The United States and Europe are currently integrating large capacity wind generation systems into the electric power grid. It is expected that by 2030, over 20% of the electric power in the U.S will be provided from wind energy. This is raising concerns about the stability of the electrical grid due to a large number of wind turbines on-line. Regulations are already being revised to ensure that the grid remains reliable with both current and future wind generating capabilities. Although there are many turbine technologies with various control philosophies, state-of-the-art wind farms are based on Doubly-Fed Induction Generators (DFIG). Due to its unique design, it has several benefits over traditional turbines. However, because of the response of the DFIG during large grid transients, it does not meet most utility company standards. Low-voltage-ride-through (LVRT), or Fault-ride-through (FRT), capability of DFIG-based wind turbines during grid faults is one of the core requirements. A vital research topic is alleviating the shortcomings of the DFIG-based wind turbine during grid faults without adding additional cost, or introducing reliability issues. In this dissertation, both system and circuit level studies are performed on the modeling, advanced control, and grid integration of the DFIG-based wind turbines during normal and FRT conditions.
First, a detailed description of the dynamic model of the DFIG-based wind turbine including both mechanical and electrical components, is presented. Then, the DFIG classical control system, including: the decoupled control of the generated active and reactive power, the voltage control of the intermediate DC bus, and the power factor control by the grid-side PWM converter are also developed.
Second, a comprehensive overview of the grid code requirements and specifications for the operation and grid-integration of wind turbines, is presented. A detailed investigation of the LVRT grid code requirement, including the LVRT characteristic and major LVRT technologies, is provided for DFIG-based wind turbines.
Third, a detailed mathematical analysis is developed to describe the transient characteristics and the complicated dynamic behavior of DFIG-based wind turbines during grid voltage sags. A generator mathematical model, both in time and Laplace domains, is used to analyze the response of wind generation systems under the influence of symmetrical and asymmetrical grid faults.
Fourth, a novel rotor-side control scheme for DFIG-based wind turbines is developed to enhance its LVRT capability during sever grid voltage sags. The proposed control strategy focuses on mitigating the rotor-side voltage and current shock during abnormal grid conditions, without any additional cost or reliability issues.
Fifth, novel sliding mode control design procedures are developed for power electronic converters that are used in DFIG-based wind turbines. This includes the DC/DC power converters, which are utilized in the energy management systems of wind turbines, and the AC/DC power converters, or the Grid-Side Converters (GSCs). Furthermore, chattering issues are also discussed and a chattering reduction design approach for power converters is proposed.
In the end, every part of the research is combined together and a unique reconfigurable grid-connected wind farm model is developed. It integrates a hybrid setup of physical hardware and real-time simulation as well as a new operation methodology called Computer Twins. The developed wind farm model provides a research setup that can help researchers, utility companies and wind turbine manufacturers to perform real-life tests and measurements to ultimately verify the success of the proposed designs and control methodologies of DFIGs during normal and abnormal grid conditions.