Hybrid full bridge-half bridge MML power converter for HVDC diode rectifier connection of large off-shore wind farms

The connection of large off-shore wind farms using diode rectifier (DR) units presents important advantages due to the simplicity of the diode rectifier converter, its robustness and weight and loss reduction. Moreover, series connected diode rectifier units allow for increased reliability as the system is capable of reduced power operation in the case of the failure of one unit. However, a DR unit outage requires a reduced DC voltage, consequently, the use of full bridge MML power converters at the on-shore station. Such converters allow for reduced HVDC-link voltage operation when one diode rectifier unit is faulty and also help to improve transient response during faults. However, the full bridge MML is more complex than its half bridge counterpart and has higher losses. In this paper a study has been carried out to ascertain the advantages and disadvantages of using a mixed full-bridge half-bridge MML power converter for the diode rectifier connection of off-shore wind farms, highlighting the operational limits of each type of MML converter under a reduced DC voltage.