• 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.

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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    • 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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Compensation Strategy to Eliminate the Effect of Current Measurement Offsets in Grid-Connected Inverters

  • Lee, Chang-Hee;Choi, Jong-Woo
    • Journal of Power Electronics
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    • v.14 no.2
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    • pp.383-391
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    • 2014
  • For the digital control of systems such as grid-connected inverters, measuring inverter output currents accurately is essential. However, current measurement offsets are inevitably generated by current measurement paths and cause DC current components in real inverter output currents. Real inverter output currents with DC components cause the DC-link capacitor voltage to oscillate at the frequency of a utility voltage. For these reasons, current measurement offsets deteriorate the overall system performance. A compensation strategy to eliminate the effect of current measurement offsets in grid-connected inverters is proposed in this study. The validity of the proposed compensation strategy is verified through simulations and experiments. Results show that the proposed compensation strategy improves the performance of grid-connected inverters.

Loss Calculation Method of Grid-Connected Photovoltaic System (계통연계형 태양광발전시스템의 손실 산출방법)

  • So, Jung-Hun;Lim, Hyun-Mook;Wang, Hye-Mi;Jung, Young-Seok;Ko, Suk-Whan;Ju, Young-Chul
    • Journal of the Korean Solar Energy Society
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    • v.33 no.5
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    • pp.18-23
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    • 2013
  • This paper presents a simple but valid loss calculation method of grid-connected photovoltaic system based on normalized yield model. The proposed method can be represented as a quantitative value for five losses and performance of grid-connected photovoltaic system with three years monitored data. These results will indicate that it is useful to investigate various loss factors causing the performance obstruction, enhance the lifetime yield for changing meteorological conditions, and determine the optimal design and performance improvement of grid-connected photovoltaic system.

A Seamless and Autonomous Mode Transfer Method of Grid-connected Inverter in Microgrid (마이크로그리드에서 계통연계 인버터의 자율적이며 끊김없는 모드전환 기법)

  • Park, Sung-Youl;Kwon, Min-Ho;Choi, Se-Wan
    • The Transactions of the Korean Institute of Power Electronics
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    • v.24 no.5
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    • pp.349-355
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    • 2019
  • A grid-connected inverter with critical loads should be able to supply a stable voltage to critical loads at mode change and during clearing time while detecting unintentional islanding. This study proposes a mode transfer method for a grid-connected inverter with critical loads. The proposed method, which integrates the grid-connected and islanded mode control loops into one control block, provides an autonomous and seamless mode transfer from the current control to the voltage control. Therefore, the proposed scheme can supply a stable voltage to critical loads at mode change and during clearing time. Experimental results are provided to validate the proposed method.

Assessment on Power Quality of Grid-Connected PV System Based on Incremental Conductance MPPT Control (증분컨덕턴스 MPPT제어 기반 계통연계형 태양광발전시스템의 전력품질 평가)

  • Seol, Jae-Woong;Jang, Jae-Jung;Kim, Dong-Min;Lee, Seung-Hyuk;Kim, Jin-O
    • The Transactions of The Korean Institute of Electrical Engineers
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    • v.56 no.1
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    • pp.8-13
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    • 2007
  • During the last years, there has been an increased interest in the new energy such as photovoltaic(PV) system from the viewpoint of environmental pollution. In this regard, this paper estimates the power quality of grid-connected PV system. As the maximum power operating point(MPOP) of photovoltaic(PV) power systems alters with changing atmospheric conditions, the efficiency of maximum power point tracking(MPPT) is important in PV power systems. Moreover, grid-connected PV system occurs some problems such as voltage inequality and harmonics. Therefore, this paper presents the results of a grid-connected PV system modeling that contains incremental conductance MPPT controller by PSCAD/EMTDC simulator and investigates the influence that can occur in the grid-connected PV system from aspect of power quality, i.e. voltage drop, total harmonic distortion(TDD) and total demand distortion(TDD). For the case study, the measured data of the PV way in Cheongwadae, Seoul, Korea is used.

Dual-Loop Power Control for Single-Phase Grid-Connected Converters with LCL Filter

  • Peng, Shuangjian;Luo, An;Chen, Yandong;Lv, Zhipeng
    • Journal of Power Electronics
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    • v.11 no.4
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    • pp.456-463
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    • 2011
  • Grid-connected converters have widely adopted LCL filters to acquire high harmonic suppression. However, the LCL filter increases the system order so that the design of the system stability would be complicated. Recently, sole-loop control strategies have been used for grid-connected converters with L or LC filters. But if the sole-loop control is directly transplanted to grid-connected converters with LCL filters, the systems may be unstable. This paper presents a novel dual-loop power control strategy composed of a power outer loop and a current inner loop. The outer loop regulates the grid-connected power. The inner loop improves the system stability margin and suppresses the resonant peak caused by the LCL filter. To obtain the control variables, a single-phase current detection is proposed based on PQ theory. The system transfer function is derived in detail and the influence of control gains on the system stability is analyzed with the root locus. Simulation and experimental results demonstrate the feasibility of the proposed control.

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.

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.