Abstract:6.1 Introduction6.2 Early geared wind turbine drive systems6.3 Direct drive generators6.4 Doubly fed induction generators (DFIGs)6.5 Low- and medium-speed (MS) geared hybrid concept6.6 Permanent magnet generators (PMGs) in direct drive wind turbines6.7 Alternative technologies and power conversion6.8 Reliability, availability and total systems efficiency6.8.1 Availability