Analysis of external rotor electric drives for an all-automatic airborne wind energy system

This paper describes the investigation and systematic identification of feasible electric machine types and inverter systems for an all-automatic airborne wind energy system. The wind energy system consists of the flying kite, which is winched out, winched in and stored fully automatically, and a ground station with the rope drum, the electric machine, the frequency converter and the control unit with grid connection. During winching out, the kite is unfolded and the electric machine acts as a generator (rating of prototype: P-{\mathrm{N}}=280\text{kW},\ U-{\mathrm{N}}=365\mathrm{V},n-{\mathrm{N}}=2.67\text{Hz}, future systems: P-{\mathrm{N}}=1\text{MW}) feeding energy to the grid. When winching in, the kite is taken out of the wind and the electric machine acts as a motor to retrieve the kite. This approach of using airborne wind energy implies some special boundary conditions for the drive train, e.g. high dynamics and overload capability due to wind gusts and strict temperature conditions, which will be discussed in this paper. Furthermore, different external rotor concepts will be investigated for the electric machine to combine the rotor directly with the rope drum in order to decrease the assembly space. Four different machine types - the electrically excited synchronous machine, the permanent magnet synchronous machine, the reluctance synchronous machine and the induction machine - are initially dimensioned and assessed according to predefined evaluation criteria. After this, the permanent magnet synchronous machine and the reluctance synchronous machine, which meet the requirements for the present application in the best possible way, are investigated in detail and finite element analysis results are presented. Thereafter, the boundary conditions regarding the converter unit are discussed. Finally, a drive system consisting of the most promising machine and converter type for a kite-based all-automatic airborne wind energy system is presented.