Closed-Loop Control Impact on the Detectability of Stator High-Resistance Connection in Doubly-Fed WRIMs Based on Rotor Power Spectral Analysis Yasser Gritli, Claudio Rossi, Angelo Tani, Domenico Casadei, Fiorenzo Filippetti Abstract— Design of modern Wind Energy Conversion Systems (WECSs) must take into account two crucial aspects: efficient control strategies and reliable monitoring and diagnosis techniques. Wind turbine generators based on Wound Rotor Induction Machine (WRIM) is actually the preferred technology mainly because of the lower rating of the power converter connected to the rotor side. With the association of a back-to back converter on the rotor side a Stator-Flux-Oriented-Control (SFOC) system can be used to obtain a decoupled control of the active and reactive power on the stator side with high efficiency. In this context, the influence of the Closed-Loop (CL) control scheme must be considered when the diagnosis of stator asymmetry is derived from the signature analysis of electrical quantities. Therefore, an examination of the spectral content of the investigated quantities for different bandwidths of CL regulators is necessary to assess the effectiveness of the derived fault indexes. In this paper, the performances of three different techniques based on rotor electrical quantities, for detecting stator asymmetry are investigated. Results with numerical simulations and experimental tests are here presented showing the validity of the established comparative study. Keywords— Ac machines, stator faults, FOC, fault diagnosis, power signature analysis. I. NOMENCLATURE f Stator frequency. s Slip. ωr Rotor speed. θr Rotor position angle. θ Stator flux phase angle. Rs Stator resistance. Rr Rotor resistance. Radd Additional resistance used during the tests. vds, vqs d-q components of the stator voltage vector in the synchronous reference frame. ids, iqs d-q components of the stator current vector in the synchronous reference frame. Lm Mutual inductance between stator and rotor windings. Y. Gritli, C. Rossi, A. Tani, D. Casadei, and F. Filippetti, are with the Department of Electrical, Electronic and Information Engineering "Guglielmo Marconi", University of Bologna, 40126 Bologna, Italy (emails: yasser.gritli@unibo.studio.it; angelo.tani@unibo.it, claudio.rossi@unibo.it; fiorenzo.filippetti@unibo.it; domenico.casadei@unibo.it). Y. Gritli is also with the Department of Electrical Engineering, University of Tunis El Manar, LA.R.A., National Engineering School of Tunis BP 37, 1002 Tunis Belvedére, Tunisia. (e-mail: yasser.gritli@ensi-uma.tn). Ls Stator self-inductance. Lr Rotor self-inductance. idms, iqms dq components of the magnetizing current in the synchronous reference frame. ias, ibs, ics Stator phase currents. iar, ibr, icr Rotor phase currents. vdr, vqr dq components of the rotor modulating signals in the synchronous reference frame. II. INTRODUCTION During the last years different motor technologies were adopted in WECSs, but wound rotor induction machine still remains the preferred technology for this category of applications, mainly because of the lower rating of the power converter connected to the rotor side. Different control strategies for WRIMs can be found in literature for efficient WECS applications, such as: accurate vector control [1]-[2], using sliding modes approach [3], fuzzy logic [4], and the so- called direct methods for torque [2], [5] and power control [6]. Nevertheless, whatever the research subject to be addressed for specific control performances, there is one common issue: the motor faults detection at incipient stages, in order to minimize maintenance cost and to prevent unscheduled downtimes. An important review [7] has revealed that 13% of wind turbine failures are related to the motor. Investigations on different failure modes in induction motors have revealed that 38% of the overall motor faults are related to the stator part [8]. Interesting details and analysis of this type of faults can be found in [9]. Specifically, each electrical fault that occurs in the stator side of a WRIM, such as resistance variations or short circuits [9]-[11], produces a phase dissymmetry because the phase impedances are no longer equal [8-9]. The increase in a phase resistance, commonly referred to as “high-resistance connection” in the literature, is a common problem that can occur in any power connection of electrical machines [9]–[13]. Operating in aggressive environments offshore or onshore, winding connections of WRIMs are subjected to corrosion, abrasion, and fretting, leading generally to local heating, which can propagate insulation damages [12]. Consequently, the detection of the first anomalies, such as resistance changes and winding imbalances, can prevent serious damages and avoid longer downtime periods. Different diagnostic methods for electrical faults in WRIM for wind turbines have been proposed [14–27]. Active power has been presented for the detection of high-resistance connection in [23], where stator active power has shown more benefits in terms of totality of 978-1-5090-5853-2/17/$31.00 ©2017 IEEE 238