High Voltage

High voltage research at LEES focuses on the fundamental research of material behavior in high electric fields and on applications to electric power and pulse power apparatus. A major concern is high voltage conduction and breakdown phenomena in solid, liquid, and gaseous dielectrics. Our work employs physical, mathematical, and computational modeling, as well as considerable experimentation. An important part of our work at LEES is the development of new sensors and devices, specifically designed and fabricated to monitor the behavior of materials used in our experiments.

We often use new, nondestructive measurement techniques for our research. These techniques can be grouped into roughly four basic methods: optical, electrical, ultrasound, and microwave absorption. For example, we apply optical and acoustic techniques to measure electric field and charge density distributions in our investigations of dielectrics. With our specially developed sensors, we can continually measure electrical and physical properties of dielectrics under representative operating conditions. We use sensor signals to provide the raw data that is the basis for interpretating and understanding the status of practical electric power apparatus. For example, a transformer diagnostic system, together with trend analysis software developed at LEES, helps predict impending problems and subsequent corrective actions.

Ultrasound provides a powerful nondestructive technique to quantify internal characteristics of materials, interfaces, and apparatus. For example, in a pioneering approach, we measure the entire charge distribution within the interior of a dielectric as a proportional acoustic wave. The wave is generated by a small added voltage stimulation, so the measurement can be made while the dielectric is energized at full voltage. We also see many applications of this technique to probe the internal charges and fields in electrolytes, biological materials, and semiconductors. Using ultrasound, we are able to observe and measure materials in a whole, functioning state.

There are many different applications of ultrasound technology useful to our work. For example, we can examine the condition of internal surfaces (e.g., pipes) for wear, erosion, or corrosion. In addition, we have ongoing projects with other high- voltage insulation systems and electron-beam effects. Through cooperative work with a local hospital, we apply new results with electron-beam technology to medical radiation therapy programs.

The monitoring of transformers has been a major project at LEES, and success in this area continues to motivate much of our current work. A basic goal of our transformer studies relies on the use of new sensors that monitor the status of the transformer's "health." We observe the effects of energization, temperature, electrical conductivity, and moisture content in transformer oil or pressboard systems. For example, we examine the electrical breakdown in transformers due to charge accumulation from transformer oil flow. This problem originates when the liquid side of pressboard-oil interfaces entrains a diffuse double-layer charge. This charge transport leads to charge accumulation on insulating surfaces, causing electrical potential to build up until the rate of charge accumulation equals the rate of charge leakage or until spark discharges occur.

Other new sensing technologies for transformers include advanced waveform detection and analysis of partial-discharge events, and the detection of oil degradation by the formation of high-speed streamer discharge events. This is enabling new instrumentation for the detection of incipient failure processes in power apparatus, such as transformers and bushings.

Students interested in pursuing research in high voltage areas need to be knowledgeable in electric and magnetic fields, and often use a background in fluids and solid mechanics, circuits, thermodynamics, optics, acoustics, or the physics of materials. Typical course work includes Electromagnetic Fields, Forces, and Motion (6.641), Electrical and Optical Processes in Gases (6.638), Continuum Electromechanics ( 6.642), and other courses on optics, acoustics, fluid mechanics, thermodynamics, and mathematics.

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