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Recent studies have found that the surface flashover strength of oil-impregnated insulation paper can be improved by the modification of nanoparticles. To reveal the working mechanism, this paper presents an experimental study on the effects of TiO2 nanoparticles on streamer propagation process at the surface of oil-based nanofluid-impregnated insulation paper under lightning impulse voltage. The results show that the surface flashover voltage of nanofluid-impregnated insulation paper is significantly increased by 34.6%. Streamer characteristics including shape and length at the surface of oil-impregnated insulation paper were investigated using the schlieren technique. It is indicated that streamer characteristics are dramatically changed by the addition of TiO2 nanoparticles. For nanofluid-impregnated insulation paper, streamers with more but much shorter branches tend to develop into adjacent nanofluid rather than clinging to the paper compared to that for pure oil-impregnated paper. The reason responsible for the shape dissimilarity lies in the change of electrical force due to the permittivity difference between oil and paper. Furthermore, more shallow traps introduced by nanoparticles contribute to the faster surface charge dissipation of nanofluid-impregnated paper, reducing the charge accumulation at the streamer head and suppressing the electric field distortion. Thus, it is hard for streamers to develop at the surface of nanofluid-impregnated paper, resulting in an increase of surface flashover strength.

Recent studies have shown that the modifications to nanofluid-impregnated pressboards by different shapes of nanoparticles are different. To investigate the effect of nanoparticles shapes on creeping flashover characteristics at the interface of nanofluid-impregnated pressboard (NP), creeping flashover behavior of spherical nanoparticle fluid/pressboard (NSFP) and nanorod fluid /pressboard (NRFP) were compared to that of oil/pressboard (OP) which was used as a reference. Creeping flashover experiments under AC and positive and negative impulse voltages were carried out. Results show that the modifications by spherical and rod nanoparticles on creeping flashover strength are distinct and different: negative creeping flashover voltages of NSFP and NRFP are increased up to 32 and 15% respectively, and the positive ones are separately improved as high as 15% and 5%. Whereas, experiments under AC voltage reveal quite a different trend: creeping flashover voltage of NSFP and NRFP are almost increased to the same extent, nearly 18%, while partial discharge inception voltage improvement in the increment of the former is 5.6 times larger than that of the latter. Charge behaviors at oil/pressboard interfaces were displayed by thermally stimulated depolarization current method. Space charge accumulation at the surface of NSFP is much less than that of NRFP and the dissipation rate of the former is faster, due to a larger trap density and a lower trap energy level, which weakens electric field distortion and results in the improvement of creeping flashover strength. The underlying mechanism is that spherical nanoparticles have a specific surface area 1.3 times as large as that of nanorods at the same concentration, and it is a source of shallow traps.

Ultra-High Frequency (UHF) method is one of the most effective methods for partial discharge (PD) detection of HV switchgear, due to its high sensitivity and strong resistance to external interference. Despite of these advantages, it is still uneasy to realize the accurate identification of the fault types and discharge stages of HV switchgear owning to the lack of a systematic research and analysis on the partial discharge characteristics of different defects in switchgear. In this paper, three typical partial discharge defect models, including needle discharge, suspended discharge, and internal discharge were designed. The PRPD spectra of different discharge defect models at various discharge stages were obtained by application of UHF method. The results demonstrated that based on the characteristics of the PRPD spectra of various discharge defect model, the type of PD could be effectively recognized by analyzing the characteristic variables of the PRPD spectra. Moreover, the discharge stages could also be roughly inferred from the diversity of different PRPD spectra. Therefore, UHF provides an effective way to evaluate the defect type and stage of PD in HV switchgear.

Nanoparticles have exhibited great potential in improving insulating property of liquid and solid dielectrics. To explore their modification mechanism on transformer oil-impregnated pressboards, TiO2 nanoparticles with different diameters of 5nm, 10nm, and 15nm were synthesized and used to prepare transformer oil-based nanofluids. The creeping flashover characteristics of both pure oil-impregnated paperboards (OPs) and nanofluid-impregnated pressboards (NPs) were examined under lightning impulse and AC voltages. It was found that the creeping flashover property of NPs is significantly improved with the decreasing of nanoparticle size. Especially, it is enhanced by 2.44 times under positive lighting impulse as nanoparticle size reduced from 15 nm to 5 nm. The results of space charge decay and trap characteristics reveal the decrease of nanoparticle size can lower trap depth and greatly enhance the space charge dissipation rate, leading to more uniform electric field distribution at the interface between nanofluid and pressboards and higher creeping flashover property.