Impartido por: Greg Stone, PhD

Transcription

1 Seminario ON-LINE CONDITION MONITORING OF MOTORS AND GENERATORS Impartido por: Greg Stone, PhD Se presentan las técnicas de Descargas Parciales en Estator, Flujo Rotórico y Vibración en cabezas de bobina del estator. Impartido en la Universidad Politécnica de Madrid el 5 de Noviembre de 2012.

2 El Dr. Stone se gradúa en Canadá en Arranca su carrera de ingeniería en la división de Investigación de Hidro Ontario donde fue responsable de ensayos en 1200 motores y generadores clave. Se convierte en uno de los pilares del desarrollo de métodos de medida de Descargas Parciales on-line para evaluación de aislamiento estatórico en bobinados estatóricos usados en motores y grandes generadores en todo en mundo. En 1990 se convierte en uno de los fundadores de Iris Power (Toronto, Canada). Ha publicado más de 150 artículos técnicos, dispone de patentes en ensayo y mantenimiento de máquinas rotativas. Ha publicado dos libros, el último: Electrical Insulation for Rotating Machines Design Evaluation, Aging, Testing and Repair. Responsable del desarrollo de varias normas en las asociaciones IEEE e IEC en el entorno de aislamiento y máquinas rotativas. unitronics electric Avenida de la Fuente Nueva, 5 San Sebastián de los Reyes MADRID Tel [email protected]

3 Rotor and Stator Winding Condition Monitoring Greg Stone Iris Power Outline Review design and failure mechanisms of rotor windings Flux monitoring for synchronous rotors on hydrogenerators and turbogenerators Design and failure of stator windings On-line partial discharge testing of stators On-line stator endwinding Vibration monitoring 1

4 Rotor Windings Synchronous rotors for turbo and hydro generator Squirrel cage rotors for induction motors Turbogenerator Round Rotor 2

5 Round Rotor Design Forged steel body, with slots machined in Turn and slot insulation Retaining ring and endwinding insulation Rotor Endwinding with Retaining Ring Removed 3

6 Round Rotor Insulation failure Thermal aging due to poor design, overexcitation or improper operation Ground insulation abrasion due to thermal cycling Copper dusting on turning gear mode Turn to Turn Insulation Failure in 600 MVA Turbo (Guangdong EPRI) 4

7 Detecting Rotor Problems Increasing 60 Hz bearing vibration Rotor ground fault relay IR/PI for groundwall (off-line) RSO (rotor spinning or off line) Air gap flux probe test (on-line) detects shorted turns Leakage Flux Measurement Pole A Slot Leakage Flux Flux Probe Quadrature Axis Pole B 5

8 Detection of Shorted Turns Distortion of leakage flux signal is minimal where the air gap flux density curve crosses through zero a function of machine load With conventional monitoring therefore take multiple readings at various generator load points for maximum sensitivity to shorted turns. Difficulties with Conventional Portable Tests Operator needs to be present to collect data at appropriate load points Unless load points are known in advance, may miss collecting data for specific slots. During normal unit load maneuvering, transient shorts may occur which are never detected. Leakage flux probe sticks into air-gap and can be damaged during rotor pull. 6

9 Newer Technology Working with a US Bureau of Reclamation a new type of probe created (TF-Probe) It is permanently mounted on rotor tooth (instead of the wedge) Measures both main flux and slot leakage flux In many retrofit cases, can be installed without a rotor pull Developed new algorithms using the main flux, that does not always require load changes to find all rotor shorts TF Probe 7

10 New Portable Measuring Instrument RFA-II 8

11 FluxTrac Remote, Automatic Rotor Shorted Turn Detector 9

12 Salient Pole Rotor Salient Pole Rotor Winding Flux Monitoring Well-established test to find turn shorts in turbo rotors Monitor flux in the air gap with a small printed circuit mounted on the stator tooth A pole with a shorted turn will induce less voltage in the sensor, compared to other poles Sounds simple - but many practical problems can lead to false indications of turn shorts or false negatives 10

13 TF Probe Installation of Flux Probes on Stator 11

14 New Portable Measuring Instrument RFA-II Shorted Turns on Poles 8 and

15 Stator Winding Monitoring On-line partial discharge (PD) On-line endwinding vibration Stator winding - 2nd most frequent reason for machine failure 13

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17 Stator Insulation System Strand Turn Groundwall (main) Semicon and grading coatings (stators 4 kv and above) Slot and endwinding support Groundwall (Main) Insulation Thickness depends on rated line to ground voltage Voltage across groundwall also depends on position of coil/bar in winding (how far from line end) Failure of groundwall causes machine failure Materials: Old cotton/mica tapes bonded with asphalt Modern mica paper bonded with epoxy or polyester 15

18 Surface Coatings to Prevent Partial Discharge Partial discharges (PD) are electrical sparks that occur in air when the electrical stress exceeds 3 kv/mm, causing breakdown of the air To prevent PD in slot between coil surface and stator core, coat coil with partly conductive (semicon) paint or tape At slot exits, use silicon carbide coating which overlaps semicon coating and extends for cm beyond core Needed on most machines >6 kv, and some modern machines >3.3 kv Semicon Coatings to Prevent PD 16

19 Stator Insulation Failure Processes Many different processes Most take years or decades to result in failure Root causes include design, manufacturing and operation Motor stator turn insulation failure 17

20 Semicon-Grading Coating PD 18

21 Loose coils in the slot PD Due to Insufficient Spacing 19

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23 Testing/Monitoring of Stator Windings Many different off-line and on-line tests Most popular and effective off-line tests are IR/PI, Hipot For stator cores: full flux and ELCID Only on-line monitor (for stator insulation) is partial discharge (PD) 21

24 What Are Partial Discharges? Small electrical sparks in air-filled cavities in or adjacent to HV electrical insulation They occur because breakdown strength of air (3 kv/mm) is much lower than that of solid insulation (~300 kv/mm) Partial Discharges create small voltage pulses PD is monitored by detecting and measuring these small voltage pulses Electrical Representation of Partial Discharge Vp-g Vair Cin Cair Cin Copper 0 V Potential difference (voltage) builds across an air-filled void PD occurs if V/Dv > 3kV/mm ( i.e., electrical stress exceeds electrical breakdown point of gas) The larger the void, the larger the discharge 22

25 Partial Discharges Over -voltage Sustain Voltage across void 60 Hz Phase to ground voltage The higher the overvoltage the higher the intensity of the discharge Discharges also affect carbon bonds in resin PD is very dependent on voltage PD pulses measured on operating generator 23

26 PD Pulse Characteristics Extremely fast rise-time current pulse = short pulse width Rise-time at discharge origin ~ 1 to 5 ns Using f=1/t (~50 to 250 MHz) Measure PD in high frequency spectrum Brief History of On-Line PD Testing 1949: First developed by Westinghouse 1978: Ontario Hydro (electric power company) develops test to separate electrical interference on hydros 1988: Ontario Hydro develops test technique for motors and turbine generators 2002: Over 50% of utility generators (>20 MW) in North America use Iris technology. 2010: test now used on 11,000 machines around the world and included in IEEE 1434 and IEC standards 24

27 Separating Electrical Noise from PD In on-line test, electrical interference from noise sources such as transmission line corona, sparking electrical connections, slip ring sparking, power tool operation This noise masks stator winding PD non specialists may assume stator is close to failure, when high levels of relatively harmless noise present false indications sensors and instruments need to separate electrical noise from PD Thus very low risk of false indications; users can perform and interpret results with only 2 days training Simultaneously Use 4 Methods to Separate PD from Noise High pass filter (noise predominantly <20 MHz) Time of pulse arrival between a pair of couplers Pulse shape analysis (risetime and degree of pulse oscillation) Surge impedance mismatch 25

28 Iris On-Line PD Applications Hydrogenerators: (PDA-IV, HydroTrac, GenGuard) Over 4000 installations Using Differential noise separation technique Motors & Small (<200MW) Turbos: (TGA-B and BusTrac/PDTrac/BusGuard) Over 6000 installations Using Directional noise separation technique Large (>200MW) H2-Cooled Turbos: (TGA-S, TurboGuard) Over 1000 installations Using Stator Slot Coupler technique PD Testing for Hydrogenerators 26

29 PD Sensors for Hydrogenerators Epoxy-Mica Capacitors (EMCs) Directly connected to HV circuit ring bus 80 pf creates a HPF with cutoff of 40 MHz Small hydro 2 per phase Large hydro 1 per parallel Differential Time-of-Arrival (Noise Elimination) 27

30 Differential Time-of-Arrival (Partial Discharge Detection) Capacitive Sensors (Hydrogenerator) 28

31 PD Testing for Motors and Smaller Turbogenerators PD Sensors Epoxy-Mica Capacitors (EMCs) Directly connected to HV circuit ring bus 80 pf creates a HPF with cutoff of 40 MHz Motors-1 per phase TGs 2 per phase 29

32 Directional Time-of-Arrival Noise Separation (Double-Ended Installation) Directional Time-of-Arrival Noise Separation (Double-Ended Installation) 30

33 Pulse Shape Noise Discrimination (Single-Ended Installation) External noise pulses are attenuated and the risetime gets longer as pulses propagate through a power cable 31

34 EMCs at machine terminals 25 kv Couplers on Large Turbo 32

35 PD Sensors for Large (>200MW) Turbogenerators Capacitive couplers on stator terminals may give false indications if sparking near the terminals or from stator core Stator Slot Couplers (SSCs) will not give such false indications SSCs MHz broadband antenna Installed in slots with line end bars 1 coupler per parallel per phase No high voltage connection Stator Slot Coupler: Cross-Sectional View 33

36 Stator Slot Coupler PD pulses are unipolar and have a width < 6 ns 34

37 Noise pulses are oscillatory and have a width >6 ns TGA-B Portable Partial Discharge Analyzer 35

38 Continuous PD Monitor Basic Interpretation of PD Results Instrumentation provides pulse magnitude analysis (2D) and pulse phase analysis (3D) plots Also produce summary indicators: Qm for peak PD magnitude and NQN for total PD activity 36

39 Importance of Noise Separation Asset Class: Combustion Turbine, Sensor Type: Epoxy Mica Capacitor (80pF) Operating load: 46.4 MW, Reactive Load: 18.9 MVar, Operating Asset Temp 81 deg C, Operating Voltage 11.5 kv Operating Gas Pressure: N/A, Ambient Temp 25 deg C, Ambient Humidity: 55 %, Freq. (Test Duration) 50 Hz. (5 sec.) Manufacturer: --- Year of Installation: 1997, Winding Manufacturer:, Re-Wind Year: Pulse Density Linear Plot Bipolar Machine PD Pulse Density Linear Plot Bipolar Total System Noise 0 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps 0 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps 100 to 316 pps 316 to 1000 pps > 1000 pps 100 to 316 pps 316 to 1000 pps > 1000 pps Pulse Magnitude [mv] Pulse Magnitude [mv] Phase Angle [deg] Phase Angle [deg] Phase: C, Sensor(s): C-M3,C-S3 Delay 11ns, Status OVR,HNPR Machine: NQN+: 144, NQN-: 63, Qm+: 77, Qm-: 36 Start Time: 6/11/2009 9:48:53 AM System: NQN+: -, NQN-: -, Qm+: -, Qm-: - Finding Deteriorated Stators Compare plots from identical machines (stator with highest PD is most deteriorated) Compare test result from a machine to a similar machine in Iris database Trend PD over time (2 times increase per year is significant rate of deterioration) 37

40 Pulse Height Analysis (PHA) 2D Graph: 13.8kV Stator with Low PD Unit PD Noise Qm: Peak PD magnitude in 10 pps Pulse Height Analysis (PHA) 2D Graph: 13.8kV Stator in Same Plant with Very High PD Unit PD Noise Pulse Magnitude (mv) Pulse Magnitude (mv) Qm: Peak PD magnitude in 10 pps 38

41 PD Database Over 272,000 test results from thousands of machines Each year Iris publishes the statistical range of Qm (peak PD) for each type of stator (voltage rating, air or hydrogen cooling, and PD sensor type) If a stator has a Qm that exceeds 90% of readings then winding is deteriorated False indication rate <1.5% PD Alarm Levels Air-Cooled Machines (80 pf sensors) Voltage Class PD Magnitude (mv) 2-4 kv kv kv kv

42 Trending Most powerful, reliable means of interpreting data Collect data over the years, if < 25% increase, winding is stable If Qm doubles in 6 to 12 months, rate of deterioration increasing Can also determine if repairs effective Need to do repeat tests under the same voltage, load, and temperatures Normally, high PD occurs for at least 2 years before failure Q m = Peak PD magnitude 10 pps PD -PD Q m (mv) Note: Indication on the Quality of maintenance can be assessed Nov-88 Jul-89 Apr-90 Dec-90 May-91 Oct-91 Apr-92 Oct-92 Jan-93 Apr-94 Nov-94 Clean & Rewedge May-95 Nov-95 Mar-96 40

43 Typical Trend in PD over Winding Life Determining Failure Mechanisms If know deterioration processes occurring, can determine what repairs to do and when If a dominant failure process, PD analysis can often determine root cause from phase position of PD, PD pulse predominance, effect of load, temperature and humidity Significant advances in the past 5 years to make the determination of the failure process more objective 41

44 On-Line Monitoring of Stator Endwinding Vibration Greg Stone Iris Power Qualitrol 42

45 Endwinding Design In two pole generators bars in EW are very long, and can not be rigidly supported by metallic structures due to high voltages 60 Hz current in two adjacent conductors create 120 Hz magnetic forces (or 50 Hz current creates 100 Hz force) Forces are in radial and circumferential (tangential) directions In addition, frame vibration can result in 50/60 Hz (once per revolution) vibration of the stator EW Support rings, blocking and bracing is needed to prevent movement in radial and circumferential directions 43

46 Causes of EW Vibration 1. In older stators, insulating blocking and bracing material shrinks, loosening up EW support 2. Large fault currents can lead to dramatic relative movement of the stator bars causing loosening of the support system, allowing vibration in normal service 3. Inadequate EW support design to reduce cost (fewer support rings, less blocking, long unsupported jumpers to circuit ring busses, use of inferior materials, not allowing for axial expansion during load changes) 4. OEM not ensuring endwinding has no 50/60 and 100/120 Hz resonances See 2011 EPRI report Generator Stator Endwinding Vibration, Report PID Examples Stator EW Vibration Problems Insulation fretting Insulation Greasing Fatigue cracks in conductors 44

47 Failure of the series connection between bars initiated by EW vibration Recent Issues In the past decade, many OEMs have reduced cost by providing less robust endwinding support Result is a dramatic increase in EW vibration problems, especially in large air-cooled, 2 pole turbo generators If EW vibration not detected before failure then considerable collateral damage can occur since it often results in phase to phase faults or broken copper conductors that open under load 45

48 Detecting EW Vibration Problems Visual inspection of EW to look for dusting, fretting, greasing Performing periodic bump testing to make sure no shift in resonant frequencies to the 50/60 Hz and 100/120 Hz regions, or changes to the damping factor Above requires an outage and removal of endshields Alternative is continuous on-line monitoring of EW vibration using fiber optic probes Requirements for On-Line EW Vibration Monitoring Accelerometers and leads must be of the optical type Fiber optic lead from probes must not be sensitive to vibration itself Probes must be installed at locations most likely to vibrate i.e. where the bars or jumpers have the longest unsupported lengths Determine optimum locations using bump testing and analysis Wide band accelerometers to cover frequency range of Hz, although response up to 1000 Hz desirable If on-line results do not show peaks at the power frequency and twice power frequency, then the results are likely inaccurate due to wrong sensor location and/or false readings due to glass fiber lead movement 46

49 Choosing Vibration Probe Locations Mainly at connection end (non-drive end) of stator, since jumpers to circuit ring bus are likely problem spots, although some failures have occurred at drive end Preferably have probes sensitive to both radial and circumferential vibration (radial normally most important) The probe locations should NOT be just placed symmetrically around the EW, or just where the fretting is occurring (maximum vibration locations are often away from blocking locations) Choosing Vibration Probe Locations In outage, temporarily install a conventional accelerometer on one stator bar away from the core, and bump bars symmetrically around the circumference in 2 or 3 planes At these worst areas do a local bump test to find which locations have the greatest vibration at 50/60 and 100/120 Hz Note that resonant frequencies will decrease as temperature increases (eg resonant frequency may drop 12 Hz over 70 C increase) 47

50 Performing a Bump Test on 18 kv, 200 MVA Stator Endwinding [(m/s^2)/n] Bump Test Results on 18 kv, 200 MVA Stator color represents axis; upper plot is phase, lower plot is amplitude ( note that resonances near 60 and 120 Hz ) Frequency Response H1(4524 B-xyz.x,8207 Ref.) - STS Measurement 1 Frequency Response H1(4524 B-xyz.y,8207 Ref.) - STS Measurement 1 Frequency Response H1(4524 B-xyz.z,8207 Ref.) - STS Measurement [Hz] Cursor values X: Hz Y(Mg):71.549m (m/s^2)/n y(ph): degrees Markers Marker1: 49Hz,0.132(m/s^ Marker2: 56Hz,0.429(m/s^ Marker3: 123Hz,0.544(m/s 48

51 Installation of fiber optic probes One Sensor - Radial Two Sensors - Radial & Tangential One Sensor - Radial One Sensor - Radial Installation of fiber optic probe 49

52 Continuous EW Vibration Monitor On-line spectrum showing 60 Hz and 120 vibrations from an operating TG 50

53 Interpretation What EW vibration is permissible has not been standardized Past published levels may have been erroneous due to issues with poor probe location and/or vibrating fiber optic probe leads In general total vibration amplitude < 4 mil peak to peak is considered acceptable; >10 mil is cause for some concern An increasing vibration trend is also of concern Conclusion Reliable, on-line methods now available to detect most rotor and stator winding faults Usually detect problems months or years before failure Enables proactive repairs avoidance of unnecessary shutdowns for testing/repairs when windings good 51

54 Notes: Notes: 52