• Journal of Electrical Engineering and information Science
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• v.2 no.3
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• pp.123-131
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• 1997

Design of single stage AC/DC converter with high power factor for low power level applications is proposed. The proposed converter gives the good power factor correction, low line current harmonic distortions, and tight output voltage regulations. This converter also has a high efficiency by employing an active clamp method and synchronous rectifiers. To verify the performances of the proposed converter, a 90W-converter has been designed. The modelling of this proposed converter is power formed using an averaging technique and based on this model a detailed analysis is carried out. This prototype meets the IEC555-2 requirements satisfactorily with nearly unity power factor and high efficiency.

• Proceedings of the KSR Conference
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• 1999.05a
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• pp.322-327
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• 1999

In most railway vehicle applications, a single phase AC/DC converter is used greatly and is essential equipment for Korea High Speed Train. A diode bridge rectifier and a phase-controlled thyristor bridge rectifier generate harmonics in power system. Nowadays, power factor and harmonics become major issue in electrical equipment for railway vehicle. Many researchers have been trying to improve the power factor and ac-side harmonics. Therefore the PWM converter is used to operate at unity Power factor and to reduce ac-side current harmonics. This paper describes the circuit for AC/DC PWM converter of Korea High Speed Train and proposes control algorithm to realize the sinusolidal input current waveform and the effective unity power factor. The validity of the proposed control method is verified through the experimental result.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.14 no.3
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• pp.251-256
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• 2001

Bi-2223 tapes have been developed for low-field power applications at liquid nitrogen temperature. When the Bi-2223 tapes are used in an application such as a power transmission cable or a power transformer, they are supplied with an AC transport current simultaneously. AC loss taking into account such real applications is a crucial issue for power applications fo the Bi-2223 tapes to be feasible. In this paper, the transport losses for different AC current levels and arrangements of the neighboring tapes have been measured in a 1./5 m long Bi-2223 tape. The significant increase of the transport losses due to neighboring tape's AC currents is observed. An increase of the transport losses caused by a decrease of the Bi-2223 tape's critical current is a minor effect. The measured trasprot losses could not be explained by a dynamic resistance loss based on DC voltage-current characteristics in combination with the neighboring tape's AC currents.The trasport losses do not depend on the frequency of the neighboring tape's AC currents but is arrangements in the range of small current especially.

• Journal of Power Electronics
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• v.10 no.3
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• pp.235-244
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• 2010

This paper presents a new isolated soft-switching bidirectional buck-boost inverter for fuel cell applications. The buck-boost inverter combines an isolated DC-DC converter with a conventional inverter to implement buck-boost DC-DC and DC-AC conversion. The main switches achieve zero voltage switching and zero current switching by using a novel synchronous switching SVPWM and the volume of the transformer in the forward and fly-back mode is also minimized. This inverter is suitable for wide input voltage applications due to its high efficiency under all conditions. An active clamping circuit reduces the switch's spike voltage and regenerates the energy stored in the leakage inductance of the transformer; therefore, the overall efficiency is improved. This paper presents the operating principle, a theoretical analysis and design guidelines. Simulation and experimental results have validated the characteristics of the buck-boost inverter.

• Journal of Power Electronics
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• v.16 no.2
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• pp.464-472
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• 2016

This study applies the selective harmonic elimination (SHE) technique to design and operate a regulated AC/DC/AC power supply suitable for maritime military applications and underground trains. The input is a single 50/60 Hz AC voltage, and the output is a 400 Hz regulated voltage. The switching angles for a multi-level inverter and transformer turns ratio are determined to operate with special connected transformers with equal power ratings and produce an almost sinusoidal current. As a result of its capability of directly controlling harmonics, the SHE technique is applicable to apparatus with congenital immunity to specific harmonics, such as series-connected transformers, which are specially designed to equally share the total load power. In the present work, a single-phase 50/60 Hz input source is rectified via a semi-controlled bridge rectifier to control DC voltage levels and thereby regulate the output load voltage at a constant level. The DC-rectified voltage then supplies six single-phase quazi-square H-bridge inverters, each of which supplies the primary of a single-phase transformer. The secondaries of the six transformers are connected in series. Through off-line calculation, the switching angles of the six inverters and the turns ratios of the six transformers are designed to ensure equal power distribution for the transformers. The SHE technique is also employed to eliminate the higher-order harmonics of the output voltage. A digital implementation is carried out to determine the switching angles. Theoretical results are demonstrated, and a scaled-down experimental 600 VA prototype is built to verify the validity of the proposed system.

• Journal of Power Electronics
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• v.14 no.5
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• pp.899-907
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• 2014

This paper proposes a single-phase bidirectional AC/DC converter topology applied in V2G systems, which consists of an inverter and a bidirectional non-inverting buck-boost converter. This topology can operate in four modes: buck charging, boost charging, buck discharging and boost discharging with high input current quality and unity input power factor. The inverter switches at line frequency, which is different from conventional voltage source inverters. A bidirectional buck-boost converter is utilized to adapt to a wider charging voltage range. The modulation and control strategy is introduced in detail, and the switching patterns are optimized to reduce the current ripple. In addition, the semiconductor losses are analyzed. Simulation and experimental results demonstrate the validity and effectiveness of the proposed topology.

• Journal of Electrical Engineering and Technology
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• v.13 no.4
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• pp.1459-1473
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• 2018

This paper focuses on voltage control schemes for multi-terminal low-voltage direct current (LVDC) distribution systems. In a multi-terminal LVDC distribution system, there can be multiple AC/DC converters that connect the LVDC distribution system to the AC grids. This configuration can provide enhanced reliability, grid-supporting functionality, and higher efficiency. The main applications of multi-terminal LVDC distribution systems include flexible power exchange between multiple power grids and integration of distributed energy resources (DERs) using DC voltages such as photovoltaics (PVs) and battery energy storage systems (BESSs). In multi-terminal LVDC distribution systems, voltage regulation is one of the most important issues for maintaining the electric power balance between demand and supply and providing high power quality to end customers. This paper focuses on a voltage control method for multi-terminal LVDC distribution system that can efficiently coordinate multiple control units, such as AC/DC converters, PVs and BESSs. In this paper, a control hierarchy is defined for undervoltage (UV) and overvoltage (OV) problems in LVDC distribution systems based on the control priority between the control units. This paper also proposes methods to determine accurate control commands for AC/DC converters and DERs. By using the proposed method, we can effectively maintain the line voltages in multi-terminal LVDC distribution systems in the normal range. The performance of the proposed voltage control method is evaluated by case studies.

• Proceedings of the KIEE Conference
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• 2001.11b
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• pp.96-100
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• 2001

Bi-2223 tapes have been developed for low-field power applications at liquid nitrogen temperature. When the Bi-2223 tapes are used in an application such as a power transmission cable or a power transformer, they are supplied with an AC transport current and exposed to an external magnetic field generated by neighboring tape's AC currents simultaneously. AC loss taking into account such real applications is a crucial issue for power applications of the Bi-2223 tapes to be feasible. In this paper, the transport losses for different AC current levels and arrangements of the neighboring tapes have been measured in a 1.5 m long Bi-2223 tape. The significant increase of the transport losses due to neighboring tape's AC currents is observed. An increase of the transport losses caused by a decrease of the Bi-2223 tape's critical current is a minor effect. The measured transport losses could not be explained by a dynamic resistance loss based on DC voltage-current characteristics in combination with the neighboring tape's AC currents. The transport losses do not depend on the frequency of the neighboring tape's AC currents but its arrangements in the range of small current especially.

• Proceedings of the KIEE Conference
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• 1994.07a
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• pp.397-399
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• 1994

In this parer, the development of input power factor and the harmonic distortion of line current were showed in the proposed three phase AC-DC converter. We identified that DC voltage in the output terminal has fast dynamic response and has nearly ripple. Moreover, the converter also possesses the regenerative capability which is useful for many applications. The switching frequency of this converter operated with constant in the range of 2KHZ. Finally, simulation and experiment results are presented.

• Journal of Electrical Engineering and Technology
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• v.11 no.1
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• pp.143-149
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• 2016

The two-stage converter is widely used in traditional DC/AC inverter. It has several disadvantages such as complex topology, large volume and high loss. In order to overcome these shortcomings, a novel half load-cycle worked dual SEPIC single-stage inverter, which is based on the analysis of the relationship between input and output voltages of SEPIC converters operating in the discontinuous conduction mode (DCM), is presented in this paper. The traditional single-stage inverter has remarkable advantages in small and medium power applications, but it can’t realize boost DC/AC output directly. Besides one pre-boost DC/DC converter is needed between the DC source and the traditional single-stage inverter. A novel DC/AC inverter without pre-boost DC/DC converter, which is comprised of two SEPIC converters, is studied. The output of dual SEPIC converters is connected with anti-parallel and half load-cycle control is used to realize boost and buck DC/AC output directly and work properly, whatever the DC input voltage is higher or lower than the AC output voltage. The working principle, parameter selection and the control strategy of the inverters are analyzed in this paper. Simulation and experiment results verify the feasibility of the new inverter.