Breakdown Characteristics Of Liquid Nitrogen Research Articles

A Study on the Electrical Breakdown Characteristics of Liquid Nitrogen for the Design of Superconducting Apparatuses

A Study on the Electrical Breakdown Characteristics of Liquid Nitrogen for the Design of Superconducting Apparatuses

Effect of barrier thickness of Polytetrafluoroethylene on the Direct Current breakdown characteristics of liquid nitrogen

When high-temperature superconducting (HTS) tapes in high voltage DC HTS devices are quenched, amounts of thermal bubbles are generated in liquid nitrogen (LN2) that will reduce the insulation strength greatly. Especially when resistive-type superconducting fault current limiters meet a short-circuit fault, the insulation strength of the LN2 decreased significantly due to the thermal bubbles and electrothermal pressure. Solid insulating barriers can improve the insulation strength of the LN2. The objective of this study is to obtain the effect of barrier thickness on the DC dielectric breakdown characteristics of LN2. Insulating barrier Polytetrafluoroethylene with three different thicknesses were tested. Three types of electrodes such as rod-plane, needle-plane, and plane-plane were applied. The results showed that the barriers can increase the negative and positive breakdown voltage both with and without thermal bubbles in a slightly non-uniform field (SNF) and non-uniform field (NF). For the SNF, the DC breakdown voltage is higher when the barrier is thinner. The effect of the barrier thickness in the NF is lower than that in the SNF field. For the NF, the positive breakdown voltage is higher when the barrier is thicker.

Barrier position effect on bubble triggered DC breakdown characteristics in liquid nitrogen

The objective of this study is to determine the effect of different barrier positions on the DC breakdown characteristics of liquid nitrogen (LN 2 ), with and without bubbles. The DC breakdown voltage is measured of a needle-to-plane electrode, with 9 mm gap distance, with and without PTFE barrier insulation of 40 μm thickness. The presence of bubbles lowers the breakdown voltage and the effect of have the barrier at various distances from the tip of the needle electrode is studied. The results show that with and without bubbles, the closer the barrier to the needle electrode, the higher the breakdown voltage. Observations using a high-speed camera show that the presence of the insulation barrier triggers the breakdown of the LN 2 /insulation barrier composite system.

Experimental investigations on power frequency electrical breakdown characteristics of liquid nitrogen for HTS power devices

Experimental investigations on power frequency electrical breakdown characteristics of liquid nitrogen for HTS power devices

Breakdown Characteristics of Liquid Nitrogen in Strongly Inhomogeneous Electric Fields

Resistive superconducting fault current limiters (R-SCFCLs) based on second-generation superconducting tapes (2G tapes) are successfully used in the medium-voltage level. To develop R-SCFCLs for the high-voltage application exceeding 100 kV, it is necessary to know the dielectric properties of liquid nitrogen more in detail, particularly if sharp edges, occurring for example at 2G tapes or at connection conductors, cannot be avoided. Consequently, in this work, the breakdown characteristics of strongly inhomogeneous electrode systems in liquid nitrogen were investigated. An electrode configuration of a plane electrode and a needle electrode with a tip radius r of approximately 0.1 mm and sphere electrodes with radii r from 1 to 3 mm were used. The distance d between the electrodes was adjusted in the range of up to 15 mm. The electrode configuration was immersed in liquid nitrogen within an open isolated vessel, and the ac breakdown voltage was measured in dependence on the rounding radius r of the high-voltage electrode and the electrode distance d. For each combination of r and d, the mean breakdown voltage was calculated from 15 single measurements. The measuring results using sphere electrodes reveal that the mean breakdown voltage VBD approximately follows the empirical law V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BD</sub> = c · d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sup> with b = 0.46. For the needle electrode with a radius of roughly 0.1 mm, the mean breakdown voltages are remarkably smaller and increase with distance d only with an exponent b = 0.28.

Evaluation of Uniform and Non-uniform Breakdown Characteristics of Liquid Nitrogen With Different Electrode Materials

High temperature superconducting devices utilize liquid nitrogen (LN2) both for cryogenic liquids and insulating media. Regarding the insulating performance of LN2, it shows excellent characteristics comparable to insulating oil which is widely used for conventional electric equipment. But, detailed complex insulating structure in LN2 composed of cryogenic liquids, solid insulating material and conductor material has not been fully considered yet. In order to commercialize superconducting devices, insulating characteristics of LN2 should be analysed and confirmed to the level of insulating oil. In this work, we focused on the breakdown characteristics of LN2 especially for different electrode materials. Aluminum, Stainless steel, and Copper electrode systems were fabricated and tested in LN2 to verify uniform and non-uniform characteristics of LN2 according to the electrode materials. AC withstand voltage test and lightning impulse test has been performed. From the test results, it was revealed that the breakdown voltages were significantly changed according to the electrode materials. Especially for Stainless steel, it shows prominent enhancement of breakdown voltage compared to copper and aluminum electrode in LN2. Consequently, it was deduced that optimum choice of conductor materials in LN2 is another critical factor to determine complex insulating performance of superconducting devices.

Breakdown Characteristics of Liquefied ${\rm SF}_{6}$ and ${\rm CF}_{4}$ Gases in Liquid Nitrogen for High Voltage Bushings in a Cryogenic Environment

High voltage cryogenic insulation issues need to be addressed in order to promote the commercialization of high temperature superconducting (HTS) equipment. One of the critical components for superconducting devices is the bushing whose role is to safely supply high current to the device. Due to a steep temperature gradient, commercial bushings which have been insulated with SF6 gas could not be directly applied to cryogenic equipment due to liquefaction of SF6 in the cryogenic environment; therefore, alternative suitable structure and insulation methods should be developed. As a fundamental step in the development of the optimum bushings for HTS devices, the breakdown characteristics of liquid nitrogen mixed with liquefied insulating gases such as N2, SF6 and CF4 have been investigated. In particular, we noted the insulation characteristics of CF4 gas whose liquefication temperature is much higher than SF6 gas. Thus, in order to investigate the possibility of substituting CF4 gas for SF6 gas for the bushings of HTS electrical equipment, impulse tests, AC withstanding voltage tests, and partial discharge (PD) tests have been performed. As a result of these tests, it was shown that mixtures of liquefied insulating gases have a much higher breakdown voltage compared to pure liquid nitrogen. Especially in a cryogenic environment, the usage of SF6 gas should be evaluated due to freezing effects. On the other hand, CF4 gas has shown excellent insulation properties even in a cryogenic environment and could be utilized as an insulation gas for high voltage bushings of HTS electrical equipment.

Experimental study on the lightning impulse dielectric strength of liquid nitrogen insulation medium according to various pressures

The lightning impulse breakdown characteristics of liquid nitrogen are necessarily considered for designing high voltage superconducting machines required high reliability. This paper investigates the dielectric capability of liquid nitrogen (LN2) to withstand lightning impulse. To gain the dielectric capability of LN2 cryogenic cooling system, lightning impulse was induced in sphere–plane electrode systems which have six different diameters of sphere electrode. The pressure is one of major condition to decide dielectric strength at LN2 cryogenic cooling system. Thus, the gaseous nitrogen was injected to control pressure in the cryostat, ranging from 100kPa to 200kPa. Moreover, field utilization factor was calculated between sphere and plane electrode systems by a finite element method analysis. The electric field criterion of LN2 as insulation media was calculated from correlation between the experimental results and the field utilization factor. In the future, these results can be applied as the design parameter of electrical insulation for developing high voltage superconducting machines.

Experimental Study on the Electrical Breakdown Characteristics of Sub-Cooled Liquid Nitrogen for Designing a High Voltage Superconducting Machine

The electrical breakdown characteristics of liquid nitrogen should be investigated for developing of a high voltage superconducting machine. This paper deals with the experimental study for the verification of dielectric characteristics of sub-cooled in accordance with utilization factors. AC dielectric experiments were carried out by using sphere-plane electrode systems. The utilization factors of simulated electrode systems were controlled by gap distance between two electrodes and diameter of a sphere electrode. Also, dielectric experiments according to various pressures, 1.25, 1.5 and 2 bar, by gaseous nitrogen were carried out. In this paper, the electrical breakdown criteria of sub-cooled conditions are derived with various sphere diameters. In addition, the sub-cooled condition and saturated condition were compared with AC dielectric characteristics. Research results could be applicable to designing of high voltage superconducting applications such as superconducting fault limiters (SFCLs), cables and large-capacity transformers.

Experimental study on the dielectric characteristics of liquid nitrogen with respect to various pressures

The electrical breakdown characteristics of liquid nitrogen (LN2) are required to design of high voltage superconducting machines which should have high reliability. This paper deals with the investigation to verify the relationships between the electrical breakdown characteristics of LN2 its pressure condition. The electrical breakdown characteristics of LN2 were measured by using sphere-plane electrode systems. The diameter of sphere electrode had five different values. The gaseous nitrogen was injected into a cryostat which contained the sphere-plane electrode system to control the pressure of LN2 from 50 to 200kPa. The AC dielectric tests in LN2 were repeated 10 times in each pressure condition. The AC breakdown voltage at sparkover with 50% probability was calculated by commercialized statics tools. From finite element method simulation results, the field utilization factor which means the field uniformity was calculated between sphere and plane electrode and its relations with AC breakdown voltage with 50% probability were analyzed. Finally, these results provided a suitable design parameter of electrical insulation for high voltage superconducting machines such as superconducting fault current limiters, superconducting cables, and superconducting transformers.

고온초전도 기기응용을 위한 모의 \ulcorner치 환경에서 액체질소의 절연파괴 특성

The electrical breakdown characteristics of liquid nitrogen(LN<TEX>$\sub$</TEX>2/) were studied under simulated quenching conditions for application of HTS apparatus. The experimental results for various quenching condition revealed that the breakdown voltage of LN<TEX>$\sub$</TEX>2/ with bubble flow velocity and gap spacing. In the case, breakdown voltage decreases gradually with the bubble velocity. When it is bubble velocity from 0 to 1 <TEX>$\ell$</TEX> /min, breakdown voltage rapidly decreases but decreases from 2 <TEX>$\ell$</TEX>/min to 10 <TEX>$\ell$</TEX>/min slowly. The breakdown voltage for vertical electrode arrangement is higher than that for horizontal electrode arrangement. Also, it did a electric field and potential distribution interpreting at the liquid nitrogen when the bubble existed. The plots of equipotential lines for three cases are also shown.

Effect of thermally induced bubbles on the breakdown characteristics of liquid nitrogen

In any practical large-scale superconducting magnet system, abnormal high voltage across the coil layer and thermally induced bubbles in the cryogenic liquid coolant appear, during the process of quenching, with the spread of the normal zone. The voltage has a rise time of at least several milliseconds, which cannot be met with the apparatus operated at the normal temperature. In the paper, the electrical breakdown characteristics of liquid nitrogen are investigated under a simulated quenching condition, i.e. in the presence of thermally induced bubbles and an electrical field with a rise time of 1–30 ms. It is found that the application of an electrical field will promote the formation of bubbles fixed on an electrode and the production of vapour locking between the electrodes, and the time required for the vapour locking is at least about 1 ms. Furthermore, to analyse quantitatively the deformation of a bubble in the presence of a time-varying electric field, a simplified theory is proposed to explain the motion of the bubble. In the theory, the existence of a thermal layer around the bubble and the use of the Clausius-Clapeyron equation, as the relationship between vapour pressure in the bubble and ambient temperature, are assumed. The theory suggests that a partial discharge inside the bubble occurs, although the light pulse from the discharge was not detected in the experiments. A bubble breakdown mechanism in the absence of thermally induced bubbles is also discussed, on the basis of the bubble deformation process under a time-varying electric field.

Partial discharge in liquid nitrogen

Results are presented of experiments on partial discharge in liquid nitrogen, including insulation breakdown characteristics of liquid nitrogen, partial discharge characteristics of composite insulation systems, lifetime of polymer films, impulse breakdown of polymer films in liquid nitrogen, etc. The partial discharge characteristics of liquid nitrogen are examined with increasing liquid-nitrogen pressure. It is concluded that a composite insulation structure of polymer film and liquid nitrogen can be used as effective electrical insulators at cryogenic temperatures.