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DC-link Control for Airborne Wind Energy Systems During Pumping Mode

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Christoph M. Hackl Research Group Leader ”Control of renewable energy systems (CRES)” Technische Universität München

Munich School of Engineering c/o Chair Windenergy

Boltzmannstr. 15 85748 Garching

Germany christoph.hackl@tum.de

www.cres.mse.tum.de

DC-link Control for Airborne Wind Energy Systems During Pumping Mode

Florian Bauer, Christoph M. Hackl, Korbinian Schechner Technische Universität München

Airborne wind energy (AWE) systems with electrical ma-chine on the ground are a promising alternative to classi-cal wind turbines. Such systems are operated in pumping mode with the reel-out phase generating electrical power and the reel-in phase dissipating power [1]. We discuss DC-link voltage control of the grid-side voltage source in-verter (VSI) which controls active power flow from the ma-chine to the grid. For pumping mode AWE systems, DC-link voltage control is a non-trivial task due to the bidirec-tional power flow: the underlying DC-link dynamics are nonlinear and non-minimum phase [2,3] during the reel-in phase. Control objective is to design a robust controller which achieves set-point tracking of the DC-link voltage for given set-point(s) under unknown loads.

We compare the classical PI controller design with con-stant gains with a PI controller design where the gains are adjusted online to the actual operating point and, hence, become state-dependent. Starting with the analy-sis of the linearised system dynamics, we derive stability bounds on the constant gains of the classical PI controller and illustrate the non-minimum phase behaviour during the reel-in phase. In a second step, we design the “adap-tive” PI controller where explicit expressions for the con-troller gains are derived based on pole placement and the physical boundaries of the system.

Based on realistic models of VSI, pulse width modula-tion, and filter [2,3], we illustrate the control performance of the classical PI controller and the ”adaptive” PI con-troller under load changes (see interval [0.0–0.5 s)) and set-point changes (see interval [0.5–0.9 s]) for decreas-ing values of the DC-link capacitance. The ”adaptive” PI

controller (blue line) shows a better tracking and distur-bance rejection capability than the classical PI controller (green line). For a small DC-link capacitance, the classi-cal PI controller even becomes unstable. The ”adaptive” PI controller remains stable and, so, allows to use smaller (cheaper) DC-link capacitors.

pm [k W ] −20 −10 0 10 20 udc [V ] 400 500 600 700 800 900 udc,ref(Cdc= 800 · 10− 6F) udc [V ] 400 500 600 700 800 900 udc,ref(Cdc= 600 · 10− 6F) time t [s] ud c [V ] 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 400 500 600 700 800 900 udc,ref(Cdc= 400 · 10− 6F) ←unstable References:

[1] Ahrens U., Diehl M., Schmehl R. (eds.): Airborne Wind Energy. Springer (2013)

[2] Dirscherl C., Hackl C., Schechner K.: Modellierung und Regelung von modernen Windkraftanlagen: Eine Einführung. In: Elektrische Antriebe – Regelung von Antriebssystemen. Springer (2015) [3] Dirscherl C., Hackl C. M., Schechner K.: Explicit model predic-tive control with disturbance observer for grid-connected voltage source power converters. Proceedings of the IEEE Int. Conference on Industrial Technology, pp. 999–1006, Seville, March 2015

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