• Journal of Power Electronics
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• v.14 no.6
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• pp.1100-1108
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• 2014

Modular multilevel converters (MMCs) have emerged as the most promising topology for high and medium voltage applications for the coming years. However, one particular negative characteristic of MMCs is the existence of circulating current, which contains a dc component and a series of low-frequency even-order ac harmonics. If not suppressed, these ac harmonics will distort the arm currents, increase the power loses, and cause higher current stresses on the semiconductor devices. Repetitive control (RC) is well known due to its distinctive capabilities in tracking periodic signals and eliminating periodic errors. In this paper, a novel circulating current control scheme base on RC is proposed to effectively track the dc component and to restrain the low-frequency ac harmonics. The integrating function is inherently embedded in the RC controller. Therefore, the proposed circulating current control only parallels the RC controller with a proportional controller. Thus, conflicts between the RC controller and the traditional proportional integral (PI) controller can be avoided. The design methodologies of the RC controller and a stability analysis are also introduced. The validity of the proposed circulating current control approach has been verified by simulation and experimental results based on a three-phase MMC downscaled prototype.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.51 no.10
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• pp.510-517
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• 2002

The use of underground transmission cables has continuously increased in densely inhabited urban and suburban area. Due to a increasing demand of underground cables, two or more circuits are installed in parallel for several kilometers. It, however, has not been realized that the sheath circulating current is generated in the system where a large number of cables are laid in the same route. In this paper, sheath circulating current is analyzed by the EMTP and compared with the measured values. Unbalance arrangement of cables or cross-bonding length causes a significant effect on the magnitude of the sheath current. Sheath circulating current could be greatly reduced by the symmetrical configuration of cables and installation of the impedance reduction system. Especially, with the impedance system of 1Ω installed, the sheath circulating current is reduced by 85.7%.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.50 no.11
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• pp.537-545
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• 2001

This paper describes the analysis of sheath circulating current and various methods to reduce the large circulating current in case of operating cable system using EMTP/ATPDraw. And also, possible methods are proposed through a detailed analysis regarding cable systems by considering various electrical and environmental factors. It is evaluated that the proposed reduction methods can be effectively applied to reduce the large sheath circulating over current with the minimized electrical problems.

• Proceedings of the KIEE Conference
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• 2002.07a
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• pp.449-451
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• 2002

Sheath circulating current rises from the change of sheath mutual impedance which is caused by imbalanced cable system, and different section length between joint boxes. However, mixed burying typo and imbalance section length take many parts of main reasons of sheath circulating current increment in domestic underground transmission power cables. Therefore, the increment reason and variation characteristics of sheath circulating current is analysed with simulation using EMTP/ATPDraw and measuring data of practical cable system which has a problem of exceeding sheath circulating current in this paper.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.52 no.4
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• pp.193-200
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• 2003

The use of underground transmission tables has continuously increased in densely inhabited urban and suburban area. Due to a increasing demand of underground cables, two or more circuits are installed in parallel for several kilometers. It, however, has not been realized that the sheath circulating current is generated in the system where a large number of cables are laid in the same route. In this paper, Author studied diversely the sheath circulating current on underground cables depending on the various length rate, the phase arrangement, and the grounding resistance of the sheath in the cross-bonded section. It was clear that very large circulating current is generated in cable systems due to unbalanced length rate and phase arrangement in the cross-bonded section.

• Proceedings of the KIEE Conference
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• summer
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• pp.394-396
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• 2004

In order to reduce the sheath circulating current, same arrangement and balanced length of cable are required for the underground cable system. But practically, changing the whole arrange of cable which is already constructed is impossible. Therefore, It is necessary to apply the restraining-unit of sheath circulating current at the cross-bonding wire of insulated joint because the impedance of restraining-unit is able to reduce sheath circulating current at a normal condition. Even at a transient state, the restraining-unit must maintain electrical and mechanical characteristics. In this paper, the features of restraining-unit developed by LG Cable as well as the electrical test results are described. It proves that the restraining-unit is applicable to the underground cable system where sheath circulating current rises.

• The Transactions of the Korean Institute of Power Electronics
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• v.18 no.3
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• pp.270-278
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• 2013

This paper proposes a control method for high voltage direct current(HVDC) with modular multilevel converter (MMC) under unbalanced voltage conditions considering the submodule(SM)'s current capacity and circulating current. It is aimed to propose a control method in which the current peak value does not exceed the maximum value of HVDC-MMC by considering the current capacity of the SM under unbalance voltage conditions. And it analyzes the effect of the unbalanced voltage on circulating currents in MMC and then proposes a control method considering each component of circulating currents under unbalanced voltages. The effectiveness of the proposed controlling method is verified through simulation results using PSCAD/EMTDC.

• Journal of Electrical Engineering and Technology
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• v.2 no.3
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• pp.320-328
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• 2007

The sheath in underground power cables serves as a layer to prevent moisture ingress into the insulation layer and provide a path for earth return current. Nowadays, owing to the maturity of manufacturing technologies, there are normally no problems for the quality of the sheath itself. However, after the cable is laid in the cable tunnel and is operating as part of the transmission network, due to network construction and some unexpected factors, some problems may be caused to the sheath. One of them is the high sheath circulating current. In a power cable system, the uniform configuration of the cables between sections is sometimes difficult to achieve because of the geometrical limitation. This will cause the increase of sheath circulating current, which results in the increase of sheath loss and the decrease of permissible current. This paper will study the various characteristics and effects of sheath circulating current, and then will prove why the sheath current rises on the underground power cable system. A newly designed device known as the Power Cable Current Analyser, as well as ATP simulation and calculation equation are used for this analysis.

• Proceedings of the KIEE Conference
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• 2008.07a
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• pp.23-24
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• 2008

For the $Y-Y-{\Delta}$ transformers operated in parallel, there exist two kinds of the circulating currents i.e. between the tanks and between the banks of the delta side. The proposed algorithm estimates the two circulating currents in the transformers in parallel in an ultra high voltage system. As the circulating current between the tanks 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. The circulating current between the banks in the delta side is estimated from the delta winding current and the line currents. 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. Test results indicate that the algorithm can estimate the two kinds of the circulating currents successfully for both cases.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.60 no.3
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• pp.479-485
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• 2011

In 22.9kV underground distribution system, power cables are provided with multiple-point ground in which each neutral line of the distribution cable(A, B, C phases) and three-wire common grounded at every connecting section. But in such grounding methods, circulating current flows between the neutral wire and grounding wire. And power loss due to circulating current also occurs in all conductors. Therefore it is getting necessary reducing circulating current in underground distribution system. This paper presents improved grounding method to overcome such problems. The proposed grounding method eliminates circulating current in the neutral line effectively and is verified that there is no electrical problem or any ineffectiveness of operating protection systems. These analyses are carried out by EMTP/ATPDraw to compare each grounding methods in steady and transient state. This grounding method suggested in this paper can be applied on real distribution system after field tests considering elimination of circulating current was implemented.