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 3kV r.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 3kV r.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?16kV r.m.s./mm, applied for periods of 1?50min. The effects of impulse stresses up to 19kV peak/mm were also studied. It was found that a sensitivity of about 0.1pC 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 30pC 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 14kV r.m.s./mm.
