Abobakr S. Ahmed

Airborne Dust Size Analysis for Tropospheric Propagation of Millimetric Waves into Dust Storms

This paper presents a study of five dust storms that occured in Riyadh together with their effect on millimeter-wave propagation. Meteorological parameters are given and airborne-dusts permittivity is estimated using relative humidity data. For large particles, the size distribution and its height dependence are investigated experimentally and the findings are presented as a functional relationship. Based on actual measurements and analysis, attenuation at 37 GHz is calculated using a newly derived expression in terms of optical visibility as a storm parameter. It is found that the fitted probability distributions are best described by normal or lognormal and vary with duststorm conditions. The average effective sizes and the 37-GHz attenuation decreases with the increase of antenna height following a power law.

Measurement of atmospheric particle size distribution during sand/duststorm in Riyadh, Saudi Arabia

Abstract This paper presents new results of measuring and analyzing atmospheric sand/dust particles during storms. Particles are collected at various heights in Riyadh, Saudi Arabia, a typical arid inland city. It is found that most of the particle size distributions can be described by a lognormal or normal distribution depending on the storm condition and height. The average diameter of sand/dust particles decreases with the increase of height according to a power law.

Effect of sub-atmospheric pressure on the conduction current in transformer oil

The effect of subatmospheric pressure on the quasi-steady-state and burst currents in transformer oil is investigated. Nonuniform high DC field is applied to aged and fresh transformer oil samples. The applied pressure is reduced from 10/sup 3/ torr to 50 torr and then increased correspondingly for each applied voltage inside a controlled vacuum chamber. Generally, it is found that at a fixed-field voltage the quasi-steady-state current remains almost unchanged with pressure variation. At a certain critical low subatmospheric pressure, the current increases sharply. The value of this critical pressure increases as the applied voltage is increased. Variation of pressure has similar effects on burst current. The results reveal that the gas bubble mechanism is the dominant factor affecting the conduction current and breakdown in insulating liquid at low pressure. A critical pressure initiates the formation of a stream of gas bubbles after which the current increases sharply, leading to a breakdown.<<ETX>>

High Field DC Conduction Current and Spectroscopy of Aged Transformer Oil

This paper studies the experimental results of the quasi-state high field dc conduction current, and changes occuring in the molecular structure of aged transformer oil, sampled from EHV transformer operating for the last five years. Aged oil was compared with fresh transformer oil and liquid paraffin. It was found that aged oil exhibits higher conduction than both of the other oils through 600 seconds of field application. However, no molecular changes were detected using different techniques of spectroscopy such as GC; UV; IR and NMR. Metallic impurities were found to be of the same order but the acidity increased manifolds to that of fresh oil.

Effect of temperature on burst phenomena in aged mineral oil under non-uniform fields

Presents the experimental results of high field conduction current burst phenomena in aged transformer oil using point-plane configuration. The effect of temperature and field strength variations on burst parameters are emphasized. It was found that bursts can be classified according to their direction. Generally, the amplitude of bursts increases with the increase in temperature, up to a certain value, after which the trend reverses. The total number of bursts per minute peaks at about 60-70°C. This observation suggests that bursts may be attributed to the electrode-film-liquid interface and not to micro bubble mechanisms.

Study of DC conduction in aged transformer oil under non-uniform high fields

This paper emphasises experimental results of DC conduction current in aged transformer oil using point-plane configuration. The effects of oil dehydration and filtration, anode material and field strength are investigated and discussed. It is found that i-t and i-E characteristics display exponential “behaviour and tunneling phenomenon, respectively. Moreover, sample preparation and anode/liquid interface play an important role in explaining the mechanism of DC conduction under non-uniform high field.