Control Design of Series-Connected PV-Powered Grid-Forming Converters via Singular Perturbation

Modular architectures that consist of several series-connected dc-ac converters have been a focal point of recent innovations in transformerless medium-voltage applications. In this article, we consider an architecture consisting of dc-ac modules containing a quadruple active bridge dc-dc converter, which generates three floating dc links that feed grid-side dc-ac inverters. Practical implementation of such a converter module in photovoltaic systems requires a variety of controllers that collectively achieve maximum power point tracking, dc-link regulation, and ac-side power control. Design of such multiloop systems is generally quite challenging due to the potential for destabilizing interactions among loops. Here, we propose a design approach where singular perturbation theory is used to decompose the timescales at which each control loop operates and provides a systematic framework for parametric selection. Our approach also ensures system stability of multiple modules with identical controls connected in series across a grid. This article concludes with experimental results of three 1000-W series-connected converter modules across a stiff grid.