• The Transactions of the Korean Institute of Electrical Engineers B
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• v.52 no.9
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• pp.434-441
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• 2003

This paper deals with the temperature characteristics according to the cooling medium and the duct size in model transformers. For the analysis and the temperature-rise tests, two 400kVA model transformers have been manufactured. One has been filled with the alpha oil as the cooling medium and constructed the duct sizes of $3\textrm{mm}$ and $5\textrm{mm}$ in the low-voltage and high-voltage windings respectively. The other has been filled the beta oil and the duct sizes were $4\textrm{mm}$ and $6\textrm{mm}$. The temperature-rise tests have been performed by the back-to-back method and the load factor has been controlled the range of 90%∼130%. The temperature values have been measured by the thermocouple and from the sixteen points in each transformer. A commercial CFD program "FLUENT" has been used for the analysis of temperature distribution. The geometry of transformer has been modeled to 3-dimensional by using the hybrid calculation mesh including the radiator. And also, the natural convection velocity has been measured at the oil top position, and compared with the calculated results.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.54 no.11
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• pp.508-514
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• 2005

In this paper, it was investigated the characteristics of PD(partial discharge) pulse shapes in oil and the possibility of TF(Time-Frequency) analysis for the discrimination of air corona in on-site PD measurement of oil transformers. For the purpose, single and multiple discharges combined with oil discharges and air corona were generated and measured by use of artificial models through the HFPD pulse detector and the oscilloscope. PD pulse shapes were different according to the type of defects, especially including air corona- pulses. Also, through the TF analysis, data clusters could be classified each other in the TF Map, even in the case of multiple discharges. Therefore, it is known that TF analysis could be aprlied for the improvement of on-site PD measurement in oil transformers.

• Journal of Electrical Engineering and Technology
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• v.9 no.5
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• pp.1686-1693
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• 2014

In this paper, oil-paper samples composed of transformer windings were used to investigate the insulation degradation process subjected to partial discharge (PD), with artificial defects inside to simulate the PD induced insulation degradation. To determine appropriate test voltages, the breakdown time obtained through a group of accelerated electrical degradation tests under high voltages was firstly fitted by two-parameter Weibull model to acquire the average breakdown time, which was then applied to establish the inverse power law life model to choose advisable test voltages. During the electrical degradation process, PD signals were synchronously detected by an ultra-high frequency (UHF) sensor from inception to breakdown. For PD analysis, the whole degradation process was divided into ten stages, and chaos theory was introduced to analyze the variation of three chaotic parameters with the development of electrical degradation, namely the largest Lyapunov exponent, correlation dimension and Komogorov entropy of PD amplitude time series. It is shown that deterministic chaos of PD is confirmed during the oil-paper degradation process, and the obtained results provide a new effective tool for the diagnosis of degradation of oil-paper insulation subjected to PD.

• Proceedings of the KIEE Conference
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• 2003.10b
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• pp.92-94
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• 2003

The autotransformer currently used on the electric railway system is made of class A insulation material and uses the paper insulation method. As a power converter supplying power to the trolley wire, the autotransformer is one of critical equipment in the railway system. In the autotransformer, load irregularly changes and overload often occurs. These cause overheating of the autotransformer and facilitate deterioration of the autotransformer resulting in burnout accidents due to insulation breakdown. Also, the current autotransformer has poor insolation and short-circuit strength which often badly affect the service life of the transformer, and needs to improve its quality urgently. To overcome one of existing shortcomings of the mold transformer, manufacturers use epoxy resins that have superior flame retardancy to get rid of fro and explosion possibilities during accidents. Currently, new mold transformers are used in indoor distribution facilities with fire-fighting equipments. Coils molded in epoxy resins do not have their insulation performance compromised by humidity, dust, etc enabling easy inspection and maintenance. Comparing to the oil immersed transformer, the mold transformer does not have any concern about environmental pollutions by oil leak or replacement Therefore, to reduce breakdowns and improve reliability of the autotransformer, it is necessary to develop a new mold autotransformer with low loss suitable for our environment to suppress breakdowns of the autotransformer and improve the reliability. This study is about development of a low-loss mold autotransformer necessitated by reasons mentioned earlier.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.57 no.11
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• pp.2011-2014
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• 2008

We measured the permittivity and electromagnetic wave propagation velocity of used insulation oil with wide frequency range including ultra-high frequency by time domain reflectometry. The permittivity or propagation velocity is essential for locating discharge faults of oil filled power transformer. We derived 2.21 as a permittivity and $2.03{\times}10^8 m/s$ as a velocity from the measurement of pulse travelling time along a coaxial line filled with used insulation oil or air. The permittivity measurement system we designed shows high measurement accuracy and the convenience for field use.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1995.11a
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• pp.361-364
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• 1995

The static charges are generated by streaming electrification phenomena when insulating oil flowing by force for the purpose of cooling at the internal of Ultra-high power transformer. In this thesis, their elimination method was studied. The falter represents a greet much electrification characteristics because falter has large interfaces with liquid. In this paper, the streaming-electrification phenomena of insulating oil by metal filter were measured by mesh number, oil flow rate, oil temperature and a kind of mesh and The effect of charge elimination by metal filter generated minus ion was measured.

• Proceedings of the KIEE Conference
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• 1989.11a
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• pp.159-163
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• 1989

The various degradation phenomena, such as the evolution of gases, tan $\delta$, moisture content and total acid number in power transformer insulation oil, have been measured and analyzed. Mineral oil has been degraded at laboratory by the forced thermal stress of $60^{\circ}C$ and $90^{\circ}C$ respectively. Thermal aging oil has been degraded about 17, 34, 72 days. Also, we extracted insulation oil from working transformers. We measured gases dissolved in samples, tan $\delta$, moisture content, total acid number. Activation energy and resistivity is calculated from them. It is found that gases and tan $\delta$ increases as partial discharge and total acid number increases and that conductivity of the sample increases as activation energy increases.

• Proceedings of the KIEE Conference
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• 1996.07c
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• pp.1630-1632
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• 1996

The static charges are generated by streaming electrification phenomena when insulating oil flowing by force for the purpose of cooling at the internal of Ultra-high power transformer. In this thesis, their elimination method was studied. In this paper the effect of Additive on the electric conductivity of Insulating oil is studied. The variation of electric conductivity disappear when Additive is molten in insulating oil, the variation of Additive is not enough to decrease streaming electrification of insulating oil(${\sigma}>10^{-12}$[S/cm]).

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1996.05a
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• pp.158-161
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• 1996

The static charges are generated by streaming electrification phenomena in insulating oil flowing by force for the purpose of cooling at the internal of Ultra-high power transformer. In this thesis, their elimination method was studied. In this paper the effect of Additive on the electric conductivity of Insulating oil is studied. The variation of electric conductivity disappear when Additive is molten in insulating oil BTA(Benzotriazole) appear more variation of electric conductivity than that of SP-S10(Sorbitan mono-stearate). But the variation is not enough to decrease streaming electrification of insulating oil($\sigma$>10$\^$-12/[S/cm]).

• Proceedings of the KIEE Conference
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• 1994.07b
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• pp.1615-1618
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• 1994

This paper studied the streaming electrification of the U.H.V. transformer with the spinning cylinder system, and compared and analyzed it with the conventional forced flowing system which uses a pump or gas for oil flowing. Also, accumulated surface voltage in its electrified material was measured with the electrostatic voltmeter, and the effect of insulation oil aging was measured. As results, in spinning cylinder system, characteristics of the streaming electrification to its rotational speed and oil temperature are same tendency as those of tile forced flowing system and it showed the spinning cylinder system is useful to study the streaming electrification. And, aging of insulation oil increases the electricity of streaming electrification at initial stage of aging.