During pumping mode, airborne wind energy systems are operated in two phases: A power generating reel-out phase and a power dissipating reel-in phase. The ground winch is connected via a DC-link voltage source converter to the grid. The control of its DC-link voltage is a challenging task due to the bidirectional power flow over the DC-link. Two PI controller designs are discussed: the classical PI controller with constant parameters and a nonlinear PI controller with online parameter adjustment. Based on a worst-case analysis of the physical properties, bounds on the constant parameters of the classical PI controller are derived leading to a conservative design to assure a stable operation also during the reel-in phase where the system dynamics are non-minimum phase. To overcome these limitations in the closed-loop bandwidth, a nonlinear PI controller is proposed which adjusts its parameters online. For controller design, the linearized system model is used and the controller parameters are computed via “online pole placement”. Simulation results illustrate robustness, stability and improved control performance of the proposed nonlinear PI controller in comparison to the classical PI controller.