Abstract
An airborne wind energy system (AWES) can harvest stronger wind streams at higher altitudes which are not accessible to conventional wind turbines. The operation of AWES requires a controller for the tethered aircraft/kite module (KM), as well as a controller for the ground station module (GSM). The literature regarding the control of AWES mostly focuses on the trajectory tracking of the KM. However, an advanced control of the GSM is also key to the successful operation of an AWES. In this paper we propose a cascaded control strategy for the GSM of an AWES during the traction or power generation phase. The GSM comprises a winch and a three-phase induction machine (IM), which acts as a generator. In the outer control-loop, an integral sliding mode control (SMC) algorithm is designed to keep the winch velocity at the prescribed level. A detailed stability analysis is also presented for the existence of the SMC for the perturbed winch system. The rotor flux-based field oriented control (RFOC) of the IM constitutes the inner control-loop. Due to the sophisticated RFOC, the decoupled and instantaneous control of torque and rotor flux is made possible using decentralized proportional integral (PI) controllers. The unknown states required to design RFOC are estimated using a discrete time Kalman filter (DKF), which is based on the quasi-linear model of the IM. The designed GSM controller is integrated with an already developed KM, and the AWES is simulated using MATLAB and Simulink. The simulation study shows that the GSM control system exhibits appropriate performance even in the presence of the wind gusts, which account for the external disturbance.
Highlights
The kinetic energy of wind tends to increase with altitude and is much higher at altitudes between 0.5–12 km above the ground than in the proximity of the surface, cf. [1]
In this research we focus on fixed GG-airborne wind energy system (AWES) that are operated in pumping cycle
It is pertinent to mention here that the unknown states of the induction machine (IM), which are used in rotor flux oriented control (RFOC) are estimated using a discrete-time Kalman filter (DKF), which is based on the quasi-linear model of IM, cf. [40]
Summary
The kinetic energy of wind tends to increase with altitude and is much higher at altitudes between 0.5–12 km above the ground than in the proximity of the surface, cf. [1]. There is a consensus that such higher-altitude winds, even 500 m, cannot be reached by any towered turbine without a radical change of concept In this scenario, airborne wind energy systems (AWES) aim at playing a vital role to convert wind at higher altitudes to electricity. In GG-AWES, the tether provides the mechanical energy to the generator housed in the ground station. In Fly-Gen systems the electricity is generated on board the flying device In this type of AWES, the tether is used to conduct electricity from the aircraft to the ground station. In traction mode the electricity is generated and in retraction phase, a small amount of energy is consumed. The lift produced by the aircraft induces tractive force on the tether and mechanical energy is provided to the electrical machine, which acts as a generator. This is guaranteed by designing a sophisticated control system for an AWES
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