On the Stability of a Minimum-Loss Controlled Dual-VSI DFIG-DC System

Recently a minimum-loss control strategy for the Dual-VSI-DFIG system was proposed. It is implemented using three rules for the determination of the optimal stator frequency, of the stator/rotor magnetizing current split and of the airgap-flux magnitude. The method uses airgap-flux orientation with direct airgap-flux and rotor-current control. This paper presents a stability analysis for the Dual-VSI-DFIG system considering two syntheses of the Proportional Integral controllers based respectively on the symmetrical optimum and on the ITAE criteria. The stability is analyzed by computing the eigenvalues of the closed-loop model. Although both tuning criteria ensure close-loop stability, the system can become unstable in case of controllergain mismatch compared to the theoretical values.

I. Introduction

The Doubly-fed Induction Generator (DFIG), interfaced with the ac mains, is one of the most popular systems in wind energy conversion systems [1]. For the interconnection of wind generators in wind farms, dc power systems are receiving increasing attention [2]. In these applications, the dc version of the DFIG, the DFIG-dc, is attractive because it simplifies dc interconnection, [3]-[5] The dual voltage source inverter (VSI) topology, Fig. 1, is also an interesting option which removes the drawbacks of the stator diode-bridge. Such a dual-VSI DFIG system comprises a wound rotor induction machine (WRIM) with the stator and rotor connected to two VSI converters hooked up to a common dc link.