Modelling the Non-equilibrium Electric Double Layer at Oil-pressboard Interface of High Voltage Transformers

In large oil-filled power transformers, cellulose-based pressboard and paper are used throughout for electrical insulation. Microscopic views have shown that pressboard insulation is a fibrous and porous structure with non-homogeneous surface. It has been recognised that the pressboard structure is more porous towards the edge [1]. The pores within the pressboard allow oil absorption during impregnation process and provide paths for oil to penetrate until saturation is reached. The ratio of fibre and oil changes as the material structure changes from a medium of bulk oil-pressboard composite toward the bulk oil medium. The porosity of pressboard can also result in impurities within the oil being drawn into the pressboard. It has also been recognised that physicochemical process of a liquid in contact with solid wall leads to the formation of electric double layer (EDL) in the liquid region [2, 3]. The material properties and geometry of pressboard thus lead to a complex oil-pressboard interface. A 2-D model of oil-pressboard interface has been constructed using Comsol Multiphysics Finite Element Analysis software and this is shown in Figure 1. The mathematical model considers the dissociation of a generic impurity in the oil into positive and negative ions and considers the role of the porous and non-homogeneous wall of pressboard in the formation of the EDL. The pressboard, which is represented by different arrays of fibre, promotes preferential adsorption and desorption processes between ions in the oil and unoccupied fibre surfaces of oil impregnated pressboard. The model studies the non-equilibrium charge density profile in the EDL at the oil-pressboard interface when the oil is in the stationary condition

The views of young people with congenital cardiac disease on designing the services for their treatment

Background and purpose: There is little documented evidence of young people with congenital cardiac disease being consulted as to what help, if any, they really need in relation to their condition. Most research concentrates on the medical aspects of the condition. There are studies of psychological and social functioning, but few have directly sought the opinions of the young people. More recent research has indicated a need for health professionals to develop services to meet both psychosocial and physical needs of young people with congenital cardiac disease. The findings of the recent Kennedy Report support this need. The purpose of this study was to explore the views of young people with a range of congenital cardiac conditions, on what would help them better deal with their condition, and when and how help might be provided. Methods: This was a qualitative study using semi-structured interviews. We interviewed 16 young people in their own home. Interviews were tape-recorded and transcribed. Analysis was conducted using the "Framework" method. Results: Whilst most of the young people interviewed actively manage their condition, they think more support from others would be beneficial. Issues of activity and communication were cited most often as areas requiring more understanding from people they interact with, for example teachers and peers. The discussion focuses on how health professionals might change or develop their practice to help young people better cope with their condition. Conclusions: Most of the young people interviewed in this study had developed their own strategies for coping with their condition. Although this is a small study, the young people provided important suggestions as to how health professionals could better develop current services

A simple screening method for determining knowledge of the appropriate levels of activity and risk behaviour in young people with congenital cardiac conditions

Objective: To assess a novel method for assessing risk and providing advice about activity to children and young people with congenital cardiac disease and their parents. Design and setting: Questionnaire survey in outpatient clinics at a tertiary centre dealing with congenital cardiac disease, and 6 peripheral clinics. Interventions: Children or their parents completed a brief questionnaire. If this indicated a desire for help, or a serious mismatch between advised and real level of activity, they were telephoned by a physiotherapist. Main measures of outcome: Knowledge about appropriate levels of activity, and identification of the number exercising at an unsafe level, the number seeking help, and the type of help required. Results: 253/258 (98.0%) questionnaires were returned, with 119/253 (47.0%) showing incorrect responses in their belief about their advised level of exercise; 17/253 (6.7%) had potentially dangerous overestimation of exercise. Asked if they wanted advice 93/253 (36.8%) said “yes”, 43/253 (17.0%) “maybe”, and 117/253 (46.2%) “no”. Of those contacted by phone to give advice, 72.7% (56/77) required a single contact and 14.3% (11/77) required an intervention that required more intensive contact lasting from 2 up to 12 weeks. Of the cohort, 3.9% (3/77) were taking part in activities that put them at significant risk. Conclusions: There is a significant lack of knowledge about appropriate levels of activity, and a desire for further advice, in children and young people with congenital cardiac disease. A few children may be at very significant risk. These needs can be identified, and clinical risk reduced, using a brief self-completed questionnaire combined with telephone follow-up from a suitably knowledgeable physiotherapist

The views of parents concerning the planning of services for rehabilitation of families of children with congenital cardiac disease

Background and purpose: Although much previous research has focused on the medical aspects of congenital cardiac disease, there is a growing body of research which suggests that families may need help and support with the wider issues associated with the medical condition. We have previously ascertained from young people with congenital cardiac disease their views on this subject. The purpose of this study was to obtain the views of their parents about the need for, and shape of, services for rehabilitation. Methods: This was a qualitative study using semi-structured interviews. We interviewed 17 parents in their own home. Interviews were tape-recorded and transcribed. Results: Parents would welcome more help and support from health professionals to enable them to manage more effectively the condition with their children. Particular areas of concern relate to the information they receive about the condition; communication between themselves and health professionals; establishing safe levels of activity; and managing the condition at school. Conclusions: This is a small study of the experiences and views of parents, which provides some important information on ways in which health professionals. could address the current lack of services for rehabilitation

Use of Machine Learning for Partial Discharge Discrimination

Partial discharge (PD) measurements are an important tool for assessing the condition of power equipment. Different sources of PD have different effects on the insulation performance of power apparatus. Therefore, discrimination between PD sources is of great interest to both system utilities and equipment manufacturers. This paper investigates the use of a wide bandwidth PD on-line measurement system to facilitate automatic PD source identification. Three artificial PD models were used to simulate typical PD sources which may exist within power systems. Wavelet analysis was applied to pre-process the obtained measurement data. This data was then processed using correlation analysis to cluster the discharges into different groups. A machine learning technique, namely the support vector machine (SVM) was then used to identify between the different PD sources. The SVM is trained to differentiate between the inherent features of each discharge source signal. Laboratory experiments indicate that this approach is applicable for use with field measurement data

Electromagnetic field application to underground power cable detection

Before commencing excavation or other work where power or other cables may be buried, it is important to determine the location of cables to ensure that they are not damaged. This paper describes a method of power-cable detection and location that uses measurements of the magnetic field produced by the currents in the cable, and presents the results of tests performed to evaluate the method. The cable detection and location program works by comparing the measured magnetic field signal with values predicted using a simple numerical model of the cable. Search coils are used as magnetic field sensors, and a measurement system is setup to measure the magnetic field of an underground power cable at a number of points above the ground so that it can detect the presence of an underground power cable and estimate its position. Experimental investigations were carried out using a model and under real site test conditions. The results show that the measurement system and cable location method give a reasonable prediction for the position of the target cable

Effect of Cross-Linking on the Electrical Properties of LDPE and its Lightning Impulse Ageing Characteristics

Cross-linked polyethylene (XLPE) is commonly used within high voltage cable insulation. It has improved thermal and mechanical resistance compared to normal low density polyethylene (LDPE). However, the cross-linking process may also vary the electrical characteristics of the material. This paper investigates changes in electrical properties of one type of LDPE before and after cross-linking. The effective lightning resistance is also considered, as the application of repetitive lightning impulse overvoltages can be a factor in insulation material ageing of high voltage cables. The material was cross-linked using trigonox-145 peroxide with controlled concentration. Samples were moulded to have a Rogowski profile and gold coated to make sure that they are evenly electrically stressed. Obtained results show that there are reductions in both space charge injection and the permittivity of the material after it is cross-linked. The breakdown strength of the material was also improved. However, the samples studied are more susceptible to ageing due to lightning impulses

Detection and Location of Underground Power Cable using Magnetic Field Technologies

The location of buried underground electricity cables is becoming a major engineering and social issue worldwide. Records of utility locations are relatively scant, and even when records are available, they almost always refer to positions relative to ground-level physical features that may no longer exist or that may have been moved or altered. The lack of accurate positioning records of existing services can cause engineering and construction delays and safety hazards when new construction, repairs, or upgrades are necessary. Hitting unknown underground obstructions has the potential to cause property damage, injuries and, even deaths. Thus, before commencing excavation or other work where power or other cables may be buried, it is important to determine the location of the cables to ensure that they are not damaged during the work. This paper describes the use of an array of passive magnetic sensors (induction coils) together with signal processing techniques to detect and locate underground power cables. The array consists of seven identical coils mounted on a support frame; one of these coils was previously tested under laboratory conditions, and relevant results have been published in [1]. A measurement system was constructed that uses a battery powered data acquisition system with two NI 9239 modules connected to the coil array, and controlled by a laptop. The system is designed to measure the magnetic field of an underground power cable at a number of points above the ground. A 3 by 3 m test area was chosen in one of our campus car parks. This area was chosen because the university’s utility map shows an isolated power cable there. Measurements were taken with the array in 16 different test positions, and compared with the values predicted for a long straight horizontal cable at various positions. Finally, error maps were plotted for different Z-coordinate values, showing the minimum fitting error for each position in this plane. One such map is shown in Figure 1; the low error values of 4-5% give a high degree of confidence that most of the measured signal is due to a cable near to these positions. This view is supported by the fact that the university’s utility map shows the cable at X = 1.4 m, and by amplitude measurements taken with a hand-held magnetic field meter

A Comparison of Electrical Breakdown Characteristics of Composite Materials Prepared With Unmodified Micro and Nano Scale Barium Titanate

High permittivity polymer matrix composites (PMCs) have been widely researched, especially in the field of microelectronics. For this study, high permittivity materials were investigated for their potential to form part of a multi-layer electric field detector. The two main requirements for such composites were high permittivity and a dielectric strength comparable to most standard polymers used as dielectric materials. Polystyrene was selected as a host polymer due to its high dielectric strength and amorphous structure. Barium titanate, a ferroelectric ceramic from the perovskite family, was selected as a high permittivity filler. Polymer permittivity in PMCs is usually orders of magnitude lower compared to the filler permittivity, although the resultant permittivity of the composite is generally markedly lower than the permittivity of the filler may suggest. This is because very little energy is stored in the ceramic filler, such that any increase in composite permittivity is due to an increase in the average field with the polymer matrix.[1]Micro and nano scale barium titanate was blended into polystyrene in an effort to discern the initial differences between composites prepared with the two different filler types. It was found that the micro scale barium titanate was well dispersed and from studying SEM micrographs, appeared to have a good particle size distribution. The nanoscale barium titanate was found to be very poorly dispersed in polystyrene, with a wide particle size distributions formed of weakly bound aggregations and some seemingly chemically bonded agglomerations which were regular in shape with a surface texture which was indicative of tightly bound primary particles. Consistent with the differences in particle dispersion within the micro and nano composites, there was a marked difference in AC breakdown strength between the different materials. All electrical breakdown data was analysed using a 2 parameter Weibull distribution. Figure 1 compares the ? values for the micro and nano composites at different filler loadings.<br/