• 한국전기전자재료학회논문지
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• 제28권5호
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• pp.326-331
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• 2015

In this paper, we study the insulating properties of the liquid nitrogen(LN2) including the thermal bubbles. The shape of the thermal bubbles in accordance with the current change was observed in the 77 K and 65 K LN2. According to the temperature of liquid nitrogen, bubbles were generated differently. The round shape of the bubble is occurred in 77 K LN2. But the layer shape of bubble is occurred in 65 K LN2. When the bubbles present, the dielectric strength of liquid nitrogen is low. However, the breakdown patterns were different according to the electrode arrangement. AC breakdown voltage(BDV) was lower than the DC BDV due to the influence of bubbles. Therefore, the design of a high-voltage superconducting equipments should consider the bubbles.

• 한국초전도ㆍ저온공학회논문지
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• 제4권2호
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• pp.1-4
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• 2002

Electrical properties of liquid nitrogen ($LN_2$) and gaseous nitrogen($GN_2$) have become of great interest again since the discovery of high temperature superconductors . It is very important from a point of superconducting apparatus protection to elucidate breakdown characteristics in $LN_2$ and $GN_2$ at atmospheric and pressurized conditions Therefore. this paper studies the effect of pressure on the breakdown characteristics in $LN_2$ and $GN_2$. Af high voltage is applied to electrode system with uniform and non-unform field in various gap length. And Breakdown voltages of $LN_2$ and $GN_2$ are investigated under AC voltage for Pressure ranging from 0 and 0.5 MPa. This research presented basis information of electrical insulation design for liquid nitrogen immersed HTS power apparatus.

• 한국전기전자재료학회논문지
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• 제13권2호
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• pp.177-182
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• 2000

Composite insulation system of liquid nitrogen and solid spacer is widely applied in high temperature superconduction power machine. This study has three step procedure. As follow, first step is composition of parallel deposited electrode and vertically deposited electrode along the direction of immersion in liquid nitrogen(LN2). Second step is investigation into surface flashover voltage of solid spacer under partially immersed in LN2, and last step is comparison the result of this research with that of fully immersed in LN2 and at cryogenic temperature gaseous nitrogen(GN2). This result presented that surface flashover voltage along solid spacer half immersed in LN2 was almost the same as that of fully immersed spacer when the thickness of spacer(t) was t<10mm. In the case of t> 10mm, however, spacer flashover voltage was equal to that obtained in GN2 at cryogenic temperature. And the immersed direction functions as role of deciding the difference of surface flashover voltage.

• 한국잠사곤충학회지
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• 제42권1호
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• pp.1-5
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• 2000

To investigate the possibility of crypreservation of mulberry seeds in liquid nitrogen(LN$_2$), characteristics of the seeds were examined after picking mulberry syncarps and drying-heat treatment. Storage in LN$_2$has the potential of providing indifinite preservation of valuable seed germplasm. Determining the tolerance of seeds among given cultivars to LN$_2$cooling and subsequent rewarming is the first step to establishing the feasibility of LN$_2$storage. Seeds of 4 mulberry varities were treated to LN$_2$(-196$\^{C}$) for 24 hours after drying heat treatment. Seed moisture content of Daeryukppong was the highest. As moisture content of mulberry seed was below 1%, storage in LN$_2$was safe. And drying heat treatment for 60 minutes was suitable to prevent decreased germination rate and germination vigor of seeds. The seeds of Cheongilppong were unsuibable to cryopreserve in LN$_2$for longterm storage.

• 한국전기전자재료학회논문지
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• 제28권6호
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• pp.396-399
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• 2015

We analyzed the correlation between breakdown voltage(BDV) of liquid nitrogen(LN2) and factors. The chosen factors affecting the breakdown are the diameter of electrode, gap length, temperature of LN2, and pressure of LN2. The BDV of LN2 was increased with increasing the diameter, the gap length and the pressure. And The BDV of LN2 was increased with decreasing the temperature. However, correlation coefficient was different from each other depending on the situation. The BDV exhibited a very high correlation coefficient of 0.92227 to dependence on the diameter. And a very high correlation coefficient of 0.94980 to dependence on the pressure under sphere(D 7.5 mm)-plane electrode. When the pressure is applied, sphere-plane electrode is the correlation coefficient was higher than that of the needle-plane electrode. It shows the dependence of a temperature coefficient of -0.758290 ~ -0.39946 under needle-plane electrode.

• 한국초전도ㆍ저온공학회논문지
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• 제5권2호
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• pp.46-50
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• 2003

In the dielectric insulation design of any high temperature superconducting (HTS) apparatus as well as HTS fault current limiter in the electrical power systems, the breakdown characteristics of cryogenic coolants such as liquid nitrogen ($LN_2$) are an important factor of the insulating engineering. Previous reports concerned with the breakdown characteristics of liquid nitrogen have pointed out that bubbles and gaseous nitrogen have a treat influence on their breakdown phenomena, However, useful data for practical insulation design of HTS fault CUITent limiter operating at subcooled L$N_2$ have not been obtained enough. Therefore, this paper presents an experimental investigate of breakdown phenomena in liquid nitrogen under AC voltage, And, we observed the breakdown voltage (BDV) of liquid nitrogen with lowering temperature. The Weibull plots of the breakdown voltage of subcooled $LN_2$ for the needle-plane electrode with d= 10 mm are studied, The dependence of breakdown voltage for needle-plane and pancake coil-pancake coil electrode on temperature is illustrated, The relationship between the AC breakdown characteristics and the temperature were clarified.

• 대한안전경영과학회:학술대회논문집
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• 대한안전경영과학회 2002년도 춘계학술대회
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• pp.221-225
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• 2002

Machinability improvement by the use of liquid nitrogen in cryogenic machining has been reported in various studies. This has been mostly attributed to the cooling effect of liquid nitrogen. However, No study has been found in discussion on whether liquid nitrogen possesses lubrication effect in cryogenic cutting. This paper presents lubrication mechanism related to chip microstructure. The friction reduction was further reflected In larger shear angle and less secondary deformation in the chip microstructures.

• 한국전기전자재료학회논문지
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• 제11권4호
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• pp.321-326
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• 1998

Electrical breakdown characteristics of liquid nitrogen($LN_2$) taking into consideration for application of high $T_c$ superconductor is very important. Also $LN_2$ will be used as both coolant and insulator in superconducting generator. In this paper, we investigated ac breakdown characteristics of cryogenic nitrogen gas above a $LN_2$ for rod-to plane electrode configuration. As result the breakdown mechanism of $LN_2$is dependence on bubble effect. And breakdown voltage is a ratio on bubble s size but electrodes arrangement is to make no difference. The breakdown voltage decreases slightly with increasing flow velocity, it again decreases abruptly with increasing flow velocity. These results were interpreted as the within pressure of rod electrode and Maxwell force.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1999년도 추계학술대회 논문집 학회본부 C
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• pp.912-914
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• 1999

Composite insulation system of liquid nitrogen and solid spacer is widely applied in high temperature superconducting power machine. This study has three step procedure. As follow, first step is composition of parallel deposited electrode and vertically deposited electrode along the direction of immersion in liquid nitrogen($LN_2$). Second step is investigation into surface flashover voltage of solid spacer under partially immersed in $LN_2$, and last step is comparison the result of this research with that of fully immersed in $LN_2$ and at cryogenic temperature gaseous nitrogen($GN_2$). This result presented that surface flashover voltage along solid spacer half immersed in $LN_2$ was almost the same as that of fully immersed spacer when the thickness of spacer(t) was t<10 mm. In the case of t>10 mm, however, spacer flashover voltage was equal to that obtained in $GN_2$ at cryogenic temperature.

• 한국전기전자재료학회논문지
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• 제15권11호
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• pp.985-990
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• 2002

The electrical breakdown characteristics of liquid nitrogen(LN$\sub$2/) were studied under simulated quenching conditions for application of HTS apparatus. The experimental results for various quenching condition revealed that the breakdown voltage of LN$\sub$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 $\ell$ /min, breakdown voltage rapidly decreases but decreases from 2 $\ell$/min to 10 $\ell$/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.