Publication: Electrical Review
New York, NY, United States
Recent Research in Manufacture of Porcelain Insulators
Thirteenth of a Series of Articles Giving a Concise Review of the Electric Power Situation—High-Tension Equipment Much Improved by Systematic Study—Routine Tests in Manufacture
By ALFRED STILL
Porcelain is used for line insulators, not because it is an ideal material for the mechanical support of overhead wires which may subject the insulators to very heavy loads, but because no better material appears to be available. It is not always to be depended upon in tension and is liable to develop cracks. Also, if made in large, thick pieces, it is difficult to avoid porosity which naturally is an undesirable feature from the electrical standpoint. A good quality of porcelain will usually stand a stress of 1500 lbs. per sq. in. in tension, but in compression it can withstand 40,000 lbs. per sq. in. Special grades of porcelain are .now obtainable with an ultimate strength as high as 12,000 lbs. per sq. in. in tension, and 65,000 lbs. in compression.
Great improvements have lately been made in the manufacture of porcelain line insulators, quite apart from the improvements in design and proportions of the parts which have resulted from a careful study and a better understanding of the flux distribution in the electrostatic field. What is known as the continuous-tunnel kiln is probably largely responsible for the production of a more uniform and reliable product. In the early days when prices were low and competition keen, the manufacturer in his efforts to avoid over-firing, would sometimes produce under-fired units which were slightly porous and, therefore, liable to absorb moisture.
During the last few years manufacturers have devoted a great deal of time to the problem of porosity, and improved methods, together with means of detecting porosity in the finished product, have done much to eliminate this cause of trouble. What is known as the fuchsine-dye test is used by at least one manufacturer to guard against porous insulators leaving the factory. In this test a full-sized unglazed piece is included with each car of 70 insulators that is put in the tunnel kiln, and when the insulators come from the kiln the car is not released until the test piece has been placed under the fuchsine test. On the slightest penetration in this test the carload is rejected. If the sample tests in a satisfactory manner the hardware is then assembled on the insulators and each unit is tested to 5000 lbs. mechanical stress, and after this the units are given a a-min. flashover test at 200,000 cycles. It is found that this mechanical stressing is not injurious to the insulator, indeed units have been placed under a 9000 lbs. mechanical stress and subjected to a dry flashover at the same time, and the stressing does not appear to injure the dielectric properties of the porcelain if kept below the rupture point.
In order to ensure penetration of the dye in porous porcelain, broken pieces of the porcelain to be tested are placed for 2 hrs. in steam vapor before being dropped into the red fuchsine dye liquid, which is subjected to a pressure of moo lbs. while the samples are cooling to the room temperature. This process requires several hours. The samples are then removed and dried, after which they are broken so that the depth of penetration of the dye may be observed. With nonporous porcelain there will be no penetration, but when it is slightly porous there will be streaks of red stain in the fracture. A test to determine porosity at the factory without destroying the units so tested would be a great advance over present methods.
As the result of research and extreme care in manufacture the difficulties due to porosity have been largely overcome during the last few years. Sometimes minute cracks in the finished insulator lead to trouble by permitting absorption of moisture, but the chief cause of trouble with suspension units on the higher voltages is the unequal expansion of parts due to the difference in the coefficients of expansion of porcelain, cement and hardware.
MOST INSULATOR TROUBLES DUE TO TEMPERATURE VARIATIONS.
A study of insulator failures among suspension units of various types manufactured within the last 6 to 8 yrs. shows that the majority of these failures have been due to temperature changes. On an average, about 20% per annum of the insulator units that have passed all factory tests satisfactorily have failed after having been in operation a few years. This is a very large percentage of failures, and it is not to be wondered at that engineers and manufacturers are much concerned over the status of the porcelain line insulator and are sparing no efforts to remedy this condition.
The cause of deterioration and failure appears very rarely to be due to the local concentration of electrical stress, which would result in more failures of the units near the line conductor than elsewhere in the string. It is found on the contrary that the units next to the tower, and also those used in dead-end positions, have a greater rate of depreciation than the units in other positions. Observations made on the Pacific coast indicate that defective top insulations; i. e. the units next to the point of attachment on the tower, amounted to about 25% of all defectives found on certain lines under observation, while only 11% of the units next to the line wire were defective. The conclusion naturally drawn from these observations is that these particular units pass through greater periodic changes of temperature than those lower down in the suspension string, which are to some extent protected from the sun by those above them, and they suffer deterioration which ultimately leads to an electrical breakdown, even when the stress in the dielectric is less than in the units that are not subject to such extreme and rapid temperature variations. It is, therefore, what may be called thermal fatigue which is the chief factor in the deterioration of suspension insulators in service.
Electrical puncture may be what puts a particular unit out of service, but this is not due directly to an excessive voltage gradient in the material of the insulator. It is frequently brought about by the unequal expansion of the hardware and the porcelain to which it is cemented, which develops small cracks and starts the trouble. The porcelain itself should be of a kind which will resist sudden changes of temperature. It is now manufactured to withstand alternate immersions in boiling and freezing water without developing cracks, and porcelain bodies have actually been made which can stand being brought to a red heat and then plunged into hot water without developing cracks.
Porcelain insulators used inside of buildings, where they are not subjected to extreme temperature variations and are not so liable to absorb moisture, invariably give better service than those on the outside which are subjected to seasonal and daily temperature changes and alternate dryness and moisture.
Observations made on the Big Creek line in California, which is about 240 mi. long running generally from north to south, show that the percentage of insulators suffering deterioration is dependent upon the climatic conditions in the different zones traversed by the line. Near the coast the humidity is high, then there is a section of mountain and desert, and in some places the line rises to an altitude of 5000 ft., where the winters are cold and damp and the summers hot and dry. The greatest number of defective insulators are found where the climate is unfavorable and where porosity and small cracks developed by temperature changes lead ultimately to electrical break-down. Heavy fogs and light drizzling rains appear to be more objectionable from the point of view of line insulation than heavy rainstorms. In valleys where the temperature is usually more even, the rate of depreciation is found to be less than in mountain and coastal sections of overhead lines.
Research work is now in progress and it is hoped to develop a temperature-cycle test by which suspension insulators may be submitted in a comparatively short interval of time to an aging effect equivalent to that produced by many years in actual service. Suggested plans for tests at Stanford University by Prof. Ryan and others include the placing of a number of units in two equal groups, one of which will be held at a uniform temperature or where the temperature changes will be very small, while the other group will be subjected to temperature changes ranging from 32 to about 140 deg. F. Some units will be tested in this manner while subjected to a load of about 2500 lbs. There would appear to be no advantage in maintaining high voltage continuously on the units under test, but it is possible that some insulators will be stressed electrically as well as mechanically while under test.
There is no doubt that the solution of the deterioration problem is in good hands and that improvements in design and manufacture will lead to both material and construction being such that insufficient or unreliable line insulation will not be an obstacle to the erection of the contemplated extra-high-voltage lines. A tough nonporous porcelain and a rugged design free so far as possible from expansion troubles will probably be the outcome of the present extensive and thorough investigations. A massive construction has the further advantage that the porcelain is less likely to be injured by the power arc following a flash-over.
FACTORY TESTS OF INSULATORS AND INSULATOR PARTS.
What are known as design tests are those made by the manufacturer to determine the properties of a particular size and shape of insulator. These tests may include mechanical loading to the point of destruction to ensure that the factor of safety is sufficiently high, but the most valuable design tests are probably those which determine the wet and dry flash-over voltage of the insulator. Voltages are usually determined by using the sphere gap, which has certain advantages over the needle gap as generally used in the past until the greater reliability of the sphere gap led to its adoption for the more accurate measurement of high voltages.
When testing for dry flash-over the insulator should be mounted to reproduce as nearly as possible the actual service conditions. This is not always easy to accomplish in the testing of pin-type insulators, but the distribution of electric stress in a string of suspension units is generally similar under test conditions to what it will be in actual service. For the rain test or wet flash-over it is customary to use a precipitation of 0.2 in. per min., the spray being directed from one or more jets on the surface of the insulator at an angle of approximately 45 deg. to the vertical. The results obtained from the rain test will depend somewhat upon the exact manner in which the water strikes the insulator, and also upon the amount of salts or impurities in the water.
Routine tests are applied for the purpose of eliminating defective materials. Assembled pin-type insulators are placed in an inverted position in a shallow dish containing water, the metal pin or water in the hole for receiving the pin being connected to the high-tension wire from a step-up transformer. This transformer should be of large capacity. For testing 200 insulators at one time a transformer capacity of 200 kv-a. is desirable. Parts of insulators, before being assembled, receive a pressure test similar to that applied to the assembled insulator. Tubes or bushings are tested by having a metal rod passed through them and flexible wires, connected to the other terminal of the transformer, wrapped around the outside in the desired position. The pressure is raised sufficiently to cause intermittent flash-over, so that even under a normal sup-ply frequency of 25 or 60 cycles the material between the two electrodes is actually subjected to high-frequency stresses. If necessary, the proper amount of reactance is put into the circuit to ensure an oscillatory discharge. These tests usually extend over a period of at least 5-min. duration.
METHODS EMPLOYED IN TESTING LINE INSU LATORS IN THE FIELD.
On account of the deterioration of porcelain insulators in service it is advisable to test them periodically, preferably without interruption to service. When duplicate lines are available insulators may be disconnected from the supply and tested with the megger. By means of this instrument the resistance in megohms of the insulator may be measured directly. In conducting such a test one man on the pole or tower holds the two leads across the insulator while another man operates the small generator (usually delivering up to 1000 volts), and reads the instrument. Each company sets its own standard for minimum resistance, and if the test reveals a resistance lower than this standard the insulator or unit is replaced. The minimum value usually lies between 500 and 5000 megohms.
Several of the large companies operating at pressures above 60,000 volts make a practice of testing insulators in the field with the megger and thereby obtain results of considerable value, although this method of testing does not reveal all defects which may ultimately cause a breakdown of the insulator.
Although the "Buzz Stick" method of testing is used on both pin-type and suspension-type insulators it has not been attended with much success on the former type, but with suspension units it appears to be more successful. The "buzz stick" consists of a long insulating handle with two metallic projections at the end forming a continuous conductor by means of which a short-circuit may be put across an individual insulator in a string of suspension units.
Before making the short-circuit test it is customary to touch the metal cap of each insulator unit with one prong of the metallic fork. A small spark occurs because of the capacity of the fork which will be charged to the potential of the cap with which it is brought in contact. The caps are touched in succession, beginning with the one nearest to the line, which should produce the largest spark since the potential to ground decreases as the units tested occupy positions progressively nearer the point of attachment to the grounded crossarm. If there is a defective insulator in the string; that is to say, a unit which has broken down and has no appreciable potential difference between the two metal fittings, the spark from the cap of such a unit will be the same as the spark from the one preceding it. Experience is needed by the tester to determine which, if any, are the defective units. He judges mainly by the noise made by the spark as it jumps from the "buzz stick" fork to the cap of the insulator.
Should this test reveal a large number of defective units it might be undesirable to proceed with the short-circuit test, but if there appear to be enough sound units in the string to permit of these being short-circuited one at a time, the metal fork is then used to connect the metal fittings of each insulator unit. The spark obtained on this second test is that due to the discharge of the condenser formed by the cap and bolt separated by the porcelain dielectric. The test proceeds as before from bottom upward, and since the number of coulombs, or the amount of dielectric flux, will be greatest for the unit near the line, this should produce the largest spark, while units nearer the tower will produce smaller sparks depending upon the distribution of potential throughout the string of units. A defective insulator is at once detected because there will be no spark, or only a very small spark, wh