Effects of Nanoparticle Materials on Prebreakdown and Breakdown Properties of Transformer Oil

Featured Application: This work is of practical value in predicting the breakdown accidents and of theoretical importance in reducing the volume of power equipment. Abstract: In order to reveal the effects of nanoparticle materials on prebreakdown and breakdown properties of transformer oil, three types of nanoparticle materials, including conductive Fe 3 O 4 , semiconductive TiO 2 and insulating Al 2 O 3 nanoparticles, were prepared with the same size and surface modiﬁcation. An experimental study on the breakdown strength and prebreakdown streamer propagation characteristics were investigated for transformer oil and three types of nanoﬂuids under positive lightning impulse voltage. The results indicate that the type of nanoparticle materials has a notable impact on breakdown strength and streamer propagation characteristics of transformer oil. Breakdown voltages of nanoﬂuids are markedly increased by 41.3% and 29.8% respectively by the presence of Fe 3 O 4 and TiO 2 nanoparticles. Whereas a slight increase of only 7.4% is observed for Al 2 O 3 nanoﬂuid. Moreover, main discharge channels with thicker and denser branches are formed and the streamer propagation velocities are greatly lowered both in Fe 3 O 4 and TiO 2 nanoﬂuids, while no obvious change appears in the propagation process of streamers in Al 2 O 3 nanoﬂuid in comparison with that in pure oil. The test results of trap characteristics reveal that the densities of shallow traps both in Fe 3 O 4 and TiO 2 nanoﬂuids are much higher than that in Al 2 O 3 nanoﬂuid and pure oil, greatly reducing the distortion of the electric ﬁeld. Thus, the propagations of positive streamers in the nanoﬂuids are signiﬁcantly suppressed by Fe 3 O 4 and TiO 2 nanoparticles, leading to the improvements of breakdown strength.


Introduction
Breakdown characteristics of transformer oil have been widely investigated for its important significance in the safe operation of power equipment [1,2]. Breakdown phenomena in transformer oil are caused by the initiation and propagation of charged gaseous channels called "streamer" under the high local electric field [3]. Positive streamers have been proved to initiate at lower applied voltages and propagate faster and further than negative streamers [4][5][6], which constitute a greater risk to oil insulated high voltage electrical equipment than negative streamers. It has been found that the positive impulse breakdown strength of transformer oil depends on the prebreakdown phenomenon [7][8][9] and it can be considerably influenced by adjusting the propagation characteristic of prebreakdown streamers with the addition of additives [10,11].
Many efforts have been devoted to improve the positive breakdown voltage of transformer oil. In the past two decades, nanofluids (NFs) have attracted much attention due to their good insulating and thermal performance [12,13]. A variety of nanoparticle (NP) materials have been used to prepare transformer oil-based nanofluids, exhibiting different modification effects. It is found that the positive lightning impulse breakdown voltage of mineral transformer oil can be improved by 82.6% with the addition of Fe 3 O 4 nanoparticles [13]. For TiO 2 nanofluid, the breakdown voltage is increased by 23.6% [14]. However, with the addition of Al 2 O 3 and SiO 2 nanoparticles, the breakdown voltage is decreased by 11.6% and 8.8%, respectively [15]. Recently, three types of nanoparticle materials with the same particle size were used to modify the switching impulse breakdown voltage of transformer oil. Based on the simulation of the potential well distribution on the nanoparticles, a possible mechanism was proposed to explain modification effects of the three kinds of nanoparticles [16]. However, there has been still dearth of experimental evidence that the type of nanoparticle materials has an effect on prebreakdown process in transformer oil, especially for nanoparticles with the same size, surface modification and concentration. The propagation process of prebreakdown streamer will give a valuable insight into the event leading to breakdown and provide a bridge between macroscopic breakdown phenomena and microscopic effect mechanism. Therefore, it is helpful to study the prebreakdown process in nanofluids for an improved understanding of the effect mechanism of nanoparticle materials.
In this paper, three types of nanoparticle materials, including conductive Fe 3 O 4 , semiconductive TiO 2 and insulating Al 2 O 3 nanoparticles, were prepared with the same size and surface modification. The breakdown strengths of transformer oil and nanofluids with the same nanoparticle concentration were measured. The propagation characteristics of positive streamers in the oil samples are studied by comparing their shapes, propagation lengths and velocities with the help of the shadowgraph technique. Moreover, the effects of nanoparticle materials on the breakdown and prebreakdown characteristics of nanofluids are discussed.

Nanofluid Preparation
In this work, naphthenic transformer oil (25# Karamay) was used as the base oil, which was filtered in order to remove the impurity particles and meet the demand of pure oil defined by CIGRE working group 12.17 [17]. Fe 3 O 4 , TiO 2 and Al 2 O 3 nanoparticles modified by oleic acid were synthesized in our lab, which were tested to be the same diameter of 10 nm, as shown in Figure 1. Nanofluids were prepared by dispersing the three types of nanoparticle materials into the pure oil with a same volume percentage concentration of 0.025% by stirring and ultrasonic treatment. This concentration is the optimal choice considering both the modification effect and dispersion stability, because the nanoparticle sedimentation happens easily under higher concentrations. The nanofluids and the pure oil were degassed at less than 1 kPa for 24 h before testing and the moisture content of each sample was around 10 ppm.  Figure 2 shows the schematic of experimental setup used to observe the propagation characteristics of prebreakdown streamers. A needle-sphere electrode system, with a high voltage tungsten needle (tip radius of 35 ± 5 µm) opposing a grounded sphere, was located within a test cell made of transparent Perspex sheet, which facilitates the streamer observation. A ten stage impulse generator with an energy of 27.5 kJ was used to provide 1.2/50 µs standard lightning impulse voltage. The positive impulse breakdown strength were measured with an electrode gap of 25 mm according to the standard procedures for testing lightning impulse breakdown strength (IEC60897-1987). Then, an Intensified Charge Coupled Device (ICCD) camera was used to capture streamer propagation images, which were performed with an electrode gap of 45 mm under peak voltage of 75 kV. The camera works with the help of the laser to meet the need of the shadowgraph technique. A trigger unit was used to synchronously trigger the impulse generator and camera. To further investigate the microscopic effect mechanism of different types of nanoparticle materials, the charge transport processes of pure oil and nanofluids were measured by the thermally stimulated current (TSC) method. The experimental setup used was shown in Figure 3. At atmosphere pressure, switch S 3 was closed and a negative direct current (DC) field of 2 kV/mm was applied to the oil sample with a thickness of 0.2 mm for 20 min at 313 K. Then, after the temperature was lowered down to 248 K, switch S 3 was opened and switch S 1 was closed to discharge the equivalent capacitor of electrodes for one minute. Finally, switch S 1 was opened and switch S 2 was closed to begin the measurement of TSC by raising temperature at a rate of 2 K/min. A Keithley 6514 electrometer provides an accuracy of 10 −16 A on the current measurement, and the accuracy of temperature measurement is about ±0.2%.

Breakdown Strength
The positive breakdown voltage and time to breakdown of pure oil and nanofluids are shown in Figure 4. Six repeat breakdown measurements for each sample were performed to generate the breakdown voltage and time to breakdown. The breakdown voltages of nanofluids are improved by the addition of three types of nanoparticles. It is worth noting that the improvements for breakdown voltage of Fe 3 O 4 and TiO 2 nanofluids are up to 41.3% and 29.8% respectively, much higher than that for Al 2 O 3 nanofluid of 7.4%. Especially, the times to breakdown in Fe 3 O 4 and TiO 2 nanofluids are significantly increased by 76.3% and 69.6%, respectively. As for Al 2 O 3 nanofluid, the time to breakdown is almost the same as that of pure oil. These indicate that the average propagation velocities of positive streamers are greatly slowed down due to the addition of Fe 3 O 4 and TiO 2 nanoparticles, which is of great importance since it is closely related to the breakdown strength.

Prebreakdown Streamers
The breakdown strength of transformer oil closely depends on the propagation process of prebreakdown streamers, including streamer shape and velocity [18][19][20], which is helpful to understand the fundamental modification effect of nanoparticles.

Shape
The propagation process of positive streamers can be divided into two stages in view of the shape development characteristics. In the initial stage, as shown in Figure 5, the streamers in pure oil and nanofluids behave similarly, exhibiting dense bush-like structures with slow velocities. While in the propagation stage, remarkable differences in the streamer shape are emerged in Fe 3 O 4 and TiO 2 nanofluids. In pure oil, certain filaments develop into main channels with a rapidly increased length accompanied by other filaments fading away. But in the case of Fe 3 O 4 and TiO 2 nanofluids, the main channels become much thicker with more lateral branches than that in pure oil during the whole propagation process, resulting in a much denser structure. In addition, the streamer propagation length is much shorter in Fe 3 O 4 and TiO 2 nanofluids, which is of notable importance due to its relationship with electric breakdown event. Whereas in the Al 2 O 3 nanofluid, streamers have no obvious change with a tree-like shape similar to that in pure oil.  Figure 6 indicates that the propagation length of streamers varies considerably across pure oil and nanofluids. It is obvious that streamers in Fe 3 O 4 and TiO 2 nanofluids keep shorter than that in pure oil during the whole development process. Moreover, it should be noted that the presence of nanoparticles extends the time to reach the maximum length, leading to a decrease of streamer velocity. The value of average velocity is calculated by dividing maximum length by arrival time. The streamers in pure oil develop fastest with a velocity of 1.40 km/s, followed by that in Al 2 O 3 , TiO 2 and Fe 3 O 4 nanofluids, with velocities of 0.90 km/s, 0.73 km/s and 0.72 km/s, respectively. The values are in accordance with the propagation velocity of 2nd streamers in mineral transformer oil [21,22]. This indicates that the addition of TiO 2 and Fe 3 O 4 nanoparticles dramatically affects the propagation velocity of streamers, which is lowered into around one half of that in pure oil.

Discussion
The electric field dependent molecular ionization has been proved to be the dominant contributor to streamer development in transformer oil, which has a great effect on breakdown strength of transformer oil [22][23][24]. In transformer oil, ionization occurs at streamer tip with a high electric field, generating positive ions and electrons. The highly mobile electrons swept back towards the positive electrode, leaving positive ions to form a net space charge region near the streamer tip, as presented in Figure 7a. These space charges slowly propagate towards the ground electrode and enhance the electric field towards the ground electrode [25]. So, the streamers tend to propagate straightly towards the ground electrode showing filamentary pattern, and breakdown happens when streamers extend throughout the gap. It is clear that the breakdown event is closely associated with charge transport process, which in turn, is influenced by the trap characteristics of transformer oil. To investigate the effects of nanoparticle materials on the trap characteristics of transformer oil, thermally stimulated currents (TSC) were measured. It is believed that TSC measurements can be used to investigate the nature and origin of charge carrier traps in dielectric, including the change of the density and energy level of the trap sites [26]. Figure 8 presents results on TSCs of pure oil and three types of nanofluids. It can be seen that the thermally stimulated currents are increased with the addition of nanoparticles. The trap characteristics including energy level and total trapped charges were calculated as shown in Table 1 [26,27]. The trap density is proportional to the total trapped charge if one-level of trap distribution is assumed [26]. It is indicated that the trap energy level of transformer oil is nearly unchanged by the addition of nanoparticles, which is considered to be shallow trap [14]. However, the trap densities are significantly increased by 65.5% and 59.0% in Fe 3 O 4 and TiO 2 nanofluids, respectively. In Al 2 O 3 nanofluid, the increase is only 14.6%. These results agree well with the effects of nanoparticle materials on breakdown and prebreakdown characteristics of nanofluids presented above.  By introducing more shallow traps, the probability of capturing electrons is more likely increased, meanwhile decreasing the energy of fast electrons during these trapping and de-trapping process in shallow traps. In all, the presence of Fe 3 O 4 and TiO 2 nanoparticles greatly increases the density of shallow traps and forms more negative charges in the oil. As a result, the electric field is changed due to the superposition of space-charge field created by these negative space charges with applied electric field, as presented in Figure 7b. The electric field towards ground electrode is weakened, making it hard for positive streamers in Fe 3 O 4 and TiO 2 nanofluids to propagate towards ground electrode. Moreover, the electric field between the space charge region and the positive electrode is enhanced, contributing to generate lateral branches. In comparison with streamers containing one or two main filaments, streamers with more branches can reduce the tip field due to mutual electrical shielding [5,28], which further slows down the propagation velocity of streamers, improving the positive breakdown strength. Whereas in Al 2 O 3 nanofluid, the increase of breakdown strength is slight due to limited increase of trap density.

Conclusions
In this paper, the effects of nanoparticle materials on prebreakdown and breakdown characteristics in nanofluids under positive lightning impulse voltage were studied, and the conclusions are drawn as follows: (1) Prebreakdown and breakdown characteristics of nanofluids are considerably affected by three types of nanoparticle materials. In Fe 3 O 4 and TiO 2 nanofluids, positive streamers with more thicker and denser branches present greatly lowered propagation velocity, resulting in much higher breakdown voltage than that in pure oil. Whereas in Al 2 O 3 nanofluid, streamers are similar with that in pure oil, just exhibiting a slight increase of only 7.4% in breakdown voltage.
(2) Densities of shallow traps in Fe 3 O 4 and TiO 2 nanofluids are significantly increased by the addition of nanoparticles. The rather higher density of shallow traps in nanofluids provides more opportunity to capture the fast electrons and convert into negative charges, inhibiting the propagation of positive streamers and improving the breakdown strength. But for Al 2 O 3 nanofluid, the density of shallow traps is only slightly increased, making a less obvious improvement of breakdown strength.