Structural influences in nitroxide mediated controlled radical polymerisation

This work describes the synthesis and application in controlled radical polymerisation of several novel pyrrolidine-N-oxyl, PROXYL, based nitroxide and alkoxyamines species bearing a variety of substituents adjacent to the nitroxide functionality. Through variation of the nitroxide electronic structure a variation in rate of styrene polymerisation was observed and one nitroxide demonstrated the ability to mediate polymerisation of w-butyl acrylate. In an attempt to rationalise this behaviour examination was carried out on model species studying the influence of nitroxide structure on thermal dissociation of alkoxyamine carbon to oxygen bond and the propensity of alkoxyamines towards hydrogen transfer disproportionation. It is suggested that the reduced propensity for this hydrogen transfer disproportionation in addition to the rate of the alkoxyamine homolysis is a key factor in determining the activity of nitroxide in mediated controlled radical polymerisation. The results of this study are placed within the context of the current state of knowledge in nitroxide mediated controlled radical polymerisation

Monitoring insulator contamination level under dry condition with a microwave reflectometer

—Build-up of surface contamination on high voltage insulators can lead to an increase in leakage current and partial discharge, which may eventually develop into flashover. Conventional contamination level monitoring systems based on leakage current, partial discharge, infrared and ultraviolet camera are only effective when the contamination layer has been wetted by rain, fog or condensation; under these conditions flashover might occur before there is time to implement remedial measures such as cleaning. This paper describes studies exploring the feasibility of applying microwave reflectometry techniques to monitor insulator contamination levels. This novel method measures the power generated by a 10.45 GHz source and reflected at the insulator contamination layer. A theoretical model establishes the relationship between equivalent salt deposit density (ESDD) levels, dielectric properties and geometry of contamination layers. Experimental results demonstrate that the output from the reflectometer is able to clearly distinguish between samples with different contamination levels under dry conditions. This contamination monitoring method could potentially provide advance warning of the future failure of wet insulators in climates where insulators can experience dry conditions for extended periods

Fault location and diagnosis in a medium voltage EPR power cable

This paper presents a case study on fault location, characterization and diagnosis in a length of shielded 11 kV medium voltage ethylene-propylene rubber (EPR) power cable. The defect was identified on-site as a low resistance fault occurring between the sheath and the core. A 43 m section was removed for further analysis. The fault resistance was characterized and the location of the defect pinpointed to within a few cm using a combination of time-difference-of-arrival location and infra-red imaging. A combination of X-ray computed tomography, scanning electron microscopy and energy dispersive X-ray spectroscopy were then applied to characterize any abnormalities in the dielectric surrounding the breakdown region. A significant number of high density contaminants were found to be embedded in the dielectric layer, having an average diameter of the order of 100 um, a maximum diameter of 310 um and an average density of 1 particle per 2.28 mm3 . Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to determine the geometry and elemental composition of some initial contaminant samples. It was concluded that contamination of the EPR layer, combined with an observed eccentricity of the cable’s core and sheath resulting in a reduced insulation gap, may have led to an electric field concentration in the region of the defect sufficient to initiate breakdown. Preventative strategies are discussed for similar families of cables, including more stringent dielectric testing requirements at the manufacturing stage and PD monitoring to detect incipient failure

Design of a microwave radiometer for monitoring high voltage insulator contamination level

Microwave radiometry is a novel method for monitoring contamination levels on high voltage insulators. The microwave radiometer described measures energy emitted from the contamination layer and could provide a safe, reliable, contactless monitoring method that is effective under dry conditions. The design of the system has focused on optimizing accuracy, stability and sensitivity using a relatively low cost architecture. Experimental results demonstrate that the output from the radiometer is able to clearly distinguish between samples with different contamination levels under dry conditions. This contamination monitoring method could potentially provide advance warning of the future failure of wet insulators in climates where insulators can experience dry conditions for extended periods

Electrical treeing in cable insulation under different HVDC operational conditions

Electrical treeing (ET) can irreversibly deteriorate the insulation of polymeric power cables leading to a complete failure. This paper presents the results of an experimental investigation into the effects of unipolar and polarity reversing DC voltages on electrical tree (ET) and partial discharge (PD) behavior within high voltage direct current (HVDC) cross linked polyethylene (XLPE) cable insulation. A double needle configuration was adopted to produce non-uniform electric fields within the insulation samples, potentially leading to electrical trees. The development of trees was monitored through an optical method and the associated partial discharge signals were measured through an electrical detection technique, simultaneously. The analysis of the results shows reasonable relation between the formation of ETs and the type of the applied voltages. The polarity reversing attribute of the test voltages has a pronounced effect on formation and growth of electrical trees. This implicates an interaction between the space charges that accumulate within polymeric materials and the operational polarity reversing electric fields, which causes insulation degradation. Therefore, study of influencing HVDC operational parameters on insulation degradations can contribute to improvements in cable design and advancement in insulation diagnostic strategies applicable in HVDC systems leading to more effective asset management

A comparison of partial discharge sensors for natural gas insulated high voltage equipment

The research in this paper consists of practical experimentation on a gas insulated section of high voltage equipment filled with carbon dioxide and technical air as a direct replacement to sulphur hexafluoride (SF6) and analyses the results of PD measurement by way of internal UHF sensors and external HFCTs. The results contribute to ongoing efforts to replace the global warming gas SF6 with an alternative such as pure carbon dioxide or technical air and are applicable to mixtures of electronegative gases that have a high content of buffer gas including carbon dioxide. The experiments undertaken involved filling a full-scale gas insulated line demonstrator with different pressures of CO2 or technical air and applying voltages up to 242 kV in both clean conditions and particle contaminated conditions. The results show that carbon dioxide and technical air can insulate a gas section normally insulated with SF6 at phase-to-earth voltage of 242 kV and that both HFCT and UHF sensors can be used to detect partial discharge with natural gases. The internal UHF sensors show the most accurate PD location results but external HFCTs offer a good compromise and very similar location accuracy

Severe, steroid-responsive, myositis mimicking necrotizing fasciitis following orthopedic surgery: a pyoderma variant with myonecrosis

Postoperative pyoderma gangrenosum is a rare neutrophilic dermatosis that may be confused for necrotizing fasciitis. The inflammatory response is triggered by the trauma of surgery and thus must be managed nonsurgically. Clinical and pathological findings in the 2 diseases can be identical, leading to misdiagnosis and massive surgical defects from the ensuing surgery. This report documents a severe case of postsurgical pyoderma following an elective rotator cuff repair presenting with myositis and myonecrosis. The patient was initially treated as having an infection, which resulted in multiple aggressive surgical debridements. Despite this, the patient continued to deteriorate and was in a critical and hemodynamically unstable condition. Following administration of high-dose intravenous corticosteroids, the patient made a dramatic recovery and went on to have internal fixation of the shoulder and closure of the wound with a combination of a free flap and a rotational flap. Extensive myositis, as seen in this case, has not been previously reported in postoperative pyoderma gangrenosum variants. Clinicians should be aware that the presence of myositis and myonecrosis should not preclude this diagnosis

Improved methods for UHF localization of partial discharge in air-insulated substations

A time-difference-of-arrival-based partial discharge (PD) location system that utilises prior knowledge of substation layout is presented. We propose a new time delay estimator that employs onset detection techniques and demonstrate, through experimental results, that it is at least 2.8 times more accurate than other conventional estimators. Using knowledge of a substation’s layout, we develop a minimum mean-square-error (MMSE)-based location estimator and an algorithm that optimises antenna placement to maximise location accuracy in regions occupied by high-voltage equipment. Simulation results show that a system that uses the proposed techniques is 5.9 times more accurate than a more conventional system in a high-noise environment

Teachers' Work in a Globalizing Economy

Extended critical case studies provide a tangible working expression of the labour process of teaching, showing how teachers are simultaneously experiencing significant changes to their work, as well as responding in ways that actively shape these processes. For teachers and researchers, this book shows what processes are at work in the global economy which impact on, and sometimes control, the role of the teacher. It also reveals how teachers accommodate, resist or redefine their working circumstances, and explores methods researchers might employ in order to increase our understanding and knowledge of the effect of globalization on teaching