Ronald T. Harrold

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.

The Relationship Between Ultrasonic and Electrical Measurements of Under-Oil Corona Sources

In order to predict the usefulness of ultrasonic techniques for detecting and locating transformer coronas or partial discharges, it is necessary to determine the relationship between electrical and ultrasonic values of different coronas within an unobstructed oil environment. Experiments are reported in which typical corona sources, ranging in value from ~5-~105 pC, were energized within an oil-filled transparent tank and the ultrasonic emissions recorded via a transducer on the tank outer wall. Several distinct modes, changing with magnitude, of the oil discharges were observed, and the relationship between charge measurements Q and ultrasonic values P were found to vary from P ¿ Q1/2 to P ¿ Q.

Ultrasonic Spectrum Signatures of Under-Oil Corona Sources

Ultrasonic techniques are useful for the detection and location of partial discharge or corona sources associated with HV power apparatus, particularly oil-filled power transformers. Measurements are commonly made using narrow-band transducers operating in the frequency range from ~20kHz to ~300 kHz, and occasionally at higher frequencies for which there is little published information. Because of the lack of available data on the spectra of ultrasonic emissions from discharges in oil-insulation systems, an investigation was initiated to determine if there was an optimum measuring frequency and whether discharges could be identified by their spectral characteristics. It was discovered that insulation voids and gap-type discharges, for example, have clearly identifiable ultrasonic frequency characteristics, and the actual physical size of voids may be estimated from their frequency spectra.

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 Youngs modulus, Poissons 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.

Physical Aspects of Vapor-Mist Dielectrics

In earlier work it was shown that by acoustically cavitating a liquid dielectric, a dense mist (~2 x 105 droplet/cm3) of micron-size droplets could be formed which considerably increased the electrical strength of an insulating gas. The high electrical strength of these vapor-mist dielectrics results from a combination of the vapor from the droplets enhancing the gas strength and the droplets collecting electrons and ions. This latter effect helps prevent the formation of electron avalanches which precede breakdown. Within the vapor-mist dielectrics, complex mechanisms are involved, ranging from the enhanced vapor pressure at the droplet surface to surface tension effects and droplet charge which help prevent droplets from both freezing and evaporating. These aspects of vapor-mist dielectrics are examined and discussed in relation to the acoustic cavitation method used for generating the mist.

Method and apparatus for selective transmission and reflection of sound through a solid

A device is disclosed which allows an object to be tuned to an incoming sound wave magnitude and wavelength, such that the sound wave will alternatively be passed therethrough without detectable reflection or be completely reflected. The apparatus is intended to be mounted within a hollow object and adjust the perceived distance between the exterior walls of that object to mimic a single thick wall. A signal processing means is supplied which detects the sound wave magnitude and wavelength impinging on the wall facing the sound wave source. The wavelength is passed to the signal processing means, which transmits a complimentary signal to the second wall of the hollow object. The complimentary signal allows the hollow object to resonate as a whole as if it were a solid of a thickness much different from its actual thickness, allowing the sound wave to pass undisturbed therethrough or be reflected therefrom.

Acoustic Waveguide Sensors For Smart Structures And Skins

Acoustic waveguides embedded within materials provide a rugged and simple means for sensing stress, strain and other phenomena within smart structures and skins. The waveguides which function well in hostile environments can be used in two signal monitoring modes - transmit/receive and receive (listen), which is helpful when monitoring in noisy environments. They can be applied for monitoring cure as a material is made and subsequently for NDE during the material lifetime. In this paper applications of acoustic waveguides for monitoring material cure, viscosity, strain, and other phenomena are outlined, and the likelihood of the merging of the technologies of embedded optical and acoustic fiber waveguides is discussed.

Acoustic waveguide measurements of the Sun and Moon's gravitational forces on Earth and of quasi-gravity related waves

Over a two year period, measurements of the daily levels of 74.91 kHz. acoustic signals transmitted between oil - immersed acoustic waveguides [AWG] correlated fairly well [20 to 70%] with daily tidal levels controlled by the sun and moons gravitational forces on earth. Consequently, it is reasoned, the AWG measurements are also in some sense or degree related to these forces. Assuming that the daily AWG measurements are indeed gravity related, then a 1 Hz. to 15 Hz. pulsatory background noise or jitter which was found to be associated with the AWG signal with a peak response at 6 Hz. could possibly be indicative of gravitational - type of waves or disturbances occurring somewhere in space. In thls regard, it is interesting to note that pulsars have been studied as possible sources of gravitational waves or disturbances, and that the pulsed radio energy emissions of >90% of pulsars catalogued occur in the <1 Hz. to 20 Hz. frequency range and peak emission also occurs at 6 Hz. This raises the possibility of a gravity - type of low frequency background noise or hum at that frequency.

Quasi-residual strain and moduli measurements in materials using embedded acoustic waveguides

Following the processing and manufacture of resin and composite parts and during their lifetime, the distribution of internal residual strain and any variations in moduli are generally unknown. Real-time information on these parameters would be valuable for improving material performance and reliability. It is believed that measurements related to material residual stresses or strain and moduli can be obtained by measuring the longitudinal wave velocities within acoustic waveguides (AWG) embedded within a material. The concept is that the wave velocities within embedded AWG are related to the material bulk modulus, density and Poissons Ratio which are all in some degree related to the material state of cure, and finally the internal residual stresses. Based on this concept it is shown that the AWG of different diameters embedded within the same resin part of uniform internal stress distribution, the AWG wave velocities should vary in relation to the square root of the AWG diameter. Experimental results using AWG of 5, 10, 16, 20, 40 and 62 mil diameter Nichrome embedded within Shell 815 clear resin with optically measured uniform strain, demonstrate a direct relationship between AWG velocities and the square root of the AWG diameter. Consequently, it is reasoned that for a part with several embedded AWG, each of the same diameter, then differences in the AWG velocities would yield information on differences in the residual strain and moduli within the part.

Attenuation of Ultrasound by Hollow Ceramic Spheres Embedded Within a Curing Resin

The basic technology of using embedded acoustic waveguides (AWG) for cure and NDE monitoring of resins and composite materials has evolved[1–11] over the last forty years. A recent paper[11] describes new applications of AWG in which acoustic wave transmissions between two embedded waveguides allows cure monitoring of large areas of a composite panel and also, in principle, the sensing of voids occurring during curing (simulated by burying hollow ceramic spheres within a composite panel). A surprising result was the system sensitivity, as the presence of only 0.16% by volume of hollow ceramic spheres could be detected. This result prompted the more detailed study reported here of ultrasound attenuation by voids (hollow ceramic spheres) embedded within a curing resin.