An accurate active islanding detection method for grid-tied inverters in distributed generation using phase disturbance based on grid synchronisation is presented in this study. To have good performance on both power quality and islanding detection, a third-harmonic injection is generated by a new kind of phase disturbance and the scope of phase disturbance coefficient is precisely defined, besides, for extracting the third-harmonic component at the point of common coupling accurately, a selective harmonic extraction based on multiple second-order generalised integrators is also provided. Performances of the proposed method have been studied and confirmed through extensive simulations using MATLAB/Simulink platform. * phase reference cos θ inv * unitised current reference of the inverter I inv(3) third harmonic in the grid-tied current Z g grid impedance Z L local load impedance Z PCC impedances at the PCC

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This study presents a four-phase interleaved high voltage conversion ratio bidirectional DC-DC converter circuit based on coupled inductors and switched capacitors, which can eliminate the defects of conventional high voltage conversion ratio bidirectional DC-DC converters in terms of high-voltage/current stress, less efficiency and low-power limitation. Parallel channels are used to reduce current stress at the low-voltage side and series connected switched capacitors are used to enlarge voltage conversion ratio, reduce voltage stress and achieve auto current sharing. This paper proposes the operation principle, feature analysis and optimization design considerations. On this basis the objectives of high voltage conversion ratio, low voltage/current stress, high power density, high efficiency and high-power applications can be achieved. Some experimental results based on a 500 W prototype converter (24 V to 48 V at low-voltage side, 400 V at high-voltage side) are given to verify the theoretical analysis and the effectiveness of the proposed converter.

DC injection is a critical issue in transformerless grid-connected inverters. DC injection control based on virtual capacitor has the advantages of low cost, low loss, high accuracy and easy implementation. In this paper, the principle of DC injection control based on virtual capacitor was analyzed. In addition, the applicable conditions, working process, steady state error and advantages were also discussed in detail. The design of the control parameters based on virtual capacitor was proposed in a grid-connected inverter with LCL filter. The robustness of the control parameters was also discussed. Simulation and experimental results verify the validity of the analysis and demonstrate that this research has a certain value in engineering applications.

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