A GENERALIZED AND FLEXIBLE CONTROL SCHEME FOR PHOTOVOLTAIC GRID-TIE MICROINVERTERS

Abstract

In this project, 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 dsPIC microcontroller. As compared to 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.

Existing System

A flyback PV microinverter operated by pulse-density-modulation and employing zero current switching technique is used.

Proposed System

In this work, various control schemes of a flyback PV microinverter have been designed and implemented based on the DDS technique, for both the standalone and the grid-connected operation. The DDS technique provides flexibility in the implementation of the control schemes such as maximum power point tracking (MPPT), phase-locked-loop (PLL), anti-islanding, and low-voltage ride-though (LVRT), by means of an integrated software run on a simple, low-cost dsPIC microcontroller. Much higher resolution is obtained in the output voltage phase angle and magnitude as compared to the conventional LUT method, owing to the adaptive nature of the look-up table (LUT) implemented within the DDS architecture, Both computer simulations and experimental work have been carried out on a 120 W PV flyback microinverter to verify the effectiveness of the proposed technique. The basic operation principles of the DDS technique, and its application to the microinverter Control are presented. The implementation of the DDS technique via a dsPIC microcontroller for the flyback PV microinverter control is given.

Architecture

Proposed control scheme of the PV microinverter based on DDS