• Proceedings of the KIPE Conference
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• 2019.07a
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• pp.380-381
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• 2019

This paper presents a grid voltage-sensorless current control design for an LCL-filtered grid-connected inverter with the purpose of enhancing the reliability and reducing the total cost of system. A disturbance observer based on the gradient steepest descent method is adopted to estimate the grid voltages with high accuracy and light computational burden even under distorted grid conditions. The grid fundamental components are effectively extracted from the estimated gird voltages by means of a least-squares algorithm to facilitate the synchronization process without using the conventional phase-locked loop. Finally, the estimated states of inverter system obtained by a discrete current-type full state observer are utilized in the state feedback current controller to realize a stable voltage-sensorless current control scheme. The effectiveness of the proposed scheme is validated through the simulation results.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.1
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• pp.89-93
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• 2017

This study proposes a grid voltage estimation scheme without a phase delay in the voltage-sensorless control of a grid-connected inverter to enhance its economic feasibility, such as manufacturing cost and system complexity. The proposed scheme estimates grid voltages using a disturbance observer (DOB)-based current controller to control the grid-connected inverter without grid-side voltage sensors. The proposed voltage-sensorless control scheme can be applied successfully to grid-connected inverters, which should be operated with synchronization to the grid, considering the phase angle of the grid can be effectively detected through estimating the grid voltages by DOB. However, a problem associated with the phase delay in estimated grid voltages remains because the DOB has dynamic behavior similar to low-pass filter. Hence, the estimated grid voltages are compensated by a phase lead compensator to overcome the limitation. The effectiveness of the proposed control and estimation schemes is proven through simulations and experiments using a 2 kVA prototype inverter.

• Journal of Power Electronics
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• v.16 no.5
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• pp.1904-1917
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• 2016

The moving average filter (MAF) is widely utilized to improve the disturbance rejection capability of phase-locked loops (PLLs). This is of vital significance for the grid-integration and stable operation of power electronic converters to electric power systems. However, the open-loop bandwidth is drastically reduced after incorporating a MAF into the PLL structure, which makes the dynamic response sluggish. To overcome this shortcoming, some new techniques have recently been proposed to improve the transient response of MAF-based PLLs. In this paper, a comprehensive performance comparison of advanced MAF-based PLL algorithms is presented. This comparison includes HPLL, MPLC-PLL, QT1-PLL, and DMAF-PLL. Various disturbances, such as grid voltage sag, voltage flicker, harmonics distortion, phase-angle and frequency jumps, DC offsets and noise, are considered to experimentally test the dynamic performances of these PLL algorithms. Finally, an improved positive sequence extraction method for a HPLL under the frequency jumps scenario is presented to compensate for the steady-state error caused by non-frequency adaptive DSC, and a satisfactory performance has been achieved.

• Journal of Power Electronics
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• v.17 no.3
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• pp.704-715
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• 2017

For three-phase synchronization applications, the synchronous reference frame phase-locked loop (SRF-PLL) is probably the most widely used technique due to its ease of implementation and satisfactory phase tracking performance under ideal grid conditions. However, under unbalanced and distorted grid conditions, its performance tends to worsen. To deal with this problem, a variety of filtering stages have been proposed and used in SRF-PLLs for the rejection of disturbance components at the cost of degrading the dynamic performance. In this paper, to improve dynamic performance without compromising the filtering capability, an effective hybrid filtering stage is proposed and incorporated into the inner loop of a quasi-type-1 PLL (QT1-PLL). The proposed filtering stage is a combination of a moving average filter (MAF) and a modified delay signal cancellation (DSC) operator in cascade. The time delay caused by the proposed filtering stage is smaller than that in the conventional MAF-based and DSC-based PLLs. A small-signal model of the proposed PLL is derived. The stability is analyzed and parameters design guidelines are given. The effectiveness of the proposed PLL is confirmed through experimental results.

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.52 no.10
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• pp.528-534
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• 2003

A grid connection algorithm is proposed for the doubly-fed induction generator (DFIG) which is widely adopted in high power variable speed wind turbine. Before the stator of DFIG is connected to grid, rotor-side converter is used to control the induced stator voltage. As a result, the stator transient current is limited below the rate value during the connection by the proposed synchronization of the stator voltage to the grid voltage. A wind power generation simulator using DC motor and wound-rotor induction generator is built and the dynamic characteristics of proposed algorithm is verified experimentally.

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

• Proceedings of the KIEE Conference
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• 2006.10d
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• pp.143-145
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• 2006

This paper presents an overall control algorithm for a grid-connected wind-power generation system using a DFIG(doubly-fed induction generator) fed by back-to-back PWM converters. The control of DFIG is based on a stator-flux oriented vector control. The system enables not only fast and smooth synchronization but also high performance regulation of active and reactive power. Experimental results shows The feasibility of the control algorithm.

• Ocean and Polar Research
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• v.23 no.2
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• pp.173-179
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• 2001

Parallel supercomputing is now a must for oceanographic numerical modelers. Most of today's parallel numerical schemes use synchronous algorithms, where some processors that have finished their tasks earlier than others must wait at synchronization points for correct computation. Hence, the load balancing is a crucial factor, however, it is, in general, difficult to achieve on heterogeneous workstation clusters. We devise an asynchronous algorithm that reduces the idle times of faster processors, and discuss application of the algorithm to some grid problems and implementation on a workstation cluster using Message Passing Interface (MPI).

• Proceedings of the KIPE Conference
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• 2008.06a
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• pp.91-93
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• 2008

In this paper, novel phase angle following algorithm for the single phase grid-connected inverter is proposed. Gird-connected inverter needs phase angle detection for synchronization grid voltage with the inverter output. In case of single phase grid-connected inverter, zero crossing detection and virtual 2-phase PLL using digital all pass filter or digital low pass filter are used conventionally. But these methods have a weakness for harmonics, noises and ripples. The proposed method of PLL achieve DFT(Discrete Fourier Transform) using Goertzel algorithm. It can extract fundamental voltage of grid. As a results, it can obtain phase angle using digital all pass filter without effect of harmonics, noises and ripples. Simulation results are presented to demonstrate the effectiveness of the proposed algorithm.

• Proceedings of the Korean Information Science Society Conference
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• 2002.10c
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• pp.322-324
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• 2002

최근에 지역적으로 분산된 컴퓨팅 자원을 어디에서나 활용할 수 있도록 해주는 GRID가 많은 주목을 받고 있다. 특히 단백질 분자모사나 고에너지 물리학 분야 둥과 같이 매우 많은 계산을 요구하는 분야에서는 GRID를 통해서 계산 자원을 제공받을 수 있다. GRID에서 제공되는 계산 능력을 잘 활용하기 위해서 각 분야에서 사용되는 어플리케이션을 병렬화 할 수도 있지만 이미 계산 방법이나 결과가 검증되어 있는 기존의 패키지를 활용하는 것도 매우 중요하므로 기존 패키지에 의한 직렬 또는 지역적으로 병렬인 프로세스를 매우 많이 생성하여 GRID를 채우는 것도 한 방법이라 하겠다. 일반적으로 이와 같은 패키지는 기동할 때에 패러미터 파일을 참조하게 되고 그 계산 결과는 매우 큰 파일로 출력이 되는데 본 논문에서는 대용량 파일에 의해서 프로세스간에 동기화 및 통신을 이루어야할 때 발생하는 문제를 해결하는 방안을 제시한다. 동기화와 통신을 동시에 다루어야 하므로 Linda 개념을 도입하였으며 기존 Linda에서는 Tuple Space안에서 대용량 파일 처리를 고려하기 어려우므로 이에 대한 해결책을 제안하였다.