• Title/Summary/Keyword: Grid-connected

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Seamless Transfer Operation Between Grid-connected and Stand-Alone Mode in the Three-phase Inverter (3상 인버터의 계통연계 및 독립운전모드 전환 연구)

  • Lee, Wujong;Jo, Hyunsik;Lee, Hak Ju;Cha, Hanju
    • The Transactions of The Korean Institute of Electrical Engineers
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    • v.62 no.2
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    • pp.201-207
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    • 2013
  • This paper propose seamless transfer operation between grid-connected and stand-alone mode in the three-phase inverter for microgrid. The inverter operates grid-connected mode and stand-alone mode. Grid-connected mode is the inverter connected to grid and stand-alone mode is to deliver energy to the load from inverter at grid fault. When conversion from gird-connected to stand-alone mode, the inverter changes current control to voltage control. When grid restored, the inverter system is conversion from stand-alone to grid-connected mode. In this case, load phase and grid phase are different. Therefore, synchronization is essential. Thus Seamless transfer operation stand-alone to grid-connected mode. In this paper, propose sealmless transfer operation between grid-connceted and stand-alome mode, and this method is verified through simulation and experiment.

Stability Analysis of Grid-Connected Inverters with an LCL Filter Considering Grid Impedance

  • Li, Xiao-Qiang;Wu, Xiao-Jie;Geng, Yi-Wen;Zhang, Qi
    • Journal of Power Electronics
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    • v.13 no.5
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    • pp.896-908
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    • 2013
  • Under high grid impedance conditions, it is difficult to guarantee the stability of grid-connected inverters with an LCL filter designed based on ideal grid conditions. In this paper, the theoretical basis for output impedance calculation is introduced. Based on the small-signal model, the d-d channel closed-loop output impedance models adopting the converter-side current control method and the grid-side current control method are derived, respectively. Specifically, this paper shows how to simplify the stability analysis which is usually complemented based on the generalized Nyquist stability criterion (GNC). The stability of each current-controlled grid-connected system is analyzed via the proposed simplified method. Moreover, the influence of the LCL parameters on the stability margin of grid-connected inverter controlled with converter-side current is studied. It is shown that the stability of grid-connected systems is fully determined by the d-d channel output admittance of the grid-connected inverter and the inductive component of the grid impedance. Experimental results validate the proposed theoretical stability analysis.

Grid Voltage Estimation Scheme without Phase Delay in Voltage-sensorless Control of a Grid-connected Inverter (전압센서를 사용하지 않는 계통연계 인버터의 제어 및 위상지연을 개선한 계통전압 추정 기법)

  • Kim, Hyun-Sou;Kim, Kyeong-Hwa
    • 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.

An Output Voltage Balance Control of Grid Connected Inverter by Phase Current Control at Critical Load Unbalanced Condition (계통연계 인버터의 주요 부하 불평형 시 상전류 제어를 통한 부하 상전압 평형 제어)

  • Tae-Hyeon Park;Hag-Wone Kim;Kwan-Yuhl Cho;Joon-Ki Min;Won-Il Choi
    • The Transactions of the Korean Institute of Power Electronics
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    • v.28 no.1
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    • pp.22-29
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    • 2023
  • A grid-connected inverter can be used in grid-connected or stand-alone modes. Generally, a grid-connected inverter operates in a grid-connected mode, but the inverter operates in stand-alone mode if grid faults occur. In the stand-alone mode, the grid-connected inverter must supply electric power to a critical load that needs to receive stable power even though grid faults occur. Generally, three-phase loads are used as critical loads, but a single phase is configured in some cases. In these conditions, the critical load is required to unbalance the load power consumption, which makes the three-phase load voltage unbalancd. This unbalanced voltage problem can cause fatal problems to the three-phase critical loads, and thus must be addressed. Hence, this paper proposes an algorithm to solve this unbalanced voltage problem by the individual phase current control. The proposed method is verified using Psim simulation and experiments.

A Review on the Grid-Connected Technology of the Distributed Energy Resources (분산형전원의 계통연계기술 리뷰)

  • Lee, Kyungsoo
    • Proceedings of the KIPE Conference
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    • 2014.07a
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    • pp.371-372
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    • 2014
  • This review focuses on grid-connected technology for distributed energy resources(DER). The grid-connected technology is categorized into three classifications: 1) protection function; 2) power quality improvement function; 3) grid stabilization fuction. Grid codes comparison of Japan, USA, EU and Korea is also described in the paper.

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PLL Control Strategy for ZVRT(Zero Voltage Ride Through) of a Grid-connected Single-phase Inverter (계통연계형 단상 인버터의 ZVRT(Zero Voltage Ride Through)를 위한 PLL 제어 전략)

  • Lee, Tae-Il;Lee, Kyung-Soo
    • The Transactions of the Korean Institute of Power Electronics
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    • v.24 no.3
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    • pp.169-180
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    • 2019
  • Grid codes for grid-connected inverters are essential considerations for bulk grid systems. In particular, a low-voltage ride-through (LVRT) function, which can contribute to the grid system's stabilization with the occurrence of voltage sag, is required by such inverters. However, when the grid voltage is under zero-voltage condition due to a grid accident, a zero-voltage ride-through (ZVRT) function is required. Grid-connected inverters typically have phase-locked loop (PLL) control to synchronize the phase of the grid voltage with that of the inverter output. In this study, the LVRT regulations of Germany, the United States, and Japan are analyzed. Then, three major PLL methods of grid-connected single-phase inverters, namely, notch filter-PLL, dq-PLL using an active power filter, and second-order generalized integrator-PLL, are reviewed. The proposed PLL method, which controls inverter output under ZVRT condition, is suggested. The proposed PLL operates better than the three major PLL methods under ZVRT condition in the simulation and experimental tests.

Control of Grid-Connected Inverters Using Adaptive Repetitive and Proportional Resonant Schemes

  • Abusara, Mohammad A.;Sharkh, Suleiman M.;Zanchetta, Pericle
    • Journal of Power Electronics
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    • v.15 no.2
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    • pp.518-529
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    • 2015
  • Repetitive and proportional-resonant controllers can effectively reject grid harmonics in grid-connected inverters because of their high gains at the fundamental frequency and the corresponding harmonics. However, the performances of these controllers can seriously deteriorate if the grid frequency deviates from its nominal value. Non-ideal proportional-resonant controllers provide better immunity to variations in grid frequency by widening resonant peaks at the expense of reducing the gains of the peaks, which reduces the effectiveness of the controller. This paper proposes a repetitive control scheme for grid-connected inverters that can track changes in grid frequencies and keep resonant peaks lined up with grid frequency harmonics. The proposed controller is implemented using a digital signal processor. Simulation and practical results are presented to demonstrate the controller capabilities. Results show that the performance of the proposed controller is superior to that of a proportional-resonant controller.

Investigation of Instability in Multiple Grid-Connected Inverters with LCL Output Filters

  • Asghari, Fariba;Safavizadeh, Arash;Karshenas, Hamid Reza
    • Journal of Power Electronics
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    • v.18 no.3
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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.

High-Efficiency Power Conditioning System for Grid-Connected Photovoltaic Modules

  • Choi, Woo-Young;Choi, Jae-Yeon
    • Journal of Power Electronics
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    • v.11 no.4
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    • pp.561-567
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    • 2011
  • This paper presents a high-efficiency power conditioning system (PCS) for grid-connected photovoltaic (PV) modules. The proposed PCS consists of a step-up DC-DC converter and a single-phase DC-AC inverter for the grid-connected PV modules. A soft-switching step-up DC-DC converter is proposed to generate a high DC-link voltage from the low PV module voltage with a high-efficiency. A DC-link voltage controller is presented for constant DC-link voltage regulation. A half-bridge inverter is used for the single-phase DC-AC inverter for grid connection. A grid current controller is suggested to supply PV electrical power to the power grid with a unity power factor. Experimental results are obtained from a 180 W grid-connected PV module system using the proposed PCS. The proposed PCS achieves a high power efficiency of 93.0 % with an unity power factor for a 60 Hz / 120 Vrms AC power grid.

Seamless Mode Transfer of Indirect Current Controlled Parallel Grid-Connected Inverters (간접전류제어방식 병렬형 계통연계 인버터의 무순단 모드절환)

  • Song, Injong;Choi, Junsoo;Lim, Kyungbae;Choi, Jaeho
    • The Transactions of the Korean Institute of Power Electronics
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    • v.24 no.5
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    • pp.334-341
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    • 2019
  • This study proposes the control strategy for the seamless mode transfer of indirect current controlled parallel grid-connected inverters. Under the abnormal grid condition, the grid-connected inverter can convert the operation mode from grid-connected to stand-alone mode to supply power to the local load. For a seamless mode transfer, the time delay problems caused by the accumulated control variable error must be solved, and the indirect current control method has been applied as one of the solutions. In this study, the design of control parameters for the proportional-resonant-based triple-loop indirect current controller and the control strategy for the seamless mode transfer of parallel grid-connected inverters are described and analyzed. The validity of the proposed mode transfer method is verified by the PSiM simulation results.