Aejaz Masood

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.

Electrical breakdown of Liquid Nitrogen viewed from area & volume effects

Liquid nitrogen (LN2) plays a crucial role in applying high temperature superconducting (HTSC) materials to the power apparatus. However limited literature is available pertaining to design of practical insulation for HTSC devices using LN2. This study attempts to investigate the area and the volume effects on the electrical breakdown strength in LN2 so as to obtain better design of electrical insulation for superconducting power apparatus. Measurements were made to assess the ac breakdown voltages in LN2 utilizing sphere-plane electrode configuration. Experimental results revealed that the breakdown voltage in LN2 did not increase monotonously but decreased partially as the sphere diameter increased at a constant gap length. The ac breakdown voltages in LN2 were also measured for coaxial cylindrical electrode geometry to examine the area & volume effects and the breakdown voltage showed a decrease with an increase in 90% stressed electrode area (SEA) 90 and 90% stressed liquid volume (SLV) 90.

High voltage dielectric diagnosis using automatic capacitance & dissipation factor test system

Failure of dielectric material in power equipment usually results into undesired outages and expensive replacements. Chances of failure increase with increase in electric stress on dielectric materials at high voltages. It is therefore; necessary to assess the electrical insulating properties of the materials before their practical use. This research work was aimed to determine different inherent properties of dielectric materials like volume resistivity; dielectric constant; loss tangent and co-relate them on an empirical basis by using variables predicted by basic theory. A simple equation to predict dielectric strength using ASTM electrode system; has been proposed for insulating materials like Leatheroid; Diamond dotted paper; Paper phenolic sheet; Glass bonded mica paper; Nomex and Epoxy resin fiber glass. Values of the parameters required in the equation may be obtained by performing non-destructive dielectric diagnostic measurements using an advance capacitance and tan delta bridge. This is expected that the suggested equation will be useful to practicing engineers and designers for quick estimation of electric stresses in dielectric materials studied in this research work.

Dielectric behaviour of insulating materials under liquid nitrogen

In practical HTSC devices, equipment and systems use of both LN/sub 2/ (liquid nitrogen) and CGN/sub 2/ (cryogenic nitrogen) is almost essential. Both LN/sub 2/ and CGN/sub 2/ are used not only as a coolant but also as an insulating material. In the present study an attempt has been made to study the dielectric behavior of solid insulating materials dipped in LN/sub 2/ as regards to change in loss index.

Advances in the field of insulating liquids used in electric power equipment and their perspectives-a review

Petroleum-based oils have been widely used as liquid insulation in electric power industry for over a century because of their ready availability, low viscosity, good aging behavior, and low cost. But, growing environmental concern, material sustainability, and desire for a safe nonflammable insulating liquid in power equipment have driven the researchers and engineers to explore alternative liquids with improved properties, and significance improvements have been achieved in this field. The objective of this review paper is to discuss about some of the developments in the field of liquid insulations, main features, applications, perspectives and engineering problems of several types of insulating liquids such as natural and synthetic organic liquids, cryogenic liquids, and liquefied gases. Properties like fire point, flash point and biodegradability for mineral oil, vegetable oil, silicon oil, and synthetic ester fluids are compared. A few recently installed liquid-filled power equipment with advanced insulating fluids are also mentioned.

Diagnosis of present-day cellulosic insulating materials used in electrical power industry

Failure of solid dielectric materials in power equipment usually results into undesired outages and expensive replacements due to absence of recovery unlike in liquid and gas counterparts. It is therefore, necessary to assess the electrical insulating properties of the solids materials. This research work was aimed to determine if a relationship could be found between breakdown strength and other properties of solid insulating material on an empirical basis by using variables predicted by basic theory. A simple equation to predict dielectric strength using ASTM electrode system, considering the effect of stray or edge capacitances has been proposed for cellulosic insulating materials like diamond dotted paper, paper phenolic sheets, cotton phenolic sheets, leatheroid, and presspaper which are in use present time. The equation defines a relationship between dielectric strength and other properties like loss tangent, volume resistivity, dielectric constant and thickness. Values of the parameters required in the equation may be obtained by performing non-destructive dielectric diagnostic measurements. For a given material, calculated electric strength using the proposed equation and experimentally measured values are also compared and found close to each other. This is expected that the suggested equation will be useful to practicing engineers and designers for quick estimation of electric stresses in cellulosic materials studied in this research work.