Image formation of space charge profile in organic electrical insulating material by means of detection of electromagnetic radiation excited by ultrasonic focusing beam

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In the field of research on high voltage electrical insulation, behaviors of space charge accumulated in insulating materials has long been studying as one of the major pending problem. Final object of this study is to form ultrasonic image of area profile of space charge accumulated in a test specimen. To begin with, a number of polyethylene (PE) specimens were prepared each of which was inserted a treeing needle to compose a needle-plate electrode system. A PE specimen was set in a measuring cell soaked in insulating oil and then applied approximately 70% of the predetermined DC breakdown voltage due to electrical treeing. The electrical stress was removed just before the initiation of treeing to form the area with accumulated space charge. Then focusing ultrasonic impulse beam was irradiated on the area whose central and repetition frequency are 25MHz and IkHz respectively, and extremely weak electromagnetic radiation from space charge excited by ultrasound was to be detected.Before … More the actual trial of the detection, electromagnetic energy of radiation was estimated on the assumption of adequate values of the amount of accumulated space cnarge, amplitude of ultrasonic vibration and distance between space charge area and detection coil by computer simulation. Though the resultant value is as small as several hundred nW, we concluded that radiation was able to be detected by preparing strict electromagnetic shield and designing electronic circuits for RF amplifier, detector, etc. with high SN ratio. Designs and constructions of detection coils mounted in measuring cell, investigation of its arrangements, study and trials of several RF circuits were made for many times. However, RF radiation signal from space charge was not yet detected owing to noise signal, mainly because SN ratio of entire measuring system was insufficient. We continued to make some trial for improvement of the grade of shield chamber, and also of electronic circuits such as adopting phase detection system.On the other hand, we have to write in addition that we tried to make real-time observation of electrical tree initiation using spatial scanning system of ultrasonic beam and succeeded to obtain some real-time images in the process of tree initiation and development. Less