Abstract
Cascaded multilevel converters based on medium-frequency (MF) AC-links have been proposed as alternatives to the traditional low-voltage inverter, which uses a bulky low-frequency transformer step-up voltage to medium voltage (MV) levels. In this paper, a three-phase cascaded DC-AC-AC converter with AC-link for medium-voltage applications is proposed. Three stages integrate each DC-AC-AC converter (cell): a MF square voltage generator; a MF transformer with four windings; and an AC-AC converter. Then, k DC-AC-AC converters are cascaded to generate the multilevel topology. This converter’s topological structure avoids the per-phase imbalance; this simplifies the control and reduces the problem only to solve the per-cell unbalance. Two sets of simulations were performed to verify the converter’s operation (off-grid and grid-connected modes). Finally, the papers present two reduced preliminary laboratory prototypes, one validating the cascaded configuration and the other validating the three-phase configuration.
Highlights
At the end of 2019, the global renewable generation capacity was 2537 GW
This paper presents a multilevel medium-voltage power converter with MF-AC links that simplifies the grid integration of large-scale PV farms
There are three windings on the secondary side; each winding plus a suitable converter generates each of the three-phase output voltages. This topology has been selected because it has some exciting features: (1) It includes medium-frequency AC links (MF transformer)
Summary
At the end of 2019, the global renewable generation capacity was 2537 GW. Solar energy was the renewable energy with the most significant growth reaching a global capacity of 586 GW (23% of the global renewable generation capacity) [1] According to the global weighted-average levelized cost of energy (LCOE), the cost for electricity from utility-scale photovoltaics (PV) plants has fallen 82% in the last decade. This proposal overcomes some drawbacks related to grid-tie PV plants: use of line-frequency transformers and filters; avoiding using high-rated switches (>6.5 kW) or the use of series connection switches; elimination of the MF rectifying stage; and promoting modularity. This three-phase multilevel converter is built cascading some converters interconnected with each other via medium-frequency multiple winding transformers. There are three windings on the secondary side; each winding plus a suitable converter generates each of the three-phase output voltages This topology has been selected because it has some exciting features: (1) It includes medium-frequency AC links (MF transformer).
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