Method of predicting the thermal loading of an oil circuit breaker

Abstract
The various factors influencing the temperature rises in bulk-oil circuit breakers are discussed and details of direct measurements on a certain circuit breaker with unplated contacts are given. A method for dretermining transient temperatures in the bushings using an electrical resistance capacitance heat-flow analogue in conjunction with the direct measurements is described. This method gives temperatures at points which are not readily measurable by direct means but which are of great importance, i.e. the hot-spot temperatures occurring in the bushing insulation. Steady-state and transient temperatures rises for points in the circuits breaker for which direct measurement and analogue predicted values are available show good agreement.The effects of various values of contact resistance on the bushing insulation temepratures are investigated over a range of contact resistance. The results illustrate the current uprating possible if contact resistances are kept to low values. The changes in the contact resistance of unplated contact with length of service are discussed from information obtained from supply authorities, and the effects of regular maintenance are shown.The relation between the bushing hot-spot temperature rises and the rises of the top oil are examined and indicate that the addition of 10°C to the oil temperature gives a safe estimate of the hot-spot temperature. The effects of eddy-current heating are also discussed.

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