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This paper presents a comparative analysis of the creeping discharge characteristics on mineral oil-impregnated pressboard (OIP) under combined AC and DC voltages. The creeping discharge behavior at various voltage levels to failure were analyzed using partial discharge (PD) measurements, camera observations, and electric field simulations. It was found that the creeping discharge distribution was significantly influenced by the DC component, and it was difficult to induce failure even on OIP with high moisture under combined AC and DC voltage. The flashover along the free-oil boundary layer above the OIP during creeping discharge occurred much more easily under combined AC and DC than under AC voltage, and the quality of the oil was important to avoid flashover. These differences were attributed to the combined influence of the electric field distribution under DC and space charge accumulation. [ABSTRACT FROM AUTHOR]

The mineral insulating oil and pressboard are important insulation dielectrics in AC-DC converter transformers. Thus, it is very significant to understand the dielectric properties of the oil and oil-impregnated pressboard. In this paper, the influences of the temperature, field strength and aging on the resistivity of oil and oil-impregnated pressboard have been investigated. Samples used in the experiments are new oil, aged oil, pressboard immersed in new oil and aged oil respectively. The measuring device of resistivity was full of oil and isolated from outside. All samples were measured at temperature range of 25-80 °C under the electric field strength range of 2-10 kV/mm for oil and 3-30 kV/mm for oil-impregnated pressboard. The results showed that the resistivity of oil-impregnated pressboard decreased along with the increase of field strength and temperature. For all oil specimens and pressboard immersed in aged oil, the resistivity decreasing rate ( σ lgρ / σE ) along with electric field strength showed a downtrend with the increase of temperature; while, for pressboard immersed in new oil, the decreasing rate showed an uptrend with the increase of temperature. For oil, the resistivity reduction rate with aging (r) decreased along with the increase of field strength and temperature. For pressboard immersed in oil, the reduction rate with aging (r) showed an uptrend with the increase of temperature. [ABSTRACT FROM AUTHOR]

The main insulation of converter transformers consists of transformer oil and oil-impregnated pressboard. Under operating conditions, the valve-side winding of the converter transformer is subject to DC voltage components. Therefore, studies on the characteristics of oil-impregnated pressboard conductivity are necessary. In this paper, the temperature, moisture and electric field dependency of pressboard conductivity are investigated based on a specially designed three-electrode experimental chamber, which allows for a variation in temperature ranging from 25 °C to 120 °C and an electric field strength ranging from 0 to 30 kV/mm. The experimental results show that, within the experimental conditions, the conductivity of oil-impregnated pressboard increases exponentially with increasing moisture and temperature. High moisture and temperature will increase both the carrier concentration and carrier mobility, which explains the exponential correspondence. Furthermore, the electric field dependency of the conductivity is more obvious for wet pressboard than for dry pressboard. Protons in the wet pressboard are more easily accelerated by the electric field than the impurity ions in the oil of the dry pressboard, which leads to an obvious electric field dependency of the wet pressboard conductivity. [ABSTRACT FROM AUTHOR]