Multi-modular cascaded DC-DC converter for HVDC grid connection of large-scale photovoltaic power systems

Large-scale grid-connected photovoltaic (PV) energy conversion systems operate at low voltage and are interfaced to medium-voltage and high-voltage ac utility grids through one or two step-up voltage transformer stages. In addition, the power conversion is performed with either a single stage dc-ac converter (central inverter) or a two stage dc-dc/dc-ac (string or multi-string inverter). However, prime solar irradiation regions in the world are not always located close to available utility lines, and in some cases are far away from main consumption areas. Furthermore, long overhead transmissions lines (>400km) and underwater transmission lines above 70kM, HVDC has become the most cost-effective solution. Among HVDC technologies, voltage source converter based HVDC system, mainly based on the modular multilevel converter (MMC), have become popular due to smaller filters, multi-network connection and decoupling of active and reactive power. This paper explores a new large-scale PV plant configuration based on a dc-dc stage interfaced directly to an MMC based HVDC system. Since PV systems are dc by nature, the proposed solution has several advantages, particularly if combined directly with the HVDC power station. Some power circuit topologies are presented, including their corresponding control schemes. Simulation results are presented to provide a preliminary evaluation on the operation and performance of the proposed system.