Modeling and Comparison of Power Converters for Doubly Fed Induction Generators in Wind Turbines

During the last decades, renewable energy resources have become an ever increasing part of the world wide power generation and especially energy produced by wind turbines has captured a significant part of this power production. This large penetration of wind power has caused increased focus on the generated power quality and controllability. A consequence of this increased focus has been an ever increased set of requirements formulated in national grid requirement. These requirements has forced wind turbines to evolve from a simple generator on a stick into complicated miniature power plants - an evolution which has taken place in a very short time. Further, besides the increased complexity of the wind turbines, the tendency during the late nineties and in the beginning of the new millennium has been, that the size of the turbines has doubled every third year -a progress putting a very high stress on the design engineers employed in the wind industry. Such a progress may force design engineers to adopt common practice from more or less related technologies rather than finding the optimum solution for the specific application. For instance when applying power electronic converters to wind turbines in order to comply with requirements, almost all manufactures has chosen a solution based on the well known two-level voltage source inverter.

The main focus in this thesis is to establish a simple, fast and accurate simulation tool for evaluating different converter topologies for use in a wind turbine based on the doubly-fed induction generator. The objective is to be able to compare the turbine efficiency when using the different converter topologies. The thesis has treated four converter topologies - the commonly used back-to-back two-level voltage source converter, the more un-matured matrix converter, the back-to-back transistor clamped three-level voltage source converter and finally the back-to-back diode clamped three-level voltage source converter.

To evaluate the consequences of applying different converter topologies in a wind turbine application based on the doubly-fed induction generator sufficiently detailed models of the surrounding wind turbine components such as the generator, the gear box, the blades and the transformer have to be derived. The first part of the thesis has treated the modeling approach applied on the surrounding components. The models are wherever possible based on the governing equations with coefficients obtainable from standard data sheets.

The most substantial part of the thesis has been dedicated to the modeling of the considered converter topologies. For each of the considered converters, analytical models has been derived representing the component losses. The component losses within the considered converters depend on the specific turbine design as well as on the operating conditions such as generator speed, generated power, inverter power factor and modulation method. Regarding modulation methods, an in-dept investigation on existing as well as new modulation methods are provided and especially for the latter three converter topologies a lot of efforts have been put into the development of new modulation methods. Actually, for the matrix converter three new modulation schemes has been derived and evaluated, whereas for the three-level inverter topologies four new modulation methods have been proposed. The functionality of most of the developed modulation methods have been demonstrated on an experimental test-setup. Apart from the loss modeling approach, models and methods to estimate the average temperature and peak temperature of the individual components without entering time consuming time-step simulations has been derived -metho ds applicable to determine the actual design margin for a specific converter design. Although not a part of the thesis these fast predicting thermal models will in a longer term be usable to estimate problems related to power cycling and thermal cycling of the power semiconductors within a specific turbine design. To obtain a fair comparison some initial design guidelines for each of the converters has been outlined concerning components ratings, filter design issues and choice of switching frequency.

Finally, all the developed wind turbine component models have been implemented in a simulation tool Drives enabling a fast and fair comparison of the considered converter topologies. Besides enabling a comparison on the turbine efficiency (and turbine component efficiency), the tool enables an investigation on the actual design margin in terms of determining the component peak temperature for a given load profile. Actually, for a fair comparison, the design margin for each converter should be in the same range. The tool has been demonstrated on selected turbine designs and for the present designs, it appears that the back-to-back transistor clamped three-level voltage source converter actually is the best choice (although insignificant) when considering the turbine efficiency -or more specific the annually generated energy. However as will appear during the thesis, several degrees of freedom exist within the turbine design and clearly, changes within the design may change the results obtained from the present design examples.