Ekram Husain

Analysis of Paschen Curves for air, N2 and SF6 Using the Townsend Breakdown Equation

It has been shown that it is possible to extend the validity of the Townsend breakdown criterion for evaluating the breakdown voltages in the complete pd range in which Paschen curves are available. Evaluation of the breakdown voltages for air (pd=0.0133 to 1400 kPa · cm), N2(pd=0.0313 to 1400 kPa · cm) and SF6 (pd=0.3000 to 1200 kPa · cm) has been done and in most cases the computed values are accurate to ±3% of the measured values. The computations show that it is also possible to estimate the secondary ionization coefficient ¿ in the pd ranges mentioned above.

Breakdown strength of solid dielectrics in liquid nitrogen

The objective of this research was to determine if a relationship could be found between dielectric strength and other properties of electrical insulating materials in the medium of liquid nitrogen on an empirical basis by using variables predicted by basic theory. A simple equation of the form E=A+Blog (rhov/xir tandelta) to predict the dielectric strength of a solid insulating material in liquid nitrogen medium has been proposed. The equation requires the values of volume resistivity (rhov), relative permittivity (xir) and loss tangent (tandelta) in the medium of liquid nitrogen, which may be obtained easily by low voltage non-destructive measurements. The values of electric strength calculated using this equation for Crepe paper, Kraft paper, varnished paper, pressboard, presspahn, mica, bakelite and asbestos are quite in agreement with the experimentally measured values. It is expected that the equations obtained will help the designers as a handy tool for quick estimation of breakdown strength of solid dielectrics dipped in liquid nitrogen.

Dielectric behavior of insulating materials under liquid nitrogen

Measurements were made to assess the AC breakdown voltages in liquid nitrogen (LN/sub 2/) with different electrode configurations such as sphere-sphere, needle-needle, hemisphere-hemisphere, plane-plane, sphere-needle, etc. Experimental results reveal that the breakdown voltage is a function of electrode geometry and gap length. This study also addresses the effect on the breakdown strength of solid insulating materials under LN/sub 2/ environment with a sphere-sphere electrode configuration. In this paper, special emphasis has been attributed to the effect on loss index of a variety of dielectrics dipped in LN/sub 2/. The measured values of breakdown strength and loss index have been compared with those obtained under atmospheric condition. The study reveals that the breakdown strength of cellulosic materials like paper or pressboard increases manifold while the loss index decreases significantly when dipped in LN/sub 2/ with variations of the order of 50% to 90%. However, for impervious noncellulosic materials like Perspex (acrylic glass) or presspahn, the increase in breakdown strength is not that pronounced and the decrease in loss index is of the order of 2 to 30%.

Surface Discharge Studies with Uniform Field Electrodes at Low Pressures

An investigation of power frequency (50 Hz) surface partial discharges in dry air, using 21r/3 Rogowski profile electrodes in the low pressure range of 0.067 to 91.333 kPa, shows that for the discharges occurring symmetrically around the electrodes and just outside the uniform field region, the breakdown voltages are 20 to 30% lower than those accounted for by the usual Paschen values. Emphasis, therefore, has been given to modified values of breakdown voltages for any useful calculations. The effect of reduced pressure on inception voltage has been discussed and an attempt has been made to explain the difference between the observed and calculated values on the basis of a pressure-dependent secondary ionization coefficient. It is shown that increasing the insulation thickness in a critical pressure range (0.067 to 0.400 kPa) does not allow any significant increase in the discharge free working stress of the insulation system. At higher pressures (>0.400 kPa) the increase in inception voltage with thickness and pressure follows an equation which is expected to hold for other insulating materials as well.

On Calculation of Breakdown Voltages of Mixtures of Electron Attaching Gases

In this paper we report the analysis of dc breakdown tests on mixtures of CC12F2, SF6, C-C4F8, 2-C4F8, N2, CO2, CF4, CHF3, and 1,1,1-CH3CF3 gases on the basis of the NKH formula Vmix=k(pd) aNbUc developed by us earlier for the binary mixtures of SF6 with air, N2, N2O, and CO2. It is shown that while a and c have the values 0.915 and 0.850 respectively as earlier, k and b depend on the component gases. There is a good agreement between the calculated values on the basis of the formula and measured values reported in the literature.

Calculation of Sparking Potentials of SF6 and SF6-GAS Mixtures in Uniform and Non-Uniform Electric Fields

A simple formula is developed to predict the sparking potentials of SF6 and SF6-gas mixture in uniform and non-uniform fields. The formula has been shown to be valid over a very wide range from 1 to 1800 kPa·cm of pressure and electrode gap separation for mixtures containing 5 to 100% SF6. The calculated values are found to be in good agreement with the previously reported measurements in the literature. The formula should aid design engineers in estimating electrode-spacings and clearances in power apparatus and systems.

Study of breakdown of transformer oil in the presence of barrier

Barriers can influence the failure-stresses by effectively breaking up a large oil volume into smaller oil volumes, and by interfering with particle motion, and partial discharge activity. However, despite the importance of insulating barriers to oil filled transformers, little has been reported on their effect on failure stresses. The results obtained by experimental investigation into breakdown characteristics of transformer oil under non-homogeneous field conditions (rod-plate electrode system) as well as moderately homogeneous field conditions (sphere-sphere electrode system) under alternating (50 Hz) voltage stresses have been reported in this work. The effect of gap length on breakdown in oil and the influence of electrode covering, and the barrier in the form of a sheet of insulation such as kraft paper, polystyrene and polypropylene is reported. It has been found that as the distance between the barrier and plate electrode increases, the breakdown strength slightly decreases and then increases remarkably, resulting in much higher strength than that with a covered ground electrode. The complicated characteristics of the breakdown have been studied under the rod-plate and sphere-sphere electrode configuration for ac voltage (50 Hz). An attempt has been made to explain the result by considering the space charge effect.<<ETX>>

Estimation of partial discharge inception voltages due to voids in solid sheet insulation

Defects like formation of voids, cavities, and cracks or inclusion of dust particles/small metallic turnings in the solid insulation of modern power cables may not be completely avoided even under the most stringent manufacturing conditions. These defects or weak points are the vulnerable sites of occurrence of Partial Discharges (PD). A continuous dissipation of energy due to discharges at afore-mentioned sites causes a gradual but definite damage to insulation and brings about its premature failure. Since PD adversely affects the insulation life, knowledge of the voltage at which PD start, Partial Discharge Inception Voltage (PDIV) is very important for proper and safe stress for design as well as reliable operation of the equipment. In this paper we report a technique for estimating the PDIV in case of discharges occurring in voids in solid sheet insulation in the ambient medium of air. Since occurrence of PD is a phenomenon dependent on upon many factors such as the type of gas in the void, the gas pressure, surface properties of the void, size and shape of the void, location of the void, dielectric constant of the surrounding medium etc; these parameters have to be taken into account. The computed values are valid in the pressure range of 0.067 kPa to 101.333 kPa. The results obtained have been compared with experimentally obtained values of PDIV for artificial voids of known dimensions in solid sheet insulation.

Effect of contaminant pH and natural fog on naturally polluted porcelain disc insulator in northern region of India

Overhead power transmission lines cross regions which are different in environmental conditions. The reliable and uninterrupted operation of modern power system depends on reliable insulation. Outdoor insulation cannot be selected without having knowledge of prevailing pollution severity. This paper reports results of natural pollution tests that have been done at different locations. Flashover voltage (FOV) of porcelain disc insulator for different pH values and conductivity has been recorded to relate FOV with pH and conductivity of the contaminants. It is found that a decrease in pH and increase in conductivity of the contaminant lowers FOV of the insulators. Regular condition monitoring of porcelain insulators can be done using pH of the contaminants as an effective pollution severity indicator, it may further be used for devising proper maintenance schedule.

A study of flashover voltage of artificially polluted porcelain disc insulator under natural fog conditions

The availability of cheap and reliable supply of electrical energy plays very important role in the economic development and social welfare of any society. Overhead power transmission lines cross regions which are different in environmental conditions. The reliable and uninterrupted operation of modern power system depends on reliable insulation. Therefore outdoor insulation cannot be selected without having knowledge of prevailing pollution severity. This paper reports results of artificial pollution tests that have been done with different contaminants. The choice of contaminants for use in the present artificial pollution test was done with respect to the occurrence of soluble and non-soluble salts in natural pollution conditions. Flashover voltage (FOV) of porcelain disc insulator for different pH values and conductivity has been recorded to relate FOV with pH and conductivity of the contaminants. It is found that an increase in pH and conductivity of the contaminants lower FOV of the insulators. Regular condition monitoring of porcelain insulators can be done using pH of the contaminants as an effective pollution severity indicator, it may further be used for devising proper maintenance schedule.