• 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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• 제15권3호
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• pp.806-818
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• 2015

Based on the inherent relationship between dc-bus voltage and grid feeding active power, two dc-bus voltage regulators with different references are adopted for a grid-connected PV inverter operating in both normal grid voltage mode and low grid voltage mode. In the proposed scheme, an additional dc-bus voltage regulator paralleled with maximum power point tracking controller is used to guarantee the reliability of the low voltage ride-through (LVRT) of the inverter. Unlike conventional LVRT strategies, the proposed strategy does not require detecting grid voltage sag fault in terms of realizing LVRT. Moreover, the developed method does not have switching operations. The proposed technique can also enhance the stability of a power system in case of varying environmental conditions during a low grid voltage period. The operation principle of the presented LVRT control strategy is presented in detail, together with the design guidelines for the key parameters. Finally, a 3 kW prototype is built to validate the feasibility of the proposed LVRT strategy.

• 전력전자학회논문지
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• 제23권4호
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• pp.294-297
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• 2018

This paper analyzes the losses of the switching device for a full bridge inverter connected to the grid. As the development of power conversion system, losses are dominant factors in judging the efficiency of a system. The losses of a switching device can be divided into switching loss and conduction loss, both of which can be estimated by analyzing periodic switching waveform. The switching loss is generated when the switch is turned on and off, while the conduction loss is generated when the switch is turned on. The estimated losses of the MOSFET switch are compared with the simulation results.

• 대한전기학회논문지:전기기기및에너지변환시스템부문B
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• 제53권11호
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• pp.671-679
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• 2004

This paper presents the on-line current controller tuning method of grid-connected inverter using PSO(particle swarm optimization) technique for minimizing the harmonic current. Synchronous frame PI current regulator is commonly used in most distributed generation. However, due to the source voltage distortion, specially in weak AC power system, current may contain large harmonic components, which increase THD(total harmonic distortion) and deteriorates power quality. Therefore, some tuning method is necessary to improve response of current controller. This paper used the PSO technique to tune the current regulator and through simulation and experiments, usefulness of the tuning method has been verified. Especially in simulating the tuning process, ASM(average switching model) of inverter is used to shorten execution time.

• Journal of Power Electronics
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• 제18권3호
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• pp.757-765
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• 2018

This paper deals with the instability and resonant phenomena in distribution systems with multiple grid-connected inverters with an LCL output filter. The penetration of roof-top and other types of small photovoltaic (PV) grid-connected systems is rapidly increasing in distribution grids due to the attractive incentives set forth by different governments. When the number of such grid-connected inverters increases, their interaction with the distribution grid may cause undesirable effects such as instability and resonance. In this paper, a grid system with several grid-connected inverters is studied. Since proportional-resonant (PR) controllers are becoming more popular, it is assumed that most inverters use this type of controller. An LCL filter is also considered at the inverters output to make the case as realistic as possible. A complete modeling of this system is presented. Consequently, it is shown that such a system is prone to instability due to the interactions of the inverter controllers. A modification of PR controllers is presented where the output capacitor is virtually decreased. As a result, the instability is avoided. Simulation results are presented and show a good agreement with the theoretical studies. Experimental results obtained on a laboratory setup show the validity of the analysis.

• 한국전자통신학회논문지
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• 제16권2호
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• pp.261-270
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• 2021

본 연구에서는 계통 연계 3상 인버터를 위한 LCL 필터를 설계 방법론을 연구하였다. 먼저, 대표적인 필터 구조들에 대해서 3상 인버터의 계통 연계를 위한 필터로서의 적용 시 장단점을 분석하였다. 다음, 3상 인버터의 계통 연계를 위한 필터 설계 방법론들을 분석하여 그리드 연계 시 고조파 요구 조건을 만족할 수 있는 효과적인 필터 설계 방법론을 결정하였다. 그리고 설계 방법론의 유효성을 확인하기 위해서 EMTP-RV를 이용하여 3상 인버터, LCL 필터 그리고 필터의 성능을 평가하기 위한 성능 평가 시스템을 모델링 하였다. 끝으로, 하나의 3상 인버터에 대한 LCL 필터를 설계하였으며, EMTP-RV 시뮬레이션 작업을 통해 3상 인버터의 출력 전류 리플의 크기가 제한됨을 확인함으로서 설계 방법론의 유효성을 검증할 수 있었다.

• 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.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2013년도 전력전자학술대회 논문집
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• pp.475-476
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• 2013

In single-phase flyback inverter for grid-connected PV-AC module, power ripple is occurred as two times grid frequency on input capacitor. So, decoupling method has attracted interest recently. Also, power generation of PV depending on irradiation is limited at particular time, so use of energy storage system can increase energy efficiency. In this paper, flyback inverter for grid-connected PV-AC module which can operate decoupling and energy storage functions is proposed and verified by PSIM simulation.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2018년도 추계학술대회
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• pp.155-156
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• 2018

This paper presents a control method for grid-connected Active-NPC inverter systems. NPC (Neutral Point Clamped) is widely used in power conversion systems. NPC has a loss of switching elements and voltage imbalance. Active NPC has been proposed to overcome these drawbacks. ANPC changed the neutral diode to IGBT to reduce the switching loss. This paper modeled a grid-connected Acitve-NPC inverter systems and analyzed its performance. DSOGI PLL was used as a phase control method for precise control of grid link voltage. The proposed method is verified by PSIM simulation.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2017년도 전력전자학술대회
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• pp.266-267
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• 2017

To synchronize the distributed generation (DG) unit with the grid, the voltage sensors are generally employed to obtain the grid phase angle. This paper presents an observer-based voltage sensorless control scheme for a three-phase inverter connected to the grid through an LCL filter. The proposed control scheme consists of an augmented state observer and a feedback controller. The augmented state observer is used to estimate the grid voltages and states of the inverter system, which are then employed to determine the grid voltage angle and to construct the feedback controller. As a result of using the observer, only the grid current sensors are required to accomplish the control scheme. The simulation results are given to prove the validity of the proposed control scheme.