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NewOnlineMotorStatorInsulationMonitor(MSIM)for3500System
C. David Whitefield, p.E.
BentlyNevadaPrincipalEngineer
dave.whitefield@ge.com
ManyofourcustomersemploymediumandlargeACmotors
(suchastherepresentativeexampleinFigure1)asprime
moversfortheirprocessmachinery,drivinglargecompressors,
pumps,blowersandfans.Traditionally,manyofthesemotors
havebeenmechanicallyprotectedandmanagedusingBently
Nevada3500vibrationandconditionmonitoringsystems.The
majorityofmotorsinthisclassemployfluidfilmbearings.
The Problem
The3500Systemiseffectivefor
vibration monitoring of motor
rotor and bearing faults, but
another common problem
with motors is the degradation
ofstatorwindinginsulation.
Statorproblems,combinedwith
bearingproblems(detected
byvibration),constituteover
75%ofmotorfailures.Theneed
to address the stator failure
modeisobvious,yetthereare
few,ifanyonlinesystemswhich
meetthatneed.Existingcondition
monitoring techniques for this class
of motors fall into one of two catego-
ries–offlineoronlinemonitoring.
Offline Monitoring
Thistypeoftestingisdonewiththe
motor shut down, cooled down, and
de-terminated, using portable test
equipment.Testsinthiscategory
include the following examples:
• Capacitance and dissipation factor
(C&DF)testing,whichareboth
conductedatambienttemperature.
• Megohmmeter(“megger”)for
InsulationResistance(IR)and
polarizationIndex(PI),ACandDC
high-potential(“hi-pot”),Partial
Discharge(PD),PowerFactoror
DissipationFactor(tip-up)and
other electrical tests designed
to assess the condition of the
statorinsulationsystem.
• PartialDischargeAnalysis:This
measurement looks for indications
oftinyarcsthatoccurwithin
voids and gaps in the winding
insulation as it deteriorates
overtime.Bothpermanently-
installed and portable versions
ofPDinstrumentsareused.
Other techniques can be used
to complement the above tests
for more effective diagnostics
andhealthassessment.
FIGURE 1:GEPegasus**MHVMedium Voltage AC induction Motor [Reference 1]
8 ORBIT Vol .32 • No.1 • Jan.2012
NEWS
Online Monitoring
Thistypeofmonitoringisdonewith
the motor energized and running,
usuallyatasignificantfractionof
fullload.Onlinemonitoringcan
beperformedwithpermanently
installed instrumentation, or
withportabletestequipment.
• Ground/phasefaultrelays:These
are classic machine protection
relays,whichwereoriginally
electromechanical devices, and
evolved into solid-state analog,
thendigitaldevices.Protective
relaysarepermanentlyinstalled
to provide real-time automatic
protective measures for detected
electricalfaults.Moderndigital
relayscanalsoprovidesome
condition monitoring data via
digitalnetworkcommunications.
• PartialDischargeAnalysis(PDA):
This measurement looks for
indicationsoftinyarcsthatoccur
within voids and gaps in the
winding insulation as it deteriorates
overtime.Bothpermanently-
installed and portable versions
ofPDinstrumentsareused.
• Temperature, moisture and
other parameters can also be
continuouslymonitored.
Tradeoffs
Offline testing is time consuming,
relativelyexpensive,andrequiresthe
process equipment to be removed
from service while the tests are
conducted.Forthesereasons,
testsareperformedinfrequently,
with inspection intervals of 3 to 6
yearsbeingcommon.Thisschedule
means the inspection interval is
the same order of magnitude as
thefailureinterval.Inaddition,the
offlinetestsaretypicallyconducted
at ambient temperatures, not at the
operatingtemperatureofthemotor.
Protection with ground/phase fault
relaysiseffectiveinshuttingthe
machine down after a fault occurs,
but does not give adequate advance
warningofinsulationdegradation.
in some instances the stator core
canbedamagedbyanelectrical
fault in spite of the shutdown
capabilitiesofprotectionrelay
systems.Coredamageresultsina
much more expensive repair than a
basic rewind, and in some cases the
motormayhavetobescrapped.
Partial discharge monitoring is the
onlycurrentlyavailablepractical
technologyforonlinecondition
monitoringofstatorinsulationhealth.
Feedbackfromourcustomerswho
haveemployedthistechnology
formanyyearsindicatesthat
partial discharge data is difficult
tointerpret,andprovidesverylittle
insight into, or advance warning of
impendingstatorinsulationfaults.
introducing a new Approach
BentlyNevadaisintroducinganew
approach to online stator insulation
conditionmonitoring.Thisapproach
is based on a new sensor developed
inconjunctionwithGE’sGlobal
ResearchCenterscientists.Itisa
completesystemconsistingofnew
transducers, a new 3500 monitor
card and the services required for
installationandcommissioning.While
it is useful as a standalone monitor,
even more value can be obtained
whenitisconnectedtoGE’sSystem
1*assetmanagementsoftware.
SystemDescription
AsshowninFigure2,thenew
statorinsulationmonitoringsystem
includes the following components
for each monitored motor:
• 3each–HighSensitivity
CurrentTransformers(HSCTs)
• 3each–HSCTinterfacemodules
• 2each–Voltagedividers
(forphasereference)
• 2each–Voltagedivider
interface modules
• 1to3each–temperatureinputs
(RTD’sorthermocouples)
• 1each–BN3500Rackand
HSCTmonitorcard
TheHSCTs,voltagedividersand
all interface modules are installed
inoronthemotorterminalbox.
Fieldwiringdirectsthesignals
tothe3500monitorcard.
Jan.2012 • No.1 • Vol .32 ORBIT
NEWS
FIGURE 2:Permanently-installedonlinemotorstatorinsulationhealthmonitor.
FIGURE 3:TheHSCTisaspecialcurrenttransformerthatis verysensitivetosmallvaluesofdifferentialcurrent.
FIGURE 4:Equivalentcircuitforstatorwindinginsulation. C = capacitance, R = resistance, V = source voltage and I=sourcecurrent.
FIGURE 5: Phase angle relationship between capacitive and resistiveleakagecurrent.
FIGURE 6:Astheinsulationsystemdegrades,thechangeincapacitanceanddissipationfactorareindicatedbythechangeinthephaseanglebetweenICandIR.Thisexampleshowsacasewhere capacitance remains unchanged, but aging increases the conductionthroughtheinsulation.
FIGURE 7:Examplephotoshowstheinsideofa4160VmotorterminationenclosureduringtestingofthenewHSCTsensors.ThelargebrownCTsarefornormaldifferentialprotection.TheHSCTsarethethinneraluminum-coveredringstotherightoftheprotectionCTs.TotherightoftheHSCTsaresometestinstrumentation CTs that were taking additional measurements aspartofthetest.
AC Voltage Source
V C R
I
IC IR
Dissipation factor = tan δ
= IR/IC
= (V/R)/(VωC)
= 1/ωRC
IC I
IR V
δ
IC IC IC IC
30º20º10º5º
IRIRIRIR
HSCT
Protection CT
neutral lead
line lead
10 ORBIT Vol .32 • No.1 • Jan.2012
NEWS
Howdoesitwork?
TheHSCT(Figure3)enables
measurementofverylowamplitude
leakagecurrent(whichleaksthrough
degradedwindinginsulation).
TheHSCTinterfacemoduleamplifies
the low level signal, which is directed
to the 3500 monitor card via field
wiring.Voltagereferencesignalsare
similarlyconditionedanddirectedto
the3500monitor.Windingtempera-
tures(fromRTD’sorthermocouples)
are the final inputs to the monitor
card.Themonitoringsystemcondi-
tions and processes these signals,
providing access, trending and alarm-
ing for values of capacitive and resis-
tive leakage currents, Capacitance
andDissipationFactor(C&DF).
TheofflineC&DFtestisusedby
manyofourcustomersasapartof
their medium and high voltage motor
preventivemaintenanceprograms.
Ournewmonitoringsystembringsthe
benefits of that assessment tool to
theonlineconditionmonitoringworld.
Figure4illustratesthebasic
relationship between capacitive
and resistive components of the
leakagecurrent.Inaneworrewound
motor,theprimaryleakagepath
iscapacitive,resultinginverylow
levelsofresistiveleakagecurrent.
Figure5showshowtheDissipation
Factordescribesthephaseangle
of the current through the stator
insulation.Iftheinsulationwere
a perfect dielectric, its resistance
would be infinite, the angle, δ,
would be zero, and dissipation
factorwouldalsobezero.
Insulationsystemsdegradeover
time because of electrical, thermal
and mechanical and environmental
stresses.Astheinsulationsystem
degrades, the resistive component
increases, appearing as a larger
dissipation factor, as shown in
Figure6.Theleakagecurrent
measurement is temperature
dependent, thus the need for the
temperatureinputsintothemonitor.
Target Machines
Our initial solution offering
targets 3-phase AC induction and
synchronousmotorsinthe1,000to
6,000 horsepower range, operating
withsupplyvoltageinthe2.3kV
to5kVrange.Themotormustbe
externallywye-connected(Figure
2).Wemusthaveaccesstoboth
phase and neutral leads in the
terminalbox,asshowninFigure7.
Value of this method
Thisnewtechnologyisthefirstcom-
merciallyavailableonlineassessment
ofstatorinsulationsystemhealth
on medium and high voltage motors
vialeakagecurrentsensing.This
meansthatyounolongerhaveto
shutyourmotordownforoffline
testing to determine if it is headed
fortrouble.Thesystemallowsyouto
realize the following advantages:
• Avoid unplanned outages
• Do more effective main-
tenance planning
• Avoid offline monitoring
downtime and associated costs
• Detectmanyproblemsthatarenot
detectedbyexistingtechnologies
• Extend time between inspections
• Reduce the cost of repair
versusaprotectiontrip,by
avoiding stator core damage
SystemIntroduction
Our current development plan calls
foravailabilityinthethirdquarter
of2012.Pleasecontactyourlocal
BentlyNevadasalesengineerfor
moreinformation.We’llalsobe
publishing more information on this
technologyinanupcomingissueof
Orbit.Staytunedformoreinformation
onmotorconditionmonitoring!
References
1.GEMotorsPegasusMHVMedium
Voltage AC induction Motors
brochure,GEA-12310C.
*DenotesatrademarkofBentlyNevada,Inc.,awhollyownedsubsidiaryofGeneralElectricCompany.
Copyright©2012GeneralElectricCompany.Allrightsreserved.
**PegasusisatrademarkofGeneralElectricCompany.
Jan.2012 • No.1 • Vol .32 ORBIT 11
NEWS
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