Wind power stabilising control : Demonstration on the Nordic grid

When unconventional types of generators such as doubly fed induction generators (DFIGs) are used in a power system, the system behaves differently under abnormal dynamic events. For example, DFIGs cause different modes of oscillation in the power system, and respond differently to changes in voltage. In order to damp oscillations in the system, it is necessary to understand the equipment causing these oscillations, and the methods of optimally damping the oscillations. Large power oscillations can occur in a power system as a result of disturbances. Ordinarily these oscillations are slow and, in principle, it is possible to damp them with the help of wind power. This suggests the use of a power oscillation damping (POD) controller for a DFIG, similar to a power system stabiliser (PSS) for a synchronous generator. Voltage stability is another important aspect of the safe operation of a power system. It has been shown that the voltage stability of a power system is affected by induction generators and also DFIGs, and we investigate some aspects of this here. In this study we develop control strategies for large wind farms comprising DFIGs, and study the impact of the wind farms on a system which is designed to reflect the dynamics of the Nordic power system. The design of multiple PODs in a wind farm is undertaken using linear matrix inequalities (LMIs). The impact of the wind turbines is investigated through the use of linear and dynamic simulations. It has been demonstrated that DFIG-based wind farms can be used for damping oscillations, even when they are not producing their rated power, and that they can also improve the critical clearing time of some faults. However, they may have an adverse impact on power systems after large voltage disturbances.QC 2012040