DFIG modelling and winding fault analysis y By S. Djurovic j School of Electrical and Electronic Engineering The University of Manchester Research • Doubly fed induction generator (DFIG) is the most commonly used type of generator in contemporary large variable speed wind turbines • One of the most common induction machine faults are winding faults due to short-circuit or open-circuit or abnormal b l connection ti off phase h windings i di • Preferred fault diagnostic method is the steady state stator current spectral analysis (MCSA) - non invasive as windings are used as search coils • Investigate the possibility of DFIG winding fault detection through monitoring stator line current frequency spectra Work so far • An analytical model for a DFIG has been developed to enable the dynamic behaviour of a generator operating with various healthy and faulty winding configurations to be simulated. • A 30 kW DFIG test-rig has been built to explore DFIG steadystate operation under a range of winding and supply fault scenarios. • The analytical model has been verified by comparison of model predictions with test-rig measurements in both time and frequency domain for a series of typical DFIG steady state operating points. • DFIG operation ti with ith a range off typical t i l winding i di f lt and faults d supply asymmetries has been explored. • Changes in the stator current spectrum arising from each fault condition, have been identified. DFIG modelling d lli for f condition diti monitoring it i purposes • Based on coupled coupled-circuit circuit approach • A circuit is defined as ‘any series connection of coils’ • Coupling inductances are calculated between circuits • This approach makes it possible to analyze an arbitrarily connected n-phase machine while taking into account higher order field space harmonics M d l equations Model i • Machine voltage and flux equations (matrix form) V s R s I s d s s L ss I s L sr I r V r R r I r d r r L rs I s L rr I r dt dt • Torque and speed equations T e T LOAD Te d J dt 1 T d L I I 2 d d dt DFIG test rig diagram DC SPEED DRIVE (MENTOR) STATOR DC MACHINE SYNCHRONIZING GRID CONTACTOR SUPPLY AC TEST MACHINE (doubly fed IM) ROTOR BACK-TO-BACK CONVERTER Test Rig Description Laboratory test bed (viewed from above) Test Rig Description DFIG Terminal box Rotor back-to-back converter Healthy and faulty DFIG winding configurations used in this presentation US 1 US 2 VS 2 WS1 VS 1 a) Stator healthy US 2 WR 1 W S2 c) Stator open-circuit fault WR 2 b) Rotor healthy US WS W S1 VS1 UR 2 VR 2 VR1 WS2 VS 2 UR 1 VS d) Stator short-circuit fault Predicted and measured DFIG current rms values for a typical operating point and a healthy DFIG measured measured predicted 15 32 14 30 13 28 12 26 24 Current [A Arms] Current [A Arms] 11 10 9 8 7 6 5 4 22 20 18 16 14 12 10 8 3 6 2 4 1 2 0 predicted sU sV Phase a) Stator line currents R 1684rpm sW 0 rU rV Phase b) Rotor line currents rW Predicted P di t d and d measured d DFIG currentt rms values for a stator open-circuit fault Measured Measured Predicted 30 28 10 26 9 24 Current [A Arms] Current [A Arms] 11 8 7 6 5 4 22 20 18 16 14 12 10 8 3 6 2 4 1 0 Predicted 32 12 2 0 sU2 sV1 sV2 sW1 sW2 Phase a) Stator winding group currents R 1556rpm rU rV Phase b) Rotor line currents rW Frequency content of predicted and measured DFIG stator line current for a DFIG operating with balanced windings and unbalanced supply 0 286 Hz -10 -5 386 Hz 150 Hz 622 Hz 250 Hz Normalized sp N pectrum [dB] Normalized spectrum [dB] N 0 722 Hz -20 -30 -40 286 Hz 386 Hz -10 -15 -20 622 Hz 722 Hz -25 -30 -35 -40 -50 -45 -60 -50 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 Frequency [Hz] Frequency [Hz] a) Experimental current spectrum b) Predicted current spectrum 800 Frequency content of predicted and measured DFIG stator line current for a stator short-circuit fault 0 0 118 Hz 218 Hz 454 Hz -10 10 454 Hz -10 Normalized spectrum [dB] N Normalized sp N pectrum [dB] 386 Hz 118 Hz 218 Hz 286 Hz 554 Hz 722 Hz 622 Hz -20 -30 -40 -50 554 Hz 622 Hz -20 722 Hz -30 -40 -50 386 Hz -60 250Hz 286 Hz 150Hz -60 0 100 200 300 400 500 600 700 800 -70 0 100 200 300 400 500 600 700 Frequency [Hz] Frequency [Hz] a) Experimental current spectrum b) Predicted current spectrum 800 Frequency content of predicted and measured DFIG stator line current for a stator open-circuit fault 0 0 264 Hz 364 Hz -10 364 Hz 521 Hz 107 Hz 578 Hz 421 Hz 678 Hz -20 -30 -40 150 Hz -50 -60 264 Hz -10 10 207 Hz Normalized spectrum [dB] N Normalized spectrum [dB] N 107 Hz 521 Hz 421 Hz 207 Hz 578 Hz -20 678 Hz -30 -40 -50 150 Hz -60 0 100 200 300 400 500 600 700 800 -70 0 100 200 300 400 500 600 700 Frequency [Hz] Frequency [Hz] a) Experimental current spectrum b) Predicted current spectrum 800 Frequency content of predicted and measured DFIG stator line current for a healthy DFIG 0 0 Model 1 -5 150 Hz 250 Hz -10 622 Hz 956 Hz 722 Hz -20 -30 -40 Normalized sp N pectrum [dB] Normalized spe N ectrum [dB] 286 Hz 386 Hz 286 Hz 386 Hz -10 -15 -20 622 Hz 722 Hz -25 956 Hz -30 -35 -40 40 -50 -45 -60 -50 0 100 200 300 400 500 600 700 800 900 1000 Frequency q y [[Hz]] a) Experimental current spectrum 0 100 200 300 400 500 600 700 800 900 1000 Frequency [Hz] b) Predicted current spectrum, Includes high order field harmonics Frequency content of predicted DFIG stator line current for a healthy DFIG 0 0 Model 1 Model 3 -5 Normalized sp N pectrum [dB] Normalized spe N ectrum [dB] -5 -10 -15 -20 -25 -30 -35 -40 40 -15 -20 622 Hz 722 Hz -25 956 Hz -30 -35 -40 40 -45 -45 -50 286 Hz 386 Hz -10 -50 0 100 200 300 400 500 600 700 800 900 Frequency q y [[Hz]] a) Predicted current spectrum, includes fundamental only 1000 0 100 200 300 400 500 600 700 800 900 1000 Frequency [Hz] b) Predicted current spectrum, Includes high order field harmonics Conclusions • A detailed DFIG analytical model developed • DFIG Test Rig built • The developed model's validity is verified through comparison i off model d l predictions di ti against i t th the experimental results obtained from the test rig • Advantages of modelling the effects of higher order field harmonics demonstrated • Some stator current fault-specific harmonic components are identified. These are slip dependant. Th k You Thank Y