With a steady increase of the wind power penetration, the demands to the wind power technology are becoming the same as those to the conventional energy sources. In order to fulfill the requirements, power electronics technology is the key for the modern wind turbine systems – both the Doubly-Fed Induction Generator (DFIG) based partial-scale wind power converter and the Permanent Magnet Synchronous Generator (PMSG) based full-scale wind power converter. Since lower cost per kWh, higher power density and longer lifetime are the main concern of power electronics, this project has been aimed to explore the reliability and cost of energy in the modern wind turbine systems. Moreover, advanced control strategies have been proposed and developed for an efficient and reliable operation during the normal condition as well as under grid faults.

The documented thesis starts with the descriptions of the DFIG system and the PMSG system. The design of the back-to-back power converters and the loss model of the power semiconductor device are discussed and established in Chapter 2. Then, Chapter 3 and Chapter 4 are dedicated to the assessment of the wind power converter in terms of reliability. Specifically, Chapter 4 estimates and compares the lifespan of the back-to-back power converters based on the thermal stress analyzed in Chapter 3. In accordance with the grid codes, Chapter 4 further evaluates the cost on reliability with various types of reactive power injection for both the configurations. The cost of energy in wind turbine system is then addressed in Chapter 5, where different wind classes and operation modes of the reactive power injection are taken into account. Finally, the internal and external challenges for power converters in the DFIG systems to ride through balanced grid faults are explored in Chapter 6.

The main contribution of this project is in developing a universal approach to evaluate and estimate the reliability and the cost of energy for modern wind turbine systems. Furthermore, simulation and experimental results validates the feasibility of an enhanced lifespan of the power electronic converters and reduced cost of energy in the DFIG system, employing a proper reactive power control method between the back-to-back power converters. In the case of grid disturbances, an optimized demagnetizing current control strategy has been proposed in order to keep the minimum thermal stress of the DFIG power converter, and thus improve its reliability.