monitoring of shaft voltage and shaft grounding current - beth galano (mps)
TRANSCRIPT
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Magnetic Products and Services, 2135 Highway 35, Holmdel, NJ 07733 USA
Introduction to MPSand
Monitoring of Shaft Voltage
and Shaft Grounding Currentin
Rotating Machinery
By
Paul Nippes, P. E. and Beth Galano
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Founder and President, Mr. Paul Nippes,
P.E. has been consulting to
Petrochemical, Power Utilities and
Industry since 1965.
IEEE C50 Chairman for many years,
Cigre, ASNT and ASTM Contributor.
A Motor and Generator designer with Allis
Chalmers and Elliott Company, Electro
Dynamic Division of General Dynamics, a
Penn State Graduate and a Professor at
the University of Wisconsin.
In Pauls Electrical Engineering consulting capacity, due to the frequency
of bearing failures in his consulting practice, Mr. Nippes championed this
research and developed products to provide solutions.
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Areas of specialization
Magnetism and the problems it can cause in Rotating
Machinery, Industrial Processes, or in any application.
Shaft Condition Monitoring - CBM using Stray Shaft
Currents and Shaft Voltages in Rotating Machinery for
Early Warning of Developing Faults.
Field Services Degaussing, Magnetizing and Shaft
Voltage Testing
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SHAFT CURRENT FROSTINGShaft Journal Bearing Pad
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SHAFT CURRENT EFFECTS
Spark tracks Gear Pitting
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Electric Current Erosion of Thrust Bearing Pads
from a Turbine Driven Natural Gas Compressor
Spark Tracks
on the Active Pad FacePad Steel Backing Erosion
on Two Adjacent Pads
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Spark tracks Developing into Heavy Current Discharge Melting into the Babbitt
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Degaussing Turbine Diaphragms Degaussing Blower Casing
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Demagnetizing Screw Compressor
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Auto Degauss
Auto Down-Cycle
Demagnetizing is SystematicRemoval of Magnetism
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Measuring Magnetism MPS Gaussometer
Avoid Field Indicators (Magnetometer)
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Red No Weld, Yellow Maybe?, Green Good to Weld
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South China Sea
On Barge Vertical Pipe Welding Magnetic Field Stabilization
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Portable Magnetic Particle InspectionsApplied to Military Impellors
Procedures Developed by an ASNT Level III (MPS Customer)
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Military Applications Degaussing Solutions
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Grounding Brush Performance ChecksShaft Voltage Surveys
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Failure Analysis and Solution Recommendations
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Carbon brush contact surface comparison
Normal Glazed
Figure #4
G
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1. Ground the Shaft
2. Check Grounding
Devices Often via
Local or Remote
Monitoring
MP G b b
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Grounding Brush Reliability is Critical
MPS G b d b
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SHAFT CONDITION MONITORING (SCM) a Patented TechnologyA continuous monitoring technology that uses shaft current and voltage profiles in
operating units for early diagnostics and prognostics to identify developing problems in
electrical and non-electrical rotating machinery.
Sensors = Grounding Brushes
Permanent Mount Monitors, Rack
Mount, Din-Rail Modules or
Hand-Held Monitors
MPS G b d b
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VCM-E disposes of numerous and varyingfrequencies and amplitudes for the purpose of
sensing only the important signal changes in
waveform fundamental frequency, peaks and
averages.
At the earliest changes and/or an excess in
these quantities, action can be taken to identify
and confirm possible problems.
Confirmation may include collection and
analysis of other unit operating parameters.
MPS Signal Conditioning
MPS G b t d t b
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Shaft Grounding And Voltage Sensing
Applied To Industrial Class Rotating Machinery
MPS G b t p d t b
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HP TURB. IP TURB. LP TURB.
GENERATOR
EXCITER
NO BRUSHES NO MONITORS AND NO PROTECTION
Shaft currents are not confined nor controlled.
Shaft current damage to bearings and seals is not prevented.
Bearing and seal oil films insulate against stray currents.
Stray voltages can damage unit or impair efficiency .
Events or oil impurities can establish electrical circuits.
MPS Ga ssb st s a p d t b
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HP TURB. IP TURB.LP TURB.
GENERATOR
EXCITER
A quality grounding brush should discharge static electrical charge before damage occurs.
A quality grounding brush should establish good shaft contact protecting the nearby bearings.
The grounding brush could provide a return path for magnetically-generated current.
The brush may require maintenance, with loss of shaft grounding without warning
Damage can occur to bearings and/or seals if the generator outboard bearing insulation fails!
Shaft current damage to bearings and seals is first indicated by excessive vibration/temperature.
SHAFT GROUNDING BRUSH
GROUNDING BRUSH, NO MONITOR, POSSIBLE PROTECTION
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Shaft Voltage Decrease And Current Increase Indicates Shaft Rub.
High Voltage At Sb#1, Inversely Proportional To Steam Temperature Indicates Wet Steam In Turbine.
High Current In Grounding Brushes And Low Voltage At Brush Sb#2 Indicates Loss Of Bearing Or Seal Insulation .
Rapid Escalation In Both Sb#2 Voltage And Grounding Currents Indicates Developing Stator Winding Fault.
Erratic And Pulsing Voltage And Current Indicate Stator Lamination Shorting And Melting .
Long Term Gradual Increases In Voltage And Current Indicate Shift In Air Gap.
Pulsing, Then Gradual Increase In Shaft Voltage And Current Indicate Rotor Winding Faulting.
Shaft Voltage Changes As Excitation Changes May Indicate The Need For A Shaft Voltage Harmonic Filter.
Trending Of Each Brush Grounding Current, With Maximum Current Alarm On Each Brush.
Each Brush Will Alarm On Low Brush Grounding Current Indicating Brush Or Grounding Maintenance Required.
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Oscilloscope recordings
taken from the identical
shaft grounding currentbut with different voltage
and time scaling.
Upper trace =
112mV @ ~ 70MHz
Lower trace =
720mV @ ~ 80Hz
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Ungrounded OB end similar to IB end
OB trace, IB end Ground brush skip
Ungrounded IB end similar to OB end
Grounded IB end skip not apparent
Erratic grounding on shaft (Voltage source = The blower motor)
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Oscilloscope trace ofshaft grounding current
composed of a 60Hz
fundamental plus a strong
common mode buckingthird harmonic.
Characteristic of
magnetic saturation
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Ungrounded (40 Volts)
1k Ohm (40 Volts)
100 Ohm (25 Volts)
10 Ohm (5 Volts)
Effect on shaft of Electromagnetic
Voltages, determined from different
resistances in the shaft grounding cable.
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Effect on shaft of electrostatic
voltages determined from different
resistances in the shaft grounding cable.
Ungrounded (30 Volts)
10k Ohm (4 Volts)
1k Ohm (0.5 Volts)
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750MW GENERATOR VCM OUTPUT INDICATING A
UNIT OR SYSTEM RECURRING CONDITION
CHART SCALES - ZERO TO 1.0 VOLTS AND 10.0 AMPERES
ONE HOUR BRUSH CURRENT
Figure #11
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Ground Fault Condition at a Nuclear Power Plant and Root Cause
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Root
Cause
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Unit 1 Shaft Peak
Currents and
Voltages for Six
Days
Grounding brush
Currents = 3.1 and
8.7 Amperes
Generator OB End
Shaft Voltage = 23.4
Volts
Turbine Shaft
Voltage = 0.2 Volts
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Unit 2 Shaft Peak
Currents and Voltages
over a Day
Grounding brush
Currents = 2.2 and 6.7
Amperes
Generator OB End
Shaft Voltage = 14.0
Volts
Turbine Shaft Voltage
= 4.8 Volts
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500 MW
Turbine
generator
Recorded Field
Voltage and
Peak ShaftGrounding
Current
During a Field
Ground Fault
Figure #14
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MPS Family of Shaft Condition MonitorsforEarly Detect ion of Rotat ing Machinery Faul ts
VCMVCM--ENEN--SIISII Single
Enclosure Housing TwoSeparate VCM-E
Boards/Circuits For
Large Or Critical Units.
SCModulesSCModules ,, Either Voltage or Current, Battery orAC Power, Local Alarm Indications; Remote Analysis
HandHand--Held VCM orHeld VCM or SCMeterSCMeter
or use with SCModules, Displays
Voltage or Current Peaks and Averages
Also Avai lable
(Explosion-proof
Models)
VCMVCM--CC,, Battery Power,Local Display, Ungrounded
Peak-Voltage And Grounded Peak and Average
Ground Current, Max I = 3 Amps. Industrial Type
Applications.
VCMVCM--EE,,AC Powered, Local
Display, Alarm LEDs of Shaft
Grounding Current And Shaft Voltage.
Continuous Signals via 4-20 mAfor
use in Control Room, via wired or
wireless connections.
VCMVCM--ENEN--RMRMRack Mount Modelconfigurable with 1 to 4
VCM-E units.
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Grounding Brush or Ground has lost continuity.
Wet steam conditions at the exhaust end of the turbine
causing charge separation with possible blade erosionand cavitation.
Dry Steam conditions at the inlet of the turbine with partial
admission, caused charge separation and spark
discharge.
Self Excitation or compounding of shaft voltages and
grounding currents from shorted rotor coil.
SHAFT MONITORS provide Early Warning
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Shaft Rubs at bearings, seals due to oil whip, coupling
misalignment, thermal growth, loss of lubrication, impurities in
the oil, etc.
Diode Failure in Generator Excitation.
Intermittent Rotor Winding Ground Fault
Loss of or Shorting of Insulation at Bearings, Seals, Gear Teeth
and Coupling, etc.
Excessive transients in pulse width modulated rotor and/or
stator electrical supply.
Stator core lamination shorting in an electrical generator.
SHAFT MONITORS provide Early Warning
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Simplicity and Value - SCM reduces data to basic terms (a fractionof the digital data), while still preserving critical data events
Supported by technology and performance of units installed inthe Utility and Petrochemical Industries.
Offers significant enhancement to PDM and Prognostic programs
of Utilities, Industry and the Military.
It could be an important input to the decision-making processas to whether a rotating machine is Operational or Service
Ready.
Alerts at earliest possible problem development requiring confirmation,
investigation
and possible parts replacement
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