Vegetable oils (VOs) are based on natural ester oil that is considered as an alternative to mineral oil for transformer applications. The dispersion of nanofillers into the VO is expected to improve the physicochemical and electrical performances. Surface-modified insulating nanoparticle is dispersed into the VO to prepare the VO-based nanofluid (NF). An open-beaker oxidative aging study is carried out at 115 °C for VO and NF at three distinct aging durations such as 100, 300, and 500 h. Alterations in the molecular level of aged VO and NF are studied through the various spectroscopic and physicochemical analyses such as nuclear magnetic resonance, Fourier transform infrared, UV-vis absorbance, fluorescence, color, acid number, interfacial tension, and flash point. The fluorescence-based analysis is carried out for VO and NF at fresh and aged condition to monitor the comparative aging degradation. Oxidative aging of VO and NF results in the formation of flammable and harmful gases, which are analyzed using dissolved gas analysis technique. A comparative analysis of the electrical properties such as ac breakdown voltage, dielectric constant, and dielectric dissipation factor for fresh and aged sample of VO and NF is studied.

Condition Assessment of Aged Ester-Based Nanofluid Through Physicochemical and Spectroscopic Measurement

This work reports an experimental and theoretical analysis of the rheological properties of mineral oil-based SiO2 nanofluid for their potential applications in transformer insulation. The flow electrification mechanism on the nanofluids with different surfactants such as cetyl trimethyl ammonium bromide (CTAB), oleic acid, and Span 80 is studied using a spinning disk technique. The results show a higher streaming current for the nanofluids with CTAB as a surfactant compared to oleic acid and Span 80. The rheological behavior of nanofluids is explored with the double gap concentric cylinder geometry. The variation of shear stress with shear rate follows a power law relationship along with a yield stress observed for all the nanofluids. A transition is seen from storage modulus to dominant loss modulus for the nanofluids during the frequency sweep analysis, whereas no transition is observed in the case of mineral oil. In addition, regression analysis using artificial neural network (ANN) algorithms are performed on the experimentally measured viscosity of the nanofluids in order to estimate theoretical parameters and provide insights into the streaming current formation. The desirable rheological characteristics of nanofluids are identified for achieving enhanced insulation performance in transformers.

Experimental Study and ANN Analysis of Rheological Behavior of Mineral Oil-Based SiO2 Nanofluids

This paper presents a detailed review of the streaming electrification phenomena of different insulating fluids for power transformers. The comparison of different techniques used to assess the charging tendency of fluids is discussed depending on the flow type (planar or centrifugal), volume of oil, and interface material. The charge separation between the insulating fluid and metallic/pressboard interfaces is explained in terms of the electrical double layer formation involving a fixed layer and diffuse layer. Based on the experimental results, the streaming electrification is observed to be a function of various factors such as speed, temperature, electric field, and surface roughness. Depending on the molecular structure of insulating liquids that come into contact with solid insulation at the interface, the streaming current can increase; hence, a suitable additive (benzotriazole, fullerene, Irgamet 39) is selected based on the type of fluid and charge polarity. The degradation of the insulating liquid upon ageing, which increases the streaming current and reclamation of such aged fluids using adsorbents (Fuller’s earth, activated carbon, bentonite, and alumina), is a possible method to suppress the static current through improving its dielectric properties. The nanofluids show a higher streaming current compared to base fluid with no change observed even after the reclamation process. The energization process using alternating current (AC) and direct current (DC) impacts the streaming phenomenon depending on its magnitude and polarity. The diffusion of sulfur compounds in the insulating liquid is another major hazard to transformers because the sulfide ions affect the physio-chemical reaction at the interface material, which is responsible for the formation of streaming current.

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Streaming Electrification of Different Insulating Fluids in Power Transformers

The article presents the results of measurements and analysis of the flow electrification process. The research is presented with a new type of measurement instrument, in which one of the electrodes moves in an oscillating manner. The arrangement comprises two parallel plates and the liquid is relocated in one dimension. One of the electrodes is in the form of a copper wire. As a result, the liquid distribution is assumed as very simple and similar to the classic Stokes boundary layer. The electrification current measurements for different oscillation frequencies were performed in such an arrangement. The basic characteristics of the measured current and the determined process models were tested. The nonlinear approach and the linear approach were used. In the case of the nonlinear mode, the displacement of electrode was employed as an input. Hammerstein's mode with a piecewise linear input nonlinearity estimator was used for identification. In the case of the linear mode, the calculated velocity magnitude was used as an input into the system. The discrete time Output Error model and the Transfer Function model were used as a linear model. In each case, very good compliance was obtained between the simulation responses and the actual current measurements.

Streaming electrification in the swinging plate system

This paper deals with flow electrification phenomenon of liquids in channels of rectangular cross section. Different theoretical models are described and compared. For all the models, it is assumed that the flow and the diffuse layer are fully developed. The space charge density conveyed by the flow is computed. First, two cases are examined in the case of weak space charge density, the exact rectangular channel solution is compared with the approximate solution of two parallel planes. This comparison shows a rather small difference between the two models. Then, in the case of two parallel planes assumption, the charge conveyed is computed without any hypothesis on the magnitude of the space charge density and compared to the solution obtained for a weak space charge density commonly assumed. This comparison shows a big difference between the two models concerning the determination of the space charge density on the wall, and, therefore, the zeta potential [1] .

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Flow Electrification of Liquids in Rectangular Channels—Comparison of Different Theoretical Models

Forced convection of insulation oil is the main heat transfer mode in large-power and ultra-high-voltage (UHV) transformers. The electrification hazard caused by physicochemical phenomena between mineral oil and pressboard has been suspected to be responsible for several failures. The electrification measurement was carried out in an oil flow system. The influences of oil temperature, flow velocity, time and applied electric field of fresh and dry transformer oil have been investigated by a measurement system. The experimental results from the oil flow system show that the streaming current increases with the increase of the temperature when the temperature is below 50°C, while it reverses when the temperature is above 50°C. The streaming current increases with the increase of AC and DC electric fields under the different flow velocities, but the streaming current polarity reverses with DC electric field polarity under the 6.9 m/s flow velocity. The electrification phenomenon can lead to reduction of the breakdown strength of the oil. It is concluded that some effective measures should be taken to restrain the possible harm caused by the streaming electrification in UHV converter transformer.

Effects of multiple parameters on static electrification and breakdown strength of transformer oil

The invention provides an amorphous alloy oil-immersed transformer employing micro-perforated panels The amorphous alloy oil-immersed transformer comprises a specially designed transformer oil tank, the micro-perforated panels, a hollow cavity, shock pads and a high-voltage bushing, a low-voltage bushing and a high-voltage tap switch, wherein the specially designed micro-perforated panels are arranged in the oil tank; the hollow cavity is used for serving as an acoustic impedance; one shock pad is arranged between a transformer body and the tank bottom; the other shock pad is arranged between a hanger plate and a clamp; transformer oil is used as an insulating and cooling medium; the high-voltage bushing, the low-voltage bushing and the high-voltage tap switch are arranged on a tank cover; and a sound absorber comprises one or more micro-perforated panels and the single hollow cavity or a plurality of hollow cavities The micro-perforated panels form a helmholtz resonant cavity after being installed on the inner side wall of the amorphous alloy oil-immersed transformer oil tank, can absorb energy of vibration caused by iron core magnetostriction and coil vibration caused by electrodynamic force, and finally reduce the noise Due to the adoption of an amorphous alloy iron core, the amorphous alloy oil-immersed transformer can save more energy than a traditional transformer; the oil tank provided with the micro-perforated panels can absorb vibration of the maximum amplitude corresponding to the iron core vibration frequency; and the noise of the micro-perforated panels is reduced

Amorphous alloy oil-immersed transformer employing micro-perforated panels

The invention provides a simulated measurement device used for simulating and measuring transformer oil-flow charged degree. The simulated measurement device comprises a simulation device used for simulating an operation environment of a transformer and a detection device used for measuring a transformer oil-flow charged degree in the operation environment. The simulation device comprises a first fuel tank, a second fuel tank, a first connecting pipe and a second connecting pipe, wherein the first connecting pipe and the second connecting pipe are connected to the first fuel tank and the second fuel tank. The first fuel tank, the first connecting pipe, the second fuel tank and the second connecting pipe form one closed path. An insulating cylinder, a copper wire and a transformer oil flow are arranged in the first fuel tank. The insulating cylinder and the copper wire are immersed into the transformer oil flow. The detection device comprises a test electrode and a picoammeter electrically connected to the test electrode. The picoammeter is used for measuring the transformer oil-flow charged degree in the operation environment so as to acquire a current signal.

Simulated measurement device

The embodiment of the invention provides a charging tendency simulation testing device for forced-oil air-cooled transformer oil paper, and belongs to the technical field of signal monitoring The charging tendency simulation testing device comprises an electrizing device and a charge measuring device; the electrizing device comprises a centrifugal speed regulating motor, a rotation disc, an oil tank containing transformer oil and a first support used for fixing the oil tank, the centrifugal speed regulating motor is connected to the rotation disc, the rotation disc is fixedly connected with the first support, and an oil draining valve is arranged at the bottom of the oil tank; the charge measuring device comprises a surface potentiometer and a second support used for fixing the surface potentiometer, and the surface potentiometer is arranged at the end, close to the oil tank, of the electrizing device and connected with a computing terminal used for processing test data measured by the surface potentiometer According to the charging tendency simulation testing device for the forced-oil air-cooled transformer oil paper, the real running condition of a transformer can be simulated, the accuracy and the validity of testing and analyzing on the charging tendency of the oil paper interface are improved, and then whether the charged oil paper arranged in the transformer oil tank affects safe running of the transformer or not is judged

Liu Feifei

Papers

Effects of multiple parameters on static electrification and breakdown strength of transformer oil

Amorphous alloy oil-immersed transformer employing micro-perforated panels

Simulated measurement device

Charging tendency simulation testing device for forced-oil air-cooled transformer oil paper