• 전력전자학회논문지
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• 제20권2호
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• pp.152-159
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• 2015

In a weak grid condition, the precise grid impedance estimation is essential to guaranteeing the high performance current control and power transfer for a grid-connected inverter. This study proposes a precise estimation method for grid impedance by PQ variations by employing the variation method of reference currents. The operation principle of grid impedance estimation is fully presented, and the negative impact of the phase locked loop is analyzed. Estimation error by a synchronization angle in the park's transformation using the phase locked loop is derived. As a result, the variation method of reference currents for accurate estimation is introduced. The validation of the proposed method is verified through several simulation results and experiments based on a 2-kW voltage source inverter prototype.

• Journal of Power Electronics
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• 제11권4호
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• pp.599-605
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• 2011

This paper proposes a gain scheduling method that improves the stability of grid-connected systems employing an LCL-filter. The method adjusts the current controller gain through an estimation of the grid impedance in order to reduce the resonance that occurs when using an LCL-filter to reduce switching harmonics. An LCL-filter typically has a frequency spectrum with a resonance peak. A change of the grid-impedance results in a change to the resonant frequency. Therefore an LCL-filter needs a damping method that is applicable when changing the grid impedance for stable system control. The proposed method instantaneously estimates the grid impedance and observes the resonant frequency at the same time. Consequently, the proposed method adjusts the current controller gain using a gain scheduling method in order to guarantee current controller stability when a change in the resonant frequency occurs. The effectiveness of the proposed method has been verified by simulations and experimental results.

• Journal of Power Electronics
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• 제11권3호
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• pp.360-368
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• 2011

This paper proposes an active frequency with a positive feedback in the d-q frame anti-islanding method suitable for a robust phase-locked loop (PLL) algorithm using the FFT concept. In general, PLL algorithms for grid-connected PV PCS use d-q transformation and controllers to make zero an imaginary part of the transformed voltage vector. In a real grid system, the grid voltage is not ideal. It may be unbalanced, noisy and have many harmonics. For these reasons, the d-q transformed components do not have a pure DC component. The controller tuning of a PLL algorithm is difficult. The proposed PLL algorithm using the FFT concept can use the strong noise cancelation characteristics of a FFT algorithm without a PI controller. Therefore, the proposed PLL algorithm has no gain-tuning of a PI controller, and it is hardly influenced by voltage drops, phase step changes and harmonics. Islanding prediction is a necessary feature of inverter-based photovoltaic (PV) systems in order to meet the stringent standard requirements for interconnection with an electrical grid. Both passive and active anti-islanding methods exist. Typically, active methods modify a given parameter, which also affects the shape and quality of the grid injected current. In this paper, the active anti-islanding algorithm for a grid-connected PV PCS uses positive feedback control in the d-q frame. The proposed PLL and anti-islanding algorithm are implemented for a 250kW PV PCS. This system has four DC/DC converters each with a 25kW power rating. This is only one-third of the total system power. The experimental results show that the proposed PLL, anti-islanding method and topology demonstrate good performance in a 250kW PV PCS.

• 전력전자학회논문지
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• 제15권2호
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• pp.159-166
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• 2010

분산전원을 계통에 연계하기 위해서는 전류제어가 필수적이며, 최근에 고성능 DSP(Digital Signal Processors)를 기반으로 빠른 동특성을 만족시키는 예측전류제어에 대한 연구가 활발히 진행되고 있다. 하지만 예측전류제어는 디지털 구현 시 발생하는 시지연, 파라미터 및 입력값의 오차, 노이즈에 의한 간섭으로 인해 제어 성능이 감소할 뿐만 아니라 시스템을 불안정하게 하는 단점을 갖고 있다. 따라서 본 논문은 계통연계형 인버터 응용에 있어서 계통전압 관측기를 이용한 예측전류제어를 제안한다. 전압 관측기 이득 선정을 위해, 계통전압에 존재하는 저차 고조파에 의한 영향을 고려하며, 필터 파라미터 오차에 의한 영향을 분석한다. 제안된 방법은 빠른 전류응답특성 뿐만 아니라, 전압센서를 사용하지 않음으로 인해 노이즈에 강인하며 시스템 구현이 간단하고 계통의 저차 고조파에 강인한 전류제어성능을 갖는다. 제안된 방법의 타당성은 시뮬레이션과 실험을 통하여 검증한다.

• Journal of Electrical Engineering and Technology
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• 제7권4호
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• pp.459-467
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• 2012

In this paper, a grid connected microgrid with multiple inverter-based distributed generators (DGs) is considered. DG in FFC mode regulates the microgrid as a controllable load from the utility point of view as long as its output is within the capacity limit. The transition mode causes a change in frequency of microgrid due to the loss of power transferred between main grid and microgrid. Frequency deviation from the nominal value can exceed the limit if the loss of power is large enough. This paper presents a coordinated control method for inverter-based DGs so that the microgrid is always regulated as a constant load from the utility viewpoint during grid connected mode, and the frequency deviation in the transition mode is minimized. DGs can share the load by changing their control modes between UPC and FFC and stabilize microgrid during transition.

• 전기학회논문지
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• 제60권8호
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• pp.1622-1626
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• 2011

Recently, interests on microgrids have been growing as clean power systems. Microgrids include small scaled distributed generation such as wind and solar power as well as diesel generators as main power sources. To operate a microgrid effectively, optimal scheduling for the microgrid is important. Especially, in the grid-connected mode, power trades between the microgrid and the power grid should be considered in optimal scheduling. In this paper, mathematic models for optimal operation of a microgrid were established based on the linear programming. In particular, the shiftable load was considered in the models to optimize it in microgrid operation. To show feasibility of the proposed models, they were applied to optimal microgrid operation and the results were discussed.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2017년도 추계학술대회
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• pp.69-70
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• 2017

This paper presents design of control system for permanent magnet synchronous generator(PMSG). The gas engine make rotating mechanical energy from gas fuel energy. The rotor of synchronous generator is connected to axis of engine. And it converts the mechanical energy to the electrical energy. The control system of PMSG helps the electrical energy to flow to grid. the single phase pfc rectifier controls the DC-link voltage by controlling the current of filter inductor. If the DC-link voltage is higher than the voltage reference, the filter current could be controlled to flow to grid. The three phase inverter controls the stator current of generator. The direction of the current is controlled depends on motoring or generating mode. The feasibility of the grid-connected PMSG is verified by the experimental results with 1kW prototype.

• 전력전자학회논문지
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• 제20권1호
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• pp.59-64
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• 2015

This paper proposes an analysis and operation strategy of a grid-connected wind turbine system using a diode rectifier. The currents of generators are the same as that of a small wind turbine system. Therefore, the analysis of generator torque is required as opposed to an analysis of blade speed. In this paper, the appropriate MPPT control method is proposed to control generator torque. Usefulness of the proposed operation strategy is verified by simulations and experiments.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2008년도 제39회 하계학술대회
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• pp.1168-1169
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• 2008

This study suggests a modeling of grid-connected wind turbine generation systems and performs simulation according to increase/decrease of real wind speed. Matlab & Simulink implemented modeling of grid-connected wind turbine generation system. Terminal voltage, grid voltage, and active/reactive power shall be observed following the performance of simulation.

• 한국수소및신에너지학회논문집
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• 제19권5호
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• pp.417-422
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• 2008

In this paper, a novel DC/DC low-voltage high-current converter circuit is proposed to improve the efficiency of power converter used in the grid-connected fuel-cell generator system. We proposed a novel high efficiency grid-connected power conditioning system for RPG fuel cell. On the result of that, the loss of system was decreased rapidly by driving stack within the condition of maximum efficiency. The peak currents of the current-type inductor and the transformer's coil are reduced by synchronizing switching frequency of Buck-type converter is increased twice as the Push-Pull converter's switching frequency. The novel structure of DC/DC converter is able to realize ZVS-ZCS in fuel-cell system is proposed. The proposed switching component of Push-Pull converter has the ZVS and ZCS function by using the circuit of new passive clamp.