• Proceedings of the KIPE Conference
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• 2020.08a
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• pp.253-254
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• 2020

High Voltage Direct Current (HVDC) 시스템은 고압 직류송전을 위한 시스템이다. 고압 직류 송전을 위해서는 전력변환기가 교류전력을 직류전력으로 변환해주어야 하는데, 최근에는 모듈형 멀티레벨 컨버터(Modular Multilevel Converter, MMC)가 많이 적용되고 있다. MMC는 다수의 서브모듈이 직렬로 구성되어 있으며 DC-link단에 대용량 커패시터가 없다. MMC의 심각한 사고 중에 하나는 DC측 전력케이블의 단락사고로 시스템에 따라서 수십 kA 정도의 사고전류가 AC측 CB(Circuit Breaker)가 열리기 전까지 수십 ms에서 수백 ms동안 흐른다. 만약 하프브릿지 회로의 서브모듈로 구성된 컨버터에 별도 보호장치가 없으면 단락전류는 서브모듈의 하단 다이오드를 통해서 흐르게 되어 소손되게 된다. 이를 방지하기 위해 단락전류를 바이패스(by-pass) 시키기 위한 별도의 사이리스터를 추가하는데 이 기기의 사양은 DC 단락 전류를 충분히 견딜 수 있어야 한다. 본 논문에서는 사이리스터의 서지 전류 내력을 평가하기 위해 사양을 분석하고 시뮬레이션과 실험을 통해서 검증하였다.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.18 no.1
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• pp.50-56
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• 2005

The electrical properties, clamping voltage characteristics, and stability of dysprosia-doped ZnO-P $r_{6}$ $O_{11}$-based varistors were investigated with different dysprosia contents from 0 to 2.0 mol%. The incorporation of dysprosia in varistor ceramics greatly increased the varistor voltage from 50 to 481.0 V/mm. It was found that the dysprosia is good additive improving a nonlinearity, in which the nonlinear exponent is above or near 50, and the leakage current is below 1.0 $\mu$A. The dysprosia-doped varistors exhibited superior clamping voltage characetristics, in which clamping voltage ratio is above or neat 2 at surge current of 50 A. The 0.5 mol% dysprosia-doped varistors only exhibited high stability, with the rate of varistor voltage of -0.9%, under DC acceleraetd aging stress, 0.95 $V_{lmA}$/15$0^{\circ}C$/24 h.h.h.h.

• Journal of Wind Energy
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• v.14 no.4
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• pp.50-56
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• 2023

With the abnormal weather phenomena caused by global warming, the frequency and intensity of lightning strikes are increasing, and lightning accidents are becoming one of the biggest causes of failures and accidents in offshore wind turbines. In order to secure generator operation reliability, effective and practical measures are needed to reduce lightning damage. Because offshore wind turbines are tall structures installed at sea, the possibility of direct lightning strikes is very high compared to other structures, and the role of surge protection devices to minimize damage to the electrical and electronic circuits inside the wind turbine is very important. In this study, a varistor, which is a key element for a class 1 surge protection device for direct lightning protection, was developed. The current density was improved by changing the varistor composition, and the distance between the electrode located on the varistor surface and the edge of the varistor was optimized through a simulation program to improve the fabrication process. Considering the combined effects of heat distribution, electric field distribution, and current density on the optimized varistor surface, silver electrodes were formed with a gap of 0.5 mm. The varistor developed in this study was confirmed to have an energy tolerance of 10/350 ㎲, 50kA, which is a representative direct lightning current waveform, and good protection characteristics with a limiting voltage of 2 kV or less.

• Proceedings of the KIEE Conference
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• 2004.07c
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• pp.1582-1584
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• 2004

The microstructure, electrical, and clamping voltage characteristics of ZPCCY-based varistor ceramics were investigated in the sintering time range of 1${\sim}$3 h. Increasing sintering time promoted the densification, in which the average grain size and density are increased in the range of $11.4{\sim}16.0{\mu}m$ and $5.34{\sim}5.54g/cm^3$, respectively. The nonlinear exponent decresed in the range of 60${\sim}$26 and the leakage current increased in the range of $1.3{\sim}10.7{\mu}A$ with increase of sintering time. The clamping voltage ratio was less than 2 for ratio surge current of 10 A over sintering times.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.57 no.10
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• pp.1818-1821
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• 2008

Several off-line diagnostic tests which include the insulation resistance(IR), polarization index(PI), low-voltage AC, and recurrent surge oscillograph(RSO) tests were performed to assess the condition of generator rotor windings. The low-voltage AC and the RSO tests were performed on the gas turbine generator rotor winding to detect shorted turns. Before intentionally applying artificial shorted faults, it was confirmed by the low voltage AC and the RSO tests that the winding was in sound condition. For simulated shorted rotor winding turns, the RSO test detected the fault in the winding. The RSO test was capable of identifying the number and pole location of the shorted turns for a number of simulated shorted coils.

• Proceedings of the KIEE Conference
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• 1999.11c
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• pp.582-584
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• 1999

Generator has synchronized with power Network after build-up output voltage. In order to prevent a current surge when synchronizing, the conditions such as identical no-load voltages identical no-load frequencies and identical phase positions must be met between generator voltage and network voltage. Hydro-Pump generators or Gas-turbine generators, Co-generation Generators that serve peak load of network are synchronized many times. So, Automatic Synchronizing Circuits are very important service. This report apply Diagnosis methods the Automatic Synchronizing Circuits(Device) by developing simulator.

• Proceedings of the KIEE Conference
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• 2005.07a
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• pp.459-461
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• 2005

This paper analyses the induced voltage characteristic of CCPU with unbalanced current from distribution line on underground transmission power cable systems. In switching surge strokes, in order to obtain the data of induced voltage/current on CCPU, the actual proof test carried out. This paper is expected to contribute the establishment of proper protection methods of CCPU against the unbalanced current from distribution line on underground transmission power cable systems.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2000.05a
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• pp.489-492
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• 2000

This paper describes a new transient voltage suppressor(TVS) with a low insertion loss and a very wide frequency bandwidth to protect antenna circuit from transient voltages. Conventional protection devices have some problems such as low frequency bandwidth and high insertion loss. In order to improve these limitations, a coaxal type TVS, which consists of a gas tube is developed. The performance of the proposed transient voltage suppressor is tested by using a combination surge generator specified in IEC 61000-4-5 standard and by using a network analyzer of 40 MHz ∼ 5GHz bandwidth. From the experimental results, it is confirmed that the proposed TVS has an enough protection performance in low insertion loss and in wide frequency bandwidth

• Proceedings of the KIPE Conference
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• 1999.07a
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• pp.319-322
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• 1999

This paper presents a novel single-stage Power Factor Corrected(PFC) AC/DC forward converter with semi-automatic current shaping in order to achieve the unity power factor and an isolated output. Since the proposed circuit is combined a boost converter used for PFCs with a forward converter used for DC to DC conversion, the over-all size of system could be reduced. And thanks to the zero voltage switching(ZVS) in both switches, the voltage stress can be reduced considerably. A simple auxiliary circuit adopted into the secondary of transformer is composed of lossless components for reducing surge voltage. A prototype which has tow IGBT(Insulated Gate Bypolar Transistor) modules as switching device is manufactured to evaluate the proposed topology. The characteristics of the proposed circuit are tested, and the validity is verified by experimental results.

• Proceedings of the KIEE Conference
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• summer
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• pp.563-565
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• 2004

It is expected to reduce stresses to components of high voltage circuit breaker and transferred switching surge from power system by applying controlled switching technique to high voltage system. This technique has at ready been applied in advanced countries. In this paper, basic principle of controlled switching technique is set up and a device to realize this technique is under developing. Controlled switching device will be improved by applying a method minimizing errors of operating time and by adopting compensation function relative to changes of ambient/operating condition.