Bushing insulators for outdoor transformers

Abstract

The paper has four sections which discuss:—(1)The electrical characteristics which outdoor bushings should possess for satisfactory operation; (2) the testing of bushings; (3) the factors which control the design of bushings; and (4) types of bushings.The paper deals particularly with bushings for outdoor transformers, but other bushings are briefly considered, e.g. those for oil circuit-breakers and also wall and roof bushings and cable terminations.(1) Electrical Characteristics.Desirable characteristics are discussed, the bushing being considered firstly as part of a transformer and therefore subject to all the transformer tests, and secondly as part of a transmission line and therefore subject to over-potentials which originate in the line.Bushing as part of transformer.—The transformer with its bushings is assumed to be tested in accordance with B.S.S. No. 171-1927. Values are thus obtained for the 1-minute test voltages, particularly for 3-phase transformers with neutral earthed or insulated. Methods of bushing failure by flash-over and by puncture are considered, and figures are given for the various flash-over voltages, viz. air-end flash-over under dry and wet conditions and also oil-end flash-over.Bushing as part of transmission line.—The transmission-line insulators are assumed to conform to the test requirements of B.S.S. No. 137-1930. The magnitudes of over-potentials which originate in the line and impinge on the transformer are considered. A law is given for the relative strengths of transformer insulation as measured by 1-minute test values and as measured under impulse conditions. The law is used to obtain flash-over characteristics for “weak links” in the line insulation. Recommended bushing flash-over characteristics are given.The two sets of flash-over figures now obtained are considered together and final recommendations are made for bushing characteristics. For comparison the characteristics are given for bushings to Central Electricity Board requirements and also for bushings in accordance with the new British Standard Specification B.S.S. No. 223-1931.Indoor transformer bushings ; bushings for oil circuitbreakers and cable terminations ; wall bushings ; roof bushings.—The differences between these and outdoor transformer bushings are considered. Suggested characteristics are given. Lightning arrestors.—These are briefly considered.(2) Testing of Bushings.Dry flash-over voltage.—Effects of barometric pressure, air temperature and humidity.Wet flash-over voltage.—The effects of intensity of rainfall and resistivity of water are considered.Duration of test.—Figures are given of the times taken to carry out various tests.Temperature-rise of metal parts.—The use of thermocouples to measure temperature-rise is described, and results of typical tests are given.Temperature-cycle test.—This test is described.(3)Design of Bushings.Features of electrical design are first considered.Temperature ; duration of test ; shape of electrodes.—These factors are considered as affecting the electric strength of insulation.Permissible voltage gradients.—These are given for porcelain, varnish-paper insulation, air, oil, solid compound, and semifluid compound.Laws of corona and puncture.—These laws are developed for bushings having two or three different dielectrics.Flash-over voltages.—Curves are given connecting axial lengths over bushing surface with flash-over voltages for dry, wet and under oil conditions.Heating of bushings.—Brief consideration is given to the heating of the centre conductor, allowing for skin effect, and to the heating of the earth flange due to hysteresis and eddy-current losses.Features of mechanical design and construction are also considered.Clamps.—For dealing with large currents.Joints.—The making of oil-tight and weather-tight joints.(4) Types of Bushings.The following types are described and discussed:—All-porcelain air-core; compound-filled; oil-filled; condenser; all-porcelain multi-tubular; all-porcelain multi-tubular embodying condenser feature.The incorporation of a current transformer as part of the bushing is described, and capacitance coupling is briefly considered.In conclusion the following Appendices are given:-I. Development of law connecting dry and wet flash-over voltages.II. Corona formation in air-core porcelain bushings.III. Radial electrostatic stresses in bushings having three different dielectrics.IV. Statistics of heavy rainfall.