Meng Huang

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.

Creeping flashover characteristics improvement of nanofluid/pressboard system with TiO2 nanoparticles

Creeping flashover easily occurs at the interface between oil and pressboard in transformer and thus results in outage of power transmission system. Investigations have shown that creeping flashover characteristics at oil/pressboard interface can be improved by the addition of TiO2 nanoparticles, but the mechanism is still not thoroughly known. In this work, creeping flashover performance at nanofluid/pressboard interface modified by different sizes of nanoparticles were studied and the mechanism was presented as well. Nanofluids with the same concentration but with different sizes of TiO2 nanoparticles were prepared, and pressboards impregnated with them were prepared as well. After that, their creeping flashover characteristics were measured and compared. Nanoparticle’s size affected the creeping flashover performance along oil/pressboard greatly under both AC and lightning impulse voltages. The highest creeping flashover voltage can be enhanced by as high as 12.2% and 32.0% respectively. The underlying...

Effect of TiO2 nanoparticles on the ion mobilities in transformer oil-based nanofluid

TiO2 nanoparticles with an average diameter of 6 nm were synthesized and well dispersed in transformer oil to prepare nanofluids (NFs). Influence of nanoparticles on the ion migration property of NFs was investigated via reversal polarity method under different applied electric fields and temperatures. It was found that the ion mobility in TiO2 nanofluid is increased to 4 times of that in pure oil (PO) under the electric field of 5 kV/m, and then decreased abruptly and keeps around one tenth of ion mobility in PO from 50 to 150 kV/m. While the case in PO keeps almost constant throughout the applied fields. The results of the conductivity and ion characteristics of PO and NFs reveal that the presence of nanoparticles can reduce the diameter of impurity ions by shrinking hydrated solvated shell and greatly enhance their mobility at low electric fields. With the increasing of electric stress, the charged nanoparticles become the major conduction contributor and drift slowly due to the higher viscous resistan...

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.

Power transformer condition assessment method based on environment and property analysis

Reliable operation of the power transformer is the basis for the security and stability of power system operation. The traditional state evaluation methods for transformer are inadequate, especially in device properties and environmental information. This paper proposed a transformers environmental and properties state evaluation method based on the defects analysis. Firstly, statistics transformer defect cases in Guangdong. According to the causes, classify the defects as leakage (from tank, oil conservator, and other main parts of transformer), moisture, and component fault, overheat, rust, filthy, exceed the standard. By the statistics, it shows that moisture, leaks, component failure, overheating defects are the main causes. Secondly, analyze the relevant between main causes and influence factors such as operational life, voltage levels, temperature, and rainfall. Then score each factor, according to the environment of the transformer, to evaluate the specific defects risk. The code of points comes from count general distribution of defect. According to the distribution law of the factors, the basic risk criteria are determined by the total defects distribution, and the different kinds of influence risk criteria are determined by the distribution characteristics of specific defects. At last, summarize specific defects score to get the corresponding operation and maintenance strategy, which are divided into 4 grades, healthy, potential, preliminary, and Critical. In the end of paper, test the assessment method with a running transformer case. The test result shows that the assessment method successfully evaluates a power transformers device properties and environmental state.

Measurement of the electric field strength in transformer oil under impulse voltage

The electric field distribution of the oil-pressboard structure under impulse voltage provides vital reference for the design of transformer insulation. The electric field distribution under impulse voltage is usually held equivalent to that of the capacitive field and calculated by simulation software. An appropriate margin to the calculation results is then selected according to the allowable strength. The design value identified in such a manner, however, falls short of support and verification of actual measurement. The present paper proposes a non-contact real-time measuring method to explore the electric field distribution of the oil-pressboard structure under impulse voltage, using a created measuring apparatus that is composed of optical system, photoelectric converter device, sealed cavity, and impulse voltage generator. Calibrated and verified by a pair of plate electrodes which generated uniform electric field along the propagating direction of the laser beam, the measured values proved to tally well with the actual values of the electric field in terms of both variation trend and numerical value. The minimum sensitivity of the established measuring apparatus was recorded as 3.5 kV/mm and the maximum deviation was 3.6%.

The assess method of validity for partial discharge sensor based on multiple criterion

Transformer is the key electrical apparatus in power system. The internal partial discharge (PD) of transformers can make insulations suffering chronic erosion and damage. At present, three main methods used for detecting partial discharge are pulse-current method, acoustic method and Ultra High Frequency (UHF) method. However, the monitoring accuracy will be affected significantly by invalid partial discharge sensors in field. This paper proposed an assess method of validity based on multiple criterion to find the invalid partial discharge sensors. Based on large amounts of data collected by partial discharge sensors in field, this paper use sliding window to acquire characteristic dataset and use the tag values to substitute the abnormal value (include negative value, null value and over-range value). Then, criterions based on zero value, tag value, consecutive same value and Coefficient of Variation (C.V) are proposed, and applying those criterions to characteristic set to obtain discriminant value. According to the difference of sensors, every discriminant value was assigned a weight value and the state value of sensor can be calculated by summing those discrimination values with weight value. Comparing the state value with tolerance value that was set in advance, the validity of sensor can be acquired. It can be concluded from the paper that the assess method of validity for partial discharge sensor is efficient and accurate. Applying this assess method into field, the invalid partial discharge sensor can be found accurately and timely.