Design of a Fault-Tolerant Tandem Converter for a Multi-MW Superconducting Offshore Wind Turbine Generator

A fault-tolerant tandem converter, based on a current-source converter and cascaded multilevel voltage-source active filter, is proposed as the grid interface for a 10MW superconducting direct-drive wind turbine generator. The converter reduces the possibility of a short-circuit fault, an issue with the extremely low generator reactance, while having a significantly lower DC capacitance requirement than an equivalent modular multilevel converter, although this is still high due to an extremely low generator frequency. Active filter DC capacitance is optimised, and a control system designed to prevent large DC voltage peaks during transients, and is verified in simulation. DC voltage balancing between the phases of the filter limits the torque control bandwidth of the converter if the capacitance is to be minimised, due to the slow speed of the balancing controller.