Large-scale grid-connected photovoltaic energy generation systems are progressing remarkably benefiting from the latest developments in solid-state semiconductors technology. In such systems, the photovoltaic arrays can be connected directly to the medium-voltage grid without employing a bulky line-frequency transformer to step up the voltage. Nano-crystalline cores with a small size and a high permeability operating at medium or high frequency can be installed in the power conversion stage. Hence, the necessary isolation as well as voltage boosting features can be provided. However, only a few power converters allow this type of isolation. This paper proposes a new modular converter structure suitable for medium-voltage grid connected systems with high-frequency isolation. The output voltages of the series-connected modules are added in order to provide the necessary voltage boosting. Four different power converter topologies with small input capacitors can be used as submodules for the presented medium-voltage configuration having different advantages and drawbacks. These different topologies are analysed in terms of power losses, footprint and functionality. To validate the mathematical analysis and the computer simulations, a scaled-down 5 kVA three-phase, 1 kV prototype is built and tested with four modules for each phase.