1 Abstract— Condition monitoring and fault diagnosis are currently considered crucial means to increase the reliability and availability of wind turbines, and consequently, to reduce the wind energy cost. With similar goals, direct drive wind turbines based on permanent magnet synchronous generators (PMSGs) with full-scale power converters are an emerging and promising technology. Numerous studies show that power converters are a significant contributor to the overall failure rate of modern wind turbines. In this context, open-circuit fault diagnosis in the two power converters of a PMSG drive for wind turbine applications is addressed in this paper. A diagnostic method is proposed for each power converter, allowing real-time detection and localization of multiple open-circuit faults. The proposed methods are suitable for integration into the drive controller and to trigger remedial actions. In order to prove the reliability and effectiveness of the proposed fault diagnostic methods, several simulation and experimental results are presented. Index Terms— Condition monitoring, fault diagnosis, multiple power switch open-circuit faults, permanent magnet machines, semiconductor device reliability, wind power generation I. INTRODUCTION ost reduction of the generated wind energy is essential for the wind power penetration to keep increasing quickly, through more efficient, reliable and cost-effective wind turbines. Hence, the wind turbines based on PMSGs drives are a promising technology [1], avoiding the use of gearboxes. Furthermore, along with the PMSG, a full-scale power converter is required, which allows variable speed operation and the fulfillment of rigorous grid codes. Several issues related to wind turbines such as power quality, systems stability [2] and ride-through capability during grid faults [3] have been considered over the last few years. More recently, reliability and availability of wind Manuscript received October 10, 2011. Accepted for publication June 18, 2012. This work was supported by the Portuguese Government through the Foundation for Science and Technology (FCT) under Project No. SFRH/BD/40286/2007, Project No. PTDC/EEA-EEL/114846/2009 and Project No. SFRH/BD/70868/2010. Copyright (c) 2012 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org. Nuno M. A. Freire and Jorge O. Estima are with the Dep. of Electrical and Computer Engineering, University of Coimbra, 3030-290 Coimbra, Portugal, and also with the Instituto de Telecomunicações (email: nunofr@ieee.org, jestima@ieee.org). A. J. Marques Cardoso is with the Dep. of Electromechanical Engineering, University of Beira Interior, 6201-001 Covilhã, Portugal, and also with the Instituto de Telecomunicações (email: ajmcardoso@ieee.org). conversion systems have gained attention, being seen as key parameters to assess their economic viability. Consequently, the condition monitoring of these systems is considered an effective tool to detect incipient failures and generate early fault alarms, assuming a paramount importance in reducing operational and maintenance costs. Faults in a wind turbine are mainly imputed to rotor blades, pitch and yaw control, bearings, gearbox, shaft, generator and power converter. After identifying the components that are prone to fail, it is necessary to choose suitable quantities to be monitored, which are commonly the vibration, torque, lubrication oils, temperature, acoustic emission, and electrical quantities [4]. Although mechanical sensors have been widely used for condition monitoring of wind turbines [5], they are expensive, sometimes difficult to install, and their signal analysis is computationally demanding. Hence, electrical quantities are currently suggested for a cost-effective condition monitoring strategy [6]-[8]. The gearbox is usually considered the most problematic component in indirect drive systems, due to long downtimes [9]-[10]. Therefore, it is expected that wind turbines reliability and availability can be improved by using direct drive PMSGs. However, the power converter required in a PMSG drive is linked to a high failure rate in wind turbines, higher than in other applications [9]. Statistical studies on failures have concluded that the aggregate failure rate of generators and converters in direct drive systems is greater than the aggregate failure rate of gearboxes, generators and converters in indirect drive ones. So, a direct drive system does not have an inherent reliability higher than the indirect one, but its availability should be greater, because a converter failure has a shorter average time of repair than a gearbox failure. Regarding statistical results from other industries, it is shown in [11] that 34% of power devices failures result from semiconductor and soldering faults and 26% from printed circuit board failures, whereas the study presented in [12] shows that about 38% of failures are attributed to power electronics and 53% to control circuits. It can then be concluded that a very high percentage of failures results in power switch faults, since a failure in a gate control circuit usually leads to a switch open-circuit fault. Besides these low reliability levels associated to the power converters, an industry-based survey also shows the current industry dissatisfaction with the reliability of power electronics monitoring methods [13]. Power switch failures can be broadly classified as short- circuit faults and open-circuit faults. Such faults can result from excessive electrical and thermal stress, lifting of the Open-Circuit Fault Diagnosis in PMSG Drives for Wind Turbine Applications Nuno M. A. Feire, Jorge O. Estima, and A. J. Marques Cardoso C