Damping of Low Frequency Power System Oscillations with Wind Power Plants

In this thesis, the focus is given to low frequency power oscillations, which have been bothering power systems from their early days of existence. Among various types of oscillations the most bothering are inter-area oscillations, which have been involved in many blackout incidents that occurred throughout 20-th and at the beginning of the 21st century. The name of the phenomenon comes from its nature - under a disturbance synchronous generators rotors swing back and forth in search for a new operating point; while swinging, generators in one geographical area usually form a coherent group which oscillates against another coherent group located in another power system area, causing high power fluctuations on the tie-lines that interconnect the areas. The reason why inter-area oscillations compromise the power system stability is their poorly damped characteristic. It means that once triggered the oscillations will last for considerable time, which may erroneously trigger the fault protection, or event might grow in amplitude leading eventually to the system collapse.
On the other hand, the renewable resource based power plants have experienced tremendous growth in recent years. This trend is expected to continue. Especially wind power is seen as one of the main players on the energy markets of the future. However, to ensure seamless integration of high amount of non-synchronous generation, transmission system operators have specified a number of connection requirements. These requirements have pushed the wind technology to the wide range utilisation of the fullconverter turbine concept, which offers excellent dynamic power control capability. In this work it is sought to employ this capability to enhance the damping of interarea oscillations. Hence, the objective of the thesis is to analyse the impact of wind power plants on power system low frequency oscillations and identify methods and limitations for potential contribution to the damping of such oscillations.
Consequently, the first part of the studies focuses on how the increased penetration of wind power into power systems affects their natural oscillatory performance. To do so, at first a generic test grid displaying a complex inter-area oscillation pattern is introduced. After the evaluation of the test grid oscillatory profile for various wind power penetration scenarios, it is concluded that full-converter based wind power plant dynamics do not participate into the inter-area modes and do not introduce new oscillations in the frequency range of concern. Nevertheless, the oscillatory characteristic of the test grid is altered under increased wind power penetration. However, it is demonstrated that similar impact would have any other non-synchronous power source.
The main body of the work is devoted to the damping control design for wind power plants with focus on the impact of such control on the plant operation. It can be expected that the referred impact is directly proportional to the control effort, which for power processing devices should be understood as the power or current capability, spent to meet to reach the control objectives. However, despite its importance, a very limited treatment of this matter has been given in the literature. Similarly, no studies have been done to identify the critical WPP parameters which may limit the feasibility or effectiveness of the WPP based concept. Consequently, these three factors are chosen as the focal points for this thesis. Comprehensive consideration is given to the selection of input signals for power oscillation damping controller. Studies show that remote signals from power system, like tie-line power flow, can provide better information on oscillation then locally available measurements. Consequently, wide area measurements are also considered as potential inputs to damping controller.
Finally, to perform the aforementioned analysis state-of-the-art methodology that is used for other applications, such as power system stabiliser and flexible ac transmission systems based damping, is recalled from the literature. However, the author's effort is not limited to mere methodology adaptation for the wind power based damping context - the limitations of conventional analysis are highlighted and accordingly new or extended methods are proposed. These new considerations are applied to derive an improved tuning methodology for power-system-stabiliser-like damping controllers.