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Incorporating inorganic nanoparticles (NPs) into polymer matrices provides a promising solution for suppressing space charge effects that can lead to premature failure of electrical insulation used in high voltage direct current engineering. However, realizing homogeneous NP dispersion is a great challenge especially in high-molecular-weight polymers. Here, we address this issue in crosslinked polyethylene by grafting matrix-compatible polymer brushes onto spherical colloidal SiO 2 NPs (10-15 nm diameter) to obtain a uniform NP dispersion, thus achieving enhanced space charge suppression, improved DC breakdown strength, and restricted internal field distortion (10.6%) over a wide range of external DC fields from À30 kV/mm to À100 kV/mm at room temperature. The NP dispersion state is the key to ensuring an optimized distribution of deep trapping sites. A well-dispersed system provides sufficient charge trapping sites and shows better performance compared to ones with large aggregates. This surface ligand strategy is attractive for future nano-modification of many engineering insulating polymers. Published by AIP Publishing.

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Dielectric barrier discharge (DBD) excitation by unipolar high voltage pulses is apromising approach for producing non-thermal plasma at atmospheric pressure. In this study, a magnetic compression solid-state pulsed power generator was used to produce repetitive nanosecond pulses for the excitation. The DBD is created using two liquid electrodes. The electrical characteristics of the discharge voltage and current are illustrated under different experimental conditions. The nanosecond-pulse discharge current is of the order of tens of amperes. This differs from common DBD current excitated by high-voltage ac sources. Compared with the characteristics of two current pulses corresponding to two discharges for unipolar pulsed-excitation, the secondary discharge in this study is minor owing to the pulsed power and discharge configuration. Under the experimental conditions, the luminous emissions from the front and side views of the liquid electrodes show that no filament is observed and the discharge is homogeneous and diffuse in the whole discharge region. The effects of applied voltage amplitude, repetition rate, and air gap spacing on the discharge characteristic are investigated. The discharge mode does not change with the variation of the investigated parameters. A comparison of high voltage ac and nanosecond-pulse excitation is also presented. In addition, discussion of the experimental results is presented. Index Terms-Gas discharge; dielectric barrier discharge; pulsed power; nanosecond pulse; magnetic compression; repetition rate; atmospheric pressure air; non-thermal plasma; discharge mode; homogeneous discharge; atmospheric pressure glow discharge.

In this paper, a high sensitivity electrometer is adopted to measure DC conduction currents across the oil gap. The effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on conduction current is considered adequately, which is very lacking in the previous references. Based on the Fower-Nordheim theory, three stages will be clearly distinguished with the increase of the electric field strength. At low electric field stage (E<0.44 kV/mm), the current through the oil I is directly proportional to the electric field E, which meets the Ohm's law. Oil conductivity mainly depends on impurity ions with only low mobility. At medium electric field stage (0.44 kV/mm1.33 kV/mm), the obtained I-V 2 characteristics is linear, which can be explained by the space charge limited current. Moreover, main factors affecting the conduction current, including oil gap, electrode geometry and materials, polarity effect, temperature, hydrostatic pressure and moisture, have been analyzed and the causes are interpreted as well.Index Terms -Dielectric conduction mechanism, electrode geometry, morphology of electrode surface, hydrostatic pressure, moisture, polarity effect, temperature, transformer oil.

Power cables operate at high temperatures over long periods of time, and the electrical behavior of silicone rubber (SIR) in the new types of extra-high-voltage prefabricated cable accessories would change as a result of thermal aging. In this study, tests were conducted to reveal the effects of thermal aging (1000 h at 60-180°C) on the electrical treeing behavior. It was found that with increasing thermal aging time, the average electrical tree initiation voltage (ATIV) of SIR initially increases to a peak value and then decreases, finally becoming stable within 1000 h. Meanwhile, the probability of pine-like trees decreases at first and then increases, whereas the probability of bush-like trees initially increases and then decreases. The thermal aging temperature affects the rate of ATIV following the Arrhenius equation. These results strongly imply the existence of a thermal aging process that greatly influences the treeing degradation process. The results obtained using differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) indicate that thermal oxidation plays a major role in the initial thermal aging process and facilitates additional crosslinking, which enhances ATIV. With increasing thermal aging time, thermal degradation and thermal crack reactions play leading roles, resulting in decreased crystallinity and ATIV. Microcracks are present after long-term thermal aging, and they are the dominant factor in ATIV stability. ATIV stability also provides a theoretical basis for the electrical strength design margin for insulating materials.

Space charge is a threat to insulation materials and oil-paper insulation gradually degrades in service. The degradation of cellulose affects space charge formation, accumulation and dissipation, and the presence of space charge affects the performance of dielectric. In this paper, the effect of thermal aging on space charge behaviors and other properties including permittivity, conductivity, and tensile strength was investigated. Oil-paper samples were aged at a hot spot temperature (180℃). During thermal aging, space charge distribution was measured at a regular interval at room temperature. And the pulse electro-acoustic (PEA) method was used for space charge measurement. The results showed that the thermal aging at hot spot temperature caused increase of conductivity and tensile strength degradation. The permittivity increased at the beginning but then it decreased, while dissipation factor showed an upward trend. Besides these, trap distribution and carriers characteristics were also affected. The formations of space charge of unaged and aged oil paper were consequently different under low electric field. Due to the generation of traps by the degradation of cellulose, more charge, especially positive charge was trapped, but the increased traps mostly were shallow traps. Based on the Schottky model, a correlation between space charge injection and the permittivity was drawn. This, together with the appearance of positive bulk charge could be used for oil paper aging status diagnosis.

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