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

在高压电气绝缘研究领域，空间电荷在绝缘材料中的积累行为一直是研究的主要问题之一。本研究的最后目的是形成测试样品中空间电荷累积的面积轮廓的超声图像。开始，制备了多个聚乙烯（PE）样品，每个样品插入一个树状针，组成针-板电极系统。将PE试样置于浸在绝缘油中的测量单元中，然后施加由于电树枝化而引起的预定DC击穿电压的约70%。电应力被删除之前开始的树枝形成的区域与积累的空间电荷。然后将聚焦超声脉冲束照射在中心频率为25 MHz、重复频率为1 kHz的区域上，检测由超声激发的空间电荷产生的极微弱电磁辐射 ...更多信息 在实际探测试验中，在空间电荷积累量、超声振动幅度和空间电荷区与探测线圈距离等适当条件下，通过计算机模拟，估算出辐射电磁能量。虽然结果值小到几百nW，但我们得出结论，通过准备严格的电磁屏蔽和设计具有高SN比的RF放大器，检测器等电子电路，可以检测到辐射。多次进行了测量池中检测线圈的设计与制作，对检测线圈的布置进行了研究，并对几种射频电路进行了研究与试验。然而，由于噪声信号的影响，空间电荷的射频辐射信号尚未被检测到，这主要是因为整个测量系统的信噪比不足。在屏蔽室等级的提高、电子线路的改进等方面进行了尝试，如采用相位检测系统等;另一方面，我们尝试用超声波束空间扫描系统对电树枝的萌生进行实时观测，并成功地获得了树枝萌生和发展过程的实时图像。少

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