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This study presents a control strategy for managing power delivered to, or absorbed from, the grid, independent of local load or grid characteristics. To achieve this objective, the strategy requires the use of three levels of control structures. The first is related to phase-locked loop and grid-tie algorithms, which are employed to minimise disturbances during the grid connection. The second, which produces voltages with reduced harmonics levels at the distributed generation (DG) terminals, comprises the cascade voltage and current control. The third structure is the active and reactive power control mechanisms, which uses the angle of displacement and the adjusting of the DG voltage amplitude to keep the power flow through the grid at a specific set-point, independent of local load characteristics. In addition, the power control structures have to operate in a decoupled operation mode, whereby the active power control has to be faster than the reactive power control, or vice versa. In terms of their physical structure, a passive LCL filter is used to connect the voltage source inverter to the grid. To verify the findings and observations discussed in this study, a set of simulations and experimental results are presented.

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Abstract-This paper presents a design procedure and a control strategy for a grid-connected single-phase multilevel converter. The proposed system uses two series connected Hbridge modules, one fed by photovoltaic panels, the other by energy storage devices. The former switches at line frequency, while the latter operates in PWM. The system is designed to minimize the voltage stress on the switches, while the control strategy is such that a constant active power is delivered to the grid with high power factor, even if the energy produced by the photovoltaic panels is variable. The paper illustrates the power converter design procedure, the hybrid PWM method and the small signal modeling used to design the controllers of each inverter. In addition, the performance of the multilevel converter is verified by means of simulation and experimental results, which show the system ability to operate as expected.

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