• Journal of Power Electronics
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• v.13 no.5
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• pp.737-745
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• 2013

Offshore wind farms are rapidly growing owing to their comparatively more stable wind conditions than onshore and land-based wind farms. The power capacity of offshore wind turbines has been increased to 5MW in order to capture a larger amount of wind energy, which results in an increase of each component's size. Furthermore, the weight of the marine turbine components installed in the nacelle directly influences the total mechanical design, as well as the operation and maintenance (O&M) costs. A reduction in the weight of the nacelle allows for cost-effective tower and foundation structures. On the other hand, longer transmission distances from an offshore wind turbine to the load leads to higher energy losses. In this regard, DC transmission is more useful than AC transmission in terms of efficiency because no reactive power is generated/consumed by DC transmission cables. This paper describes some of the challenges and difficulties faced in designing high-power-density power conversion systems (HPDPCSs) for offshore wind turbines. A new approach for high gain/high voltage systems is introduced using transformerless power conversion technologies. Finally, the proposed converter is evaluated in terms of step-up conversion ratio, device number, modulation, and costs.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.11a
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• pp.13-18
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• 2004

전력계통 분야에서는 HVDC 전력변환소, BTB, UPFC 및 SVC의 안정성 향상 및 안정적인 운용을 위한 체계적인 유지보수 및 관리가 필요하다. 특히 전력계통에 접속된 대용량 전력반도체 소자인 사이리스터 밸브는 운전중에 열적, 전기적인 스트레스를 받게 되며, 이로 인해 밸브의 수명이 감소하여 전력계통의 안정적인 운용을 어렵게 만드는 요인이 된다. 따라서 전력계통 운용의 안정성을 확보하기 위해서는 대용량 사이리스터 밸브의 열적, 전기적 스트레스에 따른 수명 변화를 예측하는 열화진단 기법의 개발이 중요하다. 본 고에서는 대용량 사이리스터 소자의 열화진단 기법에 대한 국내외 현황과 현재 연구가 진행중인 열화 진단 기법에 대해 서술하였으며, 1500V급 사이리스터 소자의 가속열화 실험을 통해 소자의 수명을 예측한 결과를 나타내었다.

• Journal of Power Electronics
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• v.18 no.6
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• pp.1920-1929
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• 2018

In order to provide a source for nine phases suitable for 18-pulse ac to dc power, this paper proposes a new structure for a step-up asymmetrical delta-connected transformer for converting three-phase ac power to nine-phase ac power. The design allows for symmetry between the nine output voltages to improve the power quality of the supply current and to minimize the THD. The results show that this new structure proves the equality between the output voltages with $40^{\circ}-{\alpha}$ and $40^{\circ}+{\alpha}$ phase shifting and produces symmetrical output currents. This result in the elimination of harmonics in the network current and provides a simulated THD that is equal to 5.12 %. An experimental prototype of the step-up asymmetrical delta-autotransformer is developed in the laboratory and the obtained results give a network current with a THD that is equal to 5.35%. Furthermore, a finite element analysis with a 3D magnetic field model is made based on the dimensions of the 4kVA, 400 V laboratory prototype three-phase with three-limb delta-autotransformer with a six-stacked-core in each limb. The magnetic distribution flux, field intensity and magnetic energy are carried out under open-circuit operation or load-loss.

• Journal of Power Electronics
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• v.19 no.6
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• pp.1496-1504
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• 2019

This paper proposes a new THD reduction algorithm for modular multilevel converters (MMCs) with offset voltage injection operated in nearest level modulation (NLM). High voltage direct current (HVDC) is actively introduced to the grid connection of offshore wind powers, and this paper deals with a voltage generation technique with an MMC for wind power generation. In the proposed method, third harmonic voltage is added for reducing the THD. The third harmonic voltage is adjusted so that each of the pole voltage magnitudes maintains a constant value with a maximum number of (N+1) levels, where N is the number of sub-modules per arm. By using the proposed method, the THD of the output voltage is mitigated without increasing the switching frequency. In addition, the proposed method has advantageous characteristics such as simple implementation. As a part of this study, this paper compares the THD results of the conventional method and the proposed method with offset voltage injection to reduce the THD. In this paper, simulations have been carried out to verify the effectiveness of the proposed scheme, and the proposed method is implemented by a HILS (Hardware in the Loop Simulation) system. The obtained results show agreement with the simulation results. It is confirmed that the new scheme achieved the maximum level output voltage and improved the THD quality.

• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.794-795
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• 2011

The DC side voltage and current of a HVDC transmission system are directly affected by non-linear switching devices such as the thyristor valve which causes real power losses, even under the superconducting conditions of a high temperature superconducting (HTS) power cable. This paper deals with the development of miniaturized superconducting DC cable system. The authors designed and fabricated two thyristor converters for DC transmission system. One is operated as a rectifier and the other is an inverter. The HTS model cable was connected between the DC side of the rectifier and inverter. Real DC transport current and voltage were applied to the miniaturized HTS DC cable. Experimental results are discussed in detail.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.63 no.12
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• pp.1640-1648
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• 2014

This paper proposes a switching frequency reduction method for MMC (Modular Multilevel Converter) utilizing redundancy operation of sub-module, which can offer reduction of voltage harmonics and switching loss. The feasibility of proposed method was verified through computer simulations with PSCAD/EMTDC software. Based on simulation analysis, a hardware scaled-model of 10kVA, DC-1000V MMC was designed and manufactured in the lab. Various experiments were conducted to verify the feasibility of proposed method in the actual hardware system. The hardware scaled-model can be effectively utilized for analyzing the performance of MMC according to the modulation scheme and redundancy operation.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.29 no.11
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• pp.66-73
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• 2015

AC-based power systems, having the advantages that voltage transformation and long distance transmission are easy, have been constructed since the last 19th century. However, DC-based power system is paid attention these days because of the development of power electronic devices as well as the increase of digital loads and distributed generation. For instance, the transmission systems using High Voltage DC (HVDC) are commercially operated in the world and the researches on distribution system using Low Voltage DC (LVDC) are gradually increased. This paper analyzes voltage sag, resulted from faults, in LVDC distribution system according to the number of poles. Modeling and simulation with various conditions are conducted by using ElectroMagnetic Transients Program (EMTP). Moreover, some countermeasures to reduce voltage sag in LVDC distribution system are suggested briefly.

• The Transactions of the Korean Institute of Power Electronics
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• v.24 no.6
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• pp.411-418
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• 2019

Light-triggered thyristors (LTTs) are essential components in high-power applications, such as HVDC transmission and several pulsed-power applications. Generally, LTT fabrication includes a deep diffusion of aluminum as a p-type dopant to form a uniform p-base region, which needs careful concern for contamination and additional facilities in silicon semiconductor manufacturing factories. We fabricated 4-inch 5,000 V LTTs with boron implantation and diffusion process as a p-type dopant. The LTT contains a main cathode region, edge termination designed with a variation of lateral doping, breakover diode, integrated resistor, photosensitive area, and dV/dt protection region. The doping concentration of each region was adjusted with different doses of boron ion implantation. The fabricated LTTs showed good light triggering characteristics for a light pulse of 905 nm and a blocking voltage (V_DRM) of 6,500 V. They drove an average on-state current (I_TAVM) of 2,270 A, peak nonrepetitive surge current (I_TSM) of 61 kA, critical rate of rise of on-state current (di/dt) of 1,010 A/㎲, and limiting load integral (I²T) of 17 MA²s without damage to the device.

• Journal of IKEEE
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• v.23 no.4
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• pp.1140-1149
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• 2019

In general, there are various pulse width modulation(PWM) methods simply using the offset voltage injection in voltage source converter(VSC). In accordance with the AC side voltage synthesis method with the offset voltage, DC side voltage utilization factor in VSC is changed. Also, this can apply equally to the MMC system. In other words, if the DC side capacity of the high voltage DC(HVDC) transmission system is determined, the maximum reactive power which can be supplied to the AC side can be changed according to the applied output voltage synthesis method with the offset voltage. In this paper, the leg energy pulsation in MMC system according to the AC side output voltage synthesis method with offset voltage which several representative PWM are applied to are mathematically analyzed and compared with each other. Finally, the above results are verified by simulation emulating the 400MVA full-scale MMC system to determine the consistency of the mathematical analysis.

• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.822-823
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• 2011

The authors analyzed harmonic current based loss of a high temperature superconducting (HTS) DC model cable. The loss in HTS DC cable is generated due to the variation of magnetic field caused by harmonic current in a HVDC transmission system. The authors designed and fabricated two meters of HTS DC model cable for verification of real loss characteristic. In this paper, the loss characteristics caused by harmonic current in the HTS DC model cable are analyzed using commercial finite element method software package. The loss of the HTS DC cable is much less than the loss of the HTS AC cable but the loss should be considered to decide a proper capacity of cooling system.