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Warwick, United Kingdom

Mitchinson P.M.,University of Southampton | Lewin P.L.,University of Southampton | Strawbridge B.D.,University of Southampton | Jarman P.,UK National Grid Corporation
IEEE Electrical Insulation Magazine | Year: 2010

Great care is taken when designing and manufacturing large oil-filled transformers. One design requirement is to ensure that all conducting surfaces have blended geometries and are covered by solid insulation material, typically a cellulose material in the form of paper or pressboard. This prevents the emission of electronic charge that will occur from any bare conducting surfaces according to the modified Schottky process when a conductor is under high electrical stress [1]. The solid insulation effectively adds an extra barrier to charge emission. However, other phenomena can introduce charge into the insulation system under certain conditions. For example, space charge can form at an oil-pressboard interface under a high electric field due to interfacial polarization arising from the difference in permittivity between adjacent materials [2]. Partial discharges (PD) are another source of electrical charge generation. Partial discharges may occur in solid, liquid, and gaseous insulations wherever there are sharp changes or discontinuities within the insulation medium and where the electric field is sufficiently high to exceed the local breakdown value. Surface contamination is a likely site for PD due to the presence of inclusions and changes in bulk material. Finally, oil flowing across pressboard can build up static charge on the pressboard surface, a phenomenon referred to as static electrification but also known as flow or streaming electrification [3]. © 2006 IEEE. Source

Awadallah S.K.E.,University of Manchester | Milanovic J.V.,University of Manchester | Jarman P.N.,UK National Grid Corporation
IEEE Transactions on Power Systems | Year: 2014

This paper presents a probabilistic framework for making decisions about the replacement of ageing power equipment. The framework involves three steps: first, to identify the most important and critical components of the system for overall system reliability; secondly, to perform Pareto analysis to relate the replacement of the components to the effect on system reliability indices; finally, to determine the optimum scenario for replacement based on a comparison between the cost of unreliability due to deferring the replacement and the saving on reinvestment cost. The proposed approach is illustrated on a meshed test system modeled using U.K. transmission system parameters, a representative transformer age profile and regulatory energy not supplied values. The results demonstrate the feasibility of the framework for application in the area of power system reliability, and show its feasibility for informing replacement decisions. © 1969-2012 IEEE. Source

Dai J.,University of Manchester | Wang Z.,University of Manchester | Jarman P.,UK National Grid Corporation
IEEE Transactions on Dielectrics and Electrical Insulation | Year: 2010

This paper presents experimental research of creepage discharge on insulation barriers in power transformers. Using point-to-plate electrode configurations under ac voltages creepage discharge is studied for both single flashover breakdown and partial discharge induced failure modes. It is confirmed that the dielectric strength of oil gap will not be reduced with introducing dry new pressboard surface into the oil gap, indeed the flashover breakdown voltage is hardly compromised by introducing aged pressboard with up to 3% moisture. However, increased moisture content in pressboard reduces partial discharge inception voltage (PDIV) significantly, i.e. ∼30% PDIV reduction for pressboard of ∼3% moisture as compared with dry pressboard. More importantly, high moisture contents in pressboard increase PD activities in oil pores which allow gasses to be trapped inside to develop gaseous channels which eventually lead creepage discharge to breakdown © 2010 IEEE. Source

Kroll M.E.,University of Oxford | Swanson J.,UK National Grid Corporation | Vincent T.J.,University of Oxford | Draper G.J.,University of Oxford
British Journal of Cancer | Year: 2010

Background: Epidemiological evidence suggests that chronic low-intensity extremely-low-frequency magnetic-field exposure is associated with increased risk of childhood leukaemia; it is not certain the association is causal. Methods:We report a national case-control study relating childhood cancer risk to the average magnetic field from high-voltage overhead power lines at the child's home address at birth during the year of birth, estimated using National Grid records. From the National Registry of Childhood Tumours, we obtained records of 28 968 children born in England and Wales during 1962-1995 and diagnosed in Britain under age 15. We selected controls from birth registers, matching individually by sex, period of birth, and birth registration district. No participation by cases or controls was required. Results:The estimated relative risk for each 0.2 T increase in magnetic field was 1.14 (95% confidence interval 0.57 to 2.32) for leukaemia, 0.80 (0.43-1.51) for CNS/brain tumours, and 1.34 (0.84-2.15) for other cancers. Conclusion:Although not statistically significant, the estimate for childhood leukaemia resembles results of comparable studies. Assuming causality, the estimated attributable risk is below one case per year. Magnetic-field exposure during the year of birth is unlikely to be the whole cause of the association with distance from overhead power lines that we previously reported. © 2010 Cancer Research UK. All rights reserved. Source

Bunch K.J.,University of Oxford | Keegan T.J.,Lancaster University | Swanson J.,UK National Grid Corporation | Vincent T.J.,University of Oxford | Murphy M.F.G.,University of Oxford
British Journal of Cancer | Year: 2014

Background: We extend our previous study of childhood leukaemia and proximity to high-voltage powerlines by including more recent data and cases and controls from Scotland, by considering 132-kV powerlines as well as 275 and 400 kV and by looking at greater distances from the powerlines. Methods: Case-control study using 53 515 children from the National Registry of Childhood Tumours 1962-2008, matched controls, and calculated distances of mother's address at child's birth to powerlines at 132, 275, and 400 kV in England, Wales and Scotland. Results: Our previous finding of an excess risk for leukaemia at distances out to 600 m declines over time. Relative risk and 95% confidence interval for leukaemia, 0-199 m compared with>1000 m, all voltages: 1960s 4.50 (0.97-20.83), 2000s 0.71 (0.49-1.03), aggregate over whole period 1.12 (0.90-1.38). Increased risk, albeit less strong, may also be present for 132-kV lines. Increased risk does not extend beyond 600 m for lines of any voltage. Conclusions: A risk declining over time is unlikely to arise from any physical effect of the powerlines and is more likely to be the result of changing population characteristics among those living near powerlines. © 2014 Cancer Research UK. Source

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