The digital computer in contamination research

The Digital Computer in Contamination Research bJJ E. W.

BOEHNE

Consultant, ITE Imperial Corporation I goo Hamilton Street, Philadelphia, Pennsyluania An introduction

ABSTRACT:

industrial

pollution

to the general problems

of the overhead insulation

attention is given to thefirst digital computer long,

contaminated

electrostatic

field parameters

voltage* insulation the electrical to control

insulator

strings

phase

distortions.

upon the testing and performance

is discussed.

The phase-angle

and thermal time constants

with the atmospheric is presented.

and

Special

study of the voltage and wattage distribution

revealing

distortion

of the column

and define the rate of application

column is energized,

associated

of high voltage systems

The

of contaminated

on

of the

extra high

of the voltage in the column and

are introduced.

and removal

i@uence

of the wetting

The possible agent,

need

while the

is suggested for testing procedures.

I. Zntroduction

High voltage transmission lines, which form the arteries of energy from generation to distribution, must be adequately insulated from the ground to overcome all types of atmospheric conditions throughout the year in order to provide a high quality electricity service. This insulation, like that of lower voltage lines, is usually provided by porcelain insulator strings, the conductors themselves being bare. The number of insulators in strings for 220 kV and below is, in general, dictated by lightning criteria. For this reason the insulation provided is usually quite adequate to withstand normal switching surges and to provide an insulation length which, even under contaminated conditions, gives a fairly satisfactory performance. Only in crucial areas of heavy marine or industrial pollution has it been necessary to increase the quality of insulation or to coat the porcelain surfaces with silicone greases. Above 230 kV, line insulation is strongly influenced by the magnitude of switching surges and/or the incidence of atmospheric contamination. Through research, including circuit breaker improvements, the magnitude of switching surges is being continually reduced. If it were not for the problems of contamination, considerable savings could be made by using insulator strings of shorter length. Above 800 kV, line insulation will be dictated essentially by contamination criteria alone. Indeed, if a solution is not found to control or overcome the contamination problem in the U.H.V. area, a voltage ceiling is possible, above which it would be impractical to try to provide overhead electric service. Underground transmission, a costly channel at best, would be the only alternative. * E.H.V., and above).

extra

high

voltage

(220-735

kV).

365

U.H.V.,

ultra

high

voltage

(765 kV

E. W. Boehne ZZ. Linearity World-wide studies of long insulator strings confirm that the insulation strength is not proportional to the string length. As additional insulators are added, a smaller and smaller increment of additional strength is provided. This fact was dramatically demonstrated in a long-hand solution (1) of the voltage distribution across an infinitely long string of uniformly contaminated insulators which had a uniform capacitance to ground (ground field) and no capacitance to the line conductor (shielding field). When a fixed 60-Hz voltage E,sinwt, was applied, the voltage distribution in the form of a phasor diagram, consisted of an equal angular spiral about the ground terminal of the applied voltage. This wonderful “sea-shell curve” is probably the most beautiful thing that will ever be found in the study of such a dirty subject. The voltage which appears across the first insulator of the infinitely long string, having a uniform ground field (a) and no shielding field (k) and a uniformly distributed contaminate (n), is given by

This long-hand solution (which is extremely simple) was confirmed both on an analog board using only 25 insulators and with a digital computer. All three solutions were characterized by indicating that “more” voltage appeared across the first 14 insulators than was applied to the entire column! This, of course, exhibits an extreme case of nonlinear voltage distribution. It reveals, however, the importance of the shielding field, i.e. the capacitance of the insulation column to the line conductor or to the bus in the case of some station insulations. As soon as a small shielding field is added the spiral nature of the phasor diagram disappears in favor of the phasor diagrams, as shown in Fig. 8 of the paper by M. Sforzini (see p. 446 of this issue). This figure (from his Ref. (15) which is identical to Ref. (1) of this paper) was programmed using the following assumptions : (1) The ground field (a) is assumed to have a “straight line variation” between 0.15 at the line terminal to 0.20 at the grounded terminal of the insulator string. This field is expressed on a dimensionless base, i.e. in terms of the unit self-capacitance of one unit of the column. (2) Similarly, the dimensionless shielding field (k) was chosen to vary “exponentially” from O-05 at the line terminal to 0.005 at the grounded terminal. (a) is also expressed dimensionlessly (3) The degree of contamination where q = (X,/R) and hence R = (X,/q) where XC is the capacitive ohms of a single insulator at the operating frequency and R, the resistance, in ohms, of the contaminant on a single insulator. A 53 in. high insulator having a lO$ in. diameter has a capacitive reactance of 80 million Sz at 60 Hz. Hence a (q) of 20 implies that the resistance of the contaminated surface for such an insulator is 80120 or 4 million Q.

366

Journal

of The Franklin

Institute

The Digital Computer in Contamination Research (4) The voltage distribution curves (5) (Fig. 8) across the 24 insulators are read as follows : Choosing, for example, the first 11 insulators in the string of 24 and a contamination level of q = 10, we find 70 per cent of the impressed voltage “across the first 11 insulators”. (5) The dotted arc (e,), at the line terminal of the string is the locus of the voltage which appears across the first insulator. It reveals that 26 per cent of the applied voltage appears across the first insulator in a clean, dry string (q = 0) under the fields imposed by (a) and (k) as shown. Here all the individual voltages are in phase, the circuit being completely capacitive. At the other extreme when the resistance dominates (q = 1000 or greater) the voltage across the first insulator drops to 4 per cent of the polluted column voltage. Here the voltages are again in phase as a resistance divider. It is to be noted that as the contamination increases the voltage distribution improves! (6) The voltage across the first insulator leads the applied voltage by a considerable angle, 30-40”, for a (q) in the range of 10-20. ZZZ. ConjSmations

The discovery of these phase relationships required some practical confirmation. In the writer’s own experience it was always puzzling to note, in the instrumented study of corona on long insulator strings, that corona bursts always appeared only on the rising or leading slope of the oscillogram or oscilloscope trace of the applied voltage to the string. The reason for this is now clear. Dr. Eugenio Brasca, then Director of the High Voltage Laboratory at CESI in Milano, upon learning of this phase displacement phenomenon on long insulator strings at a special lecture, said : “We have made thousands of voltage distribution measurements on insulator strings. After each test we sum the voltages of all the insulators in the string and invariably find that the sum exceeds the applied voltage. Then we proceeded to correct each value to bring the total to 100 per cent. And now”, he said enthusiastically, “we don’t have to do this anymore.” The digital computer model of the insulation column was tested on the beautiful hyperbolic solutions of Schwaiger (4), who never considered contamination but did give the complete voltage distribution of long insulator strings, considering both uniform ground and shielding fields. Allowing (q) to be zero (no contamination), the computer yielded curves in less than a second which were congruent with those of Schwaiger. Within the computer solutions, when the shielding field was made the mirror image of the ground field, the resulting voltage distributions were symmetrical about a point of rotation in the center of the column. This is as it should be. When the exact electrical dual of this problem was constructed it was noted that it represented the current distribution in a solid copper conductor in an iron slot of infinite depth. The current distribution pattern was known

Vol.

294,

No.

6, December

1972

367

E. W. Boehne to follow conductor. IV.

Insulator

an equal

Fields

angular

vs. Insulator

spiral

as one

progresses

down

the

solid

Testing

It is the capacitance to ground, including the capacitance to adjacent grounded objects (which defines what is termed the ground field), that disturbs and distorts the voltage distribution along the column. The so-called window insulator on a three-phase flat spacing, namely the center conductor which passes through the “window” of the transmission line tower, would have a larger disturbing ground field than its two “yard-arm” neighbors. When coated with a thin layer of foreign material which can become conducting when moistened with high humidity, dew or fog, we immediately produce a leakage current which, in turn, produces heat. The wattage loss along the column is not uniform and, in general, varies as the square of the voltage distribution. This permits unequal heating which allows the hotter regions to dry out first. This action causes the voltage and wattage distribution to become more non-uniform, leading first to sub-arcing of individual dry units which can lead to the cascading of arcs to cause insulator flashover. From the above it is to be remembered that it is the initial disturbing ground field which gives rise to the trouble. It follows, therefore, that contamination tests in the laboratory or field should specify and duplicate the representative ground fields to be found in service in order to reproduce the rates of differential drying along the surfaces of the insulator string as found in service. The reporting of tests, in view of possible duplication in other laboratories, should specify the magnitude and distribution of the ground field. In a similar manner, the corrective shielding field should be monitored, duplicated and reported with test data. Without any shielding field, as might be approximated in test by energizing the insulator column with a small wire, would permit a distorted voltage and wattage distribution as compared with the voltage and wattage distribution found when the column is energized through a bundle conductor which the insulator usually supports in service. In both cases, an effort should be made to create in the laboratory an insulator topology which simulates as closely as possible the topology found in service. In the author’s opinion, the contamination performance of long insulator strings becomes a delicate balance between the “rate” at which moisture collects on the surface and the “rate” at which each insulator in the string heats to establish the “rate” of drying. Electrically these factors control the time rate of change of conductivity for each unit in a dynamic manner. The two major fields play an important role in determining the differential rates of drying, or wetting, along the column. All of the above is influenced by the natural thermal time constants of the insulators which in turn should dictate the minimum overall time for the test. Experience teaches that the

368

Journal of The

Franklin

Institrlte

The Digital Computer in Contamination Research presence of a heavy contamination layer is not necessary in most cases. Fog alone on a relatively clean insulator can exhibit trouble. The paper by Berger (see p. 385 of this issue) adumbrates such examples. It follows that the most severe and most realistic conditions obtain when the insulator string under test is thermally stabilized by the application of voltage, well in advance of the introduction of moisture, for a period at least twice the thermal time constant of the insulator. In a similar manner, the voltage should remain on the unflashed insulator after the moisture application has been discontinued. Again the thermal time constant should determine the duration of this overlap. There is little doubt that the contamination problems should be divided into separate classes each of which would require separate treatments. Moisture, however, is common to all. To achieve only one valid, reproducible test to simulate all classes of known contaminates from agricultural sprays to desert dust would indeed be a remarkable achievement. In the author’s opinion, it would be improbable. V. The “Weather

Report”

In the spectrum of atmospheric possibilities from clean, dry air to a heavy downpour there is only a very narrow band which gives rise to our pollution problems. The dirty, smudged, salty or dusty insulator when dry presents no problem. It is principally at very high humidities or when the rate of moisture precipitation or condensation is sufficient to moisten the surface deposit causing it to become conducting that the contamination phenomena begin. That part of the moisture spectrum beginning with an insulator column which is barely dripping to a heavy downpour is again, in general, of little concern. This is true because the rate of moisture application does not permit the formation of dry zones. In the broad moisture spectrum which includes high humidity (above 90 per cent), fog, dew, mist, rain, snow, ice, sleet and hoar-frost, we are concerned primarily with only fog and dew. Snow and ice and hoar-frost present their own special problems and have been studied and reported in the literature. Essentially, up to 95 per cent of our contamination problems occur during fog and dew. The fine, salty mists (salt-fogs) associated with the sea coast are particularly troublesome. The dry salt storms which occasionally occur in our southern coastal areas present an extreme case. Such experiences are reported in this issue in some detail (see p. 375). Humidities in excess of 90 per cent in the presence of NaCl-coated insulators can produce considerable leakage and a different thermal stabilization pattern if such a condition prevailed prior to the onset of a fog. All “shakers of salt” in the southern climates are well aware of this problem. VI.

Scope

of Computer

Opportunities

The digital computer will never solve the pollution problem ; however, it becomes a powerful tool in aiding our understanding of the behavior of the

Vol. 294, No. 6, December

1972

369

E. W. Boehne contaminated column and in this manner guides our testing programs. In the first attempt it was found possible to assign a different ground capacitance to each of 25 insulators, a different shielding capacitance for each unit in the string and a different contamination level to each unit. In 14 set the computer would yield, for each unit in the string, the voltage, current, wattage and phase angle. It revealed that the wattage distribution varied as the square of the voltage distribution, as might be expected. It is the non-uniform wattage distribution that lies at the core of the poor behavior characteristics of the contaminated column. The student of contamination research should note carefully that it is not necessarily the highly polluted unit which arcs first. Indeed, it is the drier, and hence the less contaminated, unit on which arcing is first noted. This might be emphasized and exaggerated by dipping only the bottom 12 units of a 24-unit string in a wet slurry. When full voltage is applied to this “hybrid” contaminated column, the clean dry half will first flashover and be followed by a flashover of the entire column, depending upon the voltage used with respect to the total distance. No small part of sustaining an arc of this nature is the magnitude of current permitted by the source in the event of a flashover. Not less than 10 A is recommended. It follows, therefore, that more attention should be given to the control of the events leading up to the conditions which permit wide variations of wattage along the insulation column. It is here that the computer becomes most efficient in studying the alternatives. The mathematical model of the insulation column for computer study (l)-(3) expresses all (4n - 2) parameters on a dimensionless basis, where n is the number of insulators in the column. This permits great flexibility and allows the computer cards to be used to study many other structures. Among these are the vertical post insulators. It became evident that the larger the diameter of the vertical post the smaller is the disturbing ground field influence. This is true because the per unit capacitance of the column increases as the square of its diameter while the disturbing ground field increases only in proportion to the column diameter. As a result, the larger the diameter of the insulator the better is its performance. For the same reasons the performance further improves when the vertical column is solid porcelain as compared to a hollow core. Still further improvement results when the metal portions between the insulator sheds are minimized, thus further reducing the disturbing ground field. These features are all found in the multi-cone insulator column, which, when tested in parallel, side by side, with other post insulators structures, under identical environments, was found to withstand the same voltage duration which caused others to flashover. These performance features are all in keeping with the results of computer studies using the simple dimensionless model. One unusual application of the same generalized computer cards (n = 25) was the study of the distribution of voltage across each break of a two-break disconnecting switch as the center vertical column became contaminated.

370

Journal of The Franklin Institute

The Digital Computer in Contamination Research As contamination was introduced the two voltages were no longer in phase. Only moderate contamination increased the voltage of the source-side gap from 75 to 88 per cent while the gap at the grounded break changed from 25 to 18 per cent.

VIZ. The Dynamic

Phase-A

Door to the Future

In the first attempt in this area of computer research, simple models of surface evaporation and drying were introduced (2). The wattage distribution was monitored, by computer, and in this manner new values of q were obtained at the termination of a fixed time of exposure. This process was iterated six to seven times, by computer, to watch the column flashover when predetermined limits of voltage gradient were obtained. In some cases, the column would dry without flashover. In this study it became apparent that, what might be called the thermal time constant of the insulator unit, played a significant role in the differential drying along the column. This time constant centered about 15 min. From this it is not hard to see that any test program which attempted to evaluate contaminated insulators in a time less than the thermal time constant of the string is not likely to achieve reliable results in spite of any plea on reproducibility of the test. Validity should come first and should enhance, not hinder, reproducibility. It is hoped that future computer models of the insulation column can be extended to permit q to be continuously modified as a function of wattage and time and then to study the effect of controllable factors. The addition of fixed ambient values of q, which could be called q,, to represent the effect of resistance glazes, might be studied to show the reduction in differential wattage along the column as a result of this expedient. Great hope is held for the science of resistance glazing. As the above dynamic studies progressed, it became evident that there was a noticeable effect upon the performance, depending upon the rate-ofwetting of the column. For example, it was postulated for a slow rate of rise or rate of increase of fog, with respect to the thermal time constant of the column, that the non-uniform column heating would permit the wetting of the cooler insulators in the string while the hotter insulators would reject the fog and remain dry. As a result, a more severe condition would result than if, first, there were no fog and seconds later all the fog was present. This suggestion was studied by Kawai (8) in the laboratory to confirm that there was a considerable difference. For these reasons it is also clear that realistic results would dictate that the insulation column should be energized, and thermally stabilized, before the addition of the fog, to achieve practical results as might be found in the field. In a similar manner the fog should terminate well in advance of the voltage. Where this has been done there have been reports of flashover after the fog has been shut off.

Vol.

294,

No. 6, December

1972

371

E. W. Boehne Some support has come from the field in the multiple observations that more trouble has been observed at the initial onset of a fog than has occurred on the same line under a continuous heavy fog. VZZZ. Lightning

and Switching

Surges

The initial computer study revealed that the RC network of the contaminated insulator string also had a definite and recognizable electrical time constant. This is usually between 200 and 400 psec under nominal pollution. The faster the rate of rise and fall of applied voltage imposed upon an RC network, particularly when the rise and fall time is considerably less than the circuit time constant, the more the circuit performs as if only the capacitance network were present. It is for this reason that lightning, with its microsecond time rise and fall, only recognizes the capacitive network and hence essentially ignores the resistance characteristics of the wet insulator. As a result we have no wet lightning impulse tests in our standards. Matters change considerably; however, in the event an insulator column undergoes stress, at 60 Hz, due to contamination. When dry bands form along the column, due to the nonuniform drying created by the very non-uniform wattage distribution, then the column insulation characteristics are weakened and the column is vulnerable to the transient overvoltages of lightning and switching surges. Columns, all nervous with scintillations (arcing around the pins of some insulators), are particularly vulnerable to these fast transient overvoltages. Reports indicate a loss of impulse strengths from 25 to 50 per cent. Research in these areas is very current and continuing. Since q, the dimensionless contamination parameter, is equal to (l/wCB) it is clear that 60 Hz (w = 377) voltages cause more contamination trouble than 50 Hz (zu = 314). It follows that the frequencies associated with lightning and switching surges will tend to produce higher transient currents on the column. Further analysis of the lightning and switching surge behavior of long, contaminated insulator strings, by digital computer, is now before us. Excellent theses are open to the interested student of transient network theory. However, nothing will take the place of laboratory investigations which superimpose these transients on the insulator column under contamination stress. Switching impulse surges having long decay times of the wave tail should produce the greater stress. We are fortunate that fog and lightning seldom occur simultaneously in nature. IX.

Some

Notes

on Testing

One of the early results of the computer study was the observation that the current leaving the last or grounded insulator (at a point where it could be measured) is only a fraction of the current entering the column. On this basis it was felt that the current measured at the grounded point had little meaning. On the other hand, the monitoring of current surging, due to the

372

Journal

of The Franklin

Institute

The Digital Computer in Contamination Research flashing over of dry bands, is useful. Great care should be taken in interpreting the current record. In the author’s opinion the first criteria for any laboratory test is its validity. This in turn is usually discovered by extremely close attention to what is happening in the field, the nature of natural pollution phenomena. For this reason the salt-fog method appears to the author as being superior to the solid layer methods. It provides the shortest thermal time constant and has a better opportunity to exhibit dry bands on the column in a shorter time. The thermal stabilization of the column, prior to the application of a salt fog at a defined and measurable rate, would be the author’s recommended procedure. The solid layer methods in the writer’s opinion are more artificial. They do not exist, in the form used, in natural environments. They place too much emphasis upon the pollutant itself and lengthen the time to the formation of dry bands. If used, the best matching to the natural thermal time constant of the insulator would dictate that the voltage be applied to the insulation column when it is still dry and allowed to stabilize thermally under voltage before the fog is admitted at a controlled rate of rise to the test chamber. The test should last at least 20 min at full fog and the voltage left on after the fog has been terminated. The tremendous bibliography which has developed during the past 10 years is not only a measure of the interest and importance of the subject but ironically also indicates our basic ignorance concerning the nature of the problem. Many tests could be developed, each one based upon some physical model of the physics of the problem. Many more could be developed without any prior knowledge of the mechanism of flashover. It seems appropriate to suggest that laboratory studies endeavor to duplicate, as closely as possible, the pollution environment found in the field. To date, the salt-fog method comes closest to the natural conditions that has been devised. Research is accelerated when it can proceed forward on two feet: one will always be used for testing to confirm, negate or reorient a prediction ; and the other, for a straightforward analysis, understanding and sometimes rationalization of tests with field observations and analog results which can lead to directed testing with specific objectives in mind. Laboratory testing, combined with the analog or circuital model of the test, both electrical and thermal and studied with a digital computer, make a strong and promising research team. However, performance in service will always be the field judge. References An (1) E. W. Boehne and G. W. Weiner, “Contomination of EHV insulation-I. analytical study”, IEEE Summer Power Mtg., Paper 31, pp. 66-481, 1966. (2) E. W. Boehne snd G. W. Weiner “Contamination of EHV insulation-II. Power losses and their distribution”, IEEE Winter Power Mtg., Paper No. 31, pp. 67-153, 1967. (3) E. W. Boehne, “Discussion”, 1968 Trans. of C.I.G.R.E. (Int. Conf. on Large High Tension Eleclricd System), Vol. 1, Group 25, pp. 15-16, 1968.

Vol.

294, No.

6, December

1072

373

E. W. Boehne (4) A. Schwaiger, “Theory of Dielectrics”, London, New York, John Wiley, 1932. (5) M. Sforzini, “Testing of polluted insulators: the present situation, and problems of the future”, J. Franklin Inst., Vol. 294, pp. 437-468, 1972. (6) K. Berger and P. Chowdhuri, “Behavior of insulators under dew and fog”, J. Franklin Inst., Vol. 294, pp. 385-397, 1972. (7) J. R. Massey, “Control of insular contamination in substations”, J. Franklin Inst., Vol. 294, pp. 375-383, 1972. (8) M. Kawai, “Research at project UHV on the performance of contaminated insulators”, J. Franklin Inst., Vol. 294, pp. 399-436, 1972.

374

Journal of The Franklin

Institute