Optimal Control of Wind Farms with Energy Storage under Grid Code Compliance

The further integration of wind energy into electrical power systems poses operational challenges as the variability in wind power reduces transient stability. To guarantee a stable and secure power system operation, maximum allowable power ramps are defined in grid codes that transmission service operators (TSOs) specify. In this thesis it shall be investigated how a wind power system with energy storage can be optimally controlled to comply with power gradient restrictions. For this purpose, the kinetic energy in the motion of the wind turbine rotor is utilized and an external battery storage is employed. The main contributions of this thesis are the design, implementation and validation of a supervisory control scheme. The control consists of a model predictive controller and a proportional integral controller for wind turbine pitch control. The model predictive controller serves as supervisory control and provides set-points for the wind turbine control and for the battery. An existing optimal control formulation using a single order wind turbine model is further extended including a battery model capturing fast transient dynamics and a battery degradation objective. In order to assess the performance of the proposed supervisory control system, a higher order wind turbine model is used. Energy yield, power gradients and loading on the wind turbine are evaluated for different wind scenarios. It is found that the proposed supervisory control system substantially mitigates power gradients. If a maximum rotor speed of 1.5 times the rated speed is allowed and a battery with 20% of the rated wind turbine power is used, the energy yield amounts to 97.0% compared to a standard proportional integral controller. For this configuration, the maximum power gradient is reduced by 78.5% and the standard deviation by 88.7%. The damage equivalent load of the tower root bending moment increases by 20%. For the torsion moment of the rotor shaft a slight decrease by 4.5% is observed. Hence, the proposed control is able to comply with tight power ramp restrictions while keeping energy losses and increase in turbine loading to a minimum.