• The Transactions of the Korean Institute of Power Electronics
• /
• v.25 no.5
• /
• pp.385-393
• /
• 2020

This study introduces an analysis and control method for the variation of neutral point current in a grid-tied three-level neutral point clamped (NPC) converter under various grid imbalance operating conditions. Various fault cases with unbalanced amplitude and phase are systematically categorized and described using a unified metric called the imbalance factor. The fundamental component of neutral point current is generated under grid imbalance cases. The pattern and behavior of this fundamental component of neutral point current highly depend on the imbalance factor regardless of the particular type of grid fault cases. The control scheme for regulating the negative sequential component of AC input current effectively reduces the size of the fundamental component of neutral point current under a wide range of grid imbalance cases. The control scheme will enable a grid-tied three-level NPC converter to operate reliably and stably under various types of grid faults.

• Journal of Electrical Engineering and Technology
• /
• v.10 no.1
• /
• pp.102-108
• /
• 2015

This paper proposes a new fault isolation methodology for a smart protective device which plays an agent role on the smart grid distribution system with the distributed generation. It, by itself, determines accurately whether its protection zone is fault or not, identifies the fault zone and separates the fault zone through the exchange of fault information such as the current information and the voltage information with other protective devices using bi-directional communication capabilities on the smart grid distribution system. The heuristic rules are obtained from the structure and electrical characteristics determined according to the location of the fault and DG (Distributed Generation) when faults such as single-phase ground fault, phase-to-phase short fault and three-phase short fault occur on the smart grid distribution system with DG.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.58 no.11
• /
• pp.2142-2148
• /
• 2009

A microgrid is defined as two or more distributed generation or storage assets configured in a networks and capable of operation in parallel or independently form a larger electric gird, while providing continuous power to one or more end users. And when microgrid are separated from grid oprating protection devices by faults of the grid side, microsources should charge electrical power needs of loads in microgrid and operate maintaining power quality. The magnitude of the switching transients will vary based on voltage phase difference between microgrid and grid, when the microgrid is resynchronized to grid. In this paper, when microgrid is resynchronized to grid, we analyzed the existing problems for reducing switching transients of SS(Static Switch).

• Journal of Power Electronics
• /
• v.14 no.6
• /
• pp.1081-1092
• /
• 2014

Renewable energy is being spotlighted as the electric power generating source for the next generation. Due to an increase in renewable energy systems in the grid system, their impact on the grid has become non-negligible. Thus, many countries in the world, including Europe, present their own grid codes for grid power conversion devices. In order to experiment with these grid codes, grid fault test equipment is required. This paper proposes both equipment and a control method, which are constructed with a 7-level cascaded H-bridge converter, that are capable of generating various grid faults. In addition, the Pulse Width Modulation (PWM) method for multilevel converters is compared and analyzed. The proposed structure, the control method, and the PWM method are verified through simulation and experimental results.

• Proceedings of the KIPE Conference
• /
• 2011.07a
• /
• pp.423-424
• /
• 2011

본 논문에서는 불평형 전원 사고시 풍력발전 시스템의 계통 저전압 보상기법을 제안한다. 계통규정이 강화됨에 따라 풍력발전 시스템은 영전압 및 불평형 사고 시에도 안정적인 운전을 유지할 수 있는 대책이 요구된다. 제안하는 계통 저전압 보상기법은 불평형 전원사고 시에도 안정적으로 운전을 유지하며 요구되는 유, 무효 출력제어가 가능하다. 본 논문은 2MW급 풍력발전 시스템의 시뮬레이션 모델을 통하여 제안하는 보상기법의 타당성 및 강인함을 보인다.

• Journal of Power Electronics
• /
• v.8 no.1
• /
• pp.10-22
• /
• 2008

The wind-driven doubly fed induction generator (DFIG) is currently under pressure to be more grid-compatible. The main concern is the fault ride-through (FRT) requirement to keep the generator connected to the grid during faults. In response to this, the paper introduces a novel model and new control scheme for the DFIG. The model provides a means of direct stator power control and considers the stator transients. On the basis of the derived model, a robust linear quadratic (LQ) controller is synthesized. The control law has proportional and integral actions and takes account of one sample delay in the input owing to the microprocessor's execution time. Further, the influence of the grid voltage imperfection is mitigated using frequency shaped cost functional method. Compensation of the rotor current pulsations is proposed to improve the FRT capability as well as the generator performance under grid voltage unbalance. As a consequence, the control system can achieve i) fast direct power control without instability risk, ii) alleviation of the problems associated with the DFIG operation under unbalanced grid voltage, and iii) high probability of successful grid FRT. The effectiveness of the proposed solution is confirmed through simulation studies on 2MW DFIG.

• Journal of Electrical Engineering and Technology
• /
• v.12 no.3
• /
• pp.1101-1107
• /
• 2017

In this paper, a robust and fast grid synchronization method of a three-phase power converter is proposed. The amplitude and phase information of grid voltages are essential for power converters to be properly connected into the utility. The phase-lock-loop in synchronous reference frame has been widely adopted for the three-phase converter system since it shows a satisfactory performance under balanced grid voltages. However, power converters often operate under abnormal grid conditions, i.e. unbalanced by grid faults and frequency variations, and thus a proper active and reactive power control cannot be guaranteed. The proposed method adopts a second order generalized integrator in synchronous reference frame to detect positive sequence components under unbalanced grid voltages. The proposed method has a fast and robust performance due to its higher gain and frequency adaptive capability. Simulation and experimental results show the verification of the proposed synchronization algorithm and the effectiveness to detect positive sequence voltage.

• Progress in Superconductivity and Cryogenics
• /
• v.1 no.1
• /
• pp.28-32
• /
• 1999

We have performed an EMTDC simulation for the current limiting effects of a superconducting fault current limiter (SFCL). The fault currents in the 154 kV transmission line between the arbitrary S1 and S2 substations increased up to 54 KA and 60 KA during the line-to-line and three phase faults, respectively. The SFCL with 100$\omega$ of resistance after quench limited the currents to less than 17 KA within a half cycle. This limited current is well below the upper limit of a circuit breaker, suggesting that the resistance of the SFCL in the transmission line is sufficient.

• Proceedings of the KIEE Conference
• /
• 1999.07e
• /
• pp.2352-2354
• /
• 1999

The purpose of substation grounding system is to provide reference potential with power system and protect field workers from electrical shock resulted from unsymetrical power system faults. For this purpose, grounding grid should be designed to maintain max, touch voltage under safety criteria in fault conditions. It is difficult, however, to design a safe grounding grid at very resistive or narrow area. This paper describes an example of substation grounding grid design procedures in such areas with very severe design conditions. By using grounding conductors, which is located close to earth surface, earth surface potential could be controlled effectively, so that maximum touch voltages is to be maintained under safety criteria.

• Proceedings of the KIEE Conference
• /
• 2011.07a
• /
• pp.91-92
• /
• 2011

Beat is a phenomenon, where the magnitudes and frequencies of the voltage and the current fluctuate. This paper describes a quantitative analysis of the beat phenomenon of a wind farm using the envelope of a current during intertie/grid fault conditions. In this paper, the ratio of the crest to the trough of the envelope curve and the time interval between adjacent troughs are defined and used to evaluate the beat phenomenon quantitatively. Beat phenomena under various fault and wind conditions are analyzed. The proposed quantitative analysis seems simple but effective in the more understanding of beat phenomenon of a wind farm, and thus can be used as a basis for operation and/or protection of an intertie.