M.I. Qureshi

Electrical insulation in power systems

Introduction to electrical insulation in power systems gas dielectrics air insulation SF insulation liquid dielectrics solid dielectrics vacuum dielectrics composite dielectrics high voltage cables generation and measurement of testing voltages new measurement and diagnostic technologies insulation testing.

Statistical evaluation of water tree lengths in XLPE cables at different temperatures

Accelerated aging of two types of commercially produced medium voltage XLPE insulated cables was carried out under the combined stresses of three types consisting of ionic aqueous species, elevated electrical stress and temperature cycling. After 1000 h of aging, the water tree parameters were evaluated. Results show that external ionic species exert profound impact on the generation of bow tie and vented water trees, while impurities inherently present in the cable components also contribute significantly. Temperature cycling increases the bow tie trees by an order of magnitude, while morphology of insulation influences the level of water tree degradation. The water tree population and their lengths were subjected to Weibull and log-normal models of statistical distributions and the data were found to fit better on the log-normal model. For a better statistical inference, a computer based Monte Carlo simulation was devised and used to precisely discriminate the two models. Simulation results further confirm that both types of water trees fit better on the log-normal model. The most likely mechanism for water tree degradation is fatigue based and hence mechanistic by nature.

Influence of sand/dust contamination on the breakdown of asymmetrical air gaps under lightning impulses

The authors review the important parameters of particles which are present in the environment when sand/dust storms occur. Basic features of the experimental system are summarized. Extensive measurements of breakdown voltages and times to breakdown are carried out under clean and polluted conditions using rod-plane and sphere-plane gaps. It is shown that when positive lightning impulse voltages are applied to asymmetrical gaps from 5 cm to 2 m, the presence of sand/dust contamination can cause a breakdown voltage reduction. However, the presence of particles can greatly affect the breakdown characteristics of rod-plane and sphere-plane gaps under negative lightning impulses. For gaps with moderately nonuniform field distributions, the particles can decrease V/sub 50/ by >

Performance of rod-rod gaps in the presence of dust particles under lightning impulses

Investigations have been made to evaluate the performance of rod-rod gaps of two different shapes in the presence of simulated dust storms. Breakdown voltages and time lags have been determined for a range of gap spacings from 5 cm to 125 cm for standard lightning impulses. It was found: that the influence of particles in gap spacings of >or=50 cm is small and is not of much practical importance, but in spacings >

Calculation of electric field distribution at high voltage cable terminations

Crosslinked polyethylene (XLPE) insulated power cables are used for transmission and distribution of electrical power. Such cables are subjected to extensive high voltage testing for performance evaluation and quality control purposes. During such testing, the cable ends have to be prepared carefully to make a proper end termination. Usually deionized water terminations are used for this purpose. Alternatively conductive paint is used to prepare such a termination. This paper presents a method of calculating the voltage distribution across such a resistive termination when subjected to AC voltage testing. The proposed method is used to determine the effect of different design parameters on voltage and stress distribution on such cable ends. The method is simple and can be used to explain undergraduate/ graduate students the importance of stress control at a cable termination which is critical part of such cables.

Influence of sand/dust contamination on the breakdown of asymmetrical air gaps under switching impulses

This paper discusses the influence of airborne and surface-adhering sand/dust contamination on the breakdown characteristics of asymmetrical air gaps when subjected to standard switching impulses of both polarities. Extensive measurements of breakdown voltages and breakdown times are carried out under clean and polluted conditions using rod-plane and sphere-plane gaps. When positive switching impulses are applied to gaps /spl les/1.8 m, the presence of sand/dust particles /spl les/150 /spl mu/m diameter and /spl les/2 g/m/sup 3/ concentrations have insignificant influence on the breakdown characteristics of the air gaps. However, the presence of such particles can greatly affect the breakdown characteristics for negative switching impulses. Under such conditions, the particles can decrease V/sub 50/ by /spl les/40% for gaps with field factor f /spl les/3.5. For gaps with 3.5 >

Performance of rod-rod gaps in the presence of dust particles under standard switching impulses

The effects of simulated dust pollution on the breakdown characteristics of protective gaps subjected to standard switching impulses (250/2500 mu s) of both polarities are presented. Hemispherical and square cut rod gaps are used, and their performance is compared in a gap spacing varying from 5 cm to 120 cm. It is found that dust pollution either reduces the 50% breakdown voltage or increases it, depending upon the gap spacing. Formation of a stable dust particle film on the cathode is found to control these irregularities in breakdown parameters. It is also found that square cut rod gaps display immunity toward dust pollution under positive switching impulses, and are therefore best suited for applications in terrains where dust storms are common. >

Optimized pit configuration for efficient grounding of the power system in high resistivity soils using low resistivity materials

In an electrical power system, the integrity of the grounding system is very important to maintain a reference point of potential for equipment and personnel safety, to provide a discharge point for lightning currents as well as to prevent excessive high voltages on the power system. Therefore, to maintain ground system effectiveness, proper design, installation and testing of grounding system is required. In Saudi Arabia, the weather is dry and the soil resistivity varies significantly from area to area because the geodetic terrain varies from sea shore to the arid desert and dry mountains. In most of the inland desert areas, the soil resistivity is significantly high and it is difficult to get the low earth resistance with conventional methods. Therefore to get a low value of grounding resistance, a good design of the grounding pit is necessary which can be achieved by using low resistivity materials. When such materials are used, it is important to optimize the pit design in order to have an economical and efficient grounding system. This paper presents a novel technique for finding the optimum size of grounding pits commonly used in the electrical power systems. With this technique, we can easily find out an optimized pit design that can effectively reduce the grounding resistance to an acceptable value. The obtained results can be readily used by engineers to obtain a good earth pit configuration for efficient grounding of the power system in the high resistivity soils by using low resistivity materials (LRM).

Performance of Composite Insulators in Simulated Environmental Conditions Related to Central Region of Saudi-Arabia

High voltage composite insulators are being accepted increasingly for use in outdoor installations. This research work aims at assessing the experimental performance of the two types of polymeric composite insulators being prepared for use in high voltage overhead transmission and distribution networks. Experimental investigations were carried out to study the effect of ultraviolet radiations as well as heat on the performance of composite insulators simulating the environmental conditions of Central region of Saudi Arabia. To achieve this objective, modified accelerated aging test was performed as per IEC standard 61109 to simulate the field aging of the polymeric insulators and compared various electrical and optical tests results of the laboratory aged insulators with new ones. The results will help in assessing the performance and suitability of polymeric insulators for their applications in Saudi Arabia

Effects of different ionic solutions on statistical length distribution of water trees in XLPE cable insulation

Tests are conducted to investigate the role of varying concentrations of different salt solutions on the water needle and vented water trees grown in insulation discs cut from transmission class XLPE cables. An increase in salt concentration displayed different effects on the average length of the needle grown trees which depended upon the salt solution used. Statistical length distributions of vented water trees produced at the solid-liquid interface was found to be best described by a 2D Weibull stochastic model, rather than the log-normal model.