Measurement of space-charge distributions in insulators under very rapidly varying voltage

In this paper we describe a new instrument for real-time measurements of space charge distributions in insulators submitted to rapidly varying voltages. It is illustrated with the pressure wave propagation method but can also be adapted for use with the pulsed electro-acoustic method. A new acoustic generator has been designed to produce high amplitude pressure pulses, at a high rate, and with good reproducibility. This generator, made of a thin transducer coupled with a waveguide and a backing medium, responds with a pressure pulse when a step voltage is applied. A special sample holder with a signal decoupling circuit, and a storage unit have been developed. The described instrument allows for measurements at a rate of 3200 Hz. Its potentialities are illustrated by studying the evolution of space charge in polyethylene samples stressed by both pulsed and 50 Hz ac voltages.