• Progress in Superconductivity
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• 제4권1호
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• pp.86-89
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• 2002

In this paper, the design of a IMVA single-phase high temperature superconducting(HTS) power transformer with BSCCO-2223 HTS tapes is presented. The rated voltages of each sides of the transformer are 22.9 ㎸ and 6.6 ㎸, respectively The winding of 1MVA HTS transformer is consisted of double pancake type HTS windings, which have advantages of insulation and distribution of high voltage, and are cooled by subcooled liquid nitrogen of 65K. Four HTS tapes were wound in parallel for the windings of low voltage side and the four parallel conductors are transposed. The design of 1MVA HTS transformer, a shell type core made of laminated silicon steel plate is chosen, and the core is separated with the windings by a cryostat with a room temperature bore. The cryostat made of non-magnetic and non-conducting material and a liquid nitrogen sub-cooling system is designed in order to maintain the coolant's temperature of 65K. For electromagnetic analysis of 1MVA HTS transformer, a finite element method of an axis of symmetry is used. The maximum perpendicular component of magnetic flux density of pancake windings is about 0.15T. And through analyzing the magnetic field distribution, an optimal winding arrangement of 1MVA HTS transformer is obtained.

• Progress in Superconductivity
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• 제11권1호
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• pp.72-77
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• 2009

Low temperature scanning laser microscopy (LTSLM) can be used for a two-dimensional display of bolometric response arising from the localized excitation of a sample by the focused laser beam. In this study, the distribution of critical temperature ($T_c$) and critical current density ($J_c$) in YBCO coated conductor were analyzed using LTSLM. For improving the temperature stability, we have modified the system into a double-shielding type. Through the modification, the temperature stability was successfully improved from ${\pm}10mK\;to\;{\pm}2mK$. The superconducting properties of YBCO coated conductors were measured for the sample of a narrow bridge type using wet etching process. The spatial non-uniformity of the ac voltage response, ${\delta}V(x)$, which is proportional to ${\partial}\rho(x,J_B)/{\partial}T$ in the transition temperature region could be observed and displayed in a two-dimensional image.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1993년도 하계학술대회 논문집 B
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• pp.652-654
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• 1993

In this paper, time variation properties of acoustic emission pulse were studied from low density polyethylene by using the self developed measuring system of aucoustic emission. When AC 15[kVrms] were applied to specimen of needle shape voids(0.5, 1.0, 1.5, 2.0[mm]), the breakdown time of void specimen was quite shorter than that of non-void specimen, thus the quick accurance of inception discharge were confirmed. It is observed that the skewness of the phase angle for applied voltage distribution phase, the average pulse of amplitude and the number of pulse are characteristic quantity of discharge.

• 한국산학기술학회논문지
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• 제20권4호
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• pp.25-34
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• 2019

최근, 출력전압의 제어가 가능하고 양방향 운용이 가능한 PWM(Pulse Width Modulation) 정류기가 도입되고 있다. 그러나, PWM 정류기가 적용될 경우, 전력계통측에서 사고발생하면, 회생전력에 의한 사고전류 공급으로 기존의 사고전류 크기 및 방향이 바뀔 가능성이 있다. 또한, 경전철용 급전시스템에서는 장거리 지점에서 사고가 발생하는 경우, 사고전류의 크기가 크게 감소되고, 부하전류와 비슷하거나 더 작은 경우가 발생할 수 있기 때문에 이에 대한 적절한 보호협조 운용 방안이 필요한 실정이다. 따라서, 본 논문에서는 상기의 문제점들을 해결하기 위하여, 경전철용 LVDC 배전선로의 보호기기 운용 방안을 제안한다. 구체적으로는 LVDC 배전선로에서의 거리 및 단락저항별 사고특성을 분석하여, 다양한 조건에서 사고를 적절하게 판별하는 직류선택계전기의 보호협조 운용방안을 제안한다. 또한, 배전계통 상용 해석 프로그램인 PSCAD/EMTDC를 이용하여 AC 계통, PWM 정류기, LVDC 배전선로로 구성된 경전철용 급전시스템의 모델링을 제시한다. 한편, 제안한 보호협조 알고리즘과 모델링을 이용하여 LVDC 배전선로의 보호기기 특성을 분석한 결과, 제안한 운용방식에서는 전류 경사각의 급격한 감소를 판정하는 보호요소를 추가한다. 따라서, 사고지점의 단락저항이 높거나 장거리 선로인 경우에도, 보호기기가 사고전류와 부하전류를 적절히 판별할 수 있어, 본 논문에서 제안한 보호협조 운용알고리즘의 유용성을 확인하였다.

• Journal of Power Electronics
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• 제15권1호
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• pp.202-215
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• 2015

This paper investigates the stability and performance of model predictive controlled active-front-end (AFE) rectifiers for energy storage systems, which has been increasingly applied in power distribution sectors and in renewable energy sources to ensure an uninterruptable power supply. The model predictive control (MPC) algorithm utilizes the discrete behavior of power converters to determine appropriate switching states by defining a cost function. The stability of the MPC algorithm is analyzed with the discrete z-domain response and the nonlinear simulation model. The results confirms that the control method of the active-front-end (AFE) rectifier is stable, and that is operates with an infinite gain margin and a very fast dynamic response. Moreover, the performance of the MPC controlled AFE rectifier is verified with a 3.0 kW experimental system. This shows that the MPC controlled AFE rectifier operates with a unity power factor, an acceptable THD (4.0 %) level for the input current and a very low DC voltage ripple. Finally, an efficiency comparison is performed between the MPC and the VOC-based PWM controllers for AFE rectifiers. This comparison demonstrates the effectiveness of the MPC controller.

• 한국태양에너지학회 논문집
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• 제28권2호
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• pp.64-69
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• 2008

As the grid-connected photovoltaic power conditioning systems (PVPCS) are installed in many residential areas, these have raised potential problems of network protection on electrical power system. One of the numerous problems is an Islanding phenomenon. There has been an argument that it may be a non-issue in practice because the probability of islanding is extremely low. However, there are three counter-arguments: First, the low probability of islanding is based on the assumption of 100% power matching between the PVPCS and the islanded local loads. In fact, an islanding can be easily formed even without 100% power matching (the power mismatch could be up to 30% if only traditional protections are used, e.g. under/over voltage/frequency). The 30% power-mismatch condition will drastically increase the islanding probability. Second, even with a larger power mismatch, the time for voltage or frequency to deviate sufficiently to cause a trip, plus the time required to execute a trip (particularly if conventional switchgear is required to operate), can easily be greater than the typical re-close time on the distribution circuit. Third, the low-probability argument is based on the study of PVPCS. Especially, if the output power of PVPCS equals to power consumption of local loads, it is very difficult for the PVPCS to sustain the voltage and frequency in an islanding. Unintentional islanding of PVPCS may result in power-quality issues, interference to grid-protection devices, equipment damage, and even personnel safety hazards. Therefore the verification of anti-islanding performance is strongly needed. In this paper, improved RPV method is proposed through considering power quality and anti-islanding capacity of grid-connected single-phase PVPCS in IEEE Std 1547 ("Standard for Interconnecting Distributed Resources to Electric Power Systems"). And the simulation results are verified.

• Journal of Electrical Engineering and Technology
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• 제13권6호
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• pp.2412-2420
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• 2018

The thermal degradation of low density polyethylene (LDPE) will accelerate the production of carbonyl groups (C=O), which can act as the induced dipoles under high voltage. In this paper, we researched the dielectric properties and space charge behavior of LDPE after thermal aging, which can help us to understand the correlation between carbonyl groups (C=O) and electrical properties of LDPE. The spectra results show that LDPE exhibit obvious thermooxidative reactions when the aging time is 35 days and the productions mainly contain carboxylic acid, carboxylic eater and carboxylic anhydride, whose amount increase with the increasing of aging time. The dielectric properties show that the real permittivity of LDPE is inversely proportional to temperature before aging and subsequently become proportional to temperature after thermal aging. Furthermore, both the real and imaginary permittivity increase sharply with the increasing of aging time. The fitting results of imaginary permittivity show that DC conductivity become more sensitive about temperature after thermal aging. On this basis, the active energies of materials calculated from DC conductivity increase first and then decrease with the increasing of aging time. In addition, the space charge results show that the heterocharges accumulated near electrodes in LDPE change to the homocharges after thermal aging and the mean volume charge density increase with the increasing of aging time. It is considered that the overlaps caused by electrical potential area is the main reason for the increase of DC conductivity.

• 전기학회논문지P
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• 제55권1호
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• pp.6-12
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• 2006

A popular method used by primary protection for power transformer is current ratio differential relaying (RDR) with 2nd harmonic restraints. In modern power transformer due to the use of low-loss amorphous material, the 2nd harmonic component during inrush is significantly reduced. The higher the capacitance of the high voltage status and underground distribution, the more the differential current includes the 2nd harmonic component during internal fault. Thus the conventional method may not operate properly. This paper proposes an advanced relaying algorithm and the prototype IED hardware design and it's real-time experimental results. To evaluate performance of the proposed algorithm, the study is well constructed power system model including power transformer utilizing the EMTP software and the testing is made through simulation of various cases. The proposed relaying that is well constructed using DSP chip and microprocessor etc. has been developed and the prototype IED has been verified through on-line testing. The results show that an advanced relaying based prototype IED never mis-operated and correctly identified all the faults and that inrushes that are applied.

• 전력전자학회논문지
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• 제3권3호
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• pp.184-190
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• 1998

대부분의 빌딩과 공장에서는 3상 4선식 저압 배전 시스템으로 전력을 공급받고 있다. 이 방식은 중성선에 전류가 흐르게 하는데 오늘날 전력 변환 기술의 개발과 컴퓨터의 보급으로 심각한 중성 전류가 발생하고 있다. 본 논문은 병렬형 3상 4선 능동 필터를 사용하여 계통의 전원 전압이 불평형이고 불평형 부하 및 비선형 부하가 접속된 상태에서 전원쪽으로 흐르는 중성 전류를 제거시키고, 전원전류의 파형을 정현파로 개선하는데 목적이 있으며, 부하 조건을 변화시키면서 시뮬레이션을 실행하여 능동 필터의 우수한 성능을 입증하였다.

• 한국자동차공학회논문집
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• 제20권5호
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• pp.95-101
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• 2012

An electric water pump for engine cooling system has an advantage which particularly in the cold start, the use of the electric water pump saves fuel and leads to a corresponding reduction in emissions. The canned type electric water pump without mechanical sealing elements was selected to meet the requirements for operational reliability and life. However, the electric water pump for internal combustion engine generates much more heat loss than for hybrid electric vehicle since it is operated by the electric power of high current and low voltage. In this study, the fluid flow and thermal characteristics of the canned type electric water pump as an inverter integrated water pump has been investigated under the effects of heat generation. The analysis conditions such as outdoor air temperature of $125^{\circ}C$, water pump speed of 6000 rpm, coolant temperature of $106^{\circ}C$ and coolant flow rate of 120 L/min was used as a standard condition. Therefore, flow fields and temperature distribution inside the water pump were obtained. Also, we checked the feasibility of the canned type for the electric water pump in comparison with the mechanical seal type.