• Transactions of the Korean hydrogen and new energy society
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• v.33 no.3
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• pp.226-231
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• 2022

Power converter are usually equipped for fuel cell power generation system to connect alternating current (AC) electric power grid. When converting direct current (DC) of fuel cell power source into AC, the power converter has a frequency ripple, which affects the fuel cell and the grid. Therefore, an equivalent circuit having dynamic characteristics of fuel cell power, for example, impedance, is useful for designing an inverter circuit. In this study, the current, voltage and impedance characteristics were calculated through fuel cell modeling and validated by comparing them with experiments. The equivalent circuit element values according to the current density were formulated into equations so that it could be applied to the circuit design. It is expected that the process of the equivalent circuit modeling will be applied to the actual inverter circuit design and simulated fuel cell power sources.

• Journal of Electrical Engineering and Technology
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• v.12 no.2
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• pp.654-664
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• 2017

This paper proposed an unified dual-phase-shift (UDPS) control for dual active bridge (DAB) converters in order to improve efficiency for a wide output power range. Different operating modes of UDPS are characterized with respect to the reactive current distribution. The proposed UDPS has the same output power capability with conventional phase-shift (CPS) method. Furthermore, its implementation is simple since only the change of the leading phase-shift direction is required for different operating power range. The proposed UDPS control can minimize both the inductor rms current and the circulating reactive current for various voltage conversion ratios and load conditions. The optimal phase-shift pairs for two bridges of DAB converter are derived with respect to the comprehensive reactive power loss model, including the reactive components delivered from the load and back to the source. Simulation and experimental results are illustrated and explained with details. The effectiveness of the proposed method is verified in terms of reactive power losses minimization and efficiency improvement.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.58 no.6
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• pp.1138-1145
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• 2009

This paper presents an active and reactive power compensator for the wind power system with squirrel-cage induction generator. The output power of a wind power system changes irregularly according to the variation of wind speed. The developed system is able to continuously compensate the active and reactive power. The 3-phase inverter operates for the compensation of reactive power, while the DC/DC converter with super-capacitors operates for the compensation of active power. The operational feasibility of the proposed model was verified by simulations with PSCAD/EMTDC and the feasibility of hardware implementation was confirmed by experimental works with a scaled hardware model. The proposed compensator can be expected that developed system may be used to compensated the abrupt power variation due to sudden change of wind speed or sudden power-drop by tower effect. It can be also applied for the distributed generation and the Micro-Grid.

• Journal of Power Electronics
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• v.18 no.2
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• pp.615-626
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• 2018

To ensure a DC current path and avoid large voltage overshoot of the DC-link inductor, alternating PWM pulses in the current-source rectifier should be supplemented by overlap time, which generates an overlap current and causes input current distortion. In this study, the influence of overlap time is illustrated by comparing the AC-side current before and after overlap time is added. The overlap current distribution caused by overlap time is discussed under different modulation carriers, including triangle carrier, positive-going carrier, and negative-going carrier. The quantitative relationship between the extra harmonics of the AC-side current and overlap time is based on the Fourier analysis. Based on the commutation analysis, a new carrier modulation scheme that can restrain overlap current is proposed. A 3 kW prototype is established to verify the effectiveness of the influence of overlap time and the proposed restraining modulation scheme.

• The Transactions of the Korean Institute of Power Electronics
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• v.23 no.3
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• pp.161-167
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• 2018

In battery-operated systems, an isolated converter is used to interface the utility grid with the system to increase stability when charging and discharging batteries. Systems such as vehicle-to-grids (V2Gs), on-board chargers, and energy storage systems (ESSs) have recently become popular, and the roles of isolated converters have become important considerations in fabricating such devices. A fixed-frequency LLC converter, which is a type of isolated converter, presents the advantages of high efficiency and high power density by performing zero-voltage switching (ZVS) over wide frequency ranges. However, the magnetizing inductance of the LLC converter should be designed to enable ZVS in all switching devices. Therefore, in this study, the operating characteristics of the LLC circuit are analyzed, and an optimal design method for ZVS operation is established. Moreover, an 8 kW LLC high-efficiency and high-power-density resonant converter is designed and tested for ESS application. The LLC converter achieves 98% efficiency at rated power.

• Journal of Power Electronics
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• v.17 no.1
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• pp.200-211
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• 2017

Conventional boost-full-bridge and boost-hybrid-bridge two-stage inverters are widely applied in order to adapt to the wide dc input voltage range of photovoltaic arrays. However, the efficiency of the conventional topology is not fully optimized because additional switching losses are generated in the voltage conversion so that the input voltage rises and then falls. Moreover, the electrolytic capacitors in a dc-link lead to a larger volume combined with increases in both weight and cost. This paper proposes a higher efficiency inverter with time-sharing synchronous modulation. The energy transmission paths, wheeling branches and switching losses for the high-frequency switches are optimized so that the overall efficiency is greatly improved. In this paper, a contrastive analysis of the component losses for the conventional and proposed inverter topologies is carried out in MATLAB. Finally, the high-efficiency under different switching frequencies and different input voltages is verified by a 3 kW prototype.

• 한국태양에너지학회:학술대회논문집
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• 2012.03a
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• pp.427-432
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• 2012

This paper proposes efficient operation method of PV system consisted of multi-central which is suitable for large scale system. The multi-central system used switch at a DC-link and applied proposed algorithm can improve the efficiency and the reliability on the existing system. This algorithm, with advantage of Multi-Central system can minimize the effect of different characteristic of each PV array due to a shadow or damaged PV cell. Each system is analysed and maximum power point tracking control, DC-link voltage control and output current control is used commonly. The validity is verified after comparing of the existing system and proposed system by simulation.

• Journal of Power Electronics
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• v.16 no.5
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• pp.1743-1751
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• 2016

The increasing demand for high voltage DC grids resulting from the continuous installation of offshore wind farms in the North Sea has led to the concept of multi-terminal direct current (MTDC) grids, which face some challenges. Power (current) flow control is a challenge that must be addressed to realize a reliable operation of MTDC grids. This paper presents a reduced switch count topology of a current flow controller (CFC) for power flow and current limiting applications in MTDC grids. A simple control system based on hysteresis band current control is proposed for the CFC. The theory of operation and control of the CFC are demonstrated. The key features of the proposed controller, including cable current balancing, cable current limiting, and current nulling, are illustrated. An MTDC grid is simulated using MATLAB/SIMULINK software to evaluate the steady state and dynamic performance of the proposed CFC topology. Furthermore, a low power prototype is built for a CFC to experimentally validate its performance using rapid control prototyping. Simulation and experimental studies indicate the fast dynamic response and precise results of the proposed topology. Furthermore, the proposed controller offers a real solution for power flow challenges in MTDC grids.

• Journal of Power Electronics
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• v.11 no.4
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• pp.490-498
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• 2011

This paper presents a novel analysis, design, and implementation of a battery charging three-phase high frequency semi-controlled power converter feasible for plug-in hybrid electric vehicles. The main advantages of the proposed topology include high efficiency; due to lower power losses and reduced number of switching elements, high output power density realization, and reduced passive component ratings proportionally to the frequency. Additional advantages also include grid economic utilization by insuring unity power factor operation under different possible conditions and robustness since short-circuit through a leg is not possible. A high but acceptable total harmonic distortion of the generator currents is introduced in the proposed topology which can be viewed as a minor disadvantage when compared to traditional boost rectifiers. A hysteresis control algorithm is proposed to achieve lower current harmonic distortion for the rectifier operation. The rectifier topology concept, the principle of operation, and control scheme are presented. Additionally, a dc-dc converter is also employed in the rectifier-battery connection. Test results on 50-kHz power converter system are presented and discussed to confirm the effectiveness of the proposed topology for PHEV applications.

• Journal of Power Electronics
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• v.17 no.5
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• pp.1349-1362
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• 2017

In order to further improve the harmonic suppression capability of conventional 12-pulse rectifiers, this paper proposes a low harmonic 12-pulse rectifier using an Active Inter-Phase Reactor (AIPR). Through a detailed analysis of the relationship between the input current, output current and circulating current of the DC side, the mechanism where the AC grid side current harmonics can be suppressed by the DC side circulating current is revealed. On this basis, an interleaved APFC controlled by a DSP is designed and used as an AIPR along with an interphase reactor. A simulation is carried out with MATLAB/Simulink and an experiment is performed on a 9-kVA prototype. The obtained results verify the feasibility and validity of the proposed approach. Compared with a traditional 12-pulse rectifier, the THD can be reduced to 1/5 of the original value, and the capacity of the AIPR is only 2% of the load power. Thus, it is suitable for high-power applications.