Flight path control of kite power systems in a turbulent wind environment

Converting the traction power of kites into electricity can be a low cost solution for wind energy. A reliable and robust control system is considered to be crucial for the commercial success of the technology. The focus of this paper is the control of the flight path projected onto the unit sphere. The proposed algorithm is straightforward to implement because it uses mainly LPV and PID control components and is thus easy to certify by the authorities, it allows to define limits for the maximal turn rate to avoid sensor failures, and it allows to use a low gain in the feedback loop to be robust against control loop delays up to 200 ms. This is achieved by splitting the control of the flight path into two different modes of operation: Turn maneuvers and parts of the flight path, where the course angle is constant. During the turning maneuvers mainly feedforward control is used, therefore reducing stability problems. During the straight flight path segments feedback control in combination with Nonlinear Dynamic Inversion (NDI) is used and thus deviations from the planned flight path can be compensated. NDI is needed to compensate the effect of gravity on the turn rate, but also the changes of the steering sensitivity, depending on the apparent wind speed and the angle of attack. A dynamic 4-point model of the kite is used for the validation of the controller performance. The kite is flown in a turbulent 3D wind field using the Mann-model for modeling the turbulence. The results show a low tracking error even in very turbulent wind conditions and even in the presence of large sensor errors and control loop delays: At a turbulence intensity of 26.5% the elevation error was still lower than 1.5°.