System modeling and stability analysis is one of the most important issues of inverter-dominated microgrids. It is useful to determine the system stability and optimize the control parameters. The complete small signal models for the inverterdominated microgrids have been developed which are very accurate and could be found in literature. However, the modeling procedure will become very complex when the number of inverters in microgrid is large. One possible solution is to use the reduced-order small signal models for the inverterdominated microgrids. Unfortunately, the reduced-order small signal models fail to predict the system instabilities. In order to solve the problem, a new modeling approach for inverterdominated microgrids by using dynamic phasors is presented in this paper. Our findings indicate that the proposed dynamic phasor model is able to predict accurately the stability margins of the system, while the conventional reduced-order small signal model fails. In addition, the virtual ω-E frame power control method, which deals with the power coupling caused by the line impedance X/R characteristic, has also been chosen as an application example of the proposed modeling technique.Index Terms-microgrid, droop control, inverter, small signal mode, dynamic phasor, stability analysis, I.
Power oscillation and current quality are the important performance targets for the grid-connected inverter under unbalanced grid faults. Firstly, the inherent reason for the current harmonic and power oscillation of the inverter is discussed with a quantitative analysis. Secondly, a new control strategy is proposed to achieve the coordinate control of power and current quality without the need for a phase-locked loop (PLL) or voltage/current positive/negative sequence extraction calculation. Finally, the experimental tests are conducted under unbalanced grid faults, and the results verify the effectiveness of the propose method.
Eliminating the leakage current is one of the most important issues for transformerless three phase photovoltaic (PV) systems. In this paper, the leakage current elimination of a three-phase four-leg PV inverter is investigated. With the common mode loop model established, the generation mechanism of the leakage current is clearly identified. Different typical carrier-based modulation methods and their corresponding common mode voltages are discussed. A new modulation strategy with Boolean logic function is proposed to achieve the constant common mode voltage for the leakage current reduction. Finally the different modulation methods are implemented and tested on the TMS320F28335 DSP +XC3S400 FPGA digital control platform. The experimental results verify the effectiveness of the proposed solution.Index Terms-Transformerless photovoltaic system, three-phase four-leg inverter, pulse width modulation, leakage current
This paper proposes a current-fed LLC resonant converter that is able to achieve high efficiency over a wide input voltage range. It is derived by integrating a two-phase interleaved Boost circuit and a full-bridge LLC circuit together by virtue of sharing the same full-bridge switching unit. Compared with conventional full-bridge LLC converter, the gain characteristic is improved in terms of both gain range and optimal operation area, fixed-frequency PWM control is employed to achieve output voltage regulation, and the input current ripple is minimized as well. The voltage across the turned-off primary-side switch can be always clamped by the bus voltage, reducing the switch voltage stress. Besides, its other distinct features, such as single-stage configuration, and soft switching for all switches also contribute to high power conversion efficiency. The operation principles are presented, and then the main characteristics regarding gain, input current ripple, and zero voltage switching (ZVS) considering the nonlinear output capacitance of MOSFET, are investigated and compared with conventional solutions. Also, the design procedure for some key parameters is presented, and two kinds of interleaved Boost integrated resonant converter topologies are generalized. Finally, experimental results of a converter prototype with 120-240V input and 24V/25A output verify all considerations.Index Terms-Current-fed converter, LLC resonant converter, fixed-frequency PWM control, wide voltage gain range.
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