Maximum Electric Stress Research Articles

Influence of Area and Volume Effect on Dielectric Behaviour of the Mineral Oil-Based Nanofluids

Transformer oil is conventionally used as an insulating liquid for the purpose of insulation and cooling in power transformers. The rise in the power demand has put stress on the existing insulation system used for power transmission. Nanotechnology provides an advanced approach to upgrade the conventional insulation system by producing nano-oil with enhanced dielectric characteristics. The aim of the study is to present the influence of area volume effect on the dielectric performance of mineral oil and its nanofluids. In this paper, nanofluids are prepared by dispersing two different concentrations of SiO2 nanoparticles in base transformer oil using a two-step method. The effect of area and volume is investigated on nanofluids in the laboratory using coaxial electrode configurations under different test conditions. The AC breakdown voltage and maximum electric stress is determined for the pure oil and nanofluids. The results show that the addition of SiO2 nanoparticles significantly improves the dielectric characteristics of transformer oil. Moreover, the breakdown phenomenon is also discussed to analyze the effect of nanoparticle, stressed area, and stressed volume on the dielectric strength of insulating oil. Nanofluids could be an alternative to mineral oil.

Potential of Epoxy Nanocomposites for Packaging Materials of High Voltage Power Modules: A Validation Using Experiments and Simulation

This paper focuses on the potential of epoxy resin/aluminum nitride (EP/AIN) nanocomposites through experiments and simulation for the packaging material of a 15 kV SiC-IGBT module. For this purpose, space charge, AC breakdown, thermal conductivity and glass transition temperature <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(T_{\mathrm{g}})$</tex> are measured. The electric stress reduction effect in EP/AIN nanocomposites is compared to the traditional packaging material at high-frequency AC and square voltages at 150 °C using the electric field simulation. The experimental results show that the 2 wt% EP/AIN nanocomposite exhibits the least space charge accumulation, highest AC breakdown strength and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_{\mathrm{g}}$</tex> amongst the tested nanocomposites. The offset of metallization layers plays a crucial role in the impact of the electrical properties of packaging materials on the electric field of the packaging structure. When the offset of metallization layers is zero, the maximum electric stress in the EP/AIN nanocomposites is lower than the traditional packaging material, both under the AC and square voltages. In the EP/AIN nanocomposites, as the frequency or the nano-AIN content increases, the maximum electric stress decreases under the AC voltages. Traditional packaging materials have a steeper transient electric field variation than EP/AIN nanocomposites, although the transient process is smoother at higher frequencies under square voltages for all the considered materials. A simplified physical model is proposed to explain the electric field distribution variation in the packaging structure under the AC voltages.

Analysis of Deep-Trap States in GaN/InGaN Ultraviolet Light-Emitting Diodes after Electrical Stress

We analyzed the deep-trap states of GaN/InGaN ultraviolet light-emitting diodes (UV LEDs) before and after electrical stress. After electrical stress, the light output power dropped by 5.5%, and the forward leakage current was increased. The optical degradation mechanism could be explained based on the space-charge-limited conduction (SCLC) theory. Specifically, for the reference UV LED (before stress), two sets of deep-level states which were located 0.26 and 0.52 eV below the conduction band edge were present, one with a density of 2.41 × 1016 and the other with a density of 3.91×1016 cm −3. However, after maximum electrical stress, three sets of deep-level states, with respective densities of 1.82×1016, 2.32×1016 cm −3, 5.31×1016 cm −3 were found to locate at 0.21, 0.24, and 0.50 eV below the conduction band. This finding shows that the SCLC theory is useful for understanding the degradation mechanism associated with defect generation in UV LEDs.

Electrical Considerations in Establishing Working Clearances for Energized Line Maintenance

The minimum clearance requirements for live line work, initially published in 1968 as "Recommendations for Safety in Live Line Work", was based on rod gap data from 13 sources. Since that time much has been learned about the parameters affecting the electrical behavior of air, both alone and in the presence of insulation, especially for large gaps. It has been shown that there is a critical wave shape associated with different gap geometry, which raises questions about the use of rod gap critical flashover data as the basis for working clearances. There are also concerns over the effect of atmospheric conditions on clearances and how to apply the correction factors developed to correct for "non-standard" atmospheric conditions. To establish safe working clearances, the maximum electrical stress that can appear at the work site must be determined and the distance required to withstand that stress under all working conditions. This paper discusses various types of voltage stress that can appear at the work site, as well as the factors that affect work site insulation strength. If all worst case conditions are used simultaneously to determine working clearances, recent work would suggest that clearances would be increased over those required by the present Guide. However, the experience to date with both HV and EHV circuits indicates that there is no significant safety problem with the working clearances being used by operating utilities.

Methods Of Calculations Of Field Stresses In A Three-Core Power Cable

A numerical method using the charge simulation technique with only a small number of line charges is developed to calculate the electric field everywhere in a three-core high voltage, high power belted cable. Only six line charges are used of which three lines represent the three conductors and three image line charges to take account of the grounded sheath. The locations of the maximum electric stress in the cable are determined. Experimental measurements is also carried out using an electrolytic tank to verify the calculations. Good agreement is obtained. The results are presented in a normalized format so that they can readily be applied.

Aspects of electrical breakdown of liquid insulating material I

The electrical breakdown strength of insulating oil depends on the size of foreign particles which may form bridges in a place of maximum electric stress. This theoretical relation was verified with colloid suspensions of particles of known radius. Mineral oils may deteriorate if the particles unite by the process of flocculation, the occurrence of which depends on the relative magnitudes of the attractive London-Van der Waals forces acting between the particles and the repulsive forces between their ion atmospheres. The possibility of using a mineral oil as an insulator depends on the existence of an upper limit of the size of particle complexes due to the rapid fall-off of the L.-v.d.W. forces at diameters exceeding 500 A. The latter value of 2r corresponds to a breakdown strength of 1 kV/mm. If acids are being formed, the upper limit of 500 A will shift towards larger values, and correspondingly the breakdown strength may drop below 1 kV/mm.

The unit dielectric strength of oil

IN DESIGNING FOR optimum dimensions and shapes in oil circuit breakers and other oil-insulated equipment, it has been found advantageous to determine, by means of the electrolytic field analyzer, the distribution and magnitudes of the electrical stresses which exist as a result of the application of high voltage. To make use of this information, the designer must know with engineering accuracy the unit dielectric strength of the oil; that is, the maximum electrical stress that the oil will withstand anywhere in the gap without puncture. The best generally available data on this subject have consisted of a set of empirical curves published by F. W. Peek in 1915 for oil adjacent to spheres and cylinders of different radii. The fundamental reason that oil would withstand much higher unit electrical stress when adjacent to some shapes than to others has not been understood. As a consequence, when faced with a practical shape which was neither a precise sphere nor a precise cylinder, one has not known what value of dielectric strength to use, within a range of two or three to one.

The sparking voltages between sperical electrodes

(1) Simplified formulae are given by means of which the maximum electric stress between equal spherical elctrodes can be readily found.(2)Experimental results on sparkling voltages when the potentials of the equal spherical electrodes are equals and opposite are analysed. It is found that it a be the length of the radius of each electrodes in centimetres, the maximum potential gradient Rmax at the instant of discharge is given by the Rmax. = 27.4 + 14.1/√ Kilovolts per cm. very approximately when the temperature of the air is 25° C. and the height of the barometer is 76 cm.(3) The sparking voltage E between two spherical electrodes, the radius of each being a, and x being the distance between them, is given by E = x/f (21.4 + 14.1/√a) Kilovolts at 25° C. and 76 cm. pressure, where f is a factor which can be easily calculated from the geometrical dimensions.It has to be remembered that these formulae do not apply to air-gaps less than 1 mm. or air-gaps so great that brush discharges take place.(4)It follows from the formulae given above that in general for a fixed spark-gap there are certain sized spherical electrodes which offer the maximum resistance to sparking. Increasing or diminishing the size of the electrodes from this value makes the spark take place more readilly.(5) The results when one electrode is earthed given in the “Standardization Rules” of the American Institute are analysed. From the results it appears that the potentials of the electrodes were not E and o, but were 0.57 E and — 0.43 E. These values of the potentials make the values of Rmax. deduced the same as the values found when the potentials are equals and opposite.

Societies and Academies

LONDON. Royal Society, December 6.—Sir J. J. Thomson, president, in the chair.—Prof. W. H. Young: The series of Legendre.—L. Hartshorn; The discharge of gases under high pressures. It is well known that when gas discharges through an orifice from a vessel in which the pressure is p0 into one in which it is pt, the rate of discharge is approximately constant from pi = o upwards to some critical value, but then, as £, further increases, the discharge falls off, slowly at first, after wards with greater rapidity. In the present investigation, this phenomenon is examined with greater accuracy than has hitherto been obtained. In every case it was found that the flow was constant to at least one part in 10,000 for a considerable range of £,. The critical value of pit at which the flow began to change, varied widely for different nozzles, being about 0-2 pa for the convergent and parallel ones, but as high as 0-7 p0 for certain divergent ones. Thus, the theoretical value for convergent nozzles, viz., 0-527 pa, cannot be accepted as applying even approximately to all nozzles.—Lt.-Col. A. G. Hadcock: Internal ballistics. This paper deals with the burning of the explosive in the gun and the expansion of the gas, both before and after the charge has been consumed. On firing the gun the action is threefold:-(1) The driving band on projectile is forced into the rifling grooves. (2) In subsequent burning of charge, the gas from any fraction of charge expands with consequent reduction of temperature. The still burning powder gives additional heat. The expansion is thus partly adiabatic and partly isothermal. (3) After the charge is consumed the gas expands adiabatically. From expressions given in the paper, and knowing the rate of burning of cordite under various pressures, formulas are developed for finding velocity of projectile, position in gun, and pressure of gas. The magnitude and position of maximum pressure are found by a further development of formula?.—Dr. A. Russell: The electrostatic problem of a conducting sphere in a spherical cavity. The author gives formulae by means of which the capacity, the electric force between the spheres, and the maximum electric stress on the dielectric between them can be readily computed in all cases to any required degree of accuracy. The solutions of these problems are required when determining the ratio of the measure of the electrostatic to the electromagnetic unit of charge by means of a spherical condenser for the calibration of a spherical condenser of variable capacity, for the o calibration of a high-tension voltmeter, and for the determination of the electric strengths of insulating materials.—Prof. G. N. Watson; The zeros of Bessel1 functions. The paper contains a statement and discussion of some general theorems concerning the zeros of Bessel functions; the theorems are true for functions of any order, and, unlike results previously known, are of particular interest in the case of functions of high order. It appears that comparatively general considerations of a non-arithmetical type yield fairly precise information concerning the position and numbers of the zeros of the Bessel functions of the first kind. It is doubtful whether results of this character could be obtained without making use of the method of steepest descents which has been prominent in various recent investigations.

The Electric Stress at which Ionisation Begins in Air

In the Proceedings of the American Institution of Electrical Engineers for July, 1910, Prof. J. B. Whitehead publishes the values of the electric stress at which ionization begins in air. The electrodes he used in his experiments consisted of a metal tube and a cylindrical wire coaxial with it. Alternating pressures were employed, and the inner wires used had diameters varying in size from 0.089 to 0.475 cm. If a be the radius of the inner wire, the Author has found that the expression 32 + 13.4/radical a gives all Whitehead's experimental results for the maximum electric stress in kilovolts per centimetre with a maximum inaccuracy of less than 1 per cent. The experiments show that the electric stress at which ionization occurs is independent of the metals used for the electrodes and of the inner radius of the outer tube. It depends merely on the radius of the inner wire. It is shown that Steinmetz's experimental results on the sparking distances between parallel rods are in substantial agreement with Whitehead's figures. Possible reasons for the discrepancies are given. An empirical formula is given which is based on experimental results recently published by Kowalski and Rappel for the sparking voltages between equal spherical electrodes. The results indicate that the electric stress at the moment of discharge has a minimum value when the distance between the electrodes is a certain function of their radius. The author has neglected the currents of electrified air which stream round the electrodes before the discharge takes place. These currents probably modify appreciably the values obtained for the disruptive stress at the moment of discharge. The striking similarity between the formula for the heat emitted per unit area at the surface of a hot wire cooling in air and the formula for the electric stress at the surface of an electrified wire at which ionization first begins is pointed out.