Acoustical Technology Applications in Electrical Insulation and Dielectrics

Acoustical waves travel by molecular interaction, and as a consequence, wave attenuation and transit time can be extremely sensitive to changes in the media in which they propagate. With careful acoustic measurements, a wide variety of media properties can be monitored. These range from variations in gas mixtures and pressure, to changes in material density, porosity, and crystal size and orientation. Acoustical technology can also be applied in the listening mode to sense the tiny acoustic emissions from partial discharges in dielectrics, and from stress, strain, and other events. Many of these acoustic techniques are ideal for applying to the science of electrical insulation and dielectrics, and in this paper these different methods are described and evaluated. The subject is introduced with an outline of ultrasonic partial discharge (PD) diagnostics and nondestructive evaluation, with emphasis on acoustic factors which affect accuracy. It is then shown how the use of acoustic waveguides can improve diagnostic PD measurements in hostile and inaccessible locations. In a completely different application, a technique is described in which sound velocity measurements in gas mixtures are used for predicting electrical strength. Other valuable acoustical applications which are outlined are the monitoring of sounds from bouncing particles on metal surfaces and from gas bubbles generated in liquid dielectrics. From this latter phenomenon the temperature of a hot metal surface in a liquid can be estimated. A related subject which is also discussed is the attenuation of sound waves by gas bubbles in a liquid dielectric.