Abnormal pressure-rise in transformers and its remedy

In a high-tension transformer, abnormal potential gradients likely to cause a breakdown appear usually at the foot of the bushing and at the end coils. The author describes briefly a theory which is applicable to both these cases and from which he concludes that these dangers may be due to the incorrect arrangement of electrostatic capacities. Following this theory, he shows that, by properly grading the capacities, it is possible to avoid to a great extent the dangers in both the bushing and the end coils. The correct gradation ot the capacities is shown to be as follows:¿ (1) The shunt capacity (Cs) must be graded so as to increase with the distance from the earthed end of the coil, or from the top of the bushing. (2) The capacity (Cg) to earth, or to the high-tension conductor, should be graded so as to decrease with the distance. (3) The ratio Cs/Cg should be as great as possible. (4) It is preferable to use a resistance in series with the capacity Cs. Experiments show that where a transformer is exposed to excessive stress a potential gradient of about 20 times the normal value may occur at the end coil, and this can be reduced to one-third this value by the device proposed. The author´s method also prevents breakdowns due to other causes, such as the decrease in the natural frequency of the coil.