Enhanced Efficiency of DC-AC Grid-Tied Converters for Photovoltaic Systems using AC-Link Integration

Abstract

Abstract: As of the close of 2019, the global renewable energy generation capacity reached 2,537 GW, with solar energy showing the most substantial increase at 586 GW, which constitutes 23% of this total capacity. Over the past ten years, the levelized cost of electricity (LCOE) for utility-scale photovoltaic (PV) systems has dropped by 82%, while crystalline solar PV module prices in Europe have decreased by 90%. To overcome the limitations of conventional low-voltage inverters that depend on large, low-frequency transformers for voltage step-up, this paper introduces a novel three-phase cascaded DC-AC-AC converter featuring AC-link integration, specifically optimized for improved efficiency in medium-voltage applications. The proposed architecture includes three stages for each DC-AC-AC converter cell: a medium-frequency (MF) square voltage generator, an MF transformer with four windings, and an AC-AC converter. By cascading these DC-AC-AC converters, a multilevel structure is formed, which effectively addresses per-phase imbalance while simplifying control to focus on per-cell unbalance. The converter's operational efficiency is demonstrated through two sets of simulations conducted in both off-grid and grid-connected modes. Additionally, two initial laboratory prototypes are introduced: one validates the cascaded configuration, and the other confirms the three-phase integration, highlighting significant efficiency improvements for grid-tied PV systems.

  Index Terms: DC-AC-AC converter, photovoltaic systems, AC-link integration, multilevel topology, Grid-tied, renewable energy.