System-level reliability assessment for a direct-drive PMSG based wind turbine with multiple converters

System-level reliability of wind power converter has an essential effect on the operation performance and lifespan of a wind turbine system. In this paper, a wind turbine equipped with a 2 MW direct-drive permanent-magnet synchronous generator (PMSG) serves as a case study. Considering the maximum stator current limitation of the PMSG, several multiple-converter structures and their reliability block diagrams (RBDs) are constructed for the machine side converter (MSC). To investigate the reliability influence caused by the amount of semiconductor components and the current for each component, the structures with four and five bridges in parallel are both configured. Reliability evaluation between two major parallel structures, namely, bridges in parallel and converters in parallel are also compared. In addition, the effect of different wind classes on the MSC system-level reliability has been investigated. A detailed discussion regarding the system reliability cumulative distribution function (CDF) is presented, which could serve as reference for future MSC structure design. It is concluded that the component current dominates in the system-level reliability analysis of the MSC and the standby structure can also improve the reliability under the same current level. Besides, compared with the converter structure changing, various wind classes have a minor impact on the system-level reliability.