Filled high thermal conductivity epoxy composite solves the problem of the low thermal conductivity of the epoxy resin itself, but the addition of the thermal conductive filler reduces the mechanical properties of the composite, which limits its application in the field of high voltage insulation. In this work, carboxyl-terminated butadiene nitrile liquid rubber (CTBN) was used to toughen the boron nitride-epoxy hybrid system, and the effects of different contents of CTBN on the mechanical properties, thermal conductivity, glass transition temperature, thermal stability, and dielectric properties of the composites were investigated. The results showed that when the content of CTBN was 5–15 wt.%, the CTBN formed a dispersed island structure in the epoxy resin matrix. The toughness of the composite increased by about 32%, the breakdown strength was improved, and the thermal conductivity was about 160% higher than that of pure epoxy resin. As the CTBN content increased, the glass transition temperature and thermal stability of the composite decreased and the dielectric constant and the dielectric loss increased. When the CTBN content is 10–15 wt.%, a toughened epoxy composite material with better comprehensive properties is obtained.

The variation of negative dc corona inception voltages with air pressure and humidity, using a point/plane electrode system, was studied using a perspex chamber in which the pressure and humidity could be varied. A method of calculation was developed to determine the corona inception voltages over a range of air pressures and humidities. The effective ionization coefficient was calculated as a linear interpolation of the effective ionization coefficients for dry air and water vapor. The validity of these calculations was confirmed by a series of experimental determinations of the corona inception voltages for the same range of pressures and humidities. A comparison of the negative and positive corona inception mechanisms was also made. Both the negative and positive corona inception voltages decreased with a decrease in the air pressure and an increase in the humidity. The greater values of the effective ionization intensity found in the critical zone for the negative corona accounted for their lower inception voltages. The mechanisms of the affect of the photo-absorption coefficient and Townsend's second coefficient on the negative and positive corona were also considered.

The variation of positive dc corona characteristics with air pressure and humidity using a point/plane electrode system was studied in a perspex chamber allowing the pressure and humidity to be varied. A method of calculation was developed to determine the corona current for humid air over a range of pressures and humidities in which the effective ionization coefficient was calculated as a linear interpolation of the effective ionization coefficients for dry air and water vapor. The calculation method involved the solution of partial differential equations using a particle-in-cell algorithm. This also allowed the determination of the corona inception voltages for a range of pressures and humidities as the voltage at which the corona current first appeared. The validity of these calculations was confirmed by a series of experimental determinations of the corona inception voltages and of the variation of the corona current with the applied voltage for the same range of pressures and humidities. It was found that the corona current increased when the air pressure or humidity decreased (at constant applied voltage) mainly because of the rise in the space charge density or the positive ion mobility, respectively. The coefficient C, in the relationship between the corona current, the applied dc voltage and the corona inception voltage, declined with the increase of humidity. The rate of decrease of C with humidity became larger as the pressure was reduced. An approximately inversely proportional relationship was found to exist between C and the relative air density at constant humidity. This variation of C was principally associated with the variation in the positive ion mobility.

Relatively little investigation has previously been done on the effects of aging on conductor surface conditions, or on the ac corona discharges from these conductors. In the present work these were investigated by means of comparing corona on typical aged conductors with that on new ones. The aged conductors from the ac power transmission lines had been in service for 12, 15 and 20 years. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were employed to characterize the surface morphology and composition of the conductors; and a non-contact surface profiler was used to measure the surface roughness of the six samples. As a result of corrosion and the adherence of various substances to the conductors, their surfaces were far rougher than the new ones. The ac corona discharge characteristics of these conductors were studied in a small corona cage using an ultraviolet imaging detector. As a result of the rough surfaces, the corona discharge intensity for the aged conductors was found to be much stronger than that for the new ones; and the corona inception voltages for the aged conductors were 10 to 25% lower than the new ones.