Investigation of wide band Fiber Bragg grating accelerometer use for rotating AC machinery condition monitoring Sinisa Djurovic a, Peter Kung b et al. a School of Electrical and Electronic Engineering, The University of Manchester, UK; b QPS Photronics, Pointe Claire, Canada.
Outline 1. Introduction 2. Motivation---Wind generator failure statistics 3. FBG wide band vibration monitoring system 4. Generator test rig 5. Bearing fault specific vibration frequencies 6. Measured Vibration spectra under bearing fault conditions 7. Generator electrical fault specific vibration frequencies 8. Measured Vibration spectrum for stator short-circuit and open circuit fault 9. Conclusions
Motivation Vibration analysis frequently used to detect mechanical and electrical faults in rotating machines Wind generators larger than 2MW show combined bearing and stator winding failures of more than 73% Cost effective and robust wide bandwidth vibrations required to replace currently used costly piezoelectric (PE) accelerometers
Fibre optics Vibration sensing Immune to EMI and high voltages Longer life Proven for monitoring low frequency end winding vibration in large generators Direct electrical output Distributed vibration sensing using Long Gauge technology Single sensor measures both temperature and vibration
Large wind turbines failure statistics UpWind: Design limits and solutions for very large wind turbines, EWEA 2011
Wind generator failure modes 1-2 MW >2 MW
Wide Band Vibrofibre Bandwidth of 1000 Hz to cover bearing, stator/rotor electrical faults frequencies achieved by changing the diving board material to Polycarbonate Cross Section Fiber Cavity
Measured Vibration Acceleration (m/s 2 ) Measured Vibration Amplitude (um) Performance and Characteristics 20 15 10 5 Interrogator unit and software interface 0 20 40 60 80 100 Applied Vibration Amplitude (um) Response at 100 Hz vibration frequency 0.07 0.06 0.05 0.04 0.03 0.02 0.01 Accelerometer Reflection Spectrum 0 0 200 400 600 800 1000 1200 1400 Frequency (Hz) FBG accelerometer frequency response
Wind turbine generator test rig Laboratory test bed (viewed from above) Stator terminal box Drive end bearing
Sensor performance testing Benchmarked against Bruel&Kjaer Pulse vibration platform utilising piezo-electric(pe) accelerometers Both sensor types mounted on generator frame Typical winding and bearing faults artificially introduced Sensor mounting
Rolling bearing race frequencies 1 cos 2 b b o r c N D f f D 1 cos 2 b b i r c N D f f D Outer race Inner race f r =rotational frequency, N b =number of rolling elements D b =ball diameter, D c =cage diameter, β=contact angle
Bearing Fault Emulation Various severity of bearing outer race fault introduced in experiments Drive-end SKF 6313 N b = 8 f o =3.07f r f i =4.93f r Machined bearing fault (localised outer race fault) Laboratory generator bearing design data
Acceleration [m/s 2 ] Acceleration [m/s 2 ] Spectrum showing bearing fault effects 10 0 10-1 3mm fault healthy 163.3 244.8 326.6 B&K, bearing fault vibration spectrum, 1590 rpm 489.8 571.4 734.7 816.4 408.1 653.1 898 979.5 10-2 10-3 10-4 0 100 200 300 400 500 600 700 800 900 1000 Frequency [Hz] 10 0 QPS, bearing fault vibration spectrum, 1590 rpm 10-1 3mm fault healthy 163.3 244.8 326.3 408.1 489.8 571.4 653.1 734.7 816.4 898 979.5 10-2 10-3 10-4 0 100 200 300 400 500 600 700 800 900 1000 Frequency [Hz] Piezoelectric (top) and FBG (bottom) vibration frequency spectra 3mm outer race bearing fault at 1590 rpm
Acceleration [m/s 2 ] Acceleration [m/s 2 ] Zoom-in View 0.12 B&K, bearing fault vibration spectrum, 1590 rpm 0.1 healthy 3mm fault 489.8 571.4 0.08 0.06 408.1 PE sensor 0.04 0.02 0 400 420 440 460 480 500 520 540 560 580 600 Frequency [Hz] 0.08 QPS, bearing fault vibration spectrum, 1590 rpm 0.06 489.8 healthy 3mm fault 571.4 FBG sensor 0.04 408.1 0.02 0 400 420 440 460 480 500 520 540 560 580 600 Frequency [Hz]
Typical Stator Winding Faults U1 U1 U1 U2 U2 U2 U2 U2 U2 U1 U1 U1 U2 U2 U2 V2 V2 V2 W1 W1 W1 V2 V2 V2 W1 W1 W1 V2 V2 V2 W1 W1 W1 V1 V1 V1 W2 W2 W2 Winding configurations: a) a) a) V1 V1 V1 W2 W2 W2 b) b) b) V1 V1 V1 W2 W2 W2 c) c) c) Healthy Open-circuit fault Short-circuit fault Winding Balanced Unbalanced Torque frequencies 6k 1 s fs 2 6k 1s fs k 1 s fs p k 2 1s fs p Achieved experimentally using stator terminal box k=0,1,2,3 s=slip p=pole pairs f s =supply frequency
Acceleration [m/s 2 ] Acceleration [m/s 2 ] Spectrum showing short circuit fault effects 10 0 10-1 58.59 [(3),k=6] 158.8 [(2),k=6] 258.8 [(3),k=6] B&K, stator winding short-circuit fault, 1590 rpm 376.1 [(3),k=18] 476.1 [(2),k=18] 576.3 [(3),k=18] 693.6 [(3),k=30] 793.6 [(2),k=30] 893.8 [(3),k=30] 10-2 10-3 short-circuit healthy 10-4 0 100 200 300 400 500 600 700 800 900 1000 Frequency [Hz] 10 0 10-1 58.59 [(3),k=6] 158.8 [(2),k=6] 258.8 [(3),k=6] 376.1 [(3),k=18] QPS, stator winding stator short-circuit fault, 1590 rpm 476.1 [(2),k=18] 576.3 [(3),k=18] 693.6 [(3),k=30] 793.6 [(2),k=30] 893.8 [(3),k=30] 10-2 10-3 short-circuit healthy 10-4 0 100 200 300 400 500 600 700 800 900 1000 Frequency [Hz] Piezoelectric (top) and FBG (bottom) vibration frequency spectra stator short-circuit fault at 1590 rpm
Acceleration [m/s 2 ] Acceleration [m/s 2 ] Spectrum showing open circuit fault effects 10 1 10 0 58.59 [(3),k=6] 158.8 [(2),k=6] 258.8 [(3),k=6] 376.1 [(3),k=18] B&K, stator winding open-circuit fault, 1590 rpm 476.1 [(2),k=18] 576.3 [(3),k=18] 693.6 [(3),k=30] 793.6 [(2),k=30] 893.8 [(3),k=30] 10-1 10-2 10-3 open-circuit healthy 10-4 0 100 200 300 400 500 600 700 800 900 1000 Frequency [Hz] 10 1 QPS, stator winding stator open-circuit fault, 1590 rpm 10 0 58.59 [(3),k=6] 158.8 [(2),k=6] 258.8 [(3),k=6] 376.1 [(3),k=18] 476.1 [(2),k=18] 576.3 [(3),k=18] 693.6 [(3),k=30] 793.6 [(2),k=30] 893.8 [(3),k=30] 10-1 10-2 10-3 open-circuit healthy 10-4 0 100 200 300 400 500 600 700 800 900 1000 Frequency [Hz] Piezoelectric (top) and FBG (bottom) vibration frequency spectra stator open-circuit fault at 1590 rpm
Speed [RPM] 1500 1480 1460 1440 1420 1400 Variable Speed Operation Emulating realistic wind turbine variable speed operating conditions Open-circuit winding fault introduced and PE and FBG platforms compared 1380 0 2 4 6 8 10 12 14 16 18 20 Time [s] Typical measured generator speed profile
Frequency [Hz] Frequency [Hz] Transient vibration signal spectrum 1000 900 800 HEALTHY FAULTY 700 600 500 400 300 200 100 F A U L T 1000 0 0 2 4 6 8 10 12 14 16 18 Time [sec] 900 800 700 600 500 400 300 200 100 HEALTHY FAULTY 0 0 1 2 3 4 5 6 7 8 9 Time [sec] Piezoelectric (top) and FBG (bottom) vibration short-time FFT spectra for variable speed operation with and without open-circuit fault F R E Q U E N C I E S
Single fault frequency zoom-in FAULT FREQUENCY M A G N I T U D E 3d spectrogram, PE sensor FAULT FREQUENCY FREQUENCY TIME 3d spectrogram, FBG sensor M A G N I T U D E FREQUENCY TIME
Summary Wideband VibroFibre was shown to provide comparable performance to high cost PE sensor under electrical and mechanical fault conditions Improvements ongoing in sensor characteristics and signal processing to further enhance performance and bring it closer to PE benchmark Future work will show distributed vibration sensing and investigate sensor design with simultaneous temperature and vibration sensing ability VibroFibre can become a competitive alternative to current high cost sensing solutions
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