State feedback control and variable step size MPPT algorithm of three-level grid-connected photovoltaic inverter

• We apply feedback linearization control on grid-connected photovoltaic system.
• We use multilevel inverter rather than two level inverter.
• THD of grid current is considerably reduced.
• Developed method allows control of both power factor and dc link voltage.

In this paper, the state feedback linearization control technique is applied for controlling the power factor of a grid-connected multi-level photovoltaic inverter. By applying this technique, the nonlinear state model of three-level inverter – in the d–q reference frame – will be transformed into two equivalent linear subsystems. Hence, the pole placement linear control loops can be applied on these linear subsystems in order to separately control the grid power factor and the dc link voltage of the inverter. The control system includes also a Maximum Power Point Tracker (MPPT) based on variable step size incremental conductance algorithm. Compared with conventional fixed step size method, the variable step MPPT improves the speed and the tracking accuracy. It has been shown that the three-level inverter allows reduction of the THD of grid voltage and current as well as the reduction of the blocking voltage of the inverter switches.

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