Design of Transformerless Microinverter using a High Gain DC-DC Converter and PUC Inverter

Transformerless inverters offer several advantages such as system compactness, lower cost, and lower overall power loss. However, for PV applications, there is a need for a frontal high-gain step-up DC-DC converter to use the transformerless inverter as a grid-tied micro-inverter (μ-inverter). In this paper, a single switching device high gain boost DC-DC converter based on Quadratic Boost Converter (QBC) is used along a 7-level packed-U-cells (PUC) inverter to realize the transformerless μ-inverter. The MPPT uses Perturbs & Observes (P&O) algorithm to control the front-end QBC to extract the maximum power from the PV array. Finite-set model predictive control (FS-MPC) technique is used to control the grid-tied single-phase seven-level PUC inverter using a weighted cost function. In the ideal case, the PUC inverter should deliver the extracted PV power into the grid. This is accomplished automatically by keeping the voltage level of the dc-link capacitor constant. From a control point of view, this means that the front-end QBC and PUC inverter controls are decoupled as they are separated via the dc-link capacitor which must absorb the single-phase oscillating power. A simulation was carried out to validate the performance of the proposed controllers. A grid current THD less than 5% is achieved and a stable operation under various operating conditions is validated with the proposed system. Hardware implementation is under construction and experimental results will be shown in the final paper.