Contamination flashover theory and insulator design

.1

Contamination Flashover Theory and Insulator Design

by DAVID

C. JOLLY

Department of Electrical Engineering Massachusetts Institute of Technology, Cambridge, The long, unpleasant

ABSTRACT:

followed

by a discussion

theories

are critically

there is at present improve

insulator

and physical examined future

history

of contamination

of some of the physical examined,

flashover

theory,

design over the years by a judicious

insight.

Some

existing

designs

theoretical

research in areas where more complete

I. Nature

processes

flu-shover is reviewed

involved.

and recent experimental

no comprehensive

in the light of present

Massachusetts

results

are presented.

it has nevertheless combination

knowledge

Although

been possible

of intuition,

as well a.s some proposed understanding,

briefly,

The numerouaJEashover

mod@cations

and suggestions

to

experimelat are

are made for

might lead to better insulation.

of the Problem

The steady growth in the demand for electric power requires the construction of transmission lines of ever increasing capacity. This increased capacity is most economically obtained by going to higher transmission voltages. Overhead lines with a phase to phase voltage of 765 kV are now in service, and higher voltages are contemplated. Underground cables can also be used to transmit power, but they are not soon likely to compete economically with overhead lines for long distance transmission. The exposed nature of overhead lines makes their insulation a troublesome affair. The insulation must be able to resist the effects of rain, snow, ice, corrosion, airborne contaminants and gunfire, as well as lightning and switching overvoltages. To meet these requirements, the four major types of insulators shown in Fig. 1 have been developed. The first type historically was the pin insulator [Fig. l(a)], based on the earlier telegraph insulator. It consists of a porcelain shell structure with a groove on top for the conductor and a receptacle on the bottom for the mounting pin. As transmission voltages increased, nested porcelain shells of larger diameter were adopted, but eventually voltages were reached where size and cost made this type impractical. To meet the insulation needs for lines of 100 kV and above in the first decade of this century, the suspension insulator was developed [Fig. l(b)]. These insulators can be linked in series to give a string of whatever length is required for adequate insulation. The great advantage of the suspension type is its flexibility. Any number can be hooked in series for electrical strength, strings can be paralleled for mechanical strength, and various mounting arrangements such as vertical, horizontal or vee-string are possible.

483

David C. Jolly The suspension unit is the insulator most commonly used in this country on long distance lines. Another type in use up to 230 kV is the line post [Fig. l(c)]. This is simply a porcelain column with rain sheds which has sufficient mechanical and electrical strength for the intended application. Both vertical and horizontal mounting are possible. The long rod type [Fig. l(d)] is sometimes used for

(a)

Cc)

(b)

(d)

FIG. 1. The four major types of line insulators in use today. (a) Thirty-five kV pin type insulator designed around 1900 showing three nested porcelain shells. (b) Modern lo-in. diameter suspension unit typically stressed at about 10 kV per unit in service. (c) Modern 66 kV porcelain line post insulator. (d) Modern long rod insulator for use at 110 kV.

high voltage insulation, and seems to be particularly favored by the Germans. Although various mounting arrangements are possible, it is usually used to suspend the conductor below the tower cross arm, and like the suspension type can be linked in series for higher voltages. They are not popular in this country partly because they tend to drop the conductor when struck by gunfire. At the higher transmission voltages a problem known as contamination flashover becomes increasingly important relative to other insulation troubles. This type of flashover usually occurs after insulators have become coated with airborne particles containing conducting salts. if the insulator surface is then moistened, say by fog or dew, the surface becomes conducting. The power dissipation in the wet film causes dry bands to form, choking off the

484

Joumalof

The Franklin

Institute

Contamination Flashover Theory and Insulator Design current. The voltage stress is then concentrated at these narrow dry bands which often break down, causing visible scintillations on the insulator surface. If the contamination is severe enough, these scintillations can eventually bridge the insulator, triggering a power arc and interrupting service. There are many contamination sources, including sea salt, road salt, cement dust, fly ash, bird droppings, fertilizer and many types of industrial emissions. Occasionally trouble may arise from entirely unexpected sources as in one case of a 30 kV line passing through a Far East sugar plantation. The plantation had become infested with a certain type of bug which, after becoming bloated with sugar, would struggle into the air only to land on the nearest perch, which frequently turned out to be the insulators provided at convenient intervals. The insects would then doze off, and while dormant would secrete a sugary substance, eventually coating the insulator. During heavy tropical dews, the surface would turn into a sticky mass, and flashover would occur (1). The contamination flashover problem can literally be said to be as old as high voltage transmission itself. The designers of the first high voltage, long distance transmission line, the 100 mile Lauffen-Frankfort 30 kV line of 189 1, employed insulators specifically designed to prevent surface sparkover under wet conditions (2). These insulators, shown in Fig. 2, contained a

FIG. 2. Anti-contamination insulator used on the first high voltage transmission line in 1891. The insulator is sectioned through the porcelain to show the location of the annular oil reservoir which prevented current flow between the line on top and the iron support pin. There was thus no current available to maintain dangerous surface discharges, and flashover could not occur. The insulator shown was used on two-thirds of the line, while the remaining 56 km was insulated with a more complicated insulator having three oil baths in series.

Vol. 294, No. 6, December 1972

485

David C. Jolly reservoir of oil to interrupt the flow of surface current. This oil bath concept has been revived from time to time since then, usually at intervals of about one generation. Interestingly, the major insulation problem on this pioneer line turned out to be small boys tossing stones and shattering the fragile bowl-shaped insulators. Camouflaged insulators were suggested to eliminate this trouble. In the United States early troubles were experienced on the Pacific coa,st. Insulators which had performed successfully in clear air mountain districts at 60 kV frequently failed at only 11 kV along the coast (3). The only remedy was to wash the insulators by hand as often as twice yearly. The engineer involved prophesied that washing would also have to be employed on future lines in that area and, in fact, 60 years later washing is indeed being used in the very same area. As transmission voltages have increased the contamination problem has become worse, despite intensive research on the problem. In the winter of 1962-63, for example, the British National Grid suffered the most serious dislocation in its history. An abnormally dry autumn had allowed airborne deposits to accumulate on insulator surfaces. For several nights in succession during January there were freezing fogs, and hoarfrost formed on insulator surfaces. On the evening of January 25, 1963, a thaw set in and the frost melted, forming a conducting film on the insulator surface. There were 130 flashovers and the normally interconnected grid was split into four separate sections. Four thousand MW of load was shed out of a total demand of 23,000 MW, a pre-arranged load shedding schedule and unified control preventing a more serious disaster (4). The largest power failure in the United States during the critical summer months of 1970 was a load loss of 1200 MW resulting from an unsuccessful contamination washing test on an insulator at the Sylmar, California converter station of the 750 kV d.c. Pacific Intertie Line (5). Incidents such as these must be avoided in view of society’s increasing dependence on electric power. Reliable long distance transmission is particularly vital to areas suffering local power shortages due to rapidly increasing demand, plant siting difficulties and other problems. The first really systematic study of contamination flashover was undertaken by Anfossi in 1907 (6). In his study, which was triggered by flashovers on a 25 kV Italian coastal line, he showed that a salt layer on the undersurface of the insulator caused a substantial insulation reduction, and recommended a more open insulator design which would allow rainfall to wash off the deposits. He also showed by comparing insulators on energized and unenergized lines that electrostatic attraction of particles played no role in the contamination of these particular insulators. Since the time of Anfossi, the problem has been tackled by many different investigators using a variety of techniques. One investigative technique is to simply observe the behavior of actual lines. A more convenient technique is to establish an outdoor test site where a large number of energized strings can be conveniently monitored. To obtain more controlled conditions, many studies have been carried out in laboratories with deliberately contaminated

486

Journalof The FranklinInstitute

Contamination Flashover Theory and Insulator Design insulators. People have gone to great lengths to develop methods for applying a realistic contamination layer. Some of the more original contamination methods include having a switching locomotive draw alongside strings to coat them with soot (7), hanging strings over burning garden refuse (8) and suspending strings from perches in the vulture cage of a zoo (9). Often, however, realism is entirely sacrificed in order to obtain repeatable results, as in the German methyl-cellulose coating method (10). Another approach is to consider the problem from a more theoretical standpoint in order to develop some sort of understanding of the physical processes leading to flashover. Despite the great difficulties involved in any theoretical formulation of flashover, it is likely that a thorough theoretical understanding is necessary to improve insulation design and understand operating experience. The next section will consider some of the theoretical aspects, and later some of the ways of applying theoretical knowledge to insulator design will be discussed. II.

Physical

Basis

of Flashover

Although contamination flashover has been studied for over 60 years, the physical processes by which a discharge is able to bridge a moist conducting film are not yet understood. Part of this lack of understanding results from the emphasis most researchers have placed on the more practical aspects of the problem. There has always been a pressing need for better insulators, and the approach traditionally taken has been to test insulators of various types under natural and artificial conditions in order to select the best types. Accordingly, a great deal of the contamination literature deals with the development of test methods and the comparison of different designs. The urgent need for better insulators has to some extent justified this approach, although it has led to a neglect of some theoretical aspects. A more fundamental reason for our lack of understanding lies in the complexity of the flashover process itself. The manner by which a conducting film can reduce the insulation strength of a typical suspension unit from 80 kV rms dry to less than 6 kV is not even understood qualitatively, let alone in a quantitative manner suitable for improving insulator design. In a way this is not too surprising since some aspects of the breakdown of ordinary air gaps cannot yet be explained qualitatively (ll), and contamination flashover is even more complex than air breakdown because of the field distortion produced by the film, the introduction of water vapor and ions into the discharge from the film, and changes of resistance of the film due to heating and evaporation. To calculate the flashover voltage we must write the ionization and diffusion equations for the discharge, and the heat and mass transfer equations to describe heating and evaporation of the film. The resulting set of coupled nonlinear partial differential equations, even if we could write them, could not be solved since some of the quantities needed, such as the amount of water vapor injected into the discharge, are not yet known.

Vol. 294, No. 6, December 1972

487

David C. Jolly If we cannot fully explain the breakdown of ordinary air gaps, is there any point in even trying to theoretically understand contamination flashover Z The answer appears to be yes, since theories have already been constructed which give fair agreement with experiment under some conditions. In addition, some experimental work has been done in recent years which, by adjusting conditions so that certain of the complicating factors play only a subordinate role, has already given us a good deal of qualitative insight into the flashover process. The first quantitative theory of contamination flashover was proposed in 1958 by Dr. Obenaus who outlined the steps which would be required to calculate the flashover voltage (12). The actual computation was carried out a short time later by Dr. Neumarker who derived an expression relating flashover voltage to surface resistivity (13). The theory models the flashover process as a discharge in series with a resistance, the discharge representing the partial flashover of the insulator surface and the resistance representing the unbridged portion of the insulator. Figure 3 shows the modeling concept. Obenaus assumes that flashover will occur if the discharge is able to bridge the insulator without extinguishing. PARTIAL

/

bDRY

DISCHARGE

BAND (al

Insulator

RESISTANCE

(b)

Obenous’

Model

FIG. 3. The modeling concept of Obenaus which treats the breakdown process as an arc in series with a variable resistance, the resistance decreasing as the arc lengthens (see text).

In the current range of interest, about 20-1000 mA, the discharge should have a falling voltage characteristic. This corresponds to the curve labeled V,, in Fig. 4. The voltage drop across the resistance, l&, is a linear function of current. Since these two elements are in series, the characteristic seen at the insulator terminals is obtained by adding voltages, giving the solid line of Fig. 4. It can be seen that no current solutions exist below the extinction voltage, J?&. In practice, the discharge characteristic approaches the origin at very low currents, and there is always a small but negligible current flow

488

Journal

of The Franklin

Institute

Contamination Flaahover Theory and Insulator Design at voltages below I&__.At voltages above Qt the operating point will be on the positively sloping portion of the curve since the negative resistance region is unstable. Thus, at voltage V,, as shown in Fig. 4, a current 1, will flow. The characteristics shown would apply only for a single value of the discharge length. As the discharge lengthens, its voltage drop for a given current will increase, while the resistance in series will decrease.

CURRENT

FIG.

4. Voltage current characteristic of a low current discharge in series with a resistance as employed in the extinction theory of Obenaus (see text).

The role of the series resistance in preventing flashover is clear. If the resistance is high the discharge current will be choked off, causing a high voltage drop across the discharge. If this voltage drop is high enough, the discharge will be unable to bridge the insulator, and flashover cannot occur. The actual calculations for the long rod case were first done by Neumarker, but his graphical method, although correct, is difficult to use. A more convenient algebraic derivation for the long rod case is given by Alston and Zoledziowski (14). It is important to realize that Obenaus’ extinction criterion is a necessary but not a sufficient criterion for flashover. The criterion specifies the minimum voltage below which flashover is impossible due to discharge extinction, but does not say at what voltage flashover will occur. Neumarker has clearly stated that not only must the applied voltage be above the extinction voltage, but that the physical conditions necessary for discharge motion across the wet film must also be present. In practice, Obenaus and his colleagues have assumed that these physical conditions are always present whenever his critical voltage is reached, and therefore that his extinction criterion can be used to predict flashover. Several alternatives to the extinction model of Obenaus have been proposed. One approach is that of Hampton who based his theory on an experiment in which he used a water jet to simulate a contaminated long rod insulator (15). He concluded that flashover could be treated as a stability

Vol. 294, No. 8, December 1972

489

David C. Jolly problem, stating that an unstable situation exists if a current increase occurs when the discharge root is displaced in the direction of flashover. He reasoned from this that if the voltage gradient along the discharge ever falls below the gradient along the resistive column, then flashover will occur. He was able to obtain good agreement between this model and his own results on the water column. Hampton’s criterion may have been anticipated by Shkuropat (16). Hesketh was later able to prove mathematically that Hampton’s two criteria of voltage gradient and current increase are identical only in the case of a long rod insulator (17). Wilkins has proposed that flashover will occur if the current drawn from the supply increases for an incremental forward displacement of the discharge tip, and suggests that this may be related to a more general criterion that the power drawn from the supply tends toward a maximum (l&19). Thus the discharge will elongate if it knows that by doing so, it will draw more power from the supply circuit. The physical basis of this criterion is not clear. It bears some resemblance to the Steenbeck minimum principle which states that a static arc assumes a configuration whereby it dissipates a minimum amount of power (20). However, Wilkins himself states this principle has not been extended to dynamic situations, and may in fact not apply to a moving discharge. There are other objections to this viewpoint. For example, it can be demonstrated theoretically that for a concentric electrode flat plate insulator the series resistance seen by the discharge will be almost constant until the discharge approaches within a few root diameters of the outer electrode.* This implies that as the discharge begins to grow outward from the center electrode, the current will decrease. Therefore according to the &/ax> 0 criterion flashover should not occur, although in fact it does occur. Even more damaging to the hypothesis of Wilkins are the experimental observations of Zoledziowski showing both current and power decreasing during discharge elongation under certain conditions (21,22). The theories just discussed avoid consideration of the actual physical processes occurring during discharge propagation. Several possible physical mechanisms have already been suggested by various investigators, one of the most widely accepted mechanisms being that external forces simply pull the discharge across the surface. Several forces have been suggested, including electrostatic attraction (23), electromagnetic forces (24), thermal buoyant forces (24) and steam pressure (24). Order of magnitude calculations have shown that the electrostatic and buoyant forces are of similar magnitude, and that electromagnetic forces are negligible by comparison (25). It has been observed experimentally that both buoyant (26) and electrostatic (23) forces can affect the motion of discharges moving at low velocities on the order of centimeters per second. Tominaga has proposed that discharge movement is caused by the intense drying at the discharge root (27). This does not account for the flashover of * A. J. McElroy, private communication.

490

Journal of The Franklin

Institute

Contamination Flashover Theory and Insulator Design continuous water jets, but may apply in some cases where the contamination layer is a thin film. Another suggestion is that the nonuniform heating at the discharge root produces a surface tension gradient which induces a convective motion of the liquid surface, pulling the discharge root forward in much the same manner as flame propagates over a liquid fuel spill. * The driving mechanisms mentioned above all produce elongation velocities of tens of meters per second at the most. This is in agreement with velocity data taken by Nasser which showed propagation velocities up to about 40 meters/set (28). However, far higher velocities have also been observed. Hesketh has observed velocities up to 600 meters/set on a water jet (29), Obenaus has reported average velocities up to 4200 meters/set on actual insulators hooked directly to a 220 kV line (30) and Japanese workers have observed average velocities of over 50 km/set for discharges propagating over the surface of a tank of potassium chloride solution (31). The only way to account for these high velocities is to assume that contamination flashover is basically an electric breakdown process, modified by the field distorting effects of the wet layer. The current flow lines in the contaminant layer will converge at the discharge root, producing a high local electric field. If this local field is high enough, the air in front of the discharge tip will be ionized, extending the discharge. Ionization by electron impact may be aided by thermal and photoelectric ionization processes. An interesting model of this process has been devised by Wilkins who considers the electrical stability of a short discharge trying to form ahead of the main discharge tip (18).Good agreement has been obtained with flashover experiments on a water channel, and hopefully the model can be extended to cover other geometries. A clue to the flashover mechanism may be provided by the observed polarity dependence of flashover voltage. Wilkins has reported that channels containing a solution of CuSO, flashover more readily when the electrode from which the discharge leaves is negative (18). Japanese workers find just the opposite polarity effect for KC1 (31). Preliminary tests at M.I.T. have shown that the favored polarity may depend on the experimental geometry, the nature of the dissolved salt and the conductance of the solution. These effects are not easily explained by any simple theory, and a great deal of work still needs to be done. The polarity also affects the appearance of the discharge root. High speed motion pictures taken at M.I.T. have shown that when the wet film acts as the cathode, the discharge root has a pronounced branched structure. When the root acts as the anode, the root appears more diffuse. (See Figs. 5 and 6.) This is reminiscent of the pronounced branching noticed in positive streamers and positive Lichtenberg figures, and is probably due to similar causes, i.e. electron diffusion (32). The branching of positive discharges propagating over water surfaces has also been reported by Boylett and Maclean (33). * D. Q. Anderson, private communication.

Vol. 294, No. 6, December 1972

491

David C. Jolly The cathode processes at the wet film still remains something of a mystery. Wilkins has measured an electrode fall of 950 V, presumably mostly at the cathode (18).This voltage is somewhat higher than is typical for glow discharges in air, but the higher voltage may be related to the reluctance with which water is thought to emit electrons (34). Measuring the secondary and photo-emission properties of water is incidentally not easy since such measurements usually require a good vacuum at the surface. A more detailed understanding of the cathode mechanism might explain some of the observed polarity effects. In summary, based on the evidence currently available, the flashover process may be described as follows. When the voltage across the dry band is sufficient, a spark occurs, bridging only the dry region. This spark then undergoes a transition to an arc-like discharge, the current of which is determined by the applied voltage, the series resistance seen by the arc and the volt-ampere characteristic of the arc itself. If this current is great enough, the field intensification produced by the convergent current flow at the discharge root on the wet film will lead to the creation of new ionizaby electron impact, phototion, extending the discharge. Ionization ionization, thermal ionization may all be involved, their relative importance being determined by current, polarity and other variables. The slow velocities of tens of meters per second which are frequently observed may be attributable to resistive heating of the water film. For voltages near the critical flashover voltage the discharge can reach the critical current necessary for propagation only if the series resistance seen by the discharge is reduced through resistive heating of the water film near the discharge root. The speed of elongation is thus limited by the effective thermal time constant of the wet film-insulator surface system. At these slower velocities, buoyant and electrostatic forces become significant, and cannot be neglected in any realistic theory. Although the above description is probably valid in a general sort of way, we are still not in a position to even qualitatively explain polarity effects, to say nothing of being able to accurately predict insulator behavior in advance. Nevertheless, it has proven possible to draw certain general conclusions which have allowed insulator design to be improved over the years. Some of the ways of improving insulator performance will be discussed in the next section.

ZZZ. Methods

of Improving

Insulator

Design

There are so many interesting and unusual physical aspects of contamination flashover that the investigator may occasionally forget that the main goal of his research is to improve insulator performance. It may thus be appropriate at this point to discuss some concrete ways that insulator performance could be improved, particularly in light of recent theoretical advances.

492

Journal of The Franklin

Institute

Contamination Flashover Theory and Insulator Design For purposes of discussion the flashover process can be divided into distinct stages : (1) deposition of conducting salts and moisture, (2) formation of dry band(s), (3) electrical breakdown of dry band(s), (4) propagation of the discharge across the moist film, bridging insulator. Each one of these events is a necessary stage in the development of flashover, providing us with four separate points of attack, since if any of these stages can be prevented, flashover will not occur.

four

the full one

Stage One The deposition of contaminant has turned out to be the most practical point of attack, and most successful designs involve inhibiting the formation of a moist, conducting layer. This can be accomplished in several ways. Insulators can be designed with convolutions on their undersurface to provide an area sheltered from atmospheric dirt and moisture. Alternatively it is possible to design insulators so that the greatest part of the creepage path is on the exterior of the insulator where it is exposed to the washing action of rain. Both of these types are currently marketed and when used properly can give good service. The protected creepage types tend to be favored in coastal regions where protection from salt spray is important, while external creepage types are more suited to those industrial districts where the deposits take longer to reach a dangerous level than the interval between rainfalls. Another approach is to coat the insulator with a slightly conducting glaze. The current flow through this glaze warms the insulator surface several degrees above ambient, a sufficient rise to prevent deposition of dew or fog under most conditions. Several uncertainties at present prevent widespread adoption of resistance glazed insulators. Over long periods of time the glaze may undergo corrosion or deterioration, although recent progress in ceramic technology has led to vastly improved glazes, and life tests now in progress have been promising (35). Certain problems result from the negative temperature coefficient of resistance of these glazes. As current heats the glaze, the resistance decrease leads to greater current flow, producing further heating. Under unfavorable conditions this can lead to thermal runaway, eventually fracturing the insulator. The problem is most acute on suspension types where the current concentration near the pin can lead to local thermal runaway. Insulators with a small diameter variation such as the post type are ideally suited to uniform resistive glazing, and have performed well in field tests. An interesting side effect of resistance glazing is that energized resistance glazed insulators have been found to accumulate cement dust much more slowly than adjacent ordinary insulators during field trials, apparently due to surface warmth (36). Another method of attack would be to make the insulator surface water repellant so that water would bead up and not form a continuous film. It

Vol. 294, No. 6, December

1972

493

David C. Jolly is difficult to make a surface which will retain its water repellant properties over long periods of service, but progress in materials technology may eventually make such a surface possible. Stage Two The formation of dry bands is essential to the flashover process since it produces the field intensification necessary to initiate the flashover discharge. Unfortunately a heavily contaminated suspension unit completely covered with a wet film may dissipate up to about 10 kW. At this power level dry bands will form in short order no matter how the contaminant is distributed. It appears that complete prevention of dry banding is impossible, although by means of appropriate design, dry bands might be encouraged to develop in a favorable manner. For example, proponents of post and long rod insulators say that the small diameter variation of these types leads to more efficient evaporation of water than occurs on suspension units which typically have a ten to one diameter variation. Whether this occurs in practice is debatable, more research on dry banding being required. Stage Three The breakdown of the dry band has received little attention, but if this breakdown could be prevented the arc which propagates across the insulator would never form. In some preliminary work by the author it has been found that dry band breakdown can actually be prevented under certain conditions (37). On ordinary smooth glazed insulator surfaces, sparking across the dry band is often triggered by the motion of a fine filament of water which is pulled from one edge of the dry band by electrostatic forces and creeps slowly across the dry gap toward the other edge. This eventually shortens the striking distance enough so that the dry gap sparks over, triggering an arc which may then propagate over the wet film and cause flashover. If however a porous coating such as powdered clay is applied over the glaze, these events can no longer occur since the capillary forces tend to impede the motion of the filament. This gives time for corona to evaporate any narrow points of the dry band, and under proper conditions a dry band can be maintained which is wide enough to withstand the entire applied voltage. Standard suspension insulators with such a coating have withstood several hours of severe salt fog and wind in the author’s laboratory without as much as scintillating once, whereas an ordinary unit would have flashed over in about 10 min. If such a coating could be developed for service conditions, insulators might be made more resistant to flashover, or in the case of synthetic insulators, the suppression of discharge activity might lead to longer service life. Stage Four The propagation of the arc over the surface has not been adequately attacked yet, primarily because so little is known about the physics of this process. Qualitative photographic studies of insulators flashing over have

494

Journal of The Franklin

Institute

Contamination Flashover Theory and Insulator Design revealed certain effects which can affect performance. For example, thermal buoyant forces may cause arcs to drift upward from one insulator to another, shorting out segments of insulation (26). To avoid this, sufficient vertical clearance between insulators must be provided. Some designs try to take advantage of the large electrode fall associated with discharges on water surfaces. Discharges on post and rod type insulators tend to take the form of many short discharges in series. Since each short discharge has an electrode drop of about 1000 volts, the voltage “left over” for discharge propagation is greatly reduced. Rumeli has questioned the effectiveness of breaking the arc into many short segments, and more research is needed to reach any definite conclusions (38). Another possible way to hinder discharge propagation is by controlling the field on the insulator surface by means of metal inserts or auxiliary electrodes. Rumeli has shown experimentally that this technique is effective for some simple geometries, but more work is needed before it can be applied to actual insulators (38). Development of discharge resistant plastics would provide greater flexibility in hindering arc propagation than exists with porcelain or glass. The greater strength of a plastic-fiberglass insulator would make possible narrower bodies a,nd more effective sheds on rod type insulators. The favorable effects of such modifications have already been demonstrated in the laboratory (39). In order to illustrate the large amount of original thinking which has been devoted to this problem it might be worthwhile at this point to mention several novel ideas which, for various reasons, have not yet been applied in practice. One inventor has suggested an insulator consisting of a dielectric rod with metal caps at the ends (40). Enclosing the insulating rod would be a sealed cylindrical bellows made of rubber or some other insulating material as shown in Fig. 7. Variations in the temperature of the atmosphere would cause the bellows to expand and contract. This breathing action would allegedly cause any deposit buildup on the exterior of the bellows to crackle off and drop to the ground, but building a durable bellows would not be easy. Some years ago a British investigator patented an insulator which literally cleaned itself by means of small wind-driven brushes which swept away deposits before they could accumulate (7). The idea never caught on though, and the patents were allowed to lapse. Another idea is an insulator constructed of a hollow pipe of some porous dielectric material with metal caps on the ends and the hollow core filled with oil. The oil would ooze outward through the pores to the surface where it would slowly engulf and carry away contamination particles. Tension strength would be provided by a dielectric rod running through the center of the porous pipe (41). Several highly original schemes have been suggested for increasing the creepage path. One of the most unusual is the suggestion that insulators be made in the shape of a helix, similar to a giant bedspring as shown in Fig. 8 (42). This would provide a very large creepage distance to total

Vol.294,No. 6,December1972

495

David C. Jolly

FIG. 7. Concept of bellows protected insulator. The central dielectric rod is protected from the elements by a flexible bellows which expands and contracts in response to atmospheric temperature variations, causing deposits to crackle off and fall to the ground.

FIG. 8. “Bedspring” insulator concept. The helical shape gives this insulator an exceptionally long creepage path relative to its overall height. This increases the series resistance seen by the arc (see Fig. 3) and also, since the flashover discharge tends to follow the surface, the total length of the arc must be very long to cause flashover, enhancing the probability of extinction before it bridges the insulator. The principal difficulty is finding a suitable insulating material.

496

Journal of The Franklin Institute

.

Contamination Flashover Theory and Insulator Design

I

iength ratio. Finding a suitable insulating material which is durable and at the same time stiff enough to support the vertical and transverse stresses acting at the conductor tie point is no trivial task. It may also be necessary to provide dampers to prevent large amplitude, low frequency oscillations of the conductor. Hopefully advances in the theory of insulator behavior will permit accurate evaluation of designs such as those mentioned above, and suggest other modifications likely to improve performance. Some of the research areas most likely to lead to useful results will be outlined in the next section.

IV.

Suggestions

for

Future

Research

There are still many aspects of contamination flashover which are not even understood qualitatively, let alone on a quantitative basis suitable for improvements in design or application of insulators. Summarized below are a few areas where the author feels further work is needed if we hope to be able to optimize insulator design, predict required insulation levels and interpret service experience. As discussed earlier, one of the most puzzling aspects of the problem is the mechanism by which the discharge moves across the surface. The ionization processes, the cathode mechanism and the effects of buoyant and electrostatic forces acting on the arc all need clarification. In addition the effects of water vapor and contaminant ions on the discharge have not been fully studied. More accurate modeling of the heating and drying effects during discharge activity is required. In theory a model of discharge propagation could be combined with a heating and drying model on a computer to predict the performance of actual insulators. There has been almost no study on the effects of chemical reactions which may occur during discharge activity. For example, nitrogen oxides produced during long periods of scintillation may react with the water film to produce a highly conducting nitric acid layer on the insulator surface which may trigger flashover even though the insulator was not dangerously contaminated to begin with. Chemical analysis of the insulator at some later date would only reveal the original contaminant, and the flashover would be termed “mysterious”. Reactions of this type may explain why it is sometimes difficult to flash over insulators removed from service in a fog chamber at the same voltage that they had flashed over in service (43). How insulators get contaminated in the first place is still poorly understood. The importance of particle charging and electrostatic attraction is still obscure, and the cleaning action of rain and wind is only roughly understood. Work needs to continue on conductive glazes to assure their durability in service, hopefully providing a partial solution to the contamination problem.

Vol.

294. No.

6, December

1972

497

David C. Jolly The development of durable synthetic materials able to resist the effect’s’ of discharge activity would make possible strong, lightweight contaminationresistant line insulation. The erosion mechanism and the behavior of water films on contaminated plastic surfaces are not well understood. The effect of beading of the film on flashover performance has not been quantitatively explained since von Cron 20 years ago observed that flashover can be triggered when electrostatic forces pull the individual water beads into a continuous filament bridging the insulator (44). A theoretical study of this effect might also be useful in understanding the behavior of greased insulators. The effects of nonuniform evaporation of water due to the capacitive nonlinearity of voltage distribution along long strips can affect performance. This effect, first predicted by Boehne using a computer program (45,46) and later verified experimentally by Kawai at Project UHV (47), causes insulation at higher voltages to perform worse than a linear extrapolation of lower voltage experience would indicate. The implications of these findings need to be carefully studied since these higher voltage lines carry large blocks of power, and must not be subject to unexpected outages. Studies at laboratories in Great Britain, Germany, Japan, France, Italy, the Soviet Union, the United States and many other countries have in the past few years led to a great increase in our understanding of this vexing problem, and hopefully the years ahead will see these results tra’nslated into a workable solution to the contamination problem.

References (1) P. Ryle, “Two transmission line problems-suspension insulators for industrial areas in Great Britain; conductor vibration”, J. IEE, Vol. 69, pp. 805-849, 1931. (2) “Oil insulators of H. Schomburg & Sons”, ETZ, Vol. 12, pp. 691-692, 1891. (3) P. Downing, “The developed high tension network of a general power system”, AIEE Trans., Vol. 29, pp. 7055719, 1910. (4) J. Forrest, “The performance of high -voltage insulators in polluted atmospheres”, Paper 69 CP ‘I-PWR, presented at the IEEE Winter Power Mtg., 1969. p. 12. Dec. 1970, (5) “Summer power failures, reductions”, Power Engineering, (6) G. Anfossi, “Behavior of insulators in the vicinity of the sea”, Atti dellu Assoc. Electrotecn. Ital., Vol. 11, pp. 326-334, 1907. (7) W. Lloyd, “Insulator sparkover”, AIEE Tram., Vol. 51, pp. 669-676, 1932. (8) 5. Forrest, “The electrical characteristics of 132-kV line insulation under various weather conditions”, J. IEE, Vol. 79, pp. 401-423, 1936, and Vol. 80, pp. 667668, 1937. (9) A. Maikopar, “The open electrical arc of very small current”, Elektrichestvo, No. 2, pp. 22-25, 1965. (10) G. Issel and H. BBhme, “Testing insulators with cellulose contamination layers”, Elektrie, Vol. 20, pp. 13-16, 1966. (11) E. Nasser, “Development of spark in air from a negative point”, J. AppZ. Ph~s., Vol. 42, pp. 2839-2847, 1971. (12) F. Obenaus, “Contamination flashover and creepage path length”, Dtd. Elektrotechnik, Vol. 12, pp. 135-136, 1958. (13) G. Neumarker, “Contamination state and creepage path”, Deutsche Akad., Berlin, Vol. 1, pp. 352-359, 1969.

498

Journal

of The Franklin

Institute

Contamination (i4) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25)

(26)

(27) (28) (29) (30)

(31) (32) (33) (34) (35)

(36)

(37)

Flashover Theory and Insulator Design

L. Alston and S. Zoledziowski, “Growth of discharges on polluted insulation”, Proc. IEE, Vol. 110, pp. 1260-1266, 1963. B. Hampton, “Flashover mechanism of polluted insulation”, Proc. IEE, Vol. 111, pp. 985-990, 1964. P. Shkuropat, “Development of a discharge on a wet insulator surface with d.c.” N-Tekh. I$. ByuZZ. Leningrad Polytechnic Inst., Vol. 1, pp. 41-51, 1957. S. Hesketh, “General criterion for the prediction of pollution flashover”, Proc. IEE, Vol. 114, pp. 531-532, 1967. R. Wilkins, “Flashover voltage of high voltage insulators with uniform surface pollution films”, Proc. IEE, Vol. 116, pp. 457-465, 1969. R. Wilkins and A. Al-Baghdadi, “Arc propagation along an electrolyte surface”, Proc. IEE, Vol. 118, pp. 1886-1892, 1971. W. Finkelnburg and H. Maecker, “Encyclopedia of Physics”, Vol. XXII, pp. 254-444, Berlin, Springer-Verlag, 1956. “Time to flashover characteristics of polluted insulation”, S. Zoledziowski, IEEE Trans. Power App. Systems, Vol. PAS-87, pp. 1397-1404, June 1968. S. Zoledziowski, Discussion to Paper 72 TP 199-3, IEEE Trans. Power App. Systems, in press. F. Obenaus, “The influence of surface coating (dew, fog, salt, and dirt) on the flashover voltage of insulators”, Hescho-Mitt., Vol. 70, pp. l-37, 1933. W. Frischmann, “Contamination flashover and arc root motion”, Deutsche Elektrotechnik, Vol. 11, pp. 290-295, 1957. D. Jolly, “Contamination flashover-I. Theoretical aspects”, Paper 72 TP 199-3, presented at IEEE 1972 Winter Power Mtg., IEEE Trans. Power App. Systems, in press. R. Flugum and A. Karcic, “Effect of configuration on contaminated insulator string performance”, IEEE Trans. Power App. Systems, Vol. PAS-91, pp. 336344, 1972. A. Tominaga, “Characteristics of power-frequency flashover on contaminated surfaces in fog”, Ebc. Eng. Japan, Vol. 88, No. 12, pp. 53-59, 1968. E. Nasser, “The problem of contamination flashover on insulators”, ETZ-A, Vol. 83, pp. 356365, 1962. S. Hesketh, “The propagation of arcs over a water surface”, Proc. 8th International Conference on Phenomena in Ionized Cases, p. 255, Vienna, 1967. F. Obenaus and H. B&me, “Laboratory and system tests on contaminated shed insulators and the model concept of creepage flashover”, Elektrie, Vol. 10, pp. 417-422, 1966. H. Matsuo, Y. Yunoki, T. Oshige and N. Mita, “Impulse discharge on contaminated surface”, EZeo. Eng. Japan, Vol. 89, No. 9, pp. 26-34, 1969. E. Nasser, “Fundamentals of Gaseous Ionization and Plasma Electronics”, New York, Wiley-Intersoience, 1971. F. Boylett and I. Maclean, “The propagation of electric discharge across the surface of an electrolyte”, Proc. R. Sot. Lond. A, Vol. 324, pp. 469-489, 1971. L. Loeb, “Electrical Coronas, Their Basic Physical Mechanisms”, pp. 248-269, Berkeley, University of California Press, 1965. J. Moran and D. Powell, “Resistance graded insulators-the ultimate solution 1972 to the contamination problem”, Paper T72 202-5 presented at IEEE Winter Power Mtg., IEEE Trans. Power App. Systems, in press. D. Powell and T. Pinkham, “Conducting glaze-sustained insulator performance Seminar, in contaminated areas”, presented at Insulator Contamination IEEE Summer Power Mtg., Portland, Oregon, 1971. D. Jolly and H. Woodson, “Contamination flashover, Part III. Fog tests on Winter Power suspension insulators”, Paper C72 201-7, presented at IEEE xtg., 1972.

Vol. 294, No. 6, December

1972

David C. Jolly

, .

(38) A. Rumeli, “The mechanism of flashover of polluted insulation”, Ph.D. thesis, Univ. of Strathclyde, 1967. (39) K. Mathes, “Performance of simple insulator shapes under heavily contaminated conditions”, Paper 71 CP 239-PWR, presented at IEEE Winter Power Mtg.,

1971. (40) U.S. Patent (41) (42) (43) (44) (45) (46)

(47)

500

3,315,026, “Insulator covered with a protective envelope”, Filed Nov. 25, 1964. U.S. Patent 3,324,223, “Self-cleaning high tension insulator”, Filed Sept. 20, 1965. U.S. Patent 3,296,366, “Outdoor high tension insulator having long creepage path”, Filed Dec. 23, 1965. A. McElroy, “Flashover mechanisms of insulator with contaminated surfaces”, Ph.D. thesis, Dept. of Elec. Eng., M.I.T., Cambridge, Mass., 1969. W. Estorff and H. von Cron, “The high voltage insulator as a contamination problem”, ETZ, Vol. 73, pp. 57-61, 1952. E. Boehne, “Contamination of EHV insulation-I. An analytical study”, Paper 31 PP 66-481, presented at IEEE Summer Power Mtg., 1966. E. Boehne and G. Weiner, “Contamination of EHV insulators-II. Power losses and their distribution”, Paper 31 PP 67-153, presented at IEEE Winter Power Mtg., 1967. M. Kawai, “Flashover tests at project UHV on saIt contaminated insulators-II”, IEEE Trans. Power App. Syetems, PAS-89, pp. 1791-1799, 1970.

Journal of The Franklin Institute