Modeling of Wind Turbines for Power System Studies

Doctoral thesis, 2003

The power quality impact of wind turbines on the electric grid and the response of the wind turbines to faults in the electric grid are investigated in this thesis. A detailed model of a stall-regulated, fixed-speed wind turbine system capable of predicting the flicker impact on the connected grid is presented and compared with field measurements. The responses of fixed-speed and variable-speed wind turbine systems to faults in the electric grid are investigated and compared with field and laboratory measurements. Model structures suitable for grid fault response simulations of the fixed-speed and the variable-speed wind turbine systems are suggested. In the detailed model of the stall-regulated, fixed-speed system, the wind generation and the aerodynamic conversion are treated in two different ways. A detailed wind field simulation and an aerodynamic conversion based on wing properties are compared with a simplified approach that makes use of a single point wind simulation and the wind turbine power curve. A soft shaft representation is used for the description of the drive train, and the use of different generator model complexities is investigated. Results of the simulations are compared with field measurements and it is found that the measured grid voltage has a significant influence on the obtained results. The short circuit impedance angle of the connected grid has also a significant impact on the accuracy of the results. It is concluded that the grid fault response of the fixed-speed systems is mainly covered by the drive train and the generator dynamics. The aerodynamic conversion details are of little interest and do not need to be covered by the model structure in this case. Comparisons of measurements and simulations of the system responses to symmetric and asymmetric grid faults show good agreement. The fault response of variable-speed wind turbine systems is, to a high extent, influenced by the power electronic converters that are utilized in these systems. The response of the systems connected to the grid via a power electronic converter in the stator circuit is not treated in detail since the response is determined mainly by the power electronic converter control algorithm. The variable-speed system with a doubly fed induction generator is dealt with in more detail. It is found that the response of the system to smaller grid faults can be reproduced using the controlled generator description. Field measurements, as well as computer simulations, show poorly damped 50Hz oscillations in wind turbine outputs. It is concluded, however, that from the power system point of view this behavior is of negligible interest and the generator control description can be substituted with constant power source. The response of the doubly fed induction generator to more severe grid faults can be covered by the standard induction machine model initiated at a given operating point. Comparisons of measurements and simulations confirm this conclusion.