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The polarity reversal test is one of the routine tests for the evaluation of electric insulation in converter transformers. The polarity reversal time (PRT) has significant effect on the accumulation of interface charges in oil-impregnated pressboard insulation. IEC standard recommends that polarity reversal should be performed within 2 min so as to match with the dispersion rate of interface charges in uniform filed, but not necessarily in the non-uniform field. In order to investigate the influence of PRT on the dissipation rate of interface charges in non-uniform electric field, the density of interface charges was measured by an electrostatic capacitance probe in the needle-plate non-uniform field at the PRT of 2 ms, 4 ms and 2 min respectively. The results showed that the polarity of interface charges did not change with the applied voltage jump at the PRT of 2 ms and 4 ms, while the polarity of interface charges almost synchronously changed with the polarity reversal of applied voltage at the PRT of 2 min. For the PRT of 2 ms and 4 ms, the electric field nearby the needle electrode would be enhanced with applied electric field of the same direction, whilst the total electric field of insulation system would be weakened at the PRT of 2 min. It was inferred that polarity of interface charge might have not enough time to change in polarity reversal test within the PRT of 4 ms under non-uniform electric field, which was more close to the severe condition of covert transformer. The paper suggest that the value of PRT be reconsidered when the polarity reversal test is performed in non-uniform electric field. [ABSTRACT FROM AUTHOR]

In this paper, prebreakdown streamers in transformer oil and transformer oil-based TiO2 nanofluid were observed by the schlieren method under positive lightning impulse voltage. Streamers in the nanofluid have numerous branches with much shorter length, whereas in the pure oil, they exhibit only certain filaments with longer length. The discrepancy in positive streamer pattern of both oils is further investigated under a range of applied voltages. The complexity of streamer patterns was quantitatively described using the fractal analysis method. Results indicate that the fractal dimension of streamer patterns shows different changing tendencies in both oils, and it keeps higher value in the nanofluid than that in pure oil during the whole propagation process, well corresponding with streamer propagating structures. Moreover, a new parameter, the ratio of fractal dimension to propagation length (D/L), is introduced to classify the complex streamer patterns for the first time. Three propagation zones in both nanofluid and pure oil are clearly categorized by the value of D/L, providing a quantitative way to distinguish the streamer patterns. [ABSTRACT FROM PUBLISHER]

Recent experiments have shown that some nanoparticles can influence the breakdown strength of transformer oil under lightning impulse voltage. To reveal the working mechanism, this paper presents an experimental study on the effect of TiO2 nanoparticles on the impulse breakdown strength and prebreakdown streamer propagation process in transformer oil-based nanofluid under both positive and negative lightning impulse voltage. The test results verify that the modification of nanoparticles on breakdown strength of transformer oil has a distinct polar effect: positive breakdown voltage of nanofluid is increased by up to 30.8%, whereas the negative one is decreased by 6.8%. Streamer shape, propagation length and velocity in both pure oil and nanofluid were investigated using the shadowgraph technique. It is revealed that the propagation characteristics of positive and negative streamers in nanofluid are markedly affected by the addition of TiO2 nanoparticles. The positive streamers in nanofluid form a bush-like structure with thicker and denser branches, developing much slower than tree-like streamers in pure oil. While negative streamers in nanofluid have a tree-like shape with much longer branches, propagating faster than the original bush-like streamer in pure oil. These differences in streamer propagation characteristics and breakdown strength in pure oil and nanofluid are closely related to the change of space charge distribution caused by shallow trap in nanofluid. More negative charges are formed through capturing fast electrons into slow electrons in shallow traps induced by the presence of TiO2 nanoparticles, which change the local electric field in front of the streamer tip. Thus, streamer propagation process in nanofluid is dramatically modified, leading to the change in breakdown strength. [ABSTRACT FROM AUTHOR]

In order to improve dielectric performance of natural esters, TiO2 semiconductive nanoparticles were added to obtain an ester-based nanofluid. AC, lightning impulse voltage and partial discharge tests were applied to the natural ester and the ester-based nanofluid to check their dielectric performances. It was found that TiO2 nanoparticles can not only enhance AC breakdown voltage of the ester based nanofluid by a factor of 1.3 but also improve the partial discharge properties of the ester based nanofluid compared to that of the natural ester. Pulse electroacoustic (PEA) and thermally stimulated current (TSC) techniques were applied to investigate charge transport and trap characteristic in natural ester and ester-based nanofluid. It reveals that TiO2 nanoparticles can accelerate the charge dissipation rate and uniform the electric field and increase the shallow trap density in the ester-based nanofluid. It is proposed that electron trapping and de-trapping processes in the shallow traps could be one of the main charge transport processes in the ester based nanofluid. [ABSTRACT FROM AUTHOR]

In this paper, TiO2 semiconductive nanoparticles with a large relaxation time constant is added into transformer oil to form semiconductive nanofluids (SNFs), with the aim of enhancing insulating characteristics. ac, dc and lightning impulse breakdown voltage and partial discharge (PD) characteristics of oil samples before and after modification were measured according to ASTM standard methods. It was found that SNFs have ac, dc and lightning impulse breakdown voltage up to 1.2 times compared with pure oil. Meanwhile, the partial discharge resistance of SNFs was also dramatically improved. Charge trap and transportation characteristics of both samples have been measured by thermally stimulated current method (TSC) and pulse electroacoustic technique (PEA). It was found that electron shallow trap density and charge decay rate are greatly increased in semiconductive nanoparticles modified transformer oil. It is proposed that electron trapping and de-trapping processes in the shallow traps could be one of the main charge transport processes in dielectric liquids. [ABSTRACT FROM PUBLISHER]