• KEPCO Journal on Electric Power and Energy
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• v.2 no.2
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• pp.211-221
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• 2016

The increase of world's population and economic development are the keys drivers behind growing demand for energy. Especially the demand for electricity would eventually result in an increase of coal usage. Therefore ultra supercritical circulating fluidized bed boiler has been developed as solutions of economic eco-friendly technologies for coal and of increasing supplies of low grade fuels. Ultra supercritical circulating fluidized bed boiler has an once through type of steam cycle different from drum type in subcritical circulating fluidized bed boiler. Also, the duplication of a proven commercial module with 100-300 MWe subcritical circulating fluidized bed might be the key for design of 500~800 MWe ultra supercritical circulating fluidized bed boiler. After 2017, ultra supercritical circulating fluidized bed boiler might become standard model over subcritical circulating fluidized bed boiler. Therefore, this paper will help you to understand ultra super critical circulating fluidized bed (USC-CFB) through describing the background, status and prospect of the CFB technology.

• Proceedings of the KIEE Conference
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• 1994.07a
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• pp.509-511
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• 1994

Cycloconverters can be classified into two types by the main circuit configuration. One is the non- circulating current type required the dead time the other is circulating current type which has the circulating current reactors. The switching devices were highly stressed by the load voltages during dead time on non-circulating current type operation and the circulating currents on circulating current type operation. The overcome those problems, this paper proposes the switching patterns for the partial and the non circulation current type. The PWM methods, which are saw wave comparing method and load current integrating value comparing method, are presented.

• Proceedings of the KIEE Conference
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• 2015.07a
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• pp.996-997
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• 2015

This paper proposes parameter design principle of the sub-module capacitance, Arm inductance and a control method to reduced the circulating currents in modular multilevel converter(MMC) under unbalanced voltage conditions. Under balanced voltage conditions, only negative-sequence circulating currents exist. Consequently, the conventional method has considered only negative-sequence circulating currents in MMC. However, under unbalanced voltage conditions, there are positive-sequence, zero-sequence and negative-sequence circulating currents in MMC. Thus, under unbalanced voltage conditions, a control method should consider these all components. This study proposes the control method to reduced the circulating currents under the unbalanced voltage.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.51 no.10
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• pp.525-533
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• 2002

It is common to install multiple lines in the same route. Recently, excessive sheath circulating current was partially measured in underground cable systems of KEPCO. Especially, the installation type, unbalance section length between joint boxes and zero sequence current by distribution cable have an effect on the rising of sheath circulating current in the underground transmission system. If excessive current flows in sheath, sheath loss which is reduced the transmission capacity is produced. This paper describes the relation analysis of sheath circulating current and burying types. And also, a detailed analysis on rising cause and characteristic of sheath circulating current by considering various unbalanced conditions presents using analysis and measurement regarding cable systems which have the problem of excessive sheath circulation current.

• YAKHAK HOEJI
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• v.43 no.1
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• pp.98-103
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• 1999

Effects of Astragali Radix extract (AG) on the cell mediated-and nonlpecific immunotoxic responses of zinc chloride (Zn) were studied usign ICR mice. Mice were divided into 4 groups (10 mice/group), and Zn was given to the mice 1 hr after i.p. injection with 0.5g/kg of AG by i.p. injection daily for 10 days at a dose of 25 mg/kg. Immune responses on the responses on the relative weight of thymus, delayed-type hypersensitivity to SRBC (DTH), phagocytic activity and circulating leukocytes were evaluated. Zn treatment decreased body weight gain, the relative weight of thymus, DTH and circulating leukocytes compared with those in controls. AG treatment increased DTH, phagocytic activity and circulating leukocytes compared with those in controls. Combination of AG and Zn increased DTH and circulating leukocytes compared with those in controls, but decreased body weight gain and the relative weight of thymus. These findings indicated that AG decreased immunotoxicity of Zn on the DTH and circulating leukocytes.

• Journal of Electrical Engineering and Technology
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• v.9 no.4
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• pp.1137-1144
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• 2014

A parallel-feeding AC traction power system increases the power supply capacity and decreases voltage fluctuations, but the circulating power flow caused by the phase difference between the traction substations prevents the system from being widely used. A circuit analysis shows that the circulating power flow increases almost linearly as the phase difference increases, which adds extra load to the system and results in increased power dissipation and load unbalance. In this paper, we suggest a phase shifter for the parallel-feeding AC traction power system. The phase shifter regulates the phase difference and the circulating power flow by injecting quadrature voltage which can be obtained directly from the Scott-connection transformer in the traction substation. A case study involving the phase shifter applied to the traction power system of a Korean high-speed rail system shows that a three-level phase shifter can prevent circulating power flow while the phase difference between substations increases up to 12 degrees, mitigate the load unbalance, and reduce power dissipation.

• Journal of Electrical Engineering and Technology
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• v.13 no.3
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• pp.1241-1250
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• 2018

Parallel three-phase voltage source inverters in a direct connection configuration are widely used to increase system power ratings. A zero-sequence circulating current can be generated according to the switching method; however, the zero-sequence circulating current not only distorts current, but also reduces the system reliability and efficiency. In this paper, a model predictive control scheme is proposed for parallel inverters to drive an interior permanent magnet synchronous motor with zero-sequence circulating current suppression. The voltage vector of the parallel inverters is derived to predict and control the torque and stator flux components. In addition, the zero-sequence circulating current is suppressed by designing the cost function without an additional current sensor and high-impedance inductor. Simulation and experimental results are presented to verify the proposed control scheme.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.21 no.3
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• pp.56-61
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• 2007

In this paper, a circulating current reduction approach for the parallel operation of fuelcell systems with Z-source inverters is investigated. The carrier phase shifted SPWM(Sinusoidal Pulse Width Modulation) is used as a modulation method since it has an advantage in reducing output current harmonics. However, when this technique is applied to the parallel operation of Z-source inverters, it additionally produces circulating currents. A coupled circulating current reactor is used to reduce circulating current generated by the parallel operation of Z-source inverters and to reduce output current harmonics. The proposed circulating current reduction approach using coupled circulating current reactors is verified through simulation and experiment.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.64 no.8
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• pp.1193-1201
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• 2015

This paper presents a study of circulating current of modular multi-level converter (MMC) based a high voltage direct current (HVDC) system under unbalanced grid conditions. Due to the connection of a dependent DC source in each phase, the MMC system inherently generates the power ripple of double-line-frequency components in the AC-side and as a result, the additional sinusoidal current named circulating current flows through the each arm. Reliability improvement of HVDC system under an unbalanced grid condition is one of the important criteria. Generally, the modeling of the circulating current is based on the power relation between DC-side and AC-side. However, the method is not perfectly matched in the MMC system due to the difference of the structural characteristic. In this paper, improved modeling method of circulating current is proposed, which is based on the inner arm power. The proposed method is verified by several simulations to have good agreement of the circulating current components.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.57 no.12
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• pp.2147-2152
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• 2008

This paper proposes an algorithm to estimate the circulating currents in the transformers in parallel in an ultra high voltage system. For the Y-Y-${\Delta}$ transformers operated in parallel, there exist two kinds of the circulating currents i.e. one is between the tanks and the other between the banks of the delta side. As the former is 90 deg out of phase of the load current, it is estimated by decomposing the line current into the component 90 deg out of phase of the load current in the frequency domain. The latter is estimated in the time domain from applying the Kirchhoff's voltage law on the delta winding which gives a first-order differential equation in terms of the delta winding currents. To estimate the circulating currents between the tanks, the performance of the proposed algorithm is investigated when the impedances of the two transformer tanks are different or the taps of the on-load tap changer of the transformers are mismatched temporarily. To estimate the circulating currents between the banks, the performance of the proposed algorithm is also examined under magnetic inrush and over-excitation. Test results indicate that the algorithm can estimate the two kinds of the circulating currents successfully.