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]

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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.

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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

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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.

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