on-line condition monitoring of motors and generators

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.

El Dr. Stone se gradúa en Canadá en 1975. 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

www.unitronics-electric.com

Avenida de la Fuente Nueva, 5

San Sebastián de los Reyes. 28709 MADRID

Tel. 91 5400127 [email protected]


1

Rotor and Stator Winding Condition Monitoring

Greg StoneIris Power

[email protected]

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


2

Rotor Windings

• Synchronous rotors for turbo and hydro generator

• Squirrel cage rotors for induction motors

Turbogenerator Round Rotor


3

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


4

Round Rotor Insulation failure

• Thermal aging due to poor design, over-excitation 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)


5

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

Pole B

SlotLeakage

Flux

QuadratureAxis

Flux Probe


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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.


7

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


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New Portable Measuring Instrument RFA-II


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FluxTrac Remote, Automatic Rotor Shorted Turn Detector


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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


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TF Probe

Installation of Flux Probeson Stator


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New Portable Measuring Instrument RFA-II

Shorted Turns on Poles 8 and 48

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63 62 6160

5958

5756

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3635343332

31302928

2726

2524

2322

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2019

1817

16

1514

13

1211

109

87

65

4 3 2 1


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Stator Winding Monitoring

• On-line partial discharge (PD)• On-line endwinding vibration

Stator winding - 2nd most frequent reason for machine failure


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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


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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 10-15 cm beyond core

• Needed on most machines >6 kV, and some modern machines >3.3 kV

Semicon Coatings to Prevent PD


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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 Motor stator turn insulation failure


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Semicon-Grading Coating PD


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Loose coils in the slot

PD Due to Insufficient Spacing


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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)


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What Are Partial Discharges?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-gCin

Cin

CairVair

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


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Partial Discharges

• The higher the over-voltage the higher the intensity of the discharge

• Discharges also affect carbon bonds in resin

• PD is very dependent on voltage

60 Hz Phase to ground voltage

Sustain

Voltage across void

Over -voltage

PD pulses measuredon operating generator


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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 OnBrief History of On--Line PD TestingLine 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 60034-27-2 standards


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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


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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


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PD Sensors for Hydrogenerators

Epoxy-Mica Capacitors (EMCs)

Directly connected to HV circuit ring bus80 pF creates a HPF with cutoff of 40 MHzSmall hydro – 2 per phaseLarge hydro – 1 per parallel

Differential Time-of-Arrival(Noise Elimination)


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Differential Time-of-Arrival(Partial Discharge Detection)

Capacitive Sensors (Hydrogenerator)


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PD Testing for Motors and Smaller Turbogenerators

PD Sensors

Epoxy-Mica Capacitors (EMCs)

Directly connected to HV circuit ring bus80 pF creates a HPF with cutoff of 40 MHzMotors-1 per phaseTGs – 2 per phase


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Directional Time-of-ArrivalNoise Separation (Double-Ended

Installation)

Directional Time-of-ArrivalNoise Separation (Double-Ended

Installation)


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Pulse Shape Noise Discrimination(Single-Ended Installation)

External noise pulses are attenuated and the risetime gets longer as pulses propagate through a power cable


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EMCs at machine terminals

25 kV Couplers on Large Turbo


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PD Sensors for Large (>200MW) Turbogenerators

Capacitive couplers on stator terminals may give false indications if sparking near the terminals or from stator coreStator Slot Couplers (SSCs) will not give such false indicationsSSCs

1-1000 MHz broadband antennaInstalled in slots with line end bars1 coupler per parallel per phaseNo high voltage connection

Stator Slot Coupler: Cross-Sectional View


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Stator Slot Coupler

PD pulses are unipolar and have a width < 6 ns


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Noise pulses are oscillatory and have a width >6 ns

TGA-B Portable Partial Discharge Analyzer


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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


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Importance of Noise Separation

Phase: C, Sensor(s): C-M3,C-S3 Delay 11ns, Status OVR,HNPRMachine: NQN+: 144, NQN-: 63, Qm+: 77, Qm-: 36

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 kVOperating 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:

Start Time: 6/11/2009 9:48:53 AMSystem: NQN+: -, NQN-: -, Qm+: -, Qm-: -

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Pulse Density Linear PlotBipolar Machine PD

Pul

se M

agni

tude

[mV

]

Phase Angle [deg]

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

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-225 -180 -135 -90 -45 0 45 90

Pulse Density Linear PlotBipolar Total System Noise

Pul

se M

agni

tude

[mV

]Phase Angle [deg]

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

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)


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Pulse Height Analysis (PHA) 2D Graph:

13.8kV Stator with Low PD13.8kV Stator with Low PD

Qm: Peak PD magnitude in mV @ 10 ppsQm: Peak PD magnitude in mV @ 10 pps

Unit PD Noise

13.8kV Stator in Same Plant with Very High PD13.8kV Stator in Same Plant with Very High PD

Qm: Peak PD magnitude in mV @ 10 ppsQm: Peak PD magnitude in mV @ 10 pps

Pulse Height Analysis (PHA) 2D Graph:

250 500 750 1000 1250 1500Pulse Magnitude (mV)

250 500 750 1000 1250 1500

101

102

103

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101

102

103

104

Pulse Magnitude (mV)

Unit PD Noise


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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 274

6-8 kV 276

10-12 kV 401

13-15 kV 461


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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

Qm = Peak PD magnitude taken @ 10 pps

0

100

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1000

Nov-88

Jul-8

9

Apr-90

Dec-90

May-91

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Jan-9

3

Apr-94

Nov-94

Clean &

Rewed

ge

May-95

Nov-95

Mar-96

Qm

(mV)

+ PD

- PD

Note: Indication on the Quality of maintenance can be assessed


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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


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On-Line Monitoring of Stator Endwinding Vibration

Greg StoneIris Power – Qualitrol

[email protected]


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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


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Causes of EW Vibration1. In older stators, insulating blocking and bracing material

shrinks, loosening up EW support2. 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 1021774

Examples Stator EW Vibration Problems

Insulation fretting

Insulation Greasing

Fatigue cracks in conductors


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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


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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 20-400 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


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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 12-24 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)


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Performing a Bump Test on 18 kV, 200 MVA Stator Endwinding

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 )

0 40 80 120 160 200 240 280[Hz]

0.10.20.30.40.50.60.70.8

160800

-80-160

[(m/s^2)/N]

Cursor valuesX: 240.000 HzY(Mg):71.549m (m/s^2)/Ny(Ph):22.179 degreesMarkersMarker1: 49Hz,0.132(m/s^Marker2: 56Hz,0.429(m/s^Marker3: 123Hz,0.544(m/s

Frequency Response H1(4524 B-xyz.x,8207 Ref.) - STS Measurement 1Frequency Response H1(4524 B-xyz.y,8207 Ref.) - STS Measurement 1Frequency Response H1(4524 B-xyz.z,8207 Ref.) - STS Measurement 1


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Installation of fiber optic probes

One Sensor - Radial

One Sensor - Radial

One Sensor - Radial

Two Sensors -Radial & Tangential

Installation of fiber optic probe


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Continuous EW Vibration Monitor

On-line spectrum showing 60 Hz and 120 vibrations from an operating

TG


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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


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Notes:

Notes:

Documents

on-line condition monitoring of motors and generators