• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.1552-1553
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• 2011

The HVDC transformer which is one of the main equipments for HVDC(High Voltage Direct Current) electric power transmission systems is exposed to not only AC voltage but also the inflowing DC voltage which comes from the DC-AC converter systems. Therefore, the HVDC transformer insulation system is required to withstand the electric field stress under AC, DC and DC polarity reversal conditions. However the electric field distributions under those conditions are different because the AC electric field and DC electric field are governed by permittivity and conductivity, respectively. In this study, the changes of electric potential and electric field of conventional AC transformer insulation system under DC polarity reversal test condition were analyzed by FEM(Finite Element Method). The DC electric field stress was concentrated in the solid insulators while the AC electric field stress was concentrated in the mineral oil. In addition, the electric stress under that condition which is affected by the surface charge accumulation at the interfaces between insulators was evaluated. The stress in some parts could be higher than that of AC and DC condition, during polarity reversal test. The result of this study would be helpful for the HVDC transformer insulation system design.

• Corrosion Science and Technology
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• v.17 no.2
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• pp.54-59
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• 2018

The effect of external DC electric field on atmospheric corrosion behavior of zinc under a thin electrolyte layer (TEL) was investigated by measuring open circuit potential (OCP), cathodic polarization curve, and electrochemical impedance spectroscopy (EIS). Results of OCP vs. time curves indicated that the application of external DC electric field resulted in a negative shift of OCP of zinc. Results of cathodic polarization curves measurement and EIS measurement showed that the reduction current of oxygen increased while charge transfer resistance ($R_{ct}$) decreased under the external DC electric field. Variation of OCP negative shift, reduction current of oxygen, and $R_{ct}$ increase with increasing of external DC electric field strength as well as the effect of external DC electric field on double-layer structure in the electrode/electrolyte interface and ions distribution in thin electrolyte layer were analyzed. All results showed that the external DC electric field could accelerate the corrosion of zinc under a thin electrolyte layer.

• Journal of the Korean Magnetics Society
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• v.5 no.5
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• pp.606-609
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• 1995

Ferrite cores are often magnetized under DC-biased field because they have been intensively used in electronic circuits such as an inverter circuit and a switching regulator circuit. Thus we investigated the effects of DC-biased field on magnetic properties in the frequency range of DC-100kHz for two kinds of ferrite cores, TDK PC38 and TDK $H_{3}S$, which have different shapes of B-H loop from each other. The magnetic loss per cycle, W/f, in the $H_{3}S$ core decreased with increasing the strength of DC-biased field, although W/f in the PC38 core increased monotonically with DC-biased field. The observed decreasing tendency differs from the previous result for Si-Fe and ferrite cores, and can be attributed to decrease in eddy current loss as well as that in hysteresis loss.

• Progress in Superconductivity and Cryogenics
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• v.23 no.4
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• pp.35-38
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• 2021

Conventional direct current (DC) rotating machines are usually used for crane and press machine using high torque in metal and steel industries, because of a constant output power along variable rotating speed. A general DC motor with permanent field magnets could not increase a magnetic flux density at a gap between armature coils and field magnets. However, a superconducting DC motor has field magnets composed with high temperature superconducting (HTS) coils and it could increase the magnetic flux density at the gap to over 10 times than those of a general DC motor by control the excitation current into HTS coils. The superconducting DC motor could be operated with extremely high torque and constant output power at a low rotational speed. In this paper, a 500 W superconducting DC rotating machine was conceptually designed with a LN₂ (Liquid Nitrogen) cooling method and the operation characteristics results of HTS field magnets were presented. The two no-insulation HTS magnets for a 500 W superconducting DC rotating machine were fabricated. The excitation current for the HTS magnets could be controlled from 0 to 40 A. This test results will be available to design large-sized HTS magnets for a number of hundred kW class superconducting DC rotating machine under LN₂ cooling system.

• Journal of Electrical Engineering and Technology
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• v.12 no.2
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• pp.665-673
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• 2017

This paper proposes a field weakening control method for operating an induction motor with a variable DC input voltage condition. In the variable DC voltage condition such as a battery, the field weakening method are required for the maximum output power. The conventional field weakening control methods can be used for operating the induction motor over the rated speed in a constant DC-link voltage condition. However, the conventional methods for operating the motor with the variable DC voltage is not suitable for the maximum output power. To overcome this problem, this paper proposes the optimized field weakening control method to extend the operating range of the induction motor with a rated power in a limited thermal and a wide DC input voltage conditions. The optimized d-axis and q-axis current equations are derived according to the field weakening region I and II to extend the operating region. The experimental results are presented to verify the effectiveness of the proposed method.

• Journal of the Korean Magnetics Society
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• v.4 no.2
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• pp.150-153
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• 1994

Magnetic field annealing method has been used to obtain proper hysteresis loop shapes which are useful to a device using amorphous ribbon. In this study, two pairs of Helmhotz coils were used to apply longitudinal and transverse magnetic field during annealing. For the measurement of coercive field strength noise which depends on magnetic field annealing, Co-based amorphous alloy ribbon $VITROVAC^{\circledR}$ 6030 was used. For the sample which was annealed under dc transverse and dc longitudinal magnetic field, coercive field strength noise was nearly independent of magnetizing frequency ranging from 1 to 100 kHz, but dc transverse and ac longitudinal magnetic fields annealed samples show that the coercive field strength noise decreased in power of magnetizing frequency. When magnetic domain nucleation occurred, the coercive field strength noise increased remarkably and decreased in power of magnetizing frequency.

• Journal of Power Electronics
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• v.10 no.3
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• pp.228-234
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• 2010

This paper investigates the conducted-noise characteristics of a digitally-controlled randomly-switched dc-dc converter. In order to investigate the effect of the suggested digital controller on the conducted-noise characteristics of a dc-dc converter, three factors have been studied: the field-programmable gate array (FPGA) clock speed, the randomization ratio percentage, and the effect of using a closed loop feedback controller. A field-programmable gate array is much more flexible than analog control circuits, has a lower cost, and can be used for power supply applications. A novel FPGA-based implementation has been suggested for obtaining the experimental validations and realizing the studied concepts. Furthermore, the experimental results have been discussed and design guidelines have been included.

• Proceedings of the KIEE Conference
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• 2005.07b
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• pp.1637-1639
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• 2005

The measurement of magnetic field is performed about DC and AC magnetic field in test track of depot. The test point is cap, on the converter/inverter box, on the traction motor, on the APSE and on the line filter, the height of measurement is bottom and 50 cm height. In case of AC magnetic field, the selected specific frequency is measured on the converter/inverter box. The AC magnetic field is checked and analysis through RS-232C and notebook PC. The DC magnetic field is measured by using the Hall Probe, test result is saved and analysis by PXI system. On the line filter, the maximum value is 1.4 mT in case of DC magnetic field and 0.044 mT in case of AC magnetic field at 50 Hz.

• Journal of Magnetics
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• v.20 no.4
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• pp.347-352
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• 2015

The magnetoelectric (ME) characteristics for Terfenol-D/PZT laminate composite dependence on bias magnetic field is investigated. At low frequency, ME response is determined by the piezomagnetic coefficient $d_{33,m}$ and the elastic compliance $s_{33}^H$ of magnetostrictive material, $d_{33,m}$ and $s_{33}^H$ for Terfenol-D are inherently nonlinear and dependent on $H_{dc}$, leading to the influence of $H_{dc}$ on low-frequency ME voltage coefficient. At resonance, the mechanical quality factor $Q_m$ dependences on $H_{dc}$ results in the differences between the low-frequency and resonant ME voltage coefficient with $H_{dc}$. In terms of ${\Delta}E$ effect, the resonant frequency shift is derived with respect to the bias magnetic field. Considering the nonlinear effect of magnetostrictive material and $Q_m$ dependence on $H_{dc}$c, it predicts the low-frequency and resonant ME voltage coefficients as a function of the dc bias magnetic field. A good agreement between the theoretical results and experimental data is obtained and it is found that ME characteristics dependence on $H_{dc}$ are mainly influenced by the nonlinear effect of magnetostrictive material.

• KEPCO Journal on Electric Power and Energy
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• v.1 no.1
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• pp.39-46
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• 2015

A outdoor termination installed at the outdoor substation is required to connect undergroud cables and overhead transmission lines. The joint box for AC transmission system is already developed and widely used to interconnect overhead and undergroud systems. But the development of the joint box for DC transmission system was only introduced from China and Japan, but theire developemnt staus and core technologies were not fully reported. In order to implement HVDC systems connecting ovehead transmission lines and undergroud cables, a outdoor termination should be developed, but the detailed specifications and information of this device were not reported. It is estimated that the development of the joint box for DC environment has some technical obstacles including insulating materials, electric field mitigation, thermal temperature rise, and space charge accumuations. Among this, the most important one is the DC elctrical insualtion design. Therefore, in order to investigate the DC elctrical insualton design of outdoor termination, the design of AC slip-on type outdoor termination is reffered, and DC electric field analysis performed to verify the possiblity of application of AC joint box into DC joint box. Especially for DC electric field analysis, temperature rise of insualting materials of a joint box was considered, because the conductivity of materials could be changed due to temperature rise. Furthermore, DC electric field analysis considering transinet state, and polarity reversal state were also investigated to verify which state is the most severe condition for the DC joint box. From the simualtion resulsts, it was shown that the value and the position of maximum electric field was obtained comparing AC state, DC state without temperaure rise, and DC state with temperaure rise. And it was confimred that severe DC electric field was observed considing temperaure rise. Finally, in order to reduce DC eletric field intensifation, different configuration of the joint box was applied and it was not possible to obtain satisfactory results. It means that the slight change of configuration of AC joint box was not the suitable soluton for DC joint box. It is essential to establish novel DC insulaton design skills and method for DC joint box to commercialze this product in the near future.