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Unformatted text preview: Effects of internal discharges on prospective life and overvoltage-withstand behaviour of e.h.v. oil-impregnated paper bushings T. E. Constandinou, M.Sc, A.lnst.P. Abstract The paper gives the results of an experimental laboratory investigation of model bushings, with the object of predicting the behaviour of full-scale e.h.v. oil-impregnated paper bushings operating at a radial a.c. stress of 3kVr.m.s./mm. One of the aims was to determine whether discharges of up to 5pC in full-scale bushings might damage the insulation during prolonged periods in service, and so reduce the prospective life. Frequency-accelerated life tests were made on over 90 model bushings, some of which were constructed according to accepted manufacturing methods, whilst others contained defects of various kinds. It was found that, while the initial-discharge magnitude is a good indication of air trapped in the windings in a region of high electric stress, it is not a good guide to the prospective life of the bushings. The most critical factor is the ability of the oil to dissolve any trapped gas. If the trapped gas dissolves, bushings working at a stress of 3kVr.m.s./mm will have a low discharge level and a long life, provided they have no constructional defects. If not, the discharge magnitude may fall at first, but will then rise steadily until breakdown occurs after 12-20 years. A further aim of the investigation was to determine the extent to which either permanent damage or temporary impairment of the dielectric properties could be caused by overvoltages in the range 7-16kVr.m.s./mm, applied for periods of l-50min. The effects of impulse stresses up to 19kVpeak/mm were also studied. It was found that a sensitivity of about 0-1 pC would be required to detect the discharge resulting from the gas generated during a normal overvoltage test on a well made 400kV bushing, but that much larger quantities of gas produced in this way would dissolve in the oil within a few days, and the bushing would then return to a discharge-free condition, without apparent physical damage. Where a quantity of air was trapped in the winding during manufacture, it was shown that a discharge level of 30 pC at working voltage in a 400kV bushing having a moisture content not greater than 1 % would not involve a risk of degradation by a single overvoltage test having a duration of 1 min at a stress of 14kVr.m.s./mm. List of symbols a = diameter of bushing at earth foil, mm b — diameter of bushing at thermistor, mm C = capacitance of bushing, F d = dielectric thickness of one of the two bushing elements, mm df = aluminium-foil thickness, mm £, = discharge inception stress (d.i.s.), kVr.m.s./mm / = life acceleration frequency, Hz / f/ = mean discharge current, A, measured at 50 Hz and peak voltage V o K = thermal conductivity of oil-impregnated paper, W/cm °C m = moisture content in paper, % by dry weight «! = number of discharges of a given magnitude counted in t t min Q = heat conducted, W Q' = heat generated, W...
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- Spring '11