Electrical breakdown strength of polypropylene film oil-immersed under combined AC-DC voltage

The paper presents data on electrical breakdown strength and partial discharge inception voltage of polypropylene film oil-immersed under a.c., d.c., and superimposed a.c., upon d.c. voltages, conditions that may exist in HVDC converter stations equipment. In the latter case the ratio of the a.c. to d.c. voltage components (p = Va.c./Vd.c.) was varied between the limits of 0.2 ≤ d ≤ 10. The results obtained under combined a.c. plus d.c. voltages follow closely an expression which relates the breakdown voltage to the sum of arc tangent and arc cotangent function of the parameter p. The partial discharge inception voltage remained independant of the parameter p.