In this study, the space charge in oil-impregnated thick pressboard and Kraft paper were measured by the pulsed electroacoustic (PEA) method from -20 to +60 °C. A bipolar charge transport model was used to simulate the transport processes of homocharge and heterocharge, and the acoustic attenuation process in the samples. The test results show that space charge accumulates in the thick pressboard and Kraft paper increases first and then decreases with temperature, and its maximum appears at 40°C, while little space charge accumulates at -20°C. The space charge near the lower electrode shrinks in the oil-impregnated thick pressboard at high temperature and the shrinkage is larger at 60°C. In the oil-impregnated Kraft paper, the accumulation and charge density of positive charge is different from negative charge. The ion transport process model simulates the charge shrinkage near the electrode, and disappears at the upper electrode after the acoustic attenuation simulation. The model established in this paper can well simulate the space charge distribution of oil-impregnated thick pressboard and Kraft paper under different temperatures.

High-voltage direct-current (HVDC) dry bushing capacitor-core insulation is composed of epoxy resin-impregnated insulating paper (RIP). To improve the thermal conductivity, breakdown strength, and space charge characteristics of RIP, 0.1 wt % nano-cellulose fiber (CNF)-modified RIP (CNF/RIP), 2.5–30 wt % hexagonal boron nitride (h-BN)-modified RIP (h-BN/RIP), and 2.5–30 wt % h-BN + 0.1 wt % CNF-modified RIP (h-BN + 0.1 wt % CNF/RIP) were prepared. Scanning electron microscopy (SEM) was implemented; the thermal conductivity, DC conductivity, DC breakdown strength, and space charge characteristics were tested. The maximum thermal conductivity of h-BN + 0.1 wt % CNF/RIP was 0.376 W/m.K with a h-BN content of 30 wt %. The thermal conductivity was 85.2% higher than that of unmodified RIP. The breakdown strength and charge suppression were the best in the case of 10 wt % h-BN + 0.1 wt % CNF/RIP. The maximum breakdown strength was 11.2% higher than that of unmodified RIP. These results can play a significant role in the research and development of insulation materials for HVDC dry bushing.

The space charge accumulation is an important factor to design the oil-pressboard composite insulation of convertor transformer, which is greatly affected by the temperature. In this paper, the space charge accumulation and attenuation in the two layers and single layer under DC voltage were studied by the Pulsed Electro-Acoustic Method from 20°C to 60°C and the electric field distribution was calculated by the space charge distribution. The studied results have indicated that the distributions of space charge and electric field in the test results do not follow the Maxwell-Wagner polarization. The space charge distribution is affected by the surface trap, bulk trap, charge migration and electric field. The temperature affected the polarity and amount of the charge at oil-pressboard interface. The space charge accumulation of the two layers is more complex than that of the single layer, and the space charge accumulation and attenuation of two layers need longer time to reach the stable stage. The space charge accumulation in the single layer is largest at 40°C, whereas the space charge accumulation in the two layers is largest at 20°C. Moreover, the increase in the temperature increases the attenuation amplitude and speed of space charge.

An AC-DC combined voltage is applied to the oil-pressboard insulation near the valve side during the operation of a converter transformer. To study the breakdown characteristics of an oil-pressboard insulation under such voltages, a typical plate electrode structure was employed in the laboratory to conduct a breakdown test on the oil-pressboard insulation. The electrical field distribution and the DC contents of the transformer oil and the pressboard in composite insulation under the AC-DC combined voltage were simulated by their dielectric parameters. The breakdown strength of the transformer oil decreases with the increase in the DC content of the applied voltage, whereas that of the pressboard increases. For the oil-pressboard insulation, the breakdown voltage increases first and then decreases. The electric field strength decreases in the transformer oil with the increase in the DC content, whereas it increases in the pressboard. And the DC contents of the transformer and the pressboard in composite insulation were different from that of the applied voltage. Finally, based on the above results, a mathematical model was proposed to describe the breakdown characteristics of the oil-pressboard insulation under the AC-DC combined voltage; the theoretical and experimental results were in good agreement.

Oil-impregnated cellulose insulation polymer (oil-paper/pressboard insulation) has been widely used in power transformers. Establishing effective ways of improving the physical and chemical properties of the cellulose insulation polymer is currently a popular research topic. In order to improve the charge injection inhibition and hydrophobic properties of the cellulose insulation polymer used in power transformers, nano-structure zinc oxide (ZnO) and polytetrafluoroethylene (PTFE) films were fabricated on a cellulose insulation pressboard surface via reactive radio frequency (RF) magnetron sputtering. Before the fabrication of their composite film, Accelrys Materials Studio (MS) software was applied to simulate the interaction between the nanoparticles and cellulose molecules to determine the depositing sequence. Simulation results show that the ZnO nanoparticle has a better adhesion strength with cellulose molecules than the PTFE nanoparticle, so ZnO film should be sputtered at first to fabricate the ZnO/PTFE composite film for better film quality. The sputtered, thin films were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The space charge injection behavior and the hydrophobicity performance of the untreated pressboard; and the cellulose insulation pressboard with sputtered nano-structure ZnO, PTFE, and the ZnO/PTFE functional films were compared with each other. X-ray photoelectron spectroscopy results showed that ZnO, PTFE, and ZnO/PTFE functional films were all successfully fabricated on the cellulose insulation pressboard surface. Scanning electron microscopy and XRD results present the nano-structure of the sputtered ZnO, PTFE, and ZnO/PTFE functional films and their amorphous states, respectively. The ZnO/PTFE composite functional film shows an apparent space charge suppression effect and hydrophobicity. The amount of the accumulated space charge in the pressboard sputtered ZnO/PTFE composite functional film decreased by about 40% compared with that in untreated cellulose insulation pressboard, and the water contact angle (WCA) increased from 0° to 116°.

The electric field distortion caused by space charge is an important factor affecting the operation reliability of oil–paper insulation in a converter transformer. To study the accumulation and decay characteristics of the space charge within oil-impregnated pressboard under DC and polarity reversal voltage, and consider the possible operating conditions of the converter transformer, the space charge behavior of oil-impregnated pressboard was measured by the pulsed electro-acoustic (PEA) method in the temperature range from −20 °C to 60 °C. The effect of temperature on the accumulation and decay characteristics of space charge is also analyzed. The space charge accumulated within the pressboard at low temperature is mainly homocharge injected by the electrode, while heterocharge formed by ion dissociation counteracts some of the homocharge at high temperature. Thus, the space charge of pressboard first increases, then decreases, with an increase in temperature. However, slow decay of the space charge causes severe distortion of the electric field distribution in the pressboard during voltage polarity reversal.