| dc.description.abstract | In medium voltage (MV) ultra-high-power (over 10 MW) offshore wind energy
conversion systems (WECS), current source converter (CSC)-based series-connected
configurations are a good candidate. However, existing CSC-based WECS use the
bulky low-frequency transformer (LFT), have high rotor torque ripple or require the
active rectifier, which entails higher costs, lower reliability and efficiency, and larger
size and weight than the diode rectifier. To address these issues, this thesis proposes a
novel CSC-based series-connected MV WECS, combining the six-phase generator
connected to two diode rectifiers with two medium frequency transformers (MFTs)-
based modular converters on the generator side and using the dual-bridge current source
inverters (CSIs) on the grid side. The proposed WECS effectively mitigates rotor torque
ripple caused by diode rectifiers and retains all the advantages of existing MFTs-based
WECS. Additionally, the modular converter in the proposed WECS has voltage and
current imbalance issues, so a corresponding control scheme is proposed to address
these issues. Furthermore, online selective harmonic elimination (SHE) modulation is
applied to the grid-side CSIs. This approach significantly reduces the memory
requirements of the digital controller, thereby lowering costs while retaining all the
advantages of conventional SHE techniques. The feasibility and effectiveness of the
proposed WECS, its control scheme, and the online SHE modulation are validated
through simulation results. | en_US |
