Calculation of Steady-State Surface at Winding Ends of Air-Core Power Reactors

Under severe wet weather conditions, air-core power reactors may exhibit local surface discharge activities. These activities have on occasion been observed to take place on the outer surface of the "end ring" i. e. on that part of the winding encapsulation which separates the winding from the termination structure ("spider"). Occurrence of these discharges is a function of the coil electrode configuration, operating voltage levels and properties of the encapsulation material. Laboratory evidence, however, indicates that these discharge activities exist only at very high local tangential voltage stresses and that they tend to be self-limiting. It is the purpose of this paper to report on a numerical investigation into prevention of such activities through proper dimensioning of the end ring. Results obtained, in terms of comparisons among various situations, are in good agreement with experimental observations and accumulated field experience.