Abstract ：

The trial and error method is the conventional method for LCL parameter design. Its efficiency is low and it is difficult to obtain the optimal parameter. Furthermore, due to the lack of detailed analysis of the high-frequency harmonics in the inverter output voltage, the designed LCL filter cannot effectively filter them, resulting that the quality of the grid-connected current cannot fully meet IEEE 519 standard. Based on the above-mentioned problems, the high-frequency harmonics of inverter output voltage is analyzed in detail in this article, and the multiobjective optimization function, including five optimization objectives, is constructed. Combined with the particle swarm optimization, a screening method is proposed to realize the multiobjective optimization process. Under the proposed method, the LCL parameter, which happens to meet the requirement of IEEE 519 for the high-frequency harmonic distortion rate of the grid-connected current, can be obtained, and the greatest tradeoff of LCL filtering performance and filter cost is achieved. In addition, the proposed method considers the problem of weak grid and provides a complete engineering solution for LCL parameter design in different degrees of weak grid. Finally, the effectiveness and accuracy of the proposed design method are verified by contrast simulation and experiment.

Keyword ：

Double Fourier series LCL filter multiobjective optimization particle swarm optimization (PSO) phase disposition pulsewidth modulation (PD-PWM) screening method three-level grid-connected inverter weak grid

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Abstract ：

To address the power sharing accuracy problem caused by mismatched line impedances in a real distributed microgrid with dispersed sources and loads, a decentralized solution based on the injection of an extra small AC signal into the output voltage of each inverter is proposed in this paper. Without the aid of communication links, appropriate virtual impedances could be obtained to compensate the mismatched line impedances in full frequency range, and hence the active, reactive, unbalanced and harmonic powers could be shared accurately among parallel inverters under various load conditions. Moreover, the proposed method also considers the impact of complex line impedances, and this makes it a promising strategy in practical microgrid applications. Finally, a comprehensive case is provided to validate its effectiveness. © 2020 IEEE.

Keyword ：

Electric inverters Microgrids Power electronics

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Abstract ：

To address the power sharing accuracy problem caused by mismatched line impedances in a real distributed microgrid with dispersed sources and loads, a decentralized solution based on the injection of an extra small AC signal into the output voltage of each inverter is proposed in this paper. Without the aid of communication links, appropriate virtual impedances could be obtained to compensate the mismatched line impedances in full frequency range, and hence the active, reactive, unbalanced and harmonic powers could be shared accurately among parallel inverters under various load conditions. Moreover, the proposed method also considers the impact of complex line impedances, and this makes it a promising strategy in practical microgrid applications. Finally, a comprehensive case is provided to validate its effectiveness.

Keyword ：

complex line impedance decentralized control microgrid power sharing small-signal injection

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Abstract ：

Under mismatched line impedance conditions, reactive power sharing performance of droop control tends to be barely satisfactory for the absence of sharing accuracy. Therefore, to realize accurate reactive power sharing of parallel-connected inverters in microgrids, a modified adaptive virtual impedance method is proposed in this paper. Through this method, appropriate virtual impedance of DG units could be obtained to compensate the mismatched line impedances, and extra information from PCC voltage, line impedance or other inverters is not needed. After the compensation, reactive power could be shared naturally and accurately with no impact on voltage regulation. Finally, the effectiveness of proposed method is verified by simulation and experimental results.

Keyword ：

adaptive virtual impedance communication-independent microgrid reactive power sharing

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Abstract ：

The dual active bridge (DAB) converters are widely used as the pivotal interface in the distributed generation and energy storage systems. In this paper, the lookup table method of the model predictive control (MPC) DAB converter is proposed based on the discrete-time model. The iterative relationship of the state variables is obtained. Part of the complicated predictive process, which hindered the discrete-time model based MPC to be widely applied to the DAB converter before, has been calculated off-line to form a lookup table. Therefore, the accurate predictive control and optimization algorithm can be realized on-line via the proposed lookup table method within one switching cycle. The experiment has also been performed to verify the theoretical analysis and demonstrate the effectiveness of the proposed method.

Keyword ：

discrete-time model dual active bridge converter lookup table method model predictive control

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Abstract ：

CLLLC resonant converter usually adopts traditional frequency modulation (FM) control. When the gain is less than 1, the voltage gain curve of CLLLC resonant converter has low sensitivity to frequency, which makes the frequency variation range too wide. In order to achieve the wider voltage output range, this paper proposes a hybrid modulation method adopting FM under voltage gain greater than 1 and phase shift modulation (PSM) under voltage gain less than 1. Meanwhile, this paper proposed extended description function modeling method for CLLLC resonant converter. Based on the large signal model, the voltage gain expression of phase-shift modulation can be derived, which is convenient to analyze the output characteristics of the converter. Based on the small-signal model, the frequency modulation compensator and phase-shift modulation compensator of hybrid modulation are designed respectively. Matlab simulation verifies the accuracy of modeling and the experiment verifies the effectiveness of the proposed method.

Keyword ：

CLLLC resonant converter compensator design hybrid modulation modeling

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Abstract ：

The dual active bridge (DAB) converters are widely used as the pivotal interface in the distributed generation and energy storage systems. In this paper, the lookup table method of the model predictive control (MPC) DAB converter is proposed based on the discrete-time model. The iterative relationship of the state variables is obtained. Part of the complicated predictive process, which hindered the discrete-time model based MPC to be widely applied to the DAB converter before, has been calculated off-line to form a lookup table. Therefore, the accurate predictive control and optimization algorithm can be realized on-line via the proposed lookup table method within one switching cycle. The experiment has also been performed to verify the theoretical analysis and demonstrate the effectiveness of the proposed method. © 2019 IEEE.

Keyword ：

Discrete-time model; Dual active bridge converter; Lookup table method; Model predictive control

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Abstract ：

CLLLC resonant converter usually adopts traditional frequency modulation (FM) control. When the gain is less than 1, the voltage gain curve of CLLLC resonant converter has low sensitivity to frequency, which makes the frequency variation range too wide. In order to achieve the wider voltage output range, this paper proposes a hybrid modulation method adopting FM under voltage gain greater than 1 and phase shift modulation (PSM) under voltage gain less than 1. Meanwhile, this paper proposed extended description function modeling method for CLLLC resonant converter. Based on the large signal model, the voltage gain expression of phase-shift modulation can be derived, which is convenient to analyze the output characteristics of the converter. Based on the small-signal model, the frequency modulation compensator and phase-shift modulation compensator of hybrid modulation are designed respectively. Matlab simulation verifies the accuracy of modeling and the experiment verifies the effectiveness of the proposed method. © 2019 IEEE.

Keyword ：

CLLLC resonant converter; Compensator design.; Hybrid modulation; Modeling

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Abstract ：

Under mismatched line impedance conditions, reactive power sharing performance of droop control tends to be barely satisfactory for the absence of sharing accuracy. Therefore, to realize accurate reactive power sharing of parallel-connected inverters in microgrids, a modified adaptive virtual impedance method is proposed in this paper. Through this method, appropriate virtual impedance of DG units could be obtained to compensate the mismatched line impedances, and extra information from PCC voltage, line impedance or other inverters is not needed. After the compensation, reactive power could be shared naturally and accurately with no impact on voltage regulation. Finally, the effectiveness of proposed method is verified by simulation and experimental results. © 2019 IEEE.

Keyword ：

Adaptive virtual impedance; Communication-independent; Microgrid; Reactive power sharing

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Abstract ：

A network is composed of many network nodes, so the analysis and evaluation of the target node is of great importance, especially in complex networks. For this purpose, this paper established an analysis model of target nodes for a complex network to study the correlation between the two and explore the searching mechanism of target nodes in the complex network. Besides, the importance and reliability of the target nodes were simulated. The results showed that the analysis of the target nodes had a great effect on the research of the target nodes and played a positive role in the research of the whole complex network. © 2018, © 2018 Taru Publications.

Keyword ：

Analysis Analysis and evaluation Analysis models Importance Network node Node Searching mechanism Target nodes

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