Impact of Inter-Turn Short-Circuit Fault on Wind Turbine Driven Squirrel-Cage Induction Generator Systems Takwa Sellami, Hanen Berriri, Mohamed Faouzi Mimouni, Sana Jelassi, Moumen Darcherif To cite this version: Takwa Sellami, Hanen Berriri, Mohamed Faouzi Mimouni, Sana Jelassi, Moumen Darcherif. Impact of Inter-Turn Short-Circuit Fault on Wind Turbine Driven Squirrel-Cage Induction Generator Systems. Conférence Internationale en Sciences et Technologies Electriques au Maghreb CISTEM 2014, Nov 2014, Tunis, Tunisia. <hal-01214863> HAL Id: hal-01214863 https://hal.archives-ouvertes.fr/hal-01214863 Submitted on 16 Oct 2015 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Impact of Inter-Turn Short-Circuit Fault on Wind Turbine Driven Squirrel-Cage Induction Generator Systems T. Sellami, H. Berriri, M.F. Mimouni The Electrical Engineering Department of Engineering school of Monastir (ENIM) Unit of Study of Industrial Systems and Renewable Energy Monastir, Tunisia takwa.sellami.enim@gmail.com, hanen.berriri@yahoo.fr, Mimouni@enim.rnu.tn Abstract—This study aims to assess the impact of inter-turn short-circuit in stator windings of squirrel cage induction generators on variable-speed wind turbine systems. The shortcircuit fault detection is based on a steady-state technique which is Motor Current Signatures Analysis (MCSA). Thus, theoretical developments of both healthy and faulty three-phase squirrelcage induction generator of the wind turbine system models in the abc-reference frame are made to recognize the current signature variations. Simulation results are carried out to illustrate the severe consequences of inter-turn short-circuit fault on the generator signatures and moreover on the whole system. This work highlights the importance of the existence of a robust monitoring and diagnosis system for wind turbine systems. Keywords—stator inter-turn short-circuit, modeling wind turbine system, healthy mode, faulty mode, Motor Current Signatures Analysis … I. INTRODUCTION Due to the continuity of environmental degradation conditions, renewable energy is still an effective way to deal with energy crisis and global warming [1, 2]. Within this framework, wind energy capacity has become one of the most essential energy sources and expected to carry on increasing around the world [3]. Thus, high-power variable speed wind turbine systems are ones of most active research fields [4]. Wind turbine systems modeling and control has recieved considerable attention [5-8]. However, with growing concerns about a high level of reliability and the continuity of service and operating flexibility, fault tolerant control has also become an important issue during the last years [9] and monitoring and diagnosis are the most important parts of this research area [10-13]. Indeed, several failures can occur in variable speed wind turbines system. Among them, failures related to the power converter such as open-circuit faults [14, 15] and those concern sensor faults [16, 17] and [18], also failure of gearbox bearings and torque arms [19], rotor blades [20], etc. On the other hand, there are failures related to the generator, such as broken rotor bar [21] or windings inter-turn fault [22]. In this respect, inter-turn short-circuit in stator windings is the most common stator fault witch it can progress to turn-turn or turn-ground short-circuit [23], where moisture, partial discharge, m-echanical stress and breakdown of the S. Jelassi, M. Darcherif School of Electricity, Industrial Production and Management (EPMI) Laboratory of Industrial Eco-Innovation and Energy (LR2E) Paris Grand Ouest s.jelassi@epmi.fr. turn-to-turn insulation are the most important causes of this type of faults [24]. Such faults can damage the machine and immediately interrupt the wind turbine system. The Power converter and the transformer can be damaged too [25]. To avoid these impacts, fast and effective fault detection technique should be considered [26]. Many methods have been used for short-circuit fault detection in wind generators [27, 28]. Motor Current Signatures Analysis (MCSA) is a non invasive and on-line fault detection method [29, 30]. Analyzing stator current spectra had been fruitfully utilized for short-circuit fault detection without influencing the system operation [31]. This paper is structured as follows: In Section II, a description of the wind turbine system is made. The next section describes both healthy and faulty squirrel-cage induction generator models in the abc-reference frame. In Section IV stator current signatures are analyzed by considering healthy and faulty operation mode. Conclusions are presented in the last part. II. DESCRIPTION OF THE WIND TURBINE SYSTEM Despite the sprawling double-fed induction generator, a three-phase squirrel-cage induction generator (SCIG) is still widely used within wind turbine plants [32]. The use of SCIG is advantageous since they are relatively inexpensive, robust, and require low maintenance. Even for variable-speed turbines, it is used instead of synchronous generators by the ad of converters. The basic configuration of a variable-speed turbine driven SCIG is depicted in Fig. 1. As the rotational wind turbine speed is low and variable, it must be adjusted to the suitable electrical frequency by a gearbox. A SCIG is coupled to the grid through a back-to-back converter driven by vector oriented control. The back to back converter includes the stator side converter connected to the stator windings so that ensures decoupling the electrical and the mechanical frequencies, and the grid side converter connected to the grid through the transformer. Both sides are linked by a DC bus. The DC bus voltage reference and the grid voltage level are used to establish the current references which determine the voltages to be applied on the grid side converter. The aim is to control the DC bus voltage and the reactive power which is consumed or supplied by the converter of the grid side. In the stator side converter control, the torque and rotor flux cascade loops establish the current references which determine the voltages to be applied. The stator side converter include a maximal power point tracking algorithm (MPPT). The control is performed in the synchronous rotating frame, which is oriented according to the rotor flux. T [V s abc ] V s a V s b V s c ; [V r abc ] V r a V r b V r c T [i s abc ] i s a i s b i s c ; [i r abc ] i r a i r b i r c ss Where R s abc , R r abc , L rr abc , L abc T T M sr ( ) and , M rs ( ) are written as follows : Turbine Back-to-Back converter Gearbox Grid Transformer SCIG Sator side converter control Grid side converter control Figure 1. Overview the SCIG based variable speed wind turbine system III. HEALTHY AND INTER-TURN FAULT DYNAMIC MODELS OF THE SCIG A. Healthy model The SCIG equivalent circuit in healthy conditions (no stator fault) is given in Fig. 2, where s1, s2, and s3 are the stator threephases, rs the stator phase resistance (Ohms), Ls a self inductances (H), rr the rotor phase resistance (Ohms), Lr a self inductances (H). The neutral is connected so VNN’=0V. r s R s abc 0 0 r r 0 0 ; R r abc 0 0 r s 0 rs 0 0 rr 0 Ls l s Lss abc M s Ms Ms Ls l s Ms Ms Ms ; Ls l s Lr l r Lrr abc M r Mr Mr Lr l r Mr Mr Mr ; Lr l r 0 0; r r 2 4 cos( ) cos( ) cos( ) 3 3 T 4 2 sr rs M ( ) M ( ) M sr cos( ) cos( ) cos( ) 3 3 2 4 cos( ) cos( ) cos( ) 3 3 The SCIG electromagnetic torque Cem is given by equation (2). isa r , L s s r , L r s1 r , L i sb V sa V sb i V sc s c s2 r r Mr r , L r , L s Cem ( ) p iabc 2 r , L N’ Ms s s s 1 Mr Ms r s3 VNN ' s a i sb isc d M sr abc d 0 ir a irb iabc i r c (2) T Where J is the moment inertia and p is the pole pair’s number. The SCIG mechanical angular speed Ω, is got from the gearbox. N The SCIG voltage equations written in its natural reference frame (abc) are expressed by (1). d d s V R s i s Lss i s ( M sr abc i r abc ) abc abc abc abc dt abc dt V r 0 R r i r Lrr d i r d ( M rs i s ) abc abc abc abc abc abc dt abc dt T With [iabc ] i r Figure 2. Induction generator equivalent circuit. With 0 d M rs abc d r (1) B. Faulty model A stator inter-turn short-circuit fault in stator windings causes an insulation failure which affects phase windings. This failure is modeled by an insulation resistance rf. The resistance value depends on the fault severity. A graver short-circuit fault of the phase is got when the fault insulation resistance rf decreases toward zero. This decrease in most materials, from infinite toward zero is very fast. Fig. 3 represents the SCIG stator windings with a shortcircuit fault. This fault is occurred in the first phase s1. The sub-windings (as1) represent the healthy part of the phase windings s1 and (bs1) represent the faulty one. Ns is the s1 turns phase number and Nsf is the turns short-circuited number. The short-circuit report kcc= Nsf / Ns is between 0 and 1. Ms, and Msr, are the stator mutual inductances (H) and mutual inductance between stator and rotor phases (H). Mr is the rotor mutual inductances (H). rs1b and Ls1b present resistance and self-inductance of the faulty winding. M1a,2 and M1a,3 present mutual inductances between as1 and the windings s2 and s3. In addition, M1a,1b, M1b,2 and M1b,3 present respectively three mutual inductances between bs1 and the windings as1, s2 and s3. Msr1, Msr2 are the mutual inductances between as1, bs1 and rotor. rf M1a,1b i r1a , L1a s as1 a bs1r1b , L1b if r , L r s1 r v1b v1a Mr M1a ,2 r , L s i sb V sa M1a ,3 isc V sb r s2 Ms N’ s r , L r s3 VNN ' Ls l s M M 1b,2 Lss abcf 1a ,2 M 1a ,3 M 1b,3 Ls l s M 1a ,1b 1b 1b Lss abcf , Lrr abcf and M 1a ,2 M 1b ,2 M 1a ,3 M 1b ,3 Ls l s Ms Ms s L ls M 1a ,2 M 1a ,3 L1sb l1sb M 1a ,1b M 1b ,2 M 1b,3 s s L1b l1b 2 2 M sr cos( ) M sr cos( ) M sr cos( ) 3 3 2 M cos( 2 ) M cos( ) M cos( ) sr sr sr 3 3 M sr abcf 2 2 M cos( ) M sr cos( ) M sr cos( ) sr 3 3 2 2 M sr2 cos( ) M sr2 cos( ) M sr2 cos( ) 3 3 0 rs 0 0 r1sb r r 0 Rr 0 abc 0 0 (r1sb rf ) 0 0 rs 0 0 rr 0 0 0 r r r Lr l r L abc M r Mr Mr L lr Mr rr N V sc R r abc , M sr abcf are written as follows rs 0 R s abcf 0 s r1b r Mr M1b,3 R s abcf , Where r , L s r , L s M1b,2 Matrices r Mr T rs sr M r M abcf M abcf Lr l r It is admitted that Figure 3. SCIG equivalent circuit with short-circuit fault. The SCIG voltage equations under short-circuit fault conditions are d d s V R s i s Lss i s ( M sr abcf i r abc ) abcf abcf abc abcf dt abcf dt V r 0 R r i r Lrr d i r d ( M rs i s ) abc abcf abcf abc abc abc dt abc dt (3) r s1b k cc rs ; l s1b k cc 2 Ls Ms Ls L ; Mr 2 2 M sr L L ; M s r sr1 r 1 k cc M s ; M sr2 k cc M sr M1a,1b Ls 1 k cc k cc M1a,2 M s 1 k cc ; M1a,3 M s 1 k cc M1b,2 M s k cc ; M1b,3 M s k cc With T [V s abcf ] V s a V s b V s c 0 ; [V r abc ] V r a V r b V r c T [i s abcf ] i s a i s b i s c i f ; [i r abc ] i r a i r b i r c It is admitted that L1a +l1a +2M1a,1b +L1b +l1b =Ls +ls Ls L r =Lr +lr M sr1 +M sr2 =M sr M1a,2 +M1b,2 =M1a,3 +M1b,3 =M s T T IV. SIMULATIONS AND RESULTS Simulations were carried out for a wind turbine driven SCIG system. Dynamic models in healthy and faulty modes were established to study the behavior of the wind turbine system components signatures. The SCIG parameters are given in appendix. The simulation results are shown in figures Fig. 4 to Fig. 11 with a stator fault appearing at time t=0.15s. In the first time, a partially short-circuited turns as 25% of the phase s1 is applied: parameter kcc=0.25,. The other parameter rf is fixed to 0Ω as the decrease of this insulation resistance rf in most materials from infinite toward zero is very fast. Fig. 4 presents impact of this fault on the three-phase stator currents (isa, isb, isc), the three-phase rotor currents (ira, irb, irc) and the stator currents (isd, isq) in the dq-frame. Fault current if circulating in rf in the abc-frame variations is given too. Fig. 5 shows the short-circuit fault impact on the torque and the rectifier voltage. Active and reactive powers of the SCIG signatures are presented too in Fig. 6. Fault application isq isd Fault application Time(s) The detection of inter-turn short-circuit via MCSA is based on detecting components caused by the induced faulty current components in the stator windings. This technique uses the results of spectral analysis of the stator currents. Current signals are analyzed in the time-domain and the frequencydomain. For frequency-domain study, Fast Fourier Transform (FFT) analysis is done to correlate the components of current signatures in order to detect utile frequency components. Time(s) Fault application isb Irc isa isc Time(s) isq isd Irb Ira Time(s) Time(s) Time(s) isa Irc isb Figure 4. Impact of partially short-circuited turns (kcc=0.25) at t= 0.15s on currents (a) stator currents in the dq-frame (b) Fault current (c) three-phase stator currents (d) three-phase rotor currents Irb isc Time(s) Fault application Fault application Ira Irc Time(s) Fault application Figure 7. Impact of 90% short-circuited turns (kcc=0.9) at t= 0.15s on currents (a) stator currents in the dq-frame (b) Fault current (c) three-phase stator currents (d) three-phase rotor currents Time(s) Time(s) Fault application Fault application Figure 5. Impact of partially short-circuited turns (kcc=0.25) at t= 0.15s on torque and rectifier voltage Fault application Fault application Time(s) Time(s) Figure 8. Impact of 90% short-circuited turns (kcc=0.9) at t= 0.15s on torque and rectifier voltage Time(s) Time(s) Fault application Fault application Figure 6. Impact of partially short-circuited turns (kcc=0.25) at t= 0.15s on active and reactive power In the second step, wind power system behavior is observed when increasing the severity of the fault to 90% short-circuited turns. Fig. 7 shows inpact of the high number of short-circuited turns on the three-phase stator currents (isa, isb, isc), the threephase rotor currents (ira, irb, irc), the stator currents (isd, isq) in the dq-frame and the fault current if circulating in rf. Fig. 8 and Fig. 9 show the fault impact on the torque and the rectifier voltage and on the active and reactive powers of the SCIG signatures, respectively. When the severity of short-circuit fault exceeds a certain level, the imbalance of phase currents (especially isa) becomes significant. Torque ripples appear too. In addition, the active and reactive powers are disturbed. These disturbed signals transmitted from the SCIG to the rectifier affect the rectifier voltage signatures. Then the inverter and control equipment signals are troubled too. Time(s) Time(s) Figure 9. Impact of 90% short-circuited turns (kcc=0.9) at t= 0.15s on active and reactive power Fig.10 presents phase current isa spectral distribution in the healthy mode. Fig. 11 and Fig. 12 present isa spectral signatures in the faulty mode. Fig. 11 treats the case of 25% short-circuited turns of the phase s1 and Fig. 12 deals with 90% short-circuited turns. The component 170 Hz is present under normal operations (healthy case) and abnormal ones (faulty case). It cannot be utilized as an indicator for short-circuit fault. Fig. 11 shows that 330 Hz component is a good indicator for 25% short-circuited turns fault. Moreover, the highest magnitude, non fundamental frequency component, appears at the 330 Hz frequency. Fig. 12 indicates that 430 Hz component is the good indicator for 90% short circuited turns fault and its corresponding magnitude is the highest one in the spectral. isa(A) isa(A) 170 Hz 170 Hz f(Hz) f(Hz) 330 Hz f(Hz) f(Hz) Figure 10. Phase current spectral signatures healthy case with zoom at the sideband [0...400Hz] I. Figure 11. Phase current spectral signatures faulty cases Faulty case (kcc=0.25) with zoom at the sideband [200...400Hz] isa(A) CONCLUSIONS In this paper inter-turn short-circuit fault in stator windings is detected by a Motor Current Signatures Analysis (MCSA) method. The first objective of this work is the establishment of sufficiently accurate models to determine the behavior of different wind turbine components variables in the healthy case. The second objective is the detection of shortcircuit fault by analyzing generator current signatures. The technique utilized in this paper has shown its effectiveness. Furthermore wind turbine generator faults detection by vibration analysis will be the objective of a next work. 170 Hz f(Hz) APPENDIX The SCIG used is characterized by 11kW power, stator turns number =48, rotor turns number =32, J=0.1 s r Kg.m², f=0.003 N.m.s, The load current I current I s s 0= 4A and the rated 430 Hz n =11.32A. TABLE I. PARAMETRES OF THE SCIG. P rs rr Ls Lr ls lr 2 1.5(Ω) 0.7(Ω) 0.28(H) 0.28(H) 0.011(H) 0.0075(H) f(Hz) Figure 11. Phase current spectral signatures faulty cases Faulty case (kcc=0.9) with zoom at the sideband [300...450Hz] The machine is supplied by an equilibrated three-phase sinusoidal voltage source defined by V s a V 2 cos(2 freq ) 2 s V b V 2 cos(2 freq ( )) 3 4 s V c V 2 cos(2 freq ( 3 )) With V 380V and freq 50Hz . 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