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The use of an HV fog chamber in conjunction with a variety of environmental exposures to study degradation in the surface properties of polymer systems and how this affects wet electrical insulation is dealt with. Emphasis is placed on studying the conditions for leakage current inception and further current development leading to a flashover condition. The relationship between surface current, time, and voltage are plotted graphically and discussed.

The results of an experimental investigation of the surface flashover of high density alumina spacers in vacuum are presented. The insulators were subjected to a combination of a dc or 60-Hz prestress and a 1-50-us impulse stress. It is shown that the overall flashover voltage is strongly dependent upon the magnitude and polarity of the prestress when the spacers make a butt contact with the electrodes. Recessing the solid-electrode junction below the plane of the electrode at the impulse cathode makes the overall flashover voltage independent of the prestress. The effect of the prestress is shown to disappear within 50 us of the removal of the stress. The results are explained in terms of a charge injection model.

This paper reports experimental data on parameters which affect the flashover voltages of a contaminated insulation surface in fog conditions. An attempt has been made to interrelate the effect of the wetting rate with the density of the inert binder and the effect of the density of the binder itself on the flashover yoltages, the subject on which previously reported experimental data are in contradiction. A theoretical model has been developed which predicts dry band growth and temperature distribution along dry bands. It has been found possible to relate the conclusions of the experimental study with those of this model.

Switching-surge tests were performed on three basic station insulations: horizontal insulator strings, station post insulators, phase-to-phase air clearances between buses. The influence of different geometrical parameters was investigated.

The results of carefully controlled laboratory experiments on contaminated flat plate insulators are presented. Flashover voltages and salt migration effects are shown for the following nine common salts: NaCl, NaNO3, Na2SO4, MgC12, Mg(NO3)2, MgSO4, CaCl2, Ca(NO3)2, and CaSO4. Our results show that the tendency for an insulator to flashover is dependent on the type of contaminant as well as on the equivalent salt density (esd). We also demonstrate the existence of a salt migration effect and conclude that it should be given equal status with dry band formation in attempts to understand flashover and the events leading to flashover.

Various types of insulators have been energized with ±250 kV dc voltage and exposed to natural conditions by the seaside continuously. Salt deposit density and flashover phenomenon of these insulators have been investigated extensively for about two years. According to the results obtained so far, the influence of dc voltage application on the salt deposit density was not so notable as expected, while the effect of insulator types on the voltage withstand strength was greater than anticipated.

The results of artificial contamination tests on a variety of porcelain and glass suspension insulators are presented together with a discussion of the contamination and fogging procedures. The results are presented in an analytical form that can be related to a theoretical flashover criterion. Semiconductive glazed insulators were tested and compared to normally glazed insulators of the same type.

An experimental investigation of the effect of a cuprous oxide coating upon the surface flashover of high-density alumina in vacuum is reported. It is shown that such coatings improve the impulse strength of the system and eliminate the conditioning effect observed in the case of uncoated specimens at dc and 60-Hz voltages. An explanation of the observed behavior is given in terms of a surface charging model. It is postulated that the improvement in the insulation by the coatings is due to a reduction in the secondary electron emission yield.

Results of full-scale tests on V-suspension strings up to 50 standard insulators (about 24-ft long) are presented and the need of such tests is demonstrated. Because of pronounced nonlinearity on standard insulator strings, the use of large disc insulators is recommended for UHV line insulation. A practical design method for anti-contamination design is developed on a statistical basis.

This paper describes an up-to-date summary of the contamination research at Project UHV. Results of flashover tests on various types of insulators are presented with discussions of test methods. Two important factors—flashover probability on multiple strings, and nonlinearity of flashover strength with insulator length—areconcentratively discussed. The latter is a particularly important problem for UHV line insulation. A practical design method for line insulation is developed on a statistical basis.

Investigations were made of various factors which affect the flashover values across polyethylene and tetrafluoroethylene surfaces in vacuum. The factors studied included sample conditioning under sustained direct and power frequency voltages, and the effect of surge voltage waveshape. The breakdown values for a given voltage waveshape increased with the time of conditioning up to a maximum value which then remained constant within about 10%. Alternating voltages were found to be more effective in the conditioning process. The average breakdown voltage for each insulator decreased with increasing the surge front duration from 1 μs to about 150 μs and thereafter remained constant.

1. There are differences among the lightning current data obtained by various investigators. These differences appear to be a function of the height and the terminating ground impedance of the structure on which the measurements were made. 2. There is agreement among the data obtained by magnetic links when the results were obtained from direct readings on tower or diverter rods or by the summation of readings made at each end of the ground wire of a stricken span. 3. Comparison of data obtained at sea level or at a high altitude (14,000 feet) based on measurements made of strokes to tower rods or ground wires as in conclusion 2, show no appreciable difference due to altitude. This appears to be contrary to the previously drawn conclusion based on a comparison of the high-altitude diverterrod data to sea-level lightning-current data based on the summation of tower currents. 4. There is disagreement among the lightning current magnitudes obtained as in conclusion 2 and those obtained by the summation of tower currents. 5. Lightning stroke data measured at the ESB are in disagreement with those obtained as in conclusion 2 by a factor of about two to one, particularly at the higher currents. 6. Examination of oscillograms obtained at the ESB show variations in the current wave shape that may be attributed to reflections occurring from the base of the building and other reflection zones around Manhattan Island. Thus, there are indications that the lightning stroke is not a constant current source.

Lightning, shorted conductors, line switching, and flashovers create voltage surges on the power lines serving an industrial plant. Electrical equipment in the plant, particularly high-voltage motors, can be damaged by these surges. The resulting loss of production, plus repair or replacement costs, make it imperative that adequate protection be provided against surges.

The dependability of the digital calculation accuracies of switching surges is proven through several comparative studies with the field test results. It is also clarified that the resistor preinsertion closing of a circuit breaker is one of the most effective methods for reduction of switching surges. On the other hand, in factory tests, satisfactory results have been obtained on the withstand characteristics and the coordination between the across-the-open-pole insulation and the phase-to-ground of an EHV air blast circuit breaker against switching surges.

TESTS show that when a surge is applied to the primary of a distribution transformer that has a secondary ground, the isolated case will assume a low electrostatic potential above ground so that about from 80 to 95 per cent of the surge voltage will appear from primary lead to case. For simplicity in the considerations that follow, it will be assumed that the case potential under these conditions is the same as that of the secondary neutral.

The paper deals with the characteristic impulse and switching surge voltages of high-voltage equipment which only temporarily loses its dielectric strength when flashover occurs. A test procedure suitable for determining the highest withstand voltage and the lowest flashover voltage of a test object is described, and a theoretical investigation is carried out in order to show the quality of the test procedure, and also the influence of different factors on the test result. The relation between the specified withstand voltage of a test object, as defined by IEC, and the highest withstand voltage is given some consideration. A simple method for determination of the 50-percent flashover voltage is described.

This paper reports positive switching impulse flashover characteristics of thin wire-plane gaps. Flashover tests were made under both conditions that switching impulses were added with or without ac or dc corona occurring previously. Flashover distances were up to 4 m, and ac or dc bias voltage was 400 kV crest.

A quantitative research report is made here, based on tests of EHV (extra-high-voltage) insulator strings. Impulse, switching-surge, and a-c tests on vertical, horizontal, and 45-degree V-strings were carried out, using both positive and negative unidirectional impulses and 250- to 3,250-?sec (microsecond) switching surges. The switching-surge and a-c tests were made, both dry and wet, under ASA (American Standards Association) specifications. Research was instrumented in the CESI high-voltage laboratory in Milan, Italy. The full name of this laboratory is: Centro Elettrotecnico Sperimentale Italiano Giacinto Motta, the last two words referring to the late Dr. Motta, to whom the center is dedicated (see Figs. 1 through 3). The study included radio-influence (RI) measurements, determination of voltage distribution along the insulator strings, photographs, and cathode-ray records of each test. Interesting and useful data pertaining to the design of EHV lines were collected.

HIGH-voltage apparatus insulated by compressed gas requires the use of solid insulating supports. It is generally recognized that even in a uniform field the flashover voltage of such solid insulation is considerably lower than for the gas-filled gap of equal length. An increase in the allowable gradient along such insulators would in general result in a reduction in the size, weight, and cost of gas-insulated apparatus, and must therefore be considered an important and fundamental objective of high-voltage engineering. It is probable that the flashover strength of solid insulators may be brought closer to that of the gases in which they are immersed, or to their inherent volume breakdown strength, through a better understanding of the influence on insulator flashover of such factors as material, surface condition, corrugations, and gas pressure. Most of the scant literature on this subject is restricted to relatively low alternating voltages, small samples of solid dielectrics, or to special insulator shapes in non-uniform fields.1 Reher studied the flashover of plain cylinders of ebonite, paraffin, and porcelain in dry air at pressures up to 15 atmospheres, using alternating voltages up to 140 kv maximum.2 Goldman and Wul3 have investigated the flashover of plain ebonite cylinders in nitrogen with alternating voltages up to 125 kv maximum and pressures up to 20 atmospheres. The present paper reports d-c flashover values up to 250 kv across short cylindrical insulators placed in the uniform field between metallic electrodes. These studies were made on both plain and corrugated cylinders of three dissimilar insulating materials immersed in dry nitrogen at gauge pressures up to 400 lbs. per square inch.

Switching impulse tests were performed on single-column station post insulators having heights from 5 to 20 feet mounted atop grounded support structures having heights from 8 to 20 feet. Wavefronts of 120- and 1000-?s were used. Results can be used to obtain preliminary estimates of requirements for EHV and UHV stations.

The data presented may be summarized as follows: 1. When a Switchette (the trade name for a small double-break switch with 0.020-inch gaps and opening time of one to two milliseconds) was used on 250 volts and 0.4 ampere at sea level, voltage peaks as high as 28 times line voltage or 7,000 volts were observed. 2. A maximum peak of 130 times line voltage or 3,900 volts was observed while operating the Switchette double-break on 30 volts and 0.4 ampere at sea level. 3. When the Switchette was used the voltage peaks were found to decrease with altitude for all voltages tested. A maximum of seven to one in ratio of voltage at sea level to voltage at 50,000 feet was observed. 4. Using a double-break contactor which has a 1/16-inch gap per break and a gap-travel time of 12 milliseconds, the induced voltage peaks rise with altitude in every case. A maximum of two to one in the ratio of voltage at 50,000 feet to voltage at sea level was observed. The loads on which these tests were made, as the text of the paper will show, were very much less inductive than those used for the Switchette. 5. Variations of humidity over a wide range has little effect on the voltage peaks at any altitude. 6. There are no voltage peaks above normal line voltage in a purely resistive circuit.

Flashover of suspension insulators on steel tower lines, traceable to an accumulation of dirt followed by a deposit of moisture, has been the source of numerous outages of high-voltage transmission lines. The aim of the investigation described in this paper was to determine means of increasing the insulation level of existing lines under fog and dirt conditions. Methods were developed to simulate, in the laboratory, the conditions of dirt and fog which cause flashover of insulators in the field. Using these methods, a study was made of the phenomena of flashover of contaminated suspension insulators. The benefits to be derived from lengthened strings and the superiority of special fog-type units with increased leakage distance were determined. Other means of reducing the hazard from dirt accumulation are suggested.

This paper, which is a continuation of a four-year operating record of an extensive 132-kv transmission system under lightning conditions, presents and discusses the record for 1930 and 1931. Operating experience is given on: 1. Yearly line outages due to lightning for all lines. 2. Severity and frequency of lightning storms over the system for a three-year period. 3. Frequency of double circuit outages. 4. Extent of line and station apparatus damage. 5. Effect of tower footing resistance in reducing lightning outages. 6. Reduction in tower footing resistances by the use of driven ground rods and counterpoises. 7. Effect on lightning outages by overinsulation. 8. Relative frequency of single-phase, two-phase and three-phase line faults. 9. Location of lightning flashovers on the line in reference to top, middle and bottom conductors. Conclusions are drawn from the results of the operating experience on these lines.

This paper presents anti-pollution design criteria for line and station insulators based on investigation of performance of various insulators under artificial pollution tests. Field pollution areas were classified into six classes; AA, A, B, C, D and E according to pollution degree. Recommended leakage distance for disc insulators per 1 kV of nominal line-to- line voltage are 0.75", 0.9", 1.0" and 1.1 " for A, B, C and D areas respectively. Considerable increase in leakage distance is necessary for cylindrical post insulators and porcelain shells with increase in average diameter.

This paper presents the results based on detailed accumulation of lightning performance data on 140- and 250-kV lines in the Tokyo area. Data are arranged for the yearly variation of outage and the distribution of flashovers. The former changes through a rather wide range; the latter indicates the concentration of outage. The result of the investigation signifies that expression by means of an average not always indicates the real performance.

This paper presents results on flashover voltages across insulation surfaces and on the rate of surface deterioration in a vacuum of 10-5 torr under direct, alternating, and surge voltages of front duration in the range from 1-600 ?s. The insulation material used was polymethylmethacrylate (PMMA) of cylindrical shape fitted between uniform field electrodes. The measurements were carried out on insulator specimens 25 mm in diameter and 5, 10, and 20 mm in length.

ALTHOUGH wood has been known to possess the excellent properties of mechanical strength in combination with insulation strength almost since the beginning of the electrical art, its real lightning strength was not generally appreciated until within comparatively recent years. It is true that it was rather extensively used in the past to supply power frequency insulation in treated form alone or with transil oil. For example, wood either solid or laminated has been employed for years in operating rods on oil circuit breakers and in terminal and spacing blocks in transformers. The 60-cycle strength of wood under those conditions has been studied, and although the results were not made available generally, the strength factors had been known with considerable preciseness for some time. In these applications, however, the 60-cycle (i.e., power frequency) strength of wood has been of paramount interest, and the lightning or impulse insulation value of secondary interest, either because the latter was considered comparatively unimportant or because the impulse strength furnished by other insulating members or mediums in the structure was considered sufficient.

The "wet contaminant" test method, originally developed for use with alternating voltage, is extended for use with switching surges. In order to evaluate insulators under conditions where dry bands would form, it was found necessary to first energize the contaminated insulators with alternating voltage before applying the surges. Included in the investigation are the effect of front time and tail time on the critical flashover voltages, and a comparison of the switching surge performance of five insulator types under various levels of contamination.

It is shown that times to flashover obtained by von Cron and Dorsh with alternating voltage on an artificially polluted insulator can be fitted into a relation ( V2-V02) t = KR2 where V is the applied voltage, t is the time to flashover, R is the resistance of pollution, and Vo and K are experimental constants. Experiments were carried out using pulse unidirectional voltage applied to a narrow conductive path of water solution of sodium chloride, and the empirical expression obtained was V2-V02) ? CR2L3 where L is the leakage length and C is another experimental constant. These relations are explained as results of an energy equation p·?t = e·?t where?t at is the time required to extend the discharge by ?x, p being the power available for the discharge Propagation and e the energy required to elongate the discharge by a unit length.

The need for more complete data on the performance of wood, both in regard to impulse and power-frequency operation, has been recognized for some time. Although certain fundamental data are still lacking, much has been learned from laboratory tests about the use of wood as an insulating material. The economic importance of wood-pole line makes these test data valuable assets in the consideration of materials for high-voltage installations.

The technique of measuring impulse voltages with the aid of the high-voltage cathode-ray oscillograph and the resistor voltage-divider is discussed, and applied to the determination of the minimum impulse flashover characteristics of gaps between spheres ranging in diameters from 125 mm. to 1000 mm. Flashover measurements on sphere-gaps have also been made at power frequencies and compared with the corresponding figures for impulse voltages. The results of some previous workers are considered, and compared with those obtained by the authors.

Results of experimental studies on the contamination flashover strength of EHV insulator strings are presented. The results compare favorably with a theoretical analysis by Boehne and Weiner in 1966-1967. Nonlinearity of insulation strength with insulator length has been investigated, employing a steam fog method to simulate natural humidity and fog. The nonlinearity is caused by nonuniform voltage distribution along contaminated insulator strings. Such nonuniform distribution is created by the capacitance of insulator units to ground. This relationship was clearly demonstrated by several indirect methods. The results should be very important in future contamination studies.

A description is given of the facilities at Project UHV for the study of EHV and UHV ac transmission problems up to 1500 kV. These facilities include a 1500/?3 kV, single-phase autotransformer, a special 900-kV lightning arrester, and much associated UHV and EHV measuring equipment. Special stringing procedures are employed to string overhead test lines, and special test cages yield the corona characteristics of bundle conductors. Research is in progress on 1000 to 1500 kV transmission lines.

The flashover performance of post-type support spacers for concentric compressed gas-insulated transmission lines was investigated. The experiments were made with SF6 gas at 4.4 atm. abs. and low frequency AC voltage in the EHV range. Nine cast epoxy spacers with metal inserts at each end shaped to produce different electric field distributions along the solid-gas interface were tested. The results show that flashover voltage is controlled by the electric field distribution and is inversely proportional to the stress enhancement in the gas. This conclusion is shown to be consistent with ionization-initiated breakdown at a certain stress limit about 300 kV/cm in these studies. The better designed spacers were limited by sparkover of the radial gas-gap at 1300 kV peak and not by surface flashover at the insulator.

A method of computing switching transients using a finite difference scheme is presented. The scheme proposed here has the facility of accommodating any termination (including nonlinear elements like lightning arrester). Hence this method is an improvement over the method suggested by Stafford, Evans and Hingorani. Further this method can be used for the determination of the transients arising out of different types of electrical disturbances in the system i.e. component flashover, faults, lightning strokes etc. The stability analysis of the scheme proposed and the computational aspects of the procedure are discussed.