Jinsha Yuan

Effect of TiO 2 nanoparticles on streamer propagation in transformer oil under lightning impulse voltage

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.

Fractal Analysis of Positive Streamer Patterns in Transformer Oil-Based TiO 2 Nanofluid

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.

Effect of Nanoparticle Morphology on Pre-Breakdown and Breakdown Properties of Insulating Oil-Based Nanofluids

Nanoparticles currently in use are challenged in further improving the dielectric strength of insulating oil. There is a great need for a new type of nanoparticle to promote the application of insulating oil-based nanofluids in electric industries. This paper experimentally investigates the effect of nanoparticle morphology on pre-breakdown and breakdown properties of insulating oil-based nanofluids. The positive impulse breakdown voltage of insulating oil can be significantly increased by up to 55.5% by the presence of TiO2 nanorods, up to 1.23 times that of TiO2 nanospheres. Pre-breakdown streamer propagation characteristics reveal that streamer discharge channels turn into a bush-like shape with much denser and shorter branches in the nanofluid with TiO2 nanorods. Moreover, the propagation velocity of streamers is dramatically decreased to 34.7% of that in the insulating oil. The greater improvement of nanorods on the breakdown property can be attributed to the lower distortion of the electric field. Thus, when compared with nanospheres, pre-breakdown streamer propagation of nanofluid is much more suppressed with the addition of nanorods, resulting in a greater breakdown voltage.

Positive streamer characteristic at the interface of TiO 2 nanofluid/pressboard under lightning impulse voltage

Recent studies have found that the creeping flashover strength of oil-impregnated pressboard can be improved by the modification of nanoparticles. To further explore the modification effect of nanoparticles, this paper studied the effect of TiO2 nanoparticles on the creeping flashover strength of transformer oil-impregnated pressboard and its prebreakdown streamer characteristics under positive lightning impulse voltage. The test results indicate that the creeping flashover voltage is increased by 34.6% with the addition of TiO2 nanoparticles. Furthermore, positive streamer characteristic at the interface of nanofluid/pressboard including shape, propagation length and velocity was greatly changed in the presence of nanoparticles. Streamers at the interface of pure oil/pressboard tend to propagate closely along the pressboard surface with a rapidly increased length. Whereas streamers at the interface of nanofluid/pressboard have more lateral branches extending into the surrounding oil and develop with a much shorter length. This demonstrates that the propagation of positive streamers is dramatically restrained by the addition of TiO2 nanoparticles, resulting in the increase of creeping flashover strength.

Effect of electrical conductivity on the ion mobility in dielectric liquids

Conduction process in dielectric liquids has been studied for many years because of its importance in understanding the process of dielectric breakdown. As a key parameter in conduction process, ion mobility has been measured by various methods. The reversal polarity method has been widely used to accurately measure the ion mobility in the dielectric liquids. In this paper, the effect of electrical conductivity of dielectric liquids on the ion mobility measured by the reversal polarity method is studied via the ion drift model based on the Poissons Equation. The observed results indicate that ion mobility of dielectric liquids with high conductivity decrease with the increasing of the applied electric field. On the contrary, the ion mobility of those with the low conductivity nearly keeps constant. It is revealed that in dielectric liquids with high conductivity, it is easier to cause electric field distortion at the low voltage side, making the TOF (Time of Flight) value inaccurate and leading to a larger value of ion mobility than the actual one. Therefore, when measuring the ion mobility of nonpolar dielectric liquids, the distortion influence should be considered carefully. And the effect of the different conductivity on the ion mobility should be taken into account.

Fractal analysis of streamer patterns in transformer oil under lightning impulse voltage

The insulation of dielectric liquids is of great importance for safe operation of transformers. It is generally recognized that the breakdown happens when streamers completely bridge two conductors inside the transformer, leading to insulation failure. The propagation shapes of streamers can be obtained with the help of optical technology and be analyzed qualitatively. Fractal dimension has been increasingly used to quantitatively describe the complexity of a pre-breakdown phenomenon, i.e. electrical treeing in solid insulating materials. In transformer oil, there is a great need for accurate quantitative analysis on streamer propagation patterns by using fractal dimension in consideration of the deviation of visual judgment, which is commonly used at present. In this paper, fractal dimension analysis was introduced to quantitatively study the denseness of streamer propagation patterns in transformer oil under lightning impulse voltage. The fractal dimensions of streamer patterns at different stages and voltage levels were calculated based on box counting method. Results indicate that the fractal dimension is closely related to propagation time and applied voltage. With the propagation time, streamer length increases firstly and then decreases. Different from that, the curve of fractal dimension with time shows two peaks, which is the embodiment of the transformation of streamer patterns. In addition, the fractal dimension increases with the applied voltage. The relation between streamer propagation characteristic and fractal dimension is proposed.

Simulation of ion mobility in transformer oil in different electrode systems

Accurately measurement of ion mobility behavior in transformer oil is critical for reasonable designing and safely operating HVDC transformers due to its electric field distribution determined by the ion migration in the oil. In this paper, we aim to investigate the effect of electrode configuration on the testing accuracy of ion mobility via the reversal polarity method. Both parallel plane electrode and cylindrical capacitor electrode models were simulated in two-dimension ion drift physical model based on Einsteins relation and Poissons equation. The results show that electric field distortion between cylindrical capacitor electrodes has no obvious influence on the ion mobility in comparison with that obtained at the parallel plane electrode configuration. So, cylindrical capacitor electrode configuration can accurately measure the ion mobility in the transformer oil by the reversal polarity method.

Fabrication, characterisation and stability of TiO2 nanofluids

TiO2 nanoparticles modified by oleic acid were prepared and dispersed into mineral oil to synthesise TiO2 nanofluids. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analyses indicated that the TiO2 nanoparticles are single crystals with an average diameter of 6 nm. Measurements on the stability of nanoparticle dispersion in the base oil were performed by testing UV-Vis absorption spectra of nanofluids aged at a working temperature of 80°C for 100 days. In addition, an AC breakdown test was carried out to further evaluate the stability of nanofluids. The obtained results show that the intensity of the UV absorption peak of nanofluid aged for 100 days has no obvious change in comparison with that of the fresh one. In particular, the ratio of the AC breakdown voltage of the aged nanofluid to the fresh one is still up to 98.6%. TiO2 nanofluids exhibit good colloidal stability at the working temperature. Fourier transform infrared spectra (FTIR) and thermogravimetric method (TG) analyses demonstrate that oleic acid anchors on the nanoparticle surface mainly by a chemisorption. This effective surface functionalisation of nanoparticles greatly enhances the stability of TiO2 nanofluids.