Stability and Control of Wind Farms in Power Systems

This Ph.D. project is carried out at Risø National Laboratory in cooperation with Aalborg University. Ever increasing wind power penetration in power systems all over the world lead to new grid connection requirements in terms of transient stability. While old grid connection requirements demand wind turbines to be taken off the grid immediately when a grid fault occurs, new grid connection requirements demand wind turbines to ride through transient faults. In this project it is investigated how controllers of existing wind turbine designs and wind farm concepts have to be improved to become able to do ride-through control. These investigations also show what the limitations implied by current standard hardware designs are. Basing on that it has to be investigated what future concepts could be more suitable for ride-through control. The turbine types considered are active stall turbines with directly grid connected squirrel cage induction generators, and variable speed pitch-controlled turbines with doubly fed induction generators. To assess ride-through capabilities, simulation models of wind turbine controllers and controllers are created and programmed in the power system simulation tool DIgSILENT PowerFactory. Different transient stability scenarios are investigated. The fault scenarios considered are based on faults scenarios described in grid connection requirements. In order to have a realistic case for these investigations a simplified model of the Nordic power system is used. In this model part of the conventional generation is substituted by wind power. This allows the assessment of the behaviour of wind turbine models when connected to a realistic power system model. In addition to the development of control strategies and hardware improvements, also models are evaluated. Since computer simulation is the means of investigation, realistic models are an extremely important premise for meaningful results. So far the influence of the dynamic stall effect on transient fault operation of active stall turbines has been assessed. In the coming months the possibility of more comprehensive grid control tasks with active stall and variable speed pitch turbines will be investigated.   Supervisors: Birgitte Bak-Jensen (AaU), Poul E. Sørensen (Risø)