[Trade Journal]
Publication: Journal of the American Institute Of Electrical Engineers
New York, NY, United States
p. 682-686, col. 1-2
Radio Interference from Line Insulators
BY ELLIS VAN ATTA, Non-member (1) and E. L. WHITE, Associate, A. I. E. E. (2)
Synopsis.—This paper presents a discussion of the causes of present methods of eliminating this kind of disturbance are radio interference from insulators on high-voltage equipment. The explained, and the question of future design is discussed.
INTRODUCTION
RADIO broadcasting has brought with it the problem of radio interference. The radio listener is, of course, the one most affected by interference; but the broadcasting companies, the manufacturers of electrical apparatus, and the producers of electrical energy are likewise concerned since the solution of the problem devolves upon them. During the past few years each of these interests has done much to eliminate unnecessary interference; and every kind of equipment used in the supply and consumption of electrical energy has been tried and tested for interfering qualities.
Their experience has shown that radio interference may be classified under five headings, with respect to its origin. These sources are as follows:
1. Consumers' equipment.
2. Low-voltage supply circuits and apparatus (110-550 volts).
3. Intermediate-voltage circuits and equipment (1100-7500 volts).
4. High-voltage equipment (11,000-220,000 volts).
5. Atmospheric disturbances.
Ways have been devised for eliminating practically all radio interference which originates on any of the first three classes of equipment. The last item is obviously beyond human control. The fourth classification includes numerous items of equipment which can be made free from radio interference, and a few other items for which no remedy has been devised as yet. The scope of this paper is limited to the latter group, particularly line insulators of the pin and suspension types.
GENERAL
The principles underlying radio interference are similar to those of spark telegraphy and carrier current telephony. In brief, a spark occurring on electrical equipment of any kind sets up a wave train which produces damped oscillations at a multitude of frequencies. The predominating frequencies are the resonant frequencies of the associated lines and equipment, and their harmonics, including those in the radio broadcasting band. Since the electrical constants which determine the above frequencies are distributed, and several kinds of equipment may be concerned, the resonant peaks are usually broad and overlapping. Consequently a broadcast receiver which has radio interference is usually affected over the entire broadcasting range, with occasional points of greater disturbance.
The extreme sensitiveness of modern receivers, and the use of a-c. supply, make them very susceptible to radio interference. The comparatively small amounts of energy involved in the electrical discharges described later are therefore sufficient to produce a great amount of disturbance in broadcast receivers, particularly when the discharges occur along high-voltage lines.
The distinction between corona and brush discharge should be kept in mind when radio interference from line insulators is considered. Corona discharge usually occurs at lower voltages than brush discharge, and appears as a bluish glow when viewed in a dark room. Brush discharge occurs after corona discharge, and takes the form of fine white streamers. This condition is usually considered as another form of corona discharge, but will be classed separately in this case because of the different interfering characteristics of the two discharges. In a broadcast receiver, corona discharge produces a soft, hissing sound which is not ordinarily objectionable. Brush discharge, however, produces a crackling, frying noise which is very annoying.
PIN-TYPE INSULATORS
Corona and brush discharges may occur on high-voltage lines in any or all of the following ways:
1. Between metallic surfaces.
2. Between insulating surfaces.
3. Between metallic surfaces and insulating surfaces.
To entirely free a line of radio interference, all discharges must be stopped. In order to accomplish this purpose, all hardware must be tight; adjacent pieces of hardware must either have sufficient separation to prevent discharges, or must be bonded together; conductors and tie-wires must make perfect electrical contact with the tops of the insulators; and the pins must make perfect electrical contact with the entire surface of the thread in the pin holes. On lines using pin-type insulators these requirements can be met with the exception of the last two. Conductors, tie-wires, and pins do not make good electrical contact with the surfaces of the insulators, and every insulator is therefore a potential source of radio interference.
For the purpose of this discussion, a pin-type insulator will be considered as the dielectric of a condenser, with the conductor and tie wire acting as one plate and the pin acting as the other. When potential is applied to the plates, a charging current, the magnitude of which is determined by the reactance of the condenser and the applied voltage, will flow into the condenser. Since the reactance of a condenser is a function of its electrostatic capacity and the frequency of the applied voltage, it follows that the charging current is affected by the three factors, voltage, frequency, and capacity.
Consider a 66-kv. pin-type insulator, whose electrostatic capacity is approximately 10µµf. A charging current of 0.14 milliamperes will flow into it when used on a line operated at a voltage of 38.1 kv. to ground and a frequency of 60 cycles per second. If the conductor, tie-wire, and pin all made perfect electrical contact with the insulating surfaces, this charging current could easily flow into the insulator. Unfortunately, resistance is offered to the flow of charging current by insufficient contact between the wires, pins, and insulating surfaces. Due to the fact that the dielectric strength of air is lower than that of the insulator material, the potential differences at these points of poor contact are sufficient to ionize the adjacent air, with resultant corona and brush discharges.
The problem of radio interference from pin-type insulators is thus reduced to the matter of overcoming resistance to the flow of charging current into the insulator.
Since the magnitude of the charging current into the insulator is determined by the voltage and the frequency applied, and by the electrostatic capacity of the insulator, a reduction in any of these factors will decrease the charging current. In practise, the voltage and frequency are fixed, but the capacity can be reduced by overinsulating the lines. This method has been tried with only partial success, particularly on lines operated at 55 and 66 kv. If larger pin-type insulators are used, the problem of insufficient contact between wire, pin, and insulator is still present.
The best solution of the problem appears to be some method of insuring good contact between the conductors, tie-wires, and insulating surfaces. On existing pin-type insulators, this result can probably be secured by treating the insulator heads and pin-holes in some manner which will eliminate the poor contact between the wires, pins, and insulators.
Metallic paints have been tried, without success, because such paints form a coating of metal particles suspended in varnish and do not offer a good conducting surface. Metal disks, attached to the conductors above the insulators, have proved partially successful, due to the reduction in current density where the conductors and tie-wires contact the insulators. Tests have shown that the same result may be accomplished by looping the tie-wire to form a ring several inches in diameter over the head of the insulator. Tests have also shown that discharges to the heads of insulators are materially reduced by the addition of several extra turns of tie-wire in the insulator grooves. Metal gauze, placed in the tie-wire groove, has proved effective in some cases, and seems to be the best solution of the problem at the present time. Experiments are still being conducted, however, and it is hoped that a compound can be found which will fill in the air spaces between wires and insulators, will be unaffected by weather conditions, and will not be expensive to apply.
The problem of new pin-type insulators is being attacked in several ways. Some manufacturers employ a metal cap cemented on the head of a standard insulator. Another one uses solder-impregnated gauze in the tie-wire groove. Other insulators have layers of metal applied to the heads and the wire grooves. These metals are of various kinds and varying thicknesses. Most of them are too thin to be practical but all have a good contact surface. Still another insulator is treated in the wire grooves and the pin-hole with a special glaze. This last insulator proved to be the best of all when subjected to rated voltage in a comparative test.
Obviously, the use of metal-coated heads and metal caps on pin-type insulators will result in an increase in the electrostatic capacity of such insulators. The charging current will be increased and consequently the current density at the surface of the pin-hole will be increased. Tests have shown that this point is a very important one. It is therefore imperative that the pinhole be treated in some manner to insure good contact between the pin and the insulator. Metal threads, cemented into the insulator, are being used in most cases, while one insulator is treated with a special glaze, as mentioned before.
At the beginning of this discussion it was stated that corona and brush discharges may occur between insulating surfaces such as the petticoats of pin-type insulators. The presence of such discharges is an indication of faulty design or too high an applied voltage. The remedy is obvious in either case.
SUSPENSION INSULATORS
Suspension insulators can be classified under three general types, cap-and-pin, link, and spider. The cap-and-pin type, as the name implies, consists of a porcelain disk with a cap cemented to one side, and a pin to the other. Two kinds of hardware are used for attaching adjacent units, the clevis type and the ball-and-socket type. The link type of insulator consists of porcelain disks connected together by loops of metal, so that the porcelain is in compression. The spider type consists of extra-heavy porcelain disks, with the connecting hardware imbedded in both sides in the form of a spider, and secured by a metal alloy instead of cement.
Until recently, suspension insulators as a group have been considered free from radio interference. The potential impressed upon individual disks of a string, as they are used in practise, is comparatively low. On 55-kv. lines, using three units per phase wire, the maximum duty is about 11,000 volts. For 110-kv. lines using six or seven units per string, the maximum potential per unit is 14,000 volts. On 220-kv. lines, using fourteen units per string, the maximum voltage per unit is 23,000 volts without grading rings or shields, and about 15,000 volts with such devices. When individual ball-and-socket-type insulators are tested in a dark room corona discharge appears at the cap and at the pin when potentials as low as 18,000 volts are applied. Brush discharge occurs at voltages as low as 26,000. This type of insulator, therefore, should not cause interference under ordinary conditions.
Corona and brush discharges also appear on clevis-type insulators at the above voltages when the cotter key is removed from the clevis bolt. With the cotter key in place, and the pointed ends turned upward, brush discharges occur between the points of the key and the innermost petticoat at potentials as low as 11,000 volts. The cotter keys on clevis-type insulators which have been in service on 110-kv. lines for only short periods, show unmistakable evidence of brush discharge, not only from the pointed ends but from the round ends as well. Cotter keys on the units next to the line are affected most, but the keys on other units also show signs of discharge. Obviously the cotter key is at fault on the clevis-type of insulator, and ways of eliminating this source of interference will be taken up later.
Insulators of the link type are even more liable to cause interference than clevis-type insulators. In the older models, no attempt is made to obtain good contact between the links and the porcelain, and brush discharges take place at potentials as low as 2000 volts per unit. When weights are used to simulate line loading, the brush-discharge potential rises to 4000 volts.
The newer models of link-type insulators employ lead shims, soft copper links, etc., in order to get better contact between the metal and the porcelain. Without loading, radio interference starts at 6000 volts per unit. Under 340-lb. tension, interference does not begin until 14,000 volts are impressed. Since the potential across the line unit of a string of six link-type insulators used on a 110-kv. line is about 20,000 volts, interference will be present under those or similar conditions.
On the spider-type of insulator, corona discharge does not start until potentials of 21,000 volts are applied across individual disks. Brush discharge occurs at 26,000 volts. Both discharge points are higher than the corresponding points for either cap-and-pin or link-type insulators, a fact which is accounted for by the heavy mass of porcelain used in this type of insulator, and the absence of sharp points or rough edges at points of high electrostatic flux density.
Both the spider type and the ball-and-socket type of insulator are designed to have certain values of mechanical strength, flashover voltage, and leakage distance, rather than high values of corona or brush discharge voltage. Fortunately these discharge points are higher than the usual operating voltages, and the insulators are satisfactory from the point of view of radio interference.
Clevis-type insulators are also satisfactory when the cotter key is properly designed. One manufacturer has designed a clevis-type insulator in which the cap is recessed to overlap the cotter key and prevent it from turning. One of the large power companies is replacing the regular cotter key with a circular key, so designed that the ends are concealed inside the clevis bolt when in place. Comparative tests show that clevis-type insulators equipped with circular cotter keys are on a par with ball-and-socket insulators.
The link type of insulator is satisfactory if sufficient loading is applied to keep the porcelain and the links in intimate contact, and the voltage per unit does not exceed 14,000 volts. Much of the discussion pertaining to pin-type insulators is also applicable to link-type insulators. The problems involved are similar and can probably be solved by using similar methods.
Many lines using suspension insulators also use arcing horns to protect the insulator disks during flashover and to prevent burning of the conductor. Grading rings, shields, etc., also accomplish this purpose and change the potential distribution along the insulator string, so that the maximum voltage per unit is very much reduced. Tests show that the arcing horn is the only one of the above devices which ordinarily causes radio interference. Brush discharges take place at the ends of the horns, which produce an interference similar in sound to that of pin-type insulators. These discharges can be eliminated in the present design of arcing horn by adding a small metal ball to the end of the horn. The surface area is thus increased, and sharp points are avoided.
OTHER SOURCES OF INTERFERENCE
Pin-type and suspension insulators behave alike when subjected to moisture and dirt. The presence of either of these factors will usually increase the amount of interference, particularly on pin-type insulators. Tests in the laboratory show differences of 50 per cent or more in interference caused by insulators when dirty and the same insulators when cleaned. Moisture has a similar effect as shown by the curves of Fig. 1 where the noise level is three times as high for a line with insulators wet as it is for the same line when dry.
Defective, cracked, and broken insulators of either kind set up a disturbance which often affects radio receiving sets several miles away. Small projections on the surface of the porcelain sometimes create interference, especially when they are located in a heavy electrostatic field. Discharges frequently occur from the ends of tie-wires which are not bent closely enough to the conductor.
The remedy in each case is clear. Defective insulators must be replaced. Dirty insulators can be cleaned. Wet-weather conditions are sometimes minimized by overinsulation, and tie-wire ends should always be bent back as closely to the conductor as possible. Proper inspection and maintenance are therefore essential to the elimination of radio interference from high-voltage lines.
CURVES
The curves in Fig. 1 are intended to show the effect of attenuation on radio interference which is being propagated along a transmission line, to give an idea of the distances to which interference will travel before it is reduced to a non-interfering level, and to show the effect of overinsulation. The observations were made on a 55-kv. line, one mile of which is constructed with pin-type insulators, and the remainder, about 20 miles, with ball-and-socket type suspension insulators.
The origin of the curves is taken at the point where the two types of construction join, and the abscissas are measured from that point along the section using suspension insulators. The ordinates are measured by means of a milliameter coupled to the output circuit of a superheterodyne receiver through a transformer. Although the readings of this meter have no absolute value, their significance becomes apparent when it is known that signals from a 5000-watt radio broadcasting station 75 miles away could not be heard with noise levels of ten per cent or more. At ten per cent the signals were about equal in intensity to the interfering noise. At five per cent the signals were stronger than the interference. With a zero-reading on the meter the interference was not objectionable, although it could still be heard along with the signals from the broadcasting station.
The readings for the upper curve were taken during a rain-storm. The lower curve was taken about thirty minutes after the storm ceased. In the ease of the upper curve, a slight amount of interference could still be heard at a distance of four miles, which was attributed to the effect of rain on the suspension insulators.
The curves of Fig. 2 are similar to those of Fig. 1. These curves show the attenuation of radio interference at right-angles to a 55-kv. line for two conditions, (1) with no distribution circuits to radiate the disturbance, and (2) with distribution circuits paralleling the 55-lcv. line and connected to other circuits at right-angles to the 55-kv. line. The latter condition is one which occurs frequently in cities and towns, but no way of overcoming it has been devised yet. The most effective method of minimizing this kind of radio interference is the elimination of the interference at its source. In many cases, however, the cheapest remedy for the situation may be the use of radio-frequency choke coils inserted in the distribution circuits where they leave the high-voltage line. Standard lightning-arrester choke coils have been tried, but were not successful because their inductance is too low. One company is successfully preventing radio interference on high-tension lines from following its telephone circuits by inserting choke coils in the telephone leads at points where they leave the high-voltage lines. Another company is experimenting with carrier current choke coils, and still another one is trying specially constructed choke coils of about 0.5 millihenry inductance. No reports are available on these tests, however.
CONCLUSIONS
Radio interference is one of the problems which must be considered in future insulator designs. On pin-type insulators, corona and brush discharges can be eliminated by proper design of the petticoats, by using metal-coated or metal-capped heads, by using metallic threads in the pin-holes, and in some cases, by using a special glaze on the head and in the pin-hole. Suspension insulators can be improved by changing the design of the cotter key in the clevis type, by eliminating discharges between the links and the porcelain in the link type, and by redesigning all arcing horns to eliminate discharges at the ends. The corona-discharge point on cap-and-pin insulators can be raised by proper design of hardware, by elimination of sharp points, by insuring adequate clearances at the cap and the pin, and by making the shape of the porcelain conform more closely to the lines of electrostatic flux.
Existing pin-type insulators present the most difficult problem of all. Copper mesh placed under the tie-wires has proved fairly successful in eliminating discharges at the head of the insulator, but experiments are still being made to discover a process which can be easily applied, is not too expensive, and which will stand up under operating conditions during the life of the insulator.
Radio interference from line insulators will always be a problem, because corona and brush discharges occur so readily on high-voltage equipment. Much work has been done to minimize this type of disturbance, and more is contemplated. Adequate maintenance and good construction are essential to the solution of the problem, but the greatest needs are for improved designs and continued experimenting.
1. Radio Engineer, Pacific Power & Light Company, Walla Walla, Wash.
2. Communication Engineer, Puget Sound Power & Light Company, Seattle, Wash.
Presented at the Pacific Coast Convention of the A. I. E. E., Santa Monica, Calif., Sept. 3-6, 1929. Printed complete herein.
