A Generalized and Flexible Control Scheme for Photovoltaic Grid-Tie Microinverters

In this paper, design and implementation of a flyback photovoltaic (PV) microinverter based on the direct digital synthesis (DDS) technique has been described for both the standalone and the grid-connected operation. The DDS technique adopted provides flexibility in the implementation of various control schemes of the PV microinverter on a simple low-cost digital signal processing type of microprocessor (dsPIC). As compared with the conventional look-up-table method used for generating sinusoidal output voltage waveforms by digital signal processing, a much higher resolution can be obtained in the voltage phase angle and magnitude owing to the adaptive nature of the look-up table implemented within the DDS architecture. The DDS technique is used in the implementation of all control schemes of a PV microinverter, such as maximum power point tracking (MPPT), phase-locked-loop (PLL), anti-islanding, and low-voltage ride-though (LVRT), with an integrated software run on a simple microcontroller. A dedicated computer simulation model is developed, where the PV panel model, the PLL in DQ reference frame, the MPPT algorithm, and the anti-islanding and LVRT features are all taken into account. The experimental results obtained on a 120-W PV flyback microinverter have verified the validity of the proposed technique for both the steady-state and the transient-state operation. The DDS technique is thus found to be quite convenient for application to module integrated converters.