Electric Field In Oil Research Articles

Photodetector array for measuring the spatial–temporal distribution characteristics of electric field in oil–pressboard insulation based on the Kerr electro‐optic effect

AbstractAccurate measurement and effective analysis of the space electric field and charge characteristics within the oil–pressboard insulation structure of converter transformer are essential for achieving precise insulation structure design. Presently, most electric field measurement methods relying on the Kerr electro‐optical effect are limited to single‐point measurements. Given the complexity of converter transformer insulation structures, including their large scale and the use of various materials, existing technologies and findings struggle to provide comprehensive electric field observations within the oil–pressboard region. After leveraging the Kerr electro‐optic effect, a high‐precision 32‐unit photodetector array has been developed to achieve regional measurements of spatial‐temporal electric field in oil–pressboard insulation. The array boasts a measurement spatial resolution of 1.4 mm2 and a sensitivity of 0.15 kV/mm, ensuring measurement accuracy exceeding 96.50%. Through practical measurements of the spatial electric field distribution within the oil–pressboard insulation under direct current voltage, the non‐uniform distribution of the electric field is effectively captured in oil. Notably, the maximum deviation in field strength at different positions within the transformer oil can reach 18.5%. Moreover, as the applied voltage increases, the unevenness coefficient of the field strength gradually rises, peaking at 1.19, which signifies a progressive expansion in the area affected by electric field distortion. By conducting tests to assess the dispersion of volume resistivity in samples from various positions within large sheets of pressboard, the variation in volume resistivity of materials is posited as one of the contributing factors to the non‐uniform distribution of electric field within the oil. The detailed accomplishments aim to provide essential technical and theoretical support for optimising insulation structure design of converter transformers.

Theoretical reasons for rapid heating of vegetable oils by microwaves

Water and high-moisture foods are readily heated in microwaves due to their relatively high dielectric loss factors. Vegetable oil, on the other hand, has a much smaller loss factor (about 1/100th that of water), and is generally believed to be unsuitable for microwave heating. In this study, we conducted experiments to compare heating rates between vegetable oil and pure water in a 2450 MHz microwave oven. We found that the vegetable oil samples were heated rapidly in microwaves, and even faster (1.4–2.0 times) than the water samples. To provide a theoretical explanation, we developed a 3-D computer simulation model. The simulation revealed an approximately 10-fold stronger electric field in oil compared to water, resulting in a similar amount of microwave power being absorbed by the oil and water samples. As the absorbed microwave power was converted into thermal energy, the oil samples were heated faster due to their smaller specific heat (1/2 that of water). But we also found that when the dimensions of oil are smaller than half the microwave wavelength, oil is heated slower than water due to the absence of hot spot areas. This study provides a theoretical explanation for microwave heating of vegetable oils and demonstrates opportunities for utilizing microwave energy to electrify industrial heating of vegetable oils.

Space-time distribution characteristic of electric field in oil of oil-pressboard insulation structure oil under impulse voltage

The space electric field distribution in oil under impulse voltage is very important for the design of transformer insulation. The existing research only focused on the characteristics of the electric field at a certain point in oil-pressboard insulation structure. To in-depth study of the space electric field distribution in oil under impulse voltage changes with time and space, the measurement platform for the space electric field in oil under impulse voltage was built. The space electric field distribution in transformer oil under lightning impulse voltage was measured, and its changes with time and space were analysed. The results indicate that: As the applied voltage increases, the measured field strength begins to be lower than the applied, and the difference between the maximum measured and applied field strength increase. The space electric field distribution in oil in the wavefront time stage gradually becomes uniform with the increase of time. In the tail time stage, the weakening effect of the space charge on the electric field in oil gradually weakens with the increase of time and the applied field strength is first higher than the measured and then gradually lower than it.

Kerr Electro-Optic Sensor for Electric Field in Large-Scale Oil–Pressboard Insulation Structure

The design of the oil–pressboard/paper insulation structure is the key to maintain the safe and reliable operation of power transformers, which mainly relies on simulation and verification test undertaken with simple or equivalent models, falling short of the physical entity verification undertaken in the large-scale structure. Based on the Kerr electro-optic effect and the optical fiber technology, a novel electric field measuring sensor was developed in this paper for the application test in the large-scale oil–pressboard structure. The sensor is contained of collimator, total reflection prism, coupler, and customized optical elements, whose appearance is a miniature capsule-shaped configuration. The key technical performance parameters of sensor were obtained, for instance, and the effect of imported sensor on the electric field under measurement is an average 1.17% reduction in intensity; the sensitivity of designed system is 162.84 V/mm under ac voltage and 40.43 V/mm under dc voltage. Under dc voltage, the measuring accuracy of the designed system is greater than 97.42%, while in the context of ac voltage, it could reach 97.14%. The application test was conducted to capture the dynamic process of the electric field in oil in the large-scale outlet device model, which has three uniformly spaced oil spacings divided by pressboards, under ac voltage, dc voltage, and polarity reversal voltage. The experimental results verify that the developed electric field sensor could be effectively and reliably applied in the large-scale oil–pressboard insulation, and it could be confidently inferred that the larger the scale of insulation structure, the better is the performance of sensor.

Influence of moisture on the interface charge of oil–pressboard composite insulation under DC voltage

The distortion of the electric field in the oil–pressboard composite insulation caused by the accumulation of the interface charge is detrimental to both the insulation design and operation of converter transformers. The influence of moisture content on the surface charge accumulation of oil–pressboard insulation under DC voltage was studied in this study. In accordance with the Kerr electro-optic effect, the electric field strengths in transformer oil and the surface charge density were acquired after applying the positive and negative DC voltages in three oil–pressboard insulation models with different moisture content, respectively. The resistivities of the oil and pressboard in three models, namely Model 1# with 3.8–4.2 ppm moisture in oil and 0.35–0.37% moisture in pressboard, Model 2# with 7.6–7.9 ppm moisture in oil and 0.79–0.82% moisture in pressboard and Model 3# with 14.9–15.4 ppm moisture in oil and 1.39–1.42% moisture in pressboard, was also measured. The results indicate that: (i) as negative charges in oil accumulated on the pressboard surface in a much greater speed than the positive ones, the electric field in transformer oil under negative DC voltage decreases more rapidly with time than that under positive DC voltage; (ii) the increase of the moisture content in both oil and pressboard, under either positive or negative DC voltage, leads to the decrease of both the electric field strength in transformer oil and the charge density with time; and (iii) the increase of moisture content could not only decrease the resistivity of both oil and pressboard, but also the ratio of the resistivity between the pressboard and the oil. On the basis of the Maxwell–Wagner theory, the decrease of the ratio between the pressboard and oil could lead to the decrease of the interfacial charge density, leading to the slow transient process of the electric field in transformer oil under DC voltage.

油中の電場付与用電極板間における吸着剤粒子の運動挙動（吸水量の影響）

油中の電場付与用電極板間における吸着剤粒子の運動挙動（吸水量の影響）

Transient electric field characteristics in oil-pressboard composite insulation under voltage polarity reversal

Polarity reversal generally enhances the electric field intensity in oil-pressboard composite insulation, leading to partial discharge and even insulation flashover or breakdown of the HVDC converter transformer. In an attempt to study the transient electric field in oil at polarity reversal, the present research adopted the Kerr electro-optic effect technique to conduct real-time measurement of the oil electric fields at the polarity reversal time of 10, 60 and 120 s respectively. Dielectric interface conditions were also adopted to capture the dynamic development of interface space charges. Research results indicate that (i) negative charges, under the absorption of insulation pressboard, are more likely to accumulate at the oil-pressboard interface. As a result, the development of electric field in oil at polarity reversal demonstrates prominent polarity effect, with faster electric field attenuation under negative voltage; (ii) the development of electric field in oil shows asymmetric features under double polarity reversals. The first reversal witnesses a high magnitude of electric field in oil that is about 1.47 times of the initial capacitive electric field, whilst the second reversal sees only 85% of the initial capacitive electric field. Such asymmetric development is mainly attributed to different accumulation performances of the positive and negative charges; iii) the shorter the polarity reversal time is, the higher the oil electric field strength will be. The maximum magnitudes of the field strength after polarity reversals with different time, however, are not much different from one another.

Non-linear vibration of a water drop subjected to high-voltage pulsed electric field in oil: Estimation of stretching deformation and resonance frequency

A drop of water subjected to a pulsed electric field in emulsified oil undergoes periodic stretching vibration, which is very effective for promoting the coalescence of the emulsified drops. In this study, the forces that act on a single water drop during its vibration in a pulsed electric field are analysed, and a dynamics model of the vibration of the drop is developed. The expression of the model indicates that the vibration can be described as a non-linear parametric excitation vibration, and comparison of the model predictions with experimental results reveals a good similarity. In addition, a first-order approximation of the predictions of the model is used to obtain the waveforms of the vibration, and the peak of the amplitude–frequency curve reveals the occurrence of resonance. This affords a basis for determining the optimal demulsification frequency of the emulsified oil in the pulsed electric field.

Stable deformation of droplets surface subjected to a high-voltage electric field in oil

Abstract Electro-demulsification is widely applied to the dehydration of emulsified oil. A droplet exposed to an electric field is polarised in oil and then stretched to a prolate sphere. The internal electric field distributions of prolate-sphere and spherical droplets are analysed in this work. The expressions of force acting on the droplets surface are obtained by establishing a model of stable droplet stretching deformation. An experiment is conducted to determine the draw ratio of water droplets subjected to a high electric field and exposed in four types of base oil. Comparison of the results of the experiment and model shows that the stable deformation of droplets is well predicted by the model. System parameters, such as electric field strength, droplet initial radius, interfacial tension of droplet – oil, permittivity and effect on the deformation of droplets, are also discussed.

Freezing Initiation of Super-cooled Water Droplet Colliding with Electrode Plate for Applying Electric Field in Oil

Freezing Initiation of Super-cooled Water Droplet Colliding with Electrode Plate for Applying Electric Field in Oil

Charge accumulation mechanism in oil / pressboard composite insulation system based on optical measurement of electric field

It is important to investigate electrostatic charging phenomena in insulating oil pressboard (PB) composite system in electrical insulation design of oil-immersed power transformers. Our research aim was to clarify the charge accumulation mechanism at the oil/PB interface under static and flowing oil. In this paper, we directly measured the time variation of the electric field in the insulation system using a Kerr electro-optic technique. Results show the time decaying characteristics of an electric field in oil by charge accumulation. The results are influenced by oil flow conditions. Based on the measurement results, we propose a theoretical model for charge accumulation to clarify the process quantitatively. In the model we took the oil volume contributing to the charge accumulation into account, and analyzed the charge behavior. Finally, we clarified the charge accumulation mechanism at the oil/PB interface under oil flow conditions both with and without charging.

Kerr electro-optic field mapping and charge dynamics in impurity-doped transformer oil

We measured dc electric field distribution in transformer oil mixed with different impurities and additives so as to simulate practical degradation conditions of transformer oil. The electric field in the oil doped with asphalt was reduced near the cathode and enhanced near the anode. On the other hand, electric field in oil with copper oleate was enhanced near both electrodes and reduced at the center between the electrodes. The space charge density was estimated from the measured electric field profile using a 1-dimensional form of Gauss law; negative ions occurred more than positive ions by 5 to 40 pC/cm/sup 3/ in the oil with asphalt from 10 to 30 ppm. It was also found that heterocharges with 100 to 300 pC/cm/sup 3/ existed near both electrodes in the oil with copper oleate. Consequently, the electric field and the charge distribution in the oil proved to change depending on the kind and content of impurities or additives and thus depending on the degradation of the oil.