Fault-Tolerant Control for a Modular Generator-Converter Scheme for Direct-Drive Wind Turbines

A modular power electronic converter, with a cascaded H-bridge multilevel inverter, has been proposed as the grid interface for a large direct drive wind turbine generator. The present study is to investigate the potential and requirement of fault tolerant operation in such a system. As each module is connected to isolated generator coils, tolerance to a module or coil fault is possible if the affected module can be bypassed while the control of the healthy modules can be adjusted accordingly. A boost rectifier scheme on the machine side of the power conversion stage has been developed for this purpose, featuring control of the coil current without measuring the EMF. A coordinated DC-link voltage controller has been developed which can rapidly raise the DC-link voltage of the remaining modules to compensate for the loss of a module, while ignoring the ripple in the DC-link due to low-frequency inverter switching. The ability of the resultant system to tolerate module faults has been demonstrated on a small scale laboratory prototype.