Investigation of space charge accumulation processes in a chemically cross-linked polyethylene for ac usage (XLPE) and a modified XLPE for dc usage (DC-XLPE) at voltage polarity reversal was carried out using pulsed electro-acoustic (PEA) measurement system. We found that a huge amount of so called packet-like charge, some of which partially increased the electric field in the material to more than twice of the average applied stress, generated in XLPE at the polarity reversal of DC stress. It enhanced the electric stress locally in the sample by almost twice of the applied average stress. On the other hand, we also found that the packet-like charge was not observed in DC-XLPE under the same experimental condition.

The pulsed electroacoustic (PEA) method is widely known to be an excellent method for the measurement of the space charge distribution which accumulates in thick dielectric materials (1-5mm). However, because of its lower resolution in the depth direction, conventional application of this method is less suitable for thin specimens than the laser-induced pressure wave method. The authors developed a new PEA measurement system which can be applied to thin dielectric materials (_??_ 100μm). The resolution in the depth direction is improved to less than several μm by detecting signals in a wide frequency range. In addition, the system employs a deconvolution technique with FFT in order to compensate for the frequency characteristics of the piezoelectric sensor. The new PEA measurement system is expected to be useful for quantitative and nondestructive measurement of the space charge distribution in dielectric films.

The correlation between charge distribution in an aged crosslinked polyethylene (XLPE) cable and the location of water trees has been studied. The space charge radial distributions at different angle in the insulating wall of a service-aged XLPE coaxial cable, induced by application of a DC stress, were measured using the pulsed electroacoustic method. Furthermore, a cross section of sliced insulating wall was observed with an optical microscope to compare with the radial space charge distribution. The cable under investigation was electrically aged in service at 6.6 kV AC for 25 years in wet conditions. Water trees were found at the position in which space charge was observed. On the other hand, no water tree was found at the position in which the space charge was not observed. Judging from these results, the space charge measurement seems to be useful for the diagnosis of water trees in aged cables.

This research investigates the optimal region to achieve balanced thermal and electrical insulation properties of epoxy (EP) under high frequency (HF) and high temperature (HT) via integration of surface-modified hexagonal boron nitride (h-BN) nanoparticles. The effects of nanoparticle content and high temperature on various electrical (DC, AC, and high frequency) and thermal properties of EP are investigated. It is found that the nano h-BN addition enhances thermal performance and weakens electrical insulation properties. On the other side, under HF and HT stress, the presence of h-BN nanoparticles significantly improves the electrical performance of BN/EP nanocomposites. The EP has superior insulation properties at low temperature and low frequency, whereas the BN/EP nanocomposites exhibit better insulation performance than EP under HF and HT. The factors such as homogeneous nanoparticle dispersion in EP, enhanced thermal conductivity, nanoparticle surface modification, weight percent of nanoparticles, the mismatch between the relative permittivity of EP and nano h-BN, and the presence of voids in nanocomposites play the crucial role. The optimal nanoparticle content and homogenous dispersion can produce suitable EP composites for the high frequency and high temperature environment, particularly solid-state transformer applications.

A three-dimensional space-charge measurement system using the acoustic lens and PWP method has been developed. The resolutions on the x-y plane and in the z direction of this system are about and m respectively. It takes about 3 s for each sample position and 130 min for positions on the x-y plane for a mm specimen. This speed is much higher than that of the three-dimensional space-charge measurement system using the PEA method. Although the resolutions of this system on the x-y plane and along the z direction are not high, the three-dimensional charge profile of an electron-beam-irradiated PMMA specimen is successfully constructed.