Levitation Voltage Research Articles

Analysis of Breakdown Characteristics in Nanofluid Insulation Materials with Metal Particle Contamination

Optimal insulation in transformers is essential to maintain their performance and reliability. However, insulation-related problems, particularly transformer oil contamination, often result in premature transformer failure. To address this issue, a solution has been proposed in the form of a nanofluid, a mixture of transformer oil, and nanoparticles. This study aims to investigate the impact of metal particle contamination on the breakdown and levitation voltage characteristics of nanofluids as liquid insulating materials. The nanofluids were prepared by mixing mineral oil with Fe3O4 nanoparticles at three different concentrations. Tests were conducted to evaluate the AC breakdown and levitation voltages for various contaminant metal particle sizes. The experimental results demonstrate that contaminant particle size has a significant impact on the breakdown voltage. The nanofluid with a concentration of 0.008% Fe3O4 exhibited superior breakdown voltage performance compared to mineral oil and 0.016% Fe3O4 nanofluid. Furthermore, an increase in the levitation stress was observed as the contaminant particle size increased. The findings of this study emphasize the importance of controlling contamination and selecting the appropriate concentration of nanoparticles to enhance transformer insulation performance. This study also highlights the necessity of monitoring the size of contaminant particles to prevent potential damage. Consequently, this study makes a significant contribution to the advancement of more reliable and efficient transformer insulation technology.

Effect of Magnetic Field on Partial Discharge Initiated by Metallic Particle in Thermally Aged Natural Esters Under AC and Harmonic Voltages

This paper reports the experimental and theoretical investigations of particle levitation voltage on thermally aged ester fluid, under AC and harmonic AC voltages, in the presence of both electric and magnetic fields (130 mT and 160 mT). The results indicate a higher sensitivity to identify partial discharge (PD) initiated due to particle movement in aged ester fluids with an ultra-high frequency (UHF) sensor than the fluorescent fiber technique. The cause for the reduction in sensitivity of PD detection due to the fluorescent fiber technique with thermally aged fluid is analyzed using steady-state fluorescent measurement. The reduction in the levitation voltage noticed under high-frequency AC voltages is much more severe than its impact under the fundamental frequency of AC supply voltage. In addition, the presence of a magnetic field reduces the magnitude of levitation voltage substantially. The UHF signals generated due to particle movement-initiated discharges with aged ester fluids indicate a shift in its dominant frequency of 0.9 GHz under the absence of a magnetic field to around 0.6 GHz with the effect of a magnetic field.

Effect of Zeolite Addition on Partial Discharge and Dielectric Behavior of Thermally Aged Synthetic Ester Fluid Under External Magnetic Field

In recent times, ester based liquids have met high standards and are being considered as potential alternatives over mineral oil transformer insulation. This paper investigates the effect of adding micro-porous zeolite to the thermally aged synthetic ester oil (SEO) to evaluate its electrical performance under the influence of external local magnetic field variation from 0 mT to 165 mT. It is observed that the Corona Inception Voltage (CIV) and Particle Levitation Voltage (PLV) have a declining tendency with ageing, in the presence of magnetic field. Under the influence of magnetic flux density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\simeq 160$ </tex-math></inline-formula> mT), the detected UHF signal was dominant at low frequencies (400 MHz - 900 MHz). The UHF voltage signals peak-to-peak magnitude demonstrates that the discharge magnitude increases with increased ageing duration. Further, it was observed that the addition of zeolite in the synthetic ester liquid tends to improve the electrical performance of the aged oil. The fluorescence spectroscopic analysis shows a shift of the fluorescence maxima together with increased intensity in samples with zeolite as compared to the samples without zeolite addition, which shows a relatively lower concentration of the polar contamination. The frequency dielectric spectroscopy also confirms that synthetic ester specimens showed improved dielectric properties after the zeolite treatment.

Partial discharge activity due to particle movement in SF6 gas filled electrode gap under different voltage profiles

The levitation voltage of the conducting/non conducting particle in the SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas filled electrode gap is high under DC voltage compared with the AC voltage. The levitation voltage increases with frequency of the supply voltage. Particle levitation voltage marginally increases with increase in THD of the harmonic AC voltage. The levitation voltage of the particle increases as and when the barrier is moved towards the high voltage electrode. No polarity dependency is observed on levitation voltage under DC voltages. Percentage of filler content in epoxy nanocomposites has high impact on levitation voltage of the particle. The analysis of levitation voltage with epoxy nanocomposites indicates that up to 3 wt% of MgO added specimen, the levitation voltage is high and above which a marginal reduction in levitation voltage of particle in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas medium is observed. The UHF signal magnitude caused by the particle movement process increases with the applied voltage. Also, the extinction voltage of particle movement is lower than the levitation voltage. Also, with increase of applied voltage, on levitation, the magnitude of UHF signal increases. However, on reducing the applied voltage, the UHF signal magnitude is much higher than onward application of voltage. This characteristics is almost the same under AC and DC voltages. The FFT analysis of UHF signal formed due to particle under AC and DC voltages, at the point of inception has dominant frequency at around 1 GHz. When the applied voltage is increased, especially under DC voltages, the FFT analysis of UHF signal formed indicates energy content in the band 2-3 GHz is increased, indicating indirectly, the rate of charge transfer due to particle movement with opposite electrode is high under DC voltages. The FFT analysis of UHF signal formed by non-conducting particle under high frequencies have the energy content increased in the range 2-5 GHz. PRPD analysis indicates that PD activity is observed in the entire phase of the applied AC voltage, irrespective of harmonics and THDs. The number of PD occurrence is high where the rate of rise of supply voltage is high.

Understanding the partial discharge activity generated due to particle movement in a composite insulation under AC voltages

The UHF signals are generated due to partial discharges formed by particle movement in Gas Insulated Switchgears (GISs). It is observed that the bandwidth of UHF signals generated due to partial discharges formed by particle movement in GIS is independent of operating pressure and the applied voltage. The levitation voltage of the particle sitting on the dielectric barrier is high compared to that of it on the ground electrode. When a particle is present in between the barrier and an electrode, at certain voltage level the movement of particle will be in pseudo resonance with the applied voltage frequency. When the particle is on the barrier, on application of voltage the particle moves to the center of the electrode axis causes discharges thereby radiating UHF signals, with its dominant frequency in the range of 3–4GHz, which is much different from that of the UHF signal generated due to partial discharges formed by particle movement. When the particle moving between the high voltage electrode and the barrier, at some time its motion get collapses and the particle settles in the low field region away from the axis of the electrode and generates UHF signals having characteristics of UHF signals generated due to corona discharge.

Investigation of partial discharge activity by a conducting particle in transformer oil under harmonic AC voltages adopting UHF technique

A methodical experimental study was carried out to understand the partial discharges initiated due to particle movement in transformer oil under high frequency ac voltage and under harmonic ac voltages (with different THDs), by adopting UHF technique. It is also observed that irrespective of order of harmonics, increase in THD of the supply voltage shows reduction in levitation voltage. The rise time of the partial discharge current pulse formed due to particle movement is few ns. It is observed that the frequency content of the signal radiated due to partial discharges generated due to particle movement in the electrode gap lies in the UHF signal range, with its dominant frequency near 1 GHz. The supply voltage frequency and the time required for 100 UHF signals to form shows inverse relationship. When the applied voltage is increased above levitation voltage, the UHF signal magnitude formed due to particle movement increases, irrespective of supply voltage frequency. Ternary diagram clearly indicates that irrespective of frequency of the supply voltage, the UHF signal location in the ternary diagram is the same.

Influence of barrier on partial discharge activity by a conducting particle in liquid nitrogen under AC voltages adopting UHF technique

The UHF signals are generated due to PD formed by particle movement in liquid nitrogen under AC voltages. The levitation voltage of a particle in liquid nitrogen measured through UHF technique and by conventional PD measurement technique is the same, confirming the sensitivity of UHF technique for identification of PD activity. The frequency content of UHF signal generated due to particle movement in liquid nitrogen, under AC voltages, lies in the range 0.5–1.5 GHz. The characteristics of UHF signal generated due to particle movement between the barrier and high voltage/ground electrode is much similar to the signal generated by particle movement in clean electrode gap. Pseudo resonance phenomena can occur in liquid nitrogen due to particle movement. It is also observed that the partial discharge magnitude, in general, be high when the particle moves between the barrier and high voltage electrode when compared to the barrier and the ground electrode. Percentage of clay in epoxy nanocomposites has not altered the levitation voltage of the particle in the electrode gap. Zero span analysis clearly indicates that pseudo resonance occurs when particle moves (in a short gap) between the barrier and high voltage/ground electrode.

Containerless processing by single-axis electrostatic levitation

Containerless state is one of the main characteristics of the space environment, which avoids the contamination from crucibles, and provides a special condition to materials research in outer space. Various levitation techniques have been established to realize the containerless state on the ground. Electrostatic levitation applies the Coulomb force on the charged sample in an electrostatic field to counteract the gravity and achieve containerless state. Compared with other levitation techniques, electrostatic levitation has many advantages: It is applicable to various kinds of materials that holds sufficient surface electric charges, such as metals, oxides and polymers. Its levitation stability can be maintained in a very broad temperature range, since the levitation and heating are controlled independently. It exerts negligible disturbance to the inner part of the sample, and hence is favorable for the achievement of high undercooling and rapid solidification of liquid alloys. In addition, it can be conducted in vacuum and eliminates the influence of gas medium. Therefore, electrostatic levitation becomes an important approach to the study of containerless processing for advanced materials. Since there exists no three-dimensional potential well in the electrostatic field, great difficulties are brought about to the research on the control of levitation stability. In the recent decades, containerless processing of materials by electrostatic levitation were performed in NASA (National Aeronautics and Space Administration, USA), DLR (German Aerospace Center) and JAXA (Japan Aerospace Exploration Agency). Supported by the Creative Research Team Project of the National Natural Science Foundation of China, an electrostatic levitator with single-axis feedback control system was developed by Professor Wei Bingbo’s group in Northwestern Polytechnical University. Both experimental and theoretical investigations were accomplished to solve the four key problems of electric field optimization, sample position detecting, sample charging control and levitation voltage minimization for stable levitation. Various types of materials, such as metals (AZ91D magnesium alloy and ball of aluminum foil), inorganic materials (graphite, C/C composite materials, red phosphorus and silicon dioxide) and organic materials (carbon fiber reinforced resin, liquid crystal and mung bean) have been successfully levitated in the experiments. The typical levitation experiment results are illustrated in Figure 1. The levitated sample exhibits fast control response and satisfactory levitation stabilization. It takes only 0.1 s to realize the stable levitation in upright direction, and the fluctuation is smaller than 0.3 mm (Figure 2(a)). The constraining force produced by spherical electrodes ensures the horizontal stability with a small fluctuation below 0.5 mm (Figure 2(b)).

Electrostatic levitation under the single-axis feedback control condition

An electrostatic levitator with a single-axis feedback control system was developed on the basis of electric field analysis and optimum design for levitation electrodes. In order to realize the stable levitation of various types of materials such as metals, inorganic materials and polymers, we made both experimental and theoretical investigations to solve the four key problems of electric field optimization, sample position detecting, sample charging control and levitation voltage minimization. Under the capacitive induction charging condition, a sample with the size of 2.6–4.5 mm usually bears positive charges amounting to 10−9 Coulomb. Because the single-axis feedback control system responds quickly, it takes the levitated sample only 0.1 s from leaving the bottom electrode until attaining a stable levitation in the upright direction. The levitated sample displays satisfactory levitation stability in both the upright and the horizontal directions owing to the constraining force produced by spherical electrodes.

Simulation of mass transfer from an oscillating microdroplet

Mass transfer from micrometer and sub-micrometer airborne microdroplets arises in various chemical process, material science, and atmospheric phenomena, such as impinging flow reactor and unsteady pollutant diffusion, where microdroplet oscillation can substantially increase the mass transfer rate. Previous theories, however, do not adequately predict this enhancement of mass transfer, especially in the case of relatively large-amplitude oscillations. We have analyzed slow evaporation of an oscillating microdroplet having a sufficiently low vapor pressure such that it remains at the surrounding gas temperature and has a negligibly small rate of change of diameter. We solved the governing convective diffusion equation numerically to obtain the Sherwood number as a function of the system parameters. These include the oscillation frequency, the maximum velocity, and the initial microdroplet diameter. The theoretical results are compared with mass transfer data from the literature for a dodecanol microdroplet levitated in an electrodynamic balance (EDB) and oscillated by varying the dc levitation voltage and the ac amplitude and frequency. The predicted Sherwood numbers agree with the experimental results with a mean deviation of 9.2%. The analysis shows a distinct periodic change in the mass transfer rate or Sherwood number with a period that is one-half the period of oscillation of the microdroplet.

Carbon Dioxide Uptake by Hydrated Lime Aerosol Particles

The reaction of the greenhouse gas, CO2, with Ca(OH)2 was explored using single-particle Raman spectroscopy to track the chemical reaction. Single particles were levitated in an electrodynamic balance (EDB) to maintain the particle in a laser beam. The levitation voltage provided gravimetric data. The humidity in the EDB chamber was varied to determine the effects of humidity on the reaction. It is demonstrated that no appreciable reaction occurs at low relative humidities (RH < 70%). This is evidenced by no disappearance of the Raman peak associated with the [OH]− vibrational bond, but at high humidities (RH > 70%) the Raman spectrum of CaCO3 developed as the reaction proceeded. The results are consistent with results from packed-bed studies of the reaction and are in agreement with similar findings for the SO2/Ca(OH)2 reaction used for desulfurization, that is, the reaction does not proceed until multiple monolayers of water are adsorbed on the particle surface.

Digital dielectrophoretic levitation

In this article we describe a computer-controlled active feedback dielectrophoretic (DEP) levitator and its measurement capabilities for characterizing single particles and particle chains (20–500 μm in length). Edge detection using a linear array of photodiodes is employed and an adaptive controller updates the voltage required for stable levitation at a rate of 4.7 kHz. Rms voltage fluctuations are typically less than 0.2% of the levitation voltage. The most precise data to date for the effective dipole moment of conducting particle chains up to four in length are presented along with experimental measurements of the effective dipole moment of two particle chains of unequal size as tests of accuracy and precision of the apparatus.