Quasi-uniform Field Research Articles

준평등 전계 중 절연파괴 메커니즘에 의한 SF₆의 연면방전특성과 관통파괴특성의 분석

This paper describes the surface discharge characteristics and the breakdown characteristics of a solid dielectric in SF₆ in order to utilize SF₆ gas filled in high voltage power equipment. The AC breakdown experiments were conducted to measure the surface flashover voltage and the puncture breakdown voltage with the solid dielectrics (teflon, polyethylene, acrylic, and bakelite) at SF₆ pressure of 0.1㎫ and 0.3㎫ in a quasi-uniform field, respectively. The flashover voltages increased as the dielectric constant of the solid dielectrics increased. On the other hand, puncture breakdown voltages were dependent on the heat distortion temperature more than the dielectric constant of the solid dielectrics. The experimental results were analyzed based on the surface discharge mechanism and the thermal breakdown theory of solid dielectric.

Insulation Strength and Decomposition Characteristics of a C6F12O and N2 Gas Mixture

This paper explores the decomposition characteristics of a new type of environmentally friendly insulating gas C6F12O and N2 mixed gas under AC voltage. The breakdown behavior of 3% C6F12O and N2 mixed gas in quasi-uniform field was investigated through a breakdown experiment. The self-recovery of the mixed gas was analyzed by 100 breakdown experiments. The decomposition products of C6F12O and N2 under breakdown voltage were determined by gas chromatography–mass spectrometer (GC-MS). Finally, the decomposition process of the products was calculated by density functional theory, and the ionization energy, affinity, and molecular orbital gap of the decomposition products were also calculated. The properties of the decomposition products were analyzed from the aspects of insulation and environmental protection. The experimental results show that the 3% C6F12O and N2 mixed gas did not show a downward trend over 100 breakdown tests under a 0.10 MPa breakdown voltage. The decomposition products after breakdown were CF4, C2F6, C3F6, C3F8, C4F10, and C5F12. The ionization energies of several decomposition products are more than 10 eV. The Global Warming Potential (GWP) values of the main products are lower than SF6. C2F6, C3F8, and C4F10 have better insulation properties.

Breakdown characteristics of SF6 in quasi-uniform field under oscillating lightning impulse voltages

The field impulse voltage withstand test plays one of the most important roles to ensure the safety of the gas insulated equipments. It is hard to carry out the standard impulse voltage test for the equipments because the generator is very large and difficult to transport. The oscillating impulse voltage waveform has high efficiency and has been recommended by the IEC 60060-3 as the field test waveform. SF6 gas is the insulation medium of gas insulated equipments. Its insulation characteristics under different types of impulse waveforms have great influence to the design, operation and test amplitude. This paper focuses on the breakdown characteristics of SF6 gas in a quasi-uniform field under oscillating impulse voltages (OIV). Four types of oscillating impulse lightning voltage waveforms with oscillating frequency of 100, 200, 300 and 400 kHz were adopted as the test waveforms. The standard lightning impulse (SLI) voltage was also adopted as the reference waveform. A calculation model is established to explain the results. The experiment and calculation results show that the breakdown waveforms in time domain drop quickly and then have a recovery with low amplitude to zero under OIV for the influence of circuit inductor. 50% probability breakdown voltages (BDVs0%) under OIV are higher than that under SLI. The minimum breakdown voltage and BDVs0% both increase with oscillating frequency. The voltage-time distributions show that breakdowns under OIV occur in the vicinity of each wave peak of the applied voltage. OIV have a longer time lag than SLI due to the oscillation of the waveforms.

Experimental studies on the power–frequency breakdown voltage of CF3I/N2/CO2 gas mixture

Trifluoroiodomethane is a promising alternative to SF6 because of its good insulation properties and much less serious greenhouse effect than SF6. Previous studies have shown that the insulation performance of CF3I mixed with CO2 or N2 can equal that of SF6. This study explored the frequency breakdown characteristics of CF3I and SF6 mixed with two buffer gases. The effects of air pressure and field strength were analyzed. The fixed mixing ratio of CF3I and SF6 was 30% in the experiment. The breakdown experiment was conducted by changing the mixing ratio of CO2 and N2. Results showed that the CO2/N2 mixture ratio did not exert a synergetic effect, and the CF3I/CO2 breakdown performance was better than that of CF3I/N2 in the quasi-uniform and highly non-uniform electric fields. CO2 possibly provided the C atoms for the entire system to maintain a certain balance in C, and this balance inhibited the decomposition of CF3I. The breakdown performance of SF6/N2 was good in quasi-uniform field, whereas that of SF6/CO2 was good in the highly non-uniform field.

Follow phenomenon of breakdown voltage in SF6 under lightning impulse

Follow Phenomenon, defined as a series of continuous breakdowns or withstands without sufficient time lag of successive applied voltages, is observed during the research of SF6 discharge characteristics under lightning impulse. Follow Phenomenon of SF6 gas plays a decisive role for the insulation recovery time between continuous discharges, but little relevant research has been reported before. In this work, Follow Phenomenon as a function of gas pressure, time lag of applied voltages in quasiuniform and non-uniform field, under positive and negative lightning impulse is studied. It is found that, when the time lag t=1 min, Follow Phenomenon is more obvious under positive lightning than that under negative lightning, and it becomes more pronounced under larger pressure; as time lag t increases, the increasing speed of Follow Phenomenon with pressure becomes lower. When time lag t=5 min, the measure uncertainty influenced by Following Phenomenon could be avoided when P⩽0.2 MPa in quasi-uniform field and P⩽0.3 MPa in non-uniform field. As pressure rises, a longer time lag is needed to eliminate the influence of Follow Phenomenon. At the end, analysis is made which shows that Follow Phenomenon is caused by the detachment of negative ion formed at previous discharge processes and by “the Sweeping effect” of effective initial electrons.

준평등전계중 Dry Air내 산소의 농도에 따른 절연파괴 및 연면방전 특성 연구

준평등전계중 Dry Air내 산소의 농도에 따른 절연파괴 및 연면방전 특성 연구

The effect of magnetite, graphene oxide and silicone oxide nanoparticles on dielectric withstand characteristics of mineral oil

In this work, we compare the dielectric withstand performance of nanofluids prepared using Shell Diala D as a base fluid, and magnetite, graphene oxide and silicone dioxide as nanoadditives. The ac withstand capability of the nanofluids was investigated as well as partial discharge inception voltage, partial discharge and repetition rate under ac, dc positive and dc negative voltages. The results indicate that, for all nanofluids, nanoparticle concentrations around 0.2 g/l enhance dielectric withstand properties under quasi uniform fields. Under divergent fields, partial discharge characteristics are improved under ac conditions. Under dc conditions silica nanofluid performs better than mineral oil, but the other two nanofluids do not perform well.

Size effect on breakdown strength in sub-cooled liquid nitrogen for superconducting power apparatus

Toward the practical and rational insulation design of high temperature superconducting power apparatus, the ac breakdown (BD) characteristics and mechanisms of liquid nitrogen (LN2) were systematically investigated and discussed for different pressures at P = 0.1-0.3 MPa and temperatures at T = 65-77 K under quasiuniform electric field with the field utilization factor ζ= 0.986-0.674. We obtained the BD strength (EB) in sub-cooled LN2 (SLN2) and analyzed them in terms of size effect, taking account of the stressed liquid volume (SLV) with the electric field distribution in SLN2. Together with the other researchers’ data on BD characteristics of LN2, we extended the evaluation by applying the concept of α% SLV (SLV with the electric field strength higher than the decisive electric field factor, α of the maximum electric field strength). As the result, we derived that EB with the 50 % probability decreased linearly with the increase in α% SLV on a universal line, EB = 78.4 x (α% SLV)-1/8.15, which was irrespective of the pressure and temperature of LN2.

N2/O2혼합가스 중 적층된 고체유전체에 대한 연면방전의 메커니즘과 특성

This paper presents the surface flashover mechanism of a laminated solid dielectric and describes the surface flashover characteristics with the inherent capacitance of the laminated solid dielectric in a mixture gas (8:2) under an quasi uniform field. It was found that the electron emission at a cathode and the high-local electric field region around an anode were important factors to reasonably describe the surface flashover mechanism. The surface flashover voltage by the mechanism decreased with the inherent capacitance increase of the laminated solid dielectric. In addition to the surface flashover mechanism and its characteristics, the surface flashover voltage equations as a function of the inherent capacitance were derived by considering a gas pressure used in future eco-friendly GIS and the factors influencing the surface flashover.

Dielectric properties of gas mixtures with C<sub>3</sub>F<sub>8</sub>/C<sub>2</sub>F<sub>6</sub> and N<sub>2</sub>/CO<sub>2</sub>

Thanks to its excellent insulation and cutoff performances, SF6 gas has been applied to power equipment since the 1960s and is now widely used for several kV to 1,000 kV-class GIS, GCB, and GIL. However, since 1997, when SF6 was designated at COP3 as a greenhouse gas to be reduced, there has been a wish to use an alternative insulating gas to pure SF6 gas. In the present study, alternative gases were selected from among mixtures excluding SF6 with the need to reduce GWP (global warming potential) in mind. Gas mixtures containing such substances and with a boiling point of -20°C or less, chemically stable, non-toxic, and not ozone-depleting were prioritized. Furthermore, the availability and environmental performance were taken into consideration when deciding on component gases. Consequently, to launch this series of studies, four types of gas mixtures were used combining a gas in Group A (C2F6, C3F8) – electronegative gases with relatively high dielectric strength – and a gas in Group B (N2, CO2) – gases existing in the natural world. The GWP of SF6 is 22,800 whereas that of C2F6 is 12,200 and that of C3F8 is 8,830, or several times smaller than that of SF6. In the present paper, insulation characteristics were experimentally obtained while varying the mixture ratio under a quasi-uniform electric field assuming GIS. Consequently, compared to the GWP of pure SF6, the GWP was about 12% to 38% for gas mixtures with C3F8/N2 or C3F8/CO2 and 18% to 70% for gas mixtures with C2F6/N2 or C2F6/CO2. Consequently, it emerged that, while assuming breakdown voltage proportional to gas pressure, the GWP was likely to be reduced by 30% to 90% while maintaining dielectric strength. In addition, a study was conducted on the synergism of a gas mixture through analysis using the Boltzmann equation. Consequently, the synergism was confirmed while its degree varied depending on the type of each gas mixture, and the mechanism thereof was clarified.

Dry Air/O<sub>2</sub> 혼합가스의 혼합비에 따른 절연파괴 및 연면방전 특성 연구

This paper presents the discharge characteristics and economic feasibility of a Dry Air/O₂ and a N₂/O₂ mixture gas in order to review SF6 alternative. From AC discharge experiment in an quasi-uniform field, it was found that the optimal N₂/O₂ mixing ratio which breakdown voltage and surface flashover voltage were the highest was 70/30 and that the pressure dependence on the breakdown voltage was higher than that of the surface flashover voltage in the Dry Air/O₂ and the N₂/O₂ mixture gas. The mixing ratio (70/30) and the tendency of the pressure dependence were described in detail based on physical factors (impact ionization coefficient, electron attachment coefficient, secondary electron emission coefficient) involved in discharge mechanism and a electron source, respectively. In addition, the performance insulation and the economic feasibility of the Dry Air/O₂ and the N₂/O₂ mixture gas were discussed so that Dry Air/O₂ mixture gaswas more suitable than N₂/O₂ mixture gas to the SF6 alternative.

광유 중 절연파괴전압의 분산과 절연파괴진전 과정의 분석

This paper presents a breakdown voltage and a process of breakdown progress in mineral oil under an quasi-uniform field with decomposition products which occur after the oil discharge. The breakdown voltage in the oil revealed the characteristics of dispersion regardless of an electrode gap. The cumulative probability distribution was used to analyze the dispersion of the breakdown voltage. In addition, the process of breakdown progress in the oil can be reasonably described by the electron breakdown theory based on both electrons emitted from the cathode and ions by field-aided dissociation of the oil. The proposed breakdown process will be used for the basic data to explain the behavior pattern of the decomposition product to cause the dispersion of the breakdown voltage.

Surface Discharge Characteristics Study on the Laminated Solid Insulator in Quasi-Uniform Electric Field with Dry Air

Dry air is an excellent alternative to SF6 gas and is used as an insulation gas in Eco- friendly Gas Insulated Switchgears (EGISs), which has gained popularity in industry. Solid insulators in EGIS play an important role in electrical insulation. On the other hand, surface discharge can occur easily when solid insulators are used. This paper explored the surface discharge characteristics on the structure of three-layered laminated solid insulators to elevate the flashover voltage. A laminated solid insulator was inserted after the quasi-uniform electric field was formed in the test chamber. Dry air was then injected to set the internal pressure to 1 ~ 6 atm, and the AC voltage was applied. When identical solid insulators were stacked, the surface discharge characteristics were similar to those of a single solid insulator. On the other hand, the flashover voltage rose when the middle part was thicker and had lower permittivity than the top and bottom parts in the laminated solid insulator. Based on experimental results, when stacking a solid insulator in three layers, the middle part of the solid insulator should be at least four times as thick as the top and bottom parts and have lower permittivity than the others. In addition, the flashover voltage increased with increasing gas pressure on the surface of the laminated solid insulator due to the gas effect. This study may allow insulation design engineers to have useful information when using dry air for the insulation gas where the surface discharge can occur.

Dry Air 중의 준평등전계에서 노점과 고체절연물 재질에 따른 수평연면방전 특성 연구

This study investigates the Horizontal surface discharge characteristics of solid insulators by varying their materials, and the dew-point of dry-air. The methodology of this study is that a quasi-uniform field is first applied to a test chamber. Then, the chamber is filled with dry-air as an insulation gas which pressure is varied from 1 to 6atm while applying an AC voltage to the chamber. The used solid insulators are teflon, polyethlene and polyurethane. As the dew-point is lower and the pressure of dry-air is higher, the flashover voltage of all solid insulators increases more. When each characteristic of the solid insulators is compared under the same gas pressure, the flashover voltage of teflon is the highest. Then, the flashover voltage of polyethlene is higher than that of polyurethane. Moreover, it is observed that the flashover voltage increases as the horizontal distance between the electrodes of each solid insulator become larger, respectively. However, as the pressure is increased, flashover voltage of the solid insulators is saturated. Therefore, selection of cost-effective insulation is needed in order to appropriate pressure.

Imaging the dynamics of free-electron Landau states.

Landau levels and states of electrons in a magnetic field are fundamental quantum entities underlying the quantum Hall and related effects in condensed matter physics. However, the real-space properties and observation of Landau wave functions remain elusive. Here we report the real-space observation of Landau states and the internal rotational dynamics of free electrons. States with different quantum numbers are produced using nanometre-sized electron vortex beams, with a radius chosen to match the waist of the Landau states, in a quasi-uniform magnetic field. Scanning the beams along the propagation direction, we reconstruct the rotational dynamics of the Landau wave functions with angular frequency ~100 GHz. We observe that Landau modes with different azimuthal quantum numbers belong to three classes, which are characterized by rotations with zero, Larmor and cyclotron frequencies, respectively. This is in sharp contrast to the uniform cyclotron rotation of classical electrons, and in perfect agreement with recent theoretical predictions.

Observation of low-density domain in polystyrene under nanosecond pulses in quasi-uniform electric field

Low-density domains (LDDs) developed in polystyrene (PS) samples under nanosecond pulses in a quasi-uniform electric field are observed using an on-line transmission microscope. The test samples are 2 mm in thickness with a cylindrical profile, and immersed in clean transformer oil. The nanosecond pulse is trapezoidal with a pulse width of 10 ns. Images taken by the microscope show that the LDDs always appear in the vicinity of the cone electrode, expand as the pulse number increases, and gradually vanish when the output pulses are stopped, only leaving small voids and cracks in the samples. When the pulses are imposed on the sample again, the LDDs emerge and begin to expand again, and finally lead to bulk breakdown of the samples. The observation of LDDs under this condition gives a support for the pre-breakdown and breakdown model by K. C. Kao when applyied to a nanosecond time scale.

Observation of low-density domain in polystyrene under nanosecond pulses in quasi-uniform electric field

Low-density domains (LDDs) developed in polystyrene (PS) samples under nanosecond pulses in a quasi-uniform electric field are observed using an on-line transmission microscope. The test samples are 2 mm in thickness with a cylindrical profile, and immersed in clean transformer oil. The nanosecond pulse is trapezoidal with a pulse width of 10 ns. Images taken by the microscope show that the LDDs always appear in the vicinity of the cone electrode, expand as the pulse number increases, and gradually vanish when the output pulses are stopped, only leaving small voids and cracks in the samples. When the pulses are imposed on the sample again, the LDDs emerge and begin to expand again, and finally lead to bulk breakdown of the samples. The observation of LDDs under this condition gives a support for the pre-breakdown and breakdown model by K. C. Kao when applyied to a nanosecond time scale.

An investigation into the cumulative breakdown process of polymethylmethacrylate in quasi-uniform electric field under nanosecond pulses

A group of complete images on the discharge channel developed in PMMA in quasi-uniform electric field under nanosecond pulses are observed with an on-line transmission microscope. The characteristics of the cumulative breakdown process are also generalized, which include initiating from the vicinity of the cathode, developing to the anode with a branch-like shape, and taking on a wormhole appearance when final breakdown occurs. The concluded characteristics are explained by referring to the conceptions of “low density domain” and “free radical” and considering the initial discharge channel as a virtual needle. The characteristics are helpful for designers to enhance the lifetime of insulators employed on a nanosecond time scale.

Evaluation of breakdown characteristics of N2 gas for non-standard lightning impulse waveforms - breakdown characteristics in the presence of bias voltages under non-uniform electric field

SF6 gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative is required from an environmental perspective. As one potential alternative, the authors are focusing on N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas, which has relatively good insulation characteristics among gases with a low environmental impact. To use this N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (non-standard lightning impulse waveforms; non-standard-LIWs) must be obtained. The present study obtained and evaluated the insulation characteristics where a surge occurred in the presence of a bias voltage (dc component), which was considered realistic and severe under a non-uniform electric field typically represented by metallic particles. Consequently, it was concluded that the breakdown voltage was lower for positive polarity under a non-uniform electric field in the presence of a bias voltage and that it was appropriate to conduct an experiment using a positive polarity waveform when studying dielectric strength. Furthermore, it emerged that the breakdown voltage for a positive polarity waveform was 1.06 to 1.67 times higher than that for the standard lightning impulse waveform, even if the frequency and damping rate were changed. These results support the rationalization of insulation for actual surge waveforms under the quasi-uniform electric field.

준평등전계에서의 Dry-Air 노점과 고체절연물 재질 및 형상에 따른 연면방전 특성 연구

This study investigates the surface discharge characteristics of solid insulators by varying their materials, their shapes, and the dew-point of dry-air. The methodology of this study is that a quasi-uniform field is first applied to a test chamber. Then, the chamber is filled with dry-air as an insulation gas which pressure is varied from 1 to 6atm while applying an AC voltage to the chamber. The used solid insulators are teflon, polycarbonate, and bakelite. As the dew-point is lower and the pressure of dry-air is higher, the flashover voltage of all solid insulators increases more. When each characteristic of the solid insulators is compared under the same gas pressure, the flashover voltage of teflon is the highest. Then, the flashover voltage of polycarbonate is higher than that of bakelite. Moreover, it is observed that the flashover voltage increases as the diameter and the thickness of each solid insulator become larger and thicker, respectively. However, the thickness of the solid insulators is more critical for increasing the flashover voltage than their diameter.