Reliability assessment of wind turbine operating concepts using reliability block diagrams (RBDs)

This paper represents an analysis of the Reliability, Availability, and Maintainability (RAM) analysis of major wind turbine generator (WTGs) operating concepts. Three operating concepts are considered based on the acceptable speed range of generators. Elaborated RAM analysis of each WTG system associated with each operating concept is presented starting from the subassemblies level to the subsystems level then the overall system. Based on their impact on the energy production, the subassemblies are classified to primary and secondary elements. A failure in a primary element causes interruption of the energy production while the energy production efficiency is degraded due to failures in secondary components. Mathematically, various subassemblies are given weights according to their classifications. In addition, an improved Reliability Block Diagram (RBD) is presented for estimating the RAM performance of various WTG configurations, and systems. The required input data are obtained from worldwide databases of failures, and repair of various subassemblies comprising various operating concepts. Fixed speed squirrel cage induction generators (FS-SCIG) based systems are no longer manufactured in utility scale applications due to their low aerodynamic efficiency. However, this configuration is proposed in this paper for assessing the historical low-efficiency designs and compares with the current designs that have higher efficiencies from the RAM points of view. It is found that the doubly fed induction generator with a partially rated converter (DFIG-PRC) has the highest reliability and lowest maintainability but it has a medium availability. Whereas the direct drive electrically excited synchronous generator with a fully rated converter (DDSG) has the lowest reliability, lowest availability and intermediate maintainability.