Acoustic Theory Applied to the Physics of Electrical Breakdown in Dielectrics

In order to stimulate further thought and research within the science of dielectrics, this paper discusses the remarkable correlation between acoustic theory and the physics of electrical breakdown in dielectrics. Following a brief, simplified fied review of acoustic theory and the propagation and attenuation ation of soundwaves in gases, liquids and solids, three areas in dielectrics are addressed in which the breakdown mechanisms require further clarification. These are the relative electrical strengths of gases and mixtures; streamer initiation in liquid hydrocarbons; and the incubation period before treeing or breakdown own occurs within solid dielectrics. Acoustic analysis of gas breakdown clearly shows that the relative electrical strengths of gases and mixtures vary as the inverse square of sound velocity, and consequently, are closely related to both the molecular weight and specific heat ratio of the gas or mixture. For liquid hydrocarbons, an analysis of acoustic emission levels from partial discharges combined with acoustic cavitation theory, supports the hypothesis that cavitation (collapse) of microbubbles bubbles attached to dust particles may provide the conditions for streamer initiation. Finally, attention is drawn to the relationship between the velocity of sound within solid dielectrics and Young's modulus, Poisson's ratio and density, which suggests a potential application of acoustics for assessing the mechanical strength of dielectrics as they age. Thus, simple acoustic measurements performed on solid dielectrics may provide some information on the structural condition Qf these dielectrics before treeing and subsequent breakdown occurs.