• Journal of Electrical Engineering and Technology
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• v.11 no.1
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• pp.184-191
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• 2016

In recent years, there are a number of studies that have been carried out to explore the alternative for Mineral Oil (MO) as dielectric insulating fluid in transformers due to the increasing tight regulation on safety and environment. Vegetable oils have been identified as suitable candidate since it is biodegradable, non-toxic and high flash/fire points which ensure more in-service safety. Among the types of vegetable oils considered for transformers application are Palm Oil (PO) and Coconut Oil (CO). This paper presents an experimental study on the lightning breakdown voltages of PO and CO under non-uniform electric field based on needle-sphere electrodes configuration at 3 small gap distances. The type of PO used in this study is Refined Bleached and Deodorized Palm Oil (RBDPO) Olein. The main focus of this study is to examine the lightning breakdown performance of RBDPO and CO under different test conditions and assess its suitability as dielectric insulating fluid in transformers. The effect of voltage polarities (positive and negative) and testing methods (rising-voltage, up-and-down and multiple-voltage) were investigated. The data obtained from all tests were analysed by Weibull distribution in order to determine the withstand voltages for each type of oils. It was found that the breakdown voltages of RBDPO and CO are comparable with MO under positive lightning impulse. Under negative lightning impulse, the breakdown voltage of MO is slightly higher than RBDPO and CO. There is no significant effect of testing methods and voltage polarities on lightning breakdown voltages of RBDPO and CO. Based on the statistical analysis, it is found that the breakdown voltages of RBDPO and CO at 1% probability are slightly lower than MO.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.28 no.10
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• pp.82-88
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• 2014

This paper presents the results of model tests for the attachment process of lightning leader to ground which is one of poorly understood processes of cloud-to-ground lightning discharges. In order to simulate the attachment process of lightning leader to ground, we investigated the discharge characteristics of air gap between the tip of needle-shaped electrode and the soil surface as a parameter of moisture content in soils when the positive and negative $1.2/50{\mu}s$ lightning impulse voltages are applied. The breakdown voltage and the discharge light were observed. As a result, the attachment processes of lightning leader to ground are strongly dependent on the grain size and the moisture content of soils. The time to breakdown was shortened with increasing the magnitude of incident impulse voltages. The delay time from application of the highest voltage to breakdown in sand is shortened with increasing the moisture content. The delay time from application of the voltage to breakdown in gravel varied from about $0.5{\mu}s$ to several ${\mu}s$. As the moisture content in soil increases, the breakdown voltages are decreased and the breakdown voltage versus time to breakdown curves are shifted toward the lower side. The results obtained in this work are similar to those for non-uniform air gap stressed by lightning impulse voltages.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.60 no.5
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• pp.1005-1010
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• 2011

The accidents caused by dielectric instability have been increasing in power grid. It is important to enhance the dielectric reliability of a high voltage apparatus to reduce the damage from electrical hazards. To develop an electrically reliable high voltage apparatus, the experimental study on the electrical breakdown field strength is indispensable, as well as theoretical approach. In this paper, the lightning impulse breakdown characteristics considering utilization factors are studied for the establishment of insulation design criteria of an high voltage apparatus. The utilization factors are represented as the ratio of mean electric field to maximum electric field. Dielectric experiments are performed by using several kinds of sphere-plane electrode systems made of stainless steel. As a result, it is found that dielectric characteristics are affected by not only maximum electric field intensity but also utilization factors of electrode systems. The results are expected to be applicable to designing the air-insulated high voltage apparatuses.

• Proceedings of the KIEE Conference
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• 1994.11a
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• pp.284-286
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• 1994

This paper describes the dielectric characteristics of $SF_6$ gas in non-uniform electric filed under lightning under lightning impulse and oscillating impulse voltages. The breakdown voltage-time characteristics and the breakdown voltage-pressure characteristics are measured over a pressure range extending from 0.1 to 0.5 [MPa] fur the coaxial electrode with a needle protrusion. The curvature radius of needle protrusion is 0.3[mm]. Also, the growth of the predischarge is simultaneously observed. As a result the polarity effect is pronounced, and the breakdowns voltage under the oscillating impulse voltage are higher than those under the lightning impulse voltage. It is found that the breakdown mechanism md predischarge phenomena ate closely related with the polarity and waveforms of the testing voltage.

• Journal of Electrical Engineering and Technology
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• v.6 no.3
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• pp.397-401
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• 2011

The response of lightning impulse voltage was explored in dielectric liquids employing hydrodynamic modeling with three charge carriers using the finite element method. To understand the physical behavior of discharge phenomena in dielectric liquids, the response of step voltage has been extensively studied recently using numerical techniques. That of lightning impulse voltage, however, has rarely been investigated in technical literature. Therefore, in this paper, we tested impulse response with a tip-sphere electrode which is explained in IEC standard #60897 in detail. Electric field-dependent molecular ionization is a common term for the breakdown process, so two ionization factors were tested and compared for selecting a suitable coefficient with the lightning impulse voltage. To stabilize our numerical setup, the artificial diffusion technique was adopted, and finer mesh segmentation was generated along with the axial axis. We found that the velocity from the numerical result agrees with that from the experimental result on lightning impulse breakdown testing in the literature.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.50 no.9
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• pp.456-465
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• 2001

In this paper, breakdown voltages in $SF_6-N_2$ mixtures were experimentally investigated to understand characteristics of dielectric strength and physical phenomena in nonuniform field disturbed by a needle shape protrusion. The test voltages are the lightning impulse$(\pm1.2/44 \mus)$ and the damped oscillatory impulse$(\pm400 ns / 0.83 MHz)$ voltages which can be occurred by the operation of disconnecting switches in gas-insulated switchgears(GIS). The effects of the polarity and wave shape of the test voltages, and the gas pressure on the V-t characteristics were in detail examined. The V-t characteristic curves were measured in different two ways : (1) one is the method by taking the maximum voltage recorded at or prior to breakdown against the time to breakdown, that is, the Procedures recommended in IEC 60060-1, (2) the other is the method by taking the voltage at the instant of chopping against the time to breakdown. As a result, the V-t characteristics of $SF_6-N_2$ mixtures in nonuniform electric field were significantly affected by the polarity and wave shape of the applied voltages. The positive breakdown voltages resulted in lower breakdown voltages in the time ranges considered, and the V-t curves for the negative oscillatory impulse voltage were extended over the longer time range. For the lightning impulse voltages, the V-t curves obtained by IEC Pub. 60060-1 were nearly same with the V-t curves obtained by the voltage at the instant of chopping against the time to breakdown. It is clear that the actual breakdown voltages were much lower than the maximum voltages appearing at or prior to breakdown because of the displacement current produced as a result of the dV/dt during the oscillatory transient voltage app1ication. The scattering of the negative actual breakdown voltages was much larger than that of the positive.

• Proceedings of the KIEE Conference
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• 2001.07c
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• pp.1549-1551
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• 2001

Cyclic aging for 14 days is performed to remove a large amount of the volatiles found in freshly manufactured cable. In this paper, we examined lightning impulse characteristics of 22.9 kV CV power cable before and after cyclic aging for 14days. As the result, the breakdown voltage after aging under normal temperature was lower than that before aging, and the breakdown voltage after aging under hot temperature was lower than that before aging. We found that the dispersion of lightning impulse breakdown voltage before 14 cyclic aging test were reduced rather than those after 14 cyclic aging test.

• Journal of Electrical Engineering and Technology
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• v.13 no.4
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• pp.1663-1672
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• 2018

In recent times, gas insulated medium voltage (MV) circuit breakers (CB) form a vital component in power system network, considering its advantages such as reduced size and safety margins. Gas insulation characteristics of circuit breakers are generally measured by lightning impulse (LI) test according to IEC standard 60060-1 as a factory routine test. Considering the environmental issues of $SF_6$ gas, many research works are being carried out towards the mixture of $SF_6$ gases for high voltage insulation applications. However, few reports are only available regarding the LI withstand and streamer propagation characteristics (at both positive and negative polarity of waveform) of $SF_6/N_2$ gas mixture insulated medium voltage circuit breakers. In this paper, positive and negative polarity LI tests are carried out on 22 kV medium voltage circuit breaker filled with $SF_6/N_2$ gas mixture at different gas pressures (1-5 bar) and at different gas mixture ratios. Important LI parameters such as breakdown voltage, streamer velocity, time to breakdown and acceleration voltage are evaluated with IEC standard LI ($1.2/50{\mu}s$) waveform. Weibull distribution analysis of LI breakdown voltage data is carried out and 50% probability breakdown voltage, scale parameter and shape parameter are evaluated. Results illustrate that the $25%SF_6+75%N_2$ gas filled insulation considerably enhances the LI withstand and breakdown strength of MV circuit breakers. LI breakdown voltage of circuit breaker under negative polarity shows higher value when compared with positive polarity. Results show that maintaining the gas pressure at 0.3 MPa (3 bar) with 10% $SF_6$ gas mixed with 90% $N_2$ will give optimum lighting impulse withstand performance of 22 kV MV circuit breaker.

• Proceedings of the KIEE Conference
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• 2007.07a
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• pp.1455-1456
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• 2007

This paper presents the experimental results on impulse breakdown characteristics under a highly non-uniform electric field in $SF_{6}/N_{2}$ gas mixtures according to a change in temperature. Test temperature ranges from $-25^{\circ}C$ to $25^{\circ}C$. The impulse predischarge breakdown developments are investigated by the measurements of current pulse and discharge luminous events. As a result, the predischarge development mechanisms for both positive and negative polarities are same. When increasing the temperature, breakdown voltage due to lightning impulse voltage is increased in negative polarity. On the other hand, when increasing the temperature, breakdown voltage due to lightning impulse voltage is not changed in positive polarity.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.29 no.4
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• pp.95-101
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• 2015

This paper presents experimental results about characteristics of soil discharge as a function of moisture content when the $1.2/50{\mu}s$ lightning impulse voltage is applied. The laboratory experiments, for this study, were carried out based on factors affecting the transient behavior in soils. The electrical breakdown in soils was measured over a 0-6% range of moisture content for sands and a 0-4% range of moisture content for gravels. Needle-plane electrode systems was used As a result, the conduction current prior to ionization growth in dry soil is a little, but it in wet soil is increased with the applied voltage because the wet soil particles act as conductors. The soil impedance curves show an abrupt reduction just after breakdown. The general tendency measured in different soils is that the higher the water content, the lower the breakdown voltage and the shorter the time-lag to breakdown.