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，空间电荷在绝缘材料中的积聚行为一直是人们研究的主要问题之一。本研究的最终目的是形成空间电荷在试件中积累的面积分布的超声图像。首先，制备了多个聚乙烯(PE)样品，每个样品插入一个树形针组成一个针板电极系统。将PE样品放置在浸泡在绝缘油中的测量单元中，然后施加由于电树而产生的大约70%的预定直流击穿电压。在树枝形成之前，电应力被去除，形成具有累积空间电荷的区域。然后在中心频率和重复频率分别为25 MHz和1 kHz的区域上照射聚焦超声脉冲束，检测到由超声激发的空间电荷的极弱电磁辐射。在…之前结合探测的实际试验，通过计算机模拟，在假定累积空间半径、超声振动幅度和空间电荷区与探测线圈的距离适当取值的前提下，估算了电磁辐射能量。尽管合成的辐射值只有几百nW，但我们得出结论，只要准备严格的电磁屏蔽，并设计高信噪比的射频放大器、探测器等电子电路，就可以探测到辐射。对安装在测量单元中的检测线圈进行了多次设计和构造，对其布置进行了研究，并对几种射频电路进行了研究和试验。然而，由于噪声信号的存在，整个测量系统的信噪比不足，空间电荷的射频辐射信号仍未被检测到。在不断提高屏蔽室档次的同时，在采用鉴相系统等电子电路方面也做了一些尝试，同时尝试了利用超声束空间扫描系统对电树萌发过程进行实时观测，成功地获得了树木萌发过程中的一些实时图像。较少

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