Nanotechnology provides a new way to improve the insulating properties of traditional dielectric materials. In this study, three types of mineral oil based nanofluids were prepared by suspending Fe ₃ O ₄ , TiO ₂ and Al ₂ O ₃ nanoparticles all of which were surface modified by oleic acid. The inception voltage, stopping length and propagating velocity of streamers in the nanofluids under positive lightning impulse voltage were experimentally studied. It is found that nanoparticles can restrain the initiation and propagation processes of positive streamers in transformer oil depending on the types of nanoparticles. In addition, the trap characteristics in pure oil and nanofluids were comparably studied. The relationship between the trap characteristics and mobility of charge carriers in oil samples were then established. The increased trap density in nanofluids diffuses kinetic energy of ionized electrons and converts them into negative ions, resulting in the reduced electrical field strength in front of positive streamer and increased breakdown strength of nanofluids.

The insulation of mineral oil-based nanofluids was found to vary with different concentration level of nanoparticles. However, the mechanisms behind this research finding are not well studied. In this paper, mineral oil-based nanofluids were prepared by suspending TiO₂ nanoparticles with weight percentages ranging from 0.0057% to 0.0681%. The breakdown voltage and chop time of nanofluids were observed under standard lightning impulse waveform. The experimental results show that the presence of TiO₂ nanoparticles increases the breakdown voltage of mineral oil under positive polarity. The enhancement of breakdown strength tends to saturate when the concentration of nanoparticle exceeds 0.0227 wt%. Electronic traps formed at the interfacial region of nanoparticles, which could capture fast electrons in bulk oil and reduce the net density of space charge in front of prebreakdown streamers, are responsible for the breakdown strength enhancement. When the particle concentration level is higher, the overlap of Gouy–Chapman diffusion layers results in the saturation of trap density in nanofluids. Consequently, the breakdown strength of nanofluids is saturated. Under negative polarity, the electrons are likely to be scattered by the nanoparticles on the way towards the anode, resulting in enhanced electric fields near the streamer tip and the decrement of breakdown voltage.

In order to improve the electrical characteristics of transformer oil-impregnated pressboard, transformer oil-based nanofluids were used to impregnate pressboards. It is found that nanofluid-impregnated pressboard (NP) exhibits substantially higher AC and DC breakdown voltage than that of pure oil-impregnated pressboard (OP). Pulse electroacoustic measurement (PEA) was carried out to study the difference of space charge distribution and decay characteristic in OP and NP. It has demonstrated that NP has a more uniform internal electric field, which is caused by a higher charge transport rate compared to OP, resulting in the improved electrical insulating properties.

In this paper, a two-dimensional axisymmetric numerical model of streamer discharge in alternative natural ester liquid and conventional naphthenic oil under a positive sub-microsecond impulse voltage is proposed. The model is based on the charge drift-diffusion approximation in a divergent electric field. Simulation shows that streamer propagation in naphthenic oil is divided into two stages: stage I with a small streamer velocity of 0–0.91 km s⁻¹ and stage II with a rapidly rising streamer velocity up to 213 km s⁻¹. In natural ester, the streamer velocity gradually slows down from 4.84 km s⁻¹ to 2.11 km s⁻¹. The electric field at the head of the streamer in natural ester ranges from 2.4 × 10⁸ V m⁻¹ to 2.5 × 10⁸ V m⁻¹ during propagation, which is visibly lower but more consistent than that in naphthenic oil (3.5 × 10⁸ V m⁻¹ to 8.4 × 10⁸ V m⁻¹). It is revealed that the different streamer features within the two types of oil are caused by the different competition between the Laplacian electric field and space charge effect at the streamer head. For streamer propagation in naphthenic oil, stage I is dominated by the Laplacian electric field whereas stage II is dominated by the space charge effect. In natural ester, the whole streamer propagation is dominated by the Laplacian electric field. Further, it is indicated that the streamer head consists of two specific discharging regions, i.e. the front region of the streamer head which guides new space charge accumulation and the back region of streamer head which guides density reduction of accumulated space charge. The expansion of the streamer tunnel is caused by the alternating space charges in these two regions. The work in this paper provides a theoretical reference for the practical application of alternative transformer liquids.