illustration

 

If an insulator is built up of several parts, each part should be able to withstand a pressure greater than it will have to sustain when the complete insulator is tested. If it is to be tested for 100,000 volts and is made in two parts, each part might, for instance, be tested with 70,000 volts. The object of this is to have the weak parts rejected before they are assembled. A fair puncture test for an insulator is twice the potential for which it is to be employed, applied between the head and the interior for one minute. For example, the insulators for a 50,000-volt line should each stand 100,000 volts. As the potential from any wire to ground on a 50,000-volt three-phase system would only be about 30,000 volts, a 100,000-volt test gives a factor of safety of nearly three and one-half to one. If one branch were grounded, as sometimes occurs in practice, the factor of safety would be but two to one. A one-minute test is not so severe as a continuous application of an equal potential, but insulators that have passed this test stand up well in service. New types of insulators should be mounted on iron pins and tested both wet and dry, to determine the potentials which will arc over them. The dry test is of little value, as the potential at which the arc jumps from the head to the pin can be pre-determined by measuring the shortest distance between them and referring to a curve of arcing distances in air. In a wet arcing test, a stream of water from a sprinkler nozzle under a pressure of at least fifty pounds to the inch should be played on the insulator at an angle of say thirty degrees from the horizontal. This will be similar to the condition which exists in a rain and wind storm. The insulator should not arc over from the wire to the pin at less than the potential which will exist in service between any two conductors.

 

Illustration

 

In no case should wooden pins be relied on for insulation, as their value is only temporary. All wooden pins in time become dirty and absorb moisture. Eventually they burn off unless the insulator is good enough to be used with au iron pin. If an insulator is going to fail, it is better to have it do so at the start and not interrupt the service by breaking down perhaps years afterwards.

In addition to the electrical tests, it is well (if the insulator is of a type that