High field phenomena in polymer films and high frequency attenuation in shielded power cable

Date of Completion

January 2007


Physics, Electricity and Magnetism




Polymer films are too thin to be characterized by guarded needle experiments and must be studied in a quasi-uniform field configuration. In such a configuration if the field is held above about 60% of the short term breakdown field, high field aging will cause the film to break down before dielectric relaxation is complete. Thus to study such films to near the short term breakdown field, film properties must be studied using a ramp voltage (∼300 V/s) up to breakdown, as is employed to characterize the statistical breakdown characteristics. ^ An electronic system with active feedback has been developed for measuring resistive currents in polymer films up to breakdown to study the nonlinear electrical conductivity as reflected in the relaxation time as a function of electric field. Experimental results are presented and a theoretical context has been developed for the experimental results. ^ To investigate the nonlinearity in film polarizability, a system has been designed for measuring high field capacitance up to breakdown. The system measures the capacitance by reading the capacitive current caused by a small sinusoidal signal superimposed on the high voltage DC applied to the test sample and survives high voltage breakdowns. ^ An automatic breakdown tester has been developed to facilitate the large numbers of breakdown on a polymer film required to characterize statistical breakdown properties at the low probabilities relevant to capacitor design. Using this system, a large number of breakdown data can be acquired easily resulting in a better understanding of the polymer film dielectric strength, especially at low breakdown probability. ^ Shielded power cables are employed widely in electric power systems. A good understanding of high frequency dielectric properties of shielded power cable is important for diagnostic testing of such cables using partial discharge measurement and other techniques which depend on propagation of nanosecond pulses along the cable. Experiments and theoretical computations have been conducted to extend and improve understanding of high frequency propagation characteristics of shielded power cable. ^