Experimental study and rational planning of insulation coating to suppress the corona appearance on the coil surface

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Corona developement in the coil insulation has become World's concern in order to suppress the insulation deterioration and to find appropriate methode for its measurement, and also according to them it has been extended to the problem of suppressing the corona appearance from the surface of the coil. An answer to this problem up to date has been to coat the coil surface with the resistance paint. In this report, author adopted tan δ-method to find whether the coating was effectual to prevent the corona bursting on the surface, and also tan δ may become a measure of the corona loss when the voltage higher than the critical one is applied, and also in this report, tan δ is used in experimentally analysing the locations according to the slot part and the end turn part. The corona appearance is classified by location:- one is the slot and the other the end turn, each of which has different mechanism to its occurence. Author firstly calculates and discusses the mechanism of corona bursting on the surface of the end turn. From the fact that the voltage gradient has been extremly intensive at the particular place along the surface of the end turn and the corona bursting always lies at this place, we considered how the resistance along the surface should be distributed to remove the intensive one. As one of the easy ways, we chose to coat the surface with high resistance paint and the stress is relieved suitably. We finally obtained the distribution rule through the calculation from the fact that the surface should have its resistance applied according to certain power function as most favourable case, and then discuss the process to realize this idea into practice. In reference to the surface contact between the slot and the iron core, we discuss the limitation of the surface resistance, i. e. if the resistance is too high, it will not be sufficient to suppress the corona and if the resistance is too low, it will enlarge the eddy current loss on the surface.