A Set of Multilevel Modular Medium-Voltage High Power Converters for 10-MW Wind Turbines

Medium-voltage power conversion is generally favored for future large wind turbines, e.g., 10 MW, in terms of higher power density, reduced current level, associated losses, and cost of power cables, switchgears, etc. This paper has summarized a fundamental rule to construct multilevel modular high power converters for large wind turbine power conversion. Based on that, three potential multilevel modular high power converter topologies are derived and compared. The topology with a 10-kV generator, a modular power converter, and a multi-winding step-up transformer have been specifically investigated. The large dc-link capacitor (not reliable, high cost, large volume) required in the converter is identified as the key limitation of the system used for wind power conversion. The paper proposes to compensate the ripple power by the grid-side inverter of the multilevel modular converter, thus reducing the dc-link capacitor requirement. The paper has validated the effectiveness of proportional-integral-resonant controller for this purpose. Further, the thermal impact of the proposed ripple power compensation scheme on the converter device junction temperature and the transformer secondary windings has been analytically derived. The paper has also analytically revealed that the proposed ripple power compensation scheme will not affect the grid-side power quality, although there are low-frequency harmonics in the transformer secondary windings. Simulation results with a 10-MW, 10-kV system have validated the proposed converter topology and control strategy with reduced dc-link voltage ripple.