Modelling and Analysis of Variable Speed Wind Turbines with Induction Generator during Grid Fault
Publication: Research › Ph.d. thesis
During recent years wind turbine technology has undergone rapid developments. Growth in size and the optimization of wind turbines has enabled wind energy to become increasingly competitive with conventional energy sources. As a result today's wind turbines participate actively in the power production of several countries around the world. These developments raise a number of challenges to be dealt with now and in the future. The penetration of wind energy in the grid raises questions about the compatibility of the wind turbine power production with the grid. In particular, the contribution to grid stability, power quality and behaviour during fault situations plays therefore as important a role as the reliability.
The introduction of the present work briefly presents the development of wind turbine technology. Several wind turbine types are discussed and the motivations for this project are stated. The main motivations are the challenges related to the grid connection of wind turbines.
The second chapter elucidates recent thinking in the area of grid connection by discussing several grid codes or grid requirements. In the discussion it is tried to present the view of the transmission line operators as well as the challenges wind turbine manufacturers such as Vestas faced.
Modelling has an important role in the research and development of system changes because it allows many difficult questions to be answered. The varieties of challenges that must be addressed in such modelling are not met by any single modelling software program. In addition a huge range of in-house programs from different companies exist, the most widely known software for current research on the power grid are PSS/E, EMTDC/PSCAD and DigSilent. In general research and especially for control developments the software Matlab/ Simulink is used. In all these programs simplified general models for different research purposes or specific detailed models have been developed and are used today. However, since wind turbine technology only has recently had to address grid related problems, models do not satisfy the demands peculiar to this technology. In particular, the role of both the wind turbine itself and the grid with respect to their influences, interaction and behaviour under grid faults or asymmetrical operation need further investigation. The present thesis therefore deals with the development of an improved wind turbine model. For the modelling the well known and widely used program Matlab/ Simulink has been chosen. The program is a powerful tool for solving and representing mathematical differential equations and therefore provides the opportunity to combine a wide range of different modelling issues.
The improvement of the large doubly-fed induction generator model as an interface between the mechanical and electrical characteristics of a wind turbine takes a central part in this research process. Chapter 3 presents the development and implementation of a detailed analytical three-phase induction machine model. The model uses only a minimum set of parameters as supplied by the generator manufacturers of Vestas. The accuracy of the simulation is improved by including the saturation of the main and the leakage inductances, which has been validated by test bench measurements. The influence of the generator saturation effects are discussed with respect to the behaviour of the machine itself and to the complete wind turbine behaviour.
Although the generator model is of primary interest for improving the wind turbine model, the implementation of the other elements completing a total wind turbine model have been challenging as well. In chapter 4 the development of a three-phase transformer model and especially a three-phase autotransformer as used in a Vestas Turbine is shown.
Chapter 5 deals with the development of transmission lines and electrical elements connecting the transformer and generator and giving the possibility to create a small VII electrical network and thereby give the possibilities to research the dynamics introduced by a fault in the grid.
The wind turbine models are completed by modelling the mechanical and aerodynamical parts of the wind turbine and include the control system. In this thesis models of the two different wind turbine types, one with pulsed rotor resistance control (Vestas OptiSlip control) and another one with doubly-fed control (Vestas OptiSpeed control) have been implemented.
Finally two fault situations have been studied with the two developed wind turbine models. The influences on the generator as well as the behaviour of the wind turbine during these faults are briefly discussed based on the simulation results.
It can be concluded, that it has been possible to build advanced wind turbine models in Matlab/ Simulink for researching fault situations. However, one conclusion of this thesis is that Matlab/ Simulink might not be the ideal tool for studying a detailed complete model of a wind turbine including a small power system. The complexity of the differential equations combined with a huge number of iteration processes leads to instabilities, which limit the use of the model. Investigations in optimisation of the implemented models are advised. Otherwise the presented models can be seen as basis for further modelling investigations. The developed models are open for further extension for different purpose, e.g. research of the harmonics.
The introduced saturation effects of the machine definitely have an influence especially during the simulation of asymmetric fault situations and should be included in models for investigations into the fault situations for wind turbines, even though the influence is much less than expected at the beginning of this work. The reduced influence of saturation is related to the control of the machine, which basically compensates for the error caused by the machine's natural behaviour. The saturation effect is decreased particular during the doubly-fed operation with a very strong control. It does however increase again, if the control system is disabled, as it is initiated for converter protection during fault situations, the effects related to saturation of the machine increases again. Further work may be done on the investigation on the influences of the protection systems or further validation of the whole turbine system.
Furthermore it can be said, that the developed two wind turbine models can be used for the research of grid faults. The model is amenable to further development for different research interests, for instance the development of control strategies or the development of a detailed gear model for the research of stresses.
|Place of publication||Aalborg Universitet|
|Publisher||Institut for Energiteknik, Aalborg Universitet|
|Number of pages||211|