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

在高压电绝缘研究领域中，绝缘材料中空间电荷的积累行为一直是有待解决的主要问题之一。本研究的最终目的是形成试样中空间电荷积累的面积剖面超声图像。首先，制备了一些聚乙烯（PE）样品，每个样品插入一根树形针，组成针-板电极系统。将PE试样置于浸泡在绝缘油中的测量槽中，然后施加约70%的预定直流击穿电压。在树形开始之前，电应力被移除，形成具有累积空间电荷的区域。然后将聚焦超声脉冲束照射在中心频率为25MHz、重复频率为IkHz的区域，检测到超声激发空间电荷产生的极弱电磁辐射。在实际的探测试验之前，通过计算机模拟，在假定空间累计变形量、超声振动幅值和空间电荷区与探测线圈之间的距离合适的情况下，估算了辐射的电磁能量。虽然所得值只有几百nW，但我们认为，通过制作严格的电磁屏蔽，设计高SN比的射频放大器、检测器等电子电路，可以检测到辐射。对安装在测量箱内的检测线圈进行了设计和构造，对其布置方式进行了考察，并对几种射频电路进行了多次研究和试验。但是由于噪声信号的存在，没有检测到空间电荷的射频辐射信号，主要原因是整个测量系统的SN比不足。为了提高屏蔽室的等级，以及采用鉴相系统等电子电路，我们继续进行了一些尝试。另一方面，我们尝试使用超声束空间扫描系统对电树起始进行实时观察，并成功获得了树起始发育过程中的一些实时图像。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