rms queen mary 2 report no 28/2011
TRANSCRIPT
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MA
RIN
E A
CCID
ENT
INV
ESTI
GAT
ION
BRA
NCH
AC
CID
ENT
REP
OR
T
LESS SERIOUS MARINE CASUALTY REPORT NO 28/2011 December 2011
Report on the investigation of
the catastrophic failure of a capacitor
in the aft harmonic filter room on board
RMS Queen Mary 2while approaching Barcelona
23 September 2010
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Extract from The United Kingdom Merchant Shipping (Accident Reporting and Investigation)
Regulations 2005 – Regulation 5:
“The sole objective of the investigation of an accident under the Merchant Shipping (Accident Reporting and Investigation) Regulations 2005 shall be the prevention of future accidents through the ascertainment of its causes and circumstances. It shall not be the purpose of an investigation to determine liability nor, except so far as is necessary to achieve its objective, to apportion blame.”
NOTE
This report is not written with litigation in mind and, pursuant to Regulation 13(9) of the Merchant Shipping (Accident Reporting and Investigation) Regulations 2005, shall be inadmissible in any judicial proceedings whose purpose, or one of whose purposes is to attribute or apportion liability or blame.
© Crown copyright, 2011
You may re-use this document/publication (not including departmental or agency logos) free of charge in any format or medium. You must re-use it accurately and not in a misleading context. The material must be acknowledged as Crown copyright and you must give the title of the source publication. Where we have identified any third party copyright material you will need to obtain permission from the copyright holders concerned.
Front cover photograph (Queen Mary 2) courtesy of Jörn Prestien.
All MAIB publications can be found on our website: www.maib.gov.uk
For all enquiries:Marine Accident Investigation BranchMountbatten HouseGrosvenor SquareSouthampton Email: [email protected] Kingdom Telephone: +44 (0) 23 8039 5500SO15 2JU Fax: +44 (0) 23 8023 2459
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CONTENTS
PageGLOSSARY OF ABBREVIATIONS AND ACRONYMS
SYNOPSIS 1
SECTION 1- FACTUAL INFORMATION 3
1.1 ParticularsofRMS Queen Mary 2andaccident 31.2 Background 41.3 Narrative 41.4 Damage 91.5 Vesselmanagement 111.6 Electricalsystem 11
1.6.1 Powergenerationanddistribution 111.6.2 Propulsiontransformersandpowerconverters 111.6.3 Maincircuitbreakersandprotectiondevices 121.6.4 Selectivitystudy 13
1.7 Automationandpowermanagement 141.7.1 Integratedautomationsystem 141.7.2 Alarmsamplingsequenceandhistory 141.7.3 Propulsionsystemalarmlog 141.7.4 Powermanagementsystem 15
1.8 Harmonics 151.8.1 Harmonicdistortion 151.8.2 Harmonicdistortioninmarinesystems 151.8.3 Theharmfuleffectsofharmonics 171.8.4 Totalharmonicdistortion 171.8.5 QM2 seatrialsandsystemmodelling 18
1.9 Harmonicfilters 211.9.1 Harmonicmitigation 211.9.2 Acceptancetests 231.9.3 Maintenancemanualandsafetymanagementsystem 23
1.10 Capacitors 251.10.1 Design,constructionandmanufacturing 251.10.2 TestoffailedcapacitorsbyVishay 271.10.3 TestoffailedcapacitorscommissionedbyMAIB 281.10.4 Overvoltageintheelectricalnetwork 301.10.5 Dielectricoiltest 301.10.6 Supplyandreturnshistory 311.10.7 Maintenanceandfailurehistory 311.10.8 Classificationsocietysurveyrecords 321.10.9 Polypropylenevapour 321.10.10 Improvementsmadetocapacitordesign 33
1.11 Currentimbalancedetectionsystem 331.11.1 Construction 331.11.2 TestbyConverteam 351.11.3 ExaminationbyVishay 351.11.4 Internationalstandardforcurrenttransformers 361.11.5 Monitoringandprotection 361.11.6 Historyofimbalancealarms 38
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1.12 Arc-flash 381.12.1 Phenomenonofarc-flash 381.12.2 Highvoltageregulationsforenclosures 391.12.3 Testofsoot 391.12.4 Expertopinion 40
1.13 Hi-Fogfiresuppressionsystem 401.13.1 Manufactureanddesign 401.13.2 Designappraisal 411.13.3 Currentregulations 42
1.14 Popularityofelectricpropulsionandvariablefrequencydrives 421.15 Similaraccidents 43
1.15.1 Capacitorfailures 431.15.2 Arc-flashaccidents 43
SECTION 2 - ANALYSIS 44
2.1 Aim 442.2 Theaccident 44
2.2.1 Explosion 442.2.2 Sequentialblackout 45
2.3 Harmonicdistortionofcurrentandvoltage 462.3.1 Awareness 462.3.2 Simulationsandtrials 462.3.3 Monitoringandin-serviceverification 47
2.4 Capacitorfailures 492.4.1 Initiationoffailure 492.4.2 Progressiontocatastrophicfailure 502.4.3 Designchangesinnewcapacitors 512.4.4 Maintenanceandreplacementhistory 522.4.5 Currentimbalancedetectionsystem 52
2.5 Highvoltageenclosures 532.5.1 Protection 532.5.2 Awarenessofarc-flash 54
2.6 Alarmmanagement 542.7 Water-mistinhighvoltagecompartments 54
SECTION 3 - CONCLUSIONS 56
3.1 Safetyissuesdirectlycontributingtotheaccidentwhichhaveresultedinrecommendations 56
3.2 Othersafetyissuesidentifiedduringtheinvestigationalsoleadingtorecommendations 56
SECTION 4 - ACTION TAKEN 58
SECTION 5 - RECOMMENDATIONS 59
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FIGURES
Figure 1 - Propulsionsystemandharmonicfilter
Figure 2 - Highvoltageelectricalnetwork
Figure 3 - PositionofQM2atthetimeoftheaccident
Figure 4 - Locationofharmonicfilter
Figure 5 - Hi-fogglassbulbintheaftmainswitchboardroom
Figure 6 - Waterintheaftharmonicfilterandmainswitchboardrooms
Figure 7 - Failedcapacitor
Figure 8 - Topviewofbulgedcapacitor
Figure 9 - Evidenceofarcingontheharmonicfilterbusbars
Figure 10 - Powerconvertersforpropulsionmotors
Figure 11a - Generators’poweroutput
Figure 11b - Variationofvoltageatpropulsionmotorno.1,halfdriveno.1,immediatelyaftercapacitorexplosion
Figure 12a - Simulationofexpectedtotalharmonicdistortionofvoltagewithoneharmonicfilterinuse
Figure 12b - Simulationofexpectedtotalharmonicdistortionofvoltagewithoutharmonicfilters
Figure 13 - Simulationofexpectedtotalharmonicdistortionofvoltagewiththreedieselgeneratorsandat70%propulsionpoweroutput
Figure 14 - Rank11.3ofharmonicfilters
Figure 15 - Failuremodesanalysissectioninmanufacturer’smaintenancemanualforharmonicfilters
Figure 16a - Capacitorusedinharmonicfilters
Figure 16b - Internalelements
Figure 16c - Blockdiagram
Figure 16d - Singlecapacitorelement
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Figure 17 - Creaseoncapacitorelement
Figure 18 - Holesincapacitorelementfrombulgedcapacitor
Figure 19 - ExtractfromConverteam’sdrydockattendancereporttoCarnivalUKin2008
Figure 20 - Currenttransformerfortheimbalancedetectionsystem
Figure 21 - Currentimbalanceindicatorsintheaftharmonicfilterroom
Figure 22 - Currentimbalancetransformerfortheimbalancedetectionsystemwithitscasingcutopen(inset:secondarycoil)
Figure 23 - Enginecontrolroomdisplayofharmonicfilteroperatingparameters
Figure 24 - ItemisedlistingofareasfittedwithHi-fogoutlets
ANNEXES
Annex A - Alarmlist(IASandP1200)
Annex B - SafetydatasheetfordielectricoilJarylecC101
Annex C - Capacitorexaminationreport,byVishay
Annex D - DNVcircularletter,dated20December2007,regardingpolypropylenefilmvapour
Annex E - Currenttransformerfailureanalysisreport,byVishay
Annex F - MountingandmaintenanceinstructionsbyVishay
Annex G - Arc-flashanalysis,byUKMinistryofDefence
Annex H - Electricalhazardsof‘Hi-fog’,byOmegaPointLaboratoriesLtd,forMarioffCorporationOy
Annex I - Arc-flashincidentcasehistories,byIEEE
Annex J - MAIBSafetyBulletin4/2010
Annex K - MAIBFlyertotheShippingIndustry
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GLOSSARY OF ABBREVIATIONS AND ACRONYMS
µF microfarad
ABB AseaBrownBoveri
AC alternatingcurrent
C celsius
cm centimetre
COSWP CodeofSafeWorkingPracticesforMerchantSeamen
CSM Continuoussurveyofmachinery
DC directcurrent
DG Dieselenginedrivengenerator
DNV DetNorskeVeritas
ECR Enginecontrolroom
EMI Electromagneticinterference
FSS InternationalCodeforFireSafetySystems
FWBLAFF Fixedwater-basedlocalapplicationfire-fightingsystem
GTG Gasturbinedrivengenerator
GUI GraphicalUserInterface
HF Harmonicfilter
HV highvoltage
Hz hertz
IACS InternationalAssociationofClassificationSocieties
IAS Integratedautomationsystem
IEC Internationalelectrotechnicalcommittee
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IEEE TheInstitutionofElectricalandElectronicEngineers
IMO InternationalMaritimeOrganization
IO Input-output
IP Ingressprotection
ISM InternationalSafetyManagementcode
ISO InternationalOrganizationforStandardization
K kelvin
kA kiloamperes
kg kilogramme
kV kilovolts
LR Lloyd’sRegister(Europe,MiddleEastandAsia)
mA milliampere
MCA MaritimeandCoastguardAgency
mg/g milligram/gram
mH millihenry
ml millilitre
MOD MinistryofDefence
mS millisecond
MSB Mainswitchboard
MV megavolts
MVA megavoltsamperes
MW megawatts
NK NipponKijiKyokai
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PLC Privatelimitedcompany
PMS Powermanagementsystem
QM2 RoyalMailShipQueenMary2
RMS rootmeansquare
RPM revolutionsperminute
SMS Safetymanagementsystem
SOLAS InternationalConventionfortheSafetyofLifeatSea,1974
THD Totalharmonicdistortion
THDv Totalharmonicdistortionofvoltage
UPS Uninterruptiblepowersupply
UTC UniversalCo-ordinatedTime
V volts
Times: Alltimesusedinthisreportarelocal(UTC+2)unlessotherwisestated
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LIST OF DEFINITIONS
A-60division AnA-Classdivisionisasuitablystiffenedsteel(orequivalentmaterial)bulkheadordeckthatiscapableofpreventingthepassageofsmokeandflametotheendofa1hourstandardfiretest.Whenitisinsulatedsuchthattheaveragetemperatureoftheunexposedsidewillnotincreasemorethan140ºCabovetheoriginaltemperature,andthatthetemperatureatanyonepointwillnotincreasemorethan180ºCabovetheoriginaltemperaturewithin60minutes,thedivisionisclassifiedasA-60.
Arcing Uncontrolledconductionofelectricalcurrentfromphasetoground,phasetoneutral,and/orphasetophaseaccompaniedbytheionizationofthesurroundingair.
Busbars Anelectricalconductor,maintainedataspecificvoltageandcapableofcarryingahighcurrent,usuallymadeofcopperoraluminiumandusedtodistributecurrenttomultipledevices.
Capacitor AnACdevicethatstoresenergyintheformofanelectricfield.
Deltaandstarconnections Whenathreephasevoltagesupplyisconnectedsuchthatitterminatesinasinglepoint,itiscalledastarconnection.
Whenathreephasevoltagesupplyisconnectedinatrianglesuchthateachphaseformsonesideofthetriangle,itiscalledadeltaconnection.
Dielectricmedium Amediumthatisapoorconductorofelectricityusedbetweentheconductingelementsofacapacitor,havingthepropertyofretainingelectrostaticchargeandtherebyincreasingthecapacitanceofthecapacitor.
VR
VB
Vy
VR
VB
Vy
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Electricaldegrees Theunitusedtorepresentthephasedifference(indegreesortime)betweentwoelectricalwaveformsofthesamefrequency.
Impedance AmeasureofoppositiontotheflowofcurrentinanACcircuit.Itisthecomplexsumofresistance,capacitivereactanceandinductivereactance.
Inductor Aninductor(also,reactor)isapassiveelectricalcomponentthatcanstoreenergyinamagneticfieldcreatedbytheelectriccurrentpassingthroughit.
Inversetimecharacteristic Thebehaviourofarelayorotherelectronicswitchingdevicewhichrespondswithinaperiodoftimeinverselyproportionaltothemagnitudeofthemeasuredsignal.
Non-sinusoidalornon-linearcurrent Acurrentorvoltagewaveformwhichdoesnotcorrespondexactlytothesinewaveshapeofthesupplyvoltage.
Partialdischarge Alocalisedbreakdownofdielectricinsulationinahighvoltageenvironment,affectingonlyasmallareaandwhichdoesnotextendtothefulldistanceofthetwoconductors.
Powerconverter Anelectricalorelectro-mechanicaldevicewhichconvertselectricpowerfromoneformtoanother.
Pulse AtermusedintheACtoDCrectificationcircuitindicatingthenumberofDCoutputsproducedfromonecompleteACcycle.
Reactance Theresistancetocurrentflowinacapacitivecircuitiscalledcapacitivereactancewhiletheresistancetocurrentflowinaninductivecircuitiscalledinductivereactance.
Reactivepower Thecomponentofelectricalpowerflowwhichdoesnotperformusefulworkbutwhichisneverthelessrequiredtodrivetherealpowerthroughcapacitiveandinductivecircuitsisknownasreactivepower.
Reactor Seeinductor.
Resonance Thepropensityofasystemtooscillatewithincreasingamplitude,limitedonlybythesystem’sdampingcharacteristics.Resonanceoccursonlyatcertainfrequenciesandisoftentriggeredbysubtleandminorchangesinthesystem.
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Sub-transientreactance Thereactancevalueinmotorsandgeneratorsduringthefirstcycleofafaultcurrentusedtocalculatethemaximumfaultcurrentthemachinecantolerate.Inabout0.1secondthesub-transientreactanceincreasestotransientreactance,andafter0.5to2secondsitreachesthesteadystatereactanceofthemachine.
Synchronousmotors AnACmotorwhosespeedvariesindirectproportiontothefrequencyofthesuppliedvoltage.
Thyristor Asemiconductordeviceusedtoswitchlargeamountsofelectricpowerwithasmalltriggeringcurrentorvoltage.
Transformer Adevicewithtwoormoreelectricalwindingscoupledbyamutuallymagneticfield.
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1
SYNOPSIS
At0425on23September2010,asRMSQueenMary2(QM2)wasapproachingBarcelona,anexplosionoccurredinthevessel’saftmainswitchboardroom.Withinafewseconds,allfourpropulsionmotorsshutdown,andthevesselblackedoutshortlyafterwards.Fortunately,thevesselwasclearofnavigationalhazardsanddriftedinopensea.
Theemergencygeneratorstartedautomaticallyandprovidedessentialsuppliestothevessel,anditwasquicklyestablishedthattheexplosionhadtakenplaceintheaftharmonicfilter(HF)room,situatedwithintheaftmainswitchboard.Theaftmainswitchboardwasisolated,maingeneratorswererestartedandtheshipwasabletoresumepassageat0523,subsequentlyberthinginBarcelonaatabout0900.Noonewasinjured.
TheaccidentcausedextensivedamagetotheaftHFandsurroundingstructure.Twowater-mistfiresuppressionsprayheadswereactivated,oneintheaftharmonicfilterroomandtheotherintheaftmainswitchboardroom.
TheexplosionwastriggeredbydeteriorationinthecapacitorsintheaftHF.Internalarcingbetweenthecapacitorplatesdeveloped,whichvaporisedthedielectricmediumcausingtheinternalpressuretoincrease,untilitcausedthecapacitorcasingtorupture.Dielectricfluidvapoursprayedout,ignitingandcreatingthelikelyconditionsforanarc-flashtooccurbetweenthe11000voltbusbarsthatfedpowertotheaftHF.
Acurrentimbalancedetectionsystem,whichwastheonlymeanstowarnagainstcapacitordeterioration,wasfoundtobeinoperable,anditwasevidentthatithadnotworkedforseveralyears.
TheelectricaldisturbancefromthecapacitorfailurecauseditscircuitbreakertoopenandisolatetheaftHFfromtheelectricalnetwork.Itwasnotpossibletodeterminetheexactcauseofthesubsequentblackoutbecausetheoptionforstoringhistoricaldataconcerningblackoutswasnotchosenatbuild.However,itisconsideredmostlikelythatthedisruptionwithintheaftHFatthetimeoftheaccidentcausedgeneralinstabilityintheelectricalnetworkwhichcouldnotbecontainedandledtothegeneratorsshuttingdown.
Lloyd’sRegister(Europe,MiddleEastandAsia)(LR)hasbeenrecommendedtotakeforwardproposalstotheInternationalAssociationofClassificationSocietiesto:
• Establisharequirementforallnewvesselsfittedwithharmonicmitigationequipmenttomodeltheeffectofitslossandprovidedatatocrewsothatappropriatecorrectiveactioncanbetakeninsuchcircumstances.
• Requireon-lineorperiodicmonitoringofharmonicdistortionofvoltageonallvesselswithhighvoltagepowersystemstogiveearlywarningagainstpotentialproblems.
• Developrequirementstodetectandmitigateagainstthefailureofhigh-energystoragedevicesandtoensurethatprotectiondevicesofcriticalitemsarefailsafe.
LRhasalsobeenrecommendedtoreviewitsrulesontheuseofwater-basedfire-fightingsystemsinareascontaininghighvoltageequipmentandtoworkwiththeInternationalAssociationofClassificationSocietiestoproposeappropriateguidelinestotheInternational
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2
MaritimeOrganizationforinclusionintheInternationalCodeforFireSafetySystems.TheMaritimeandCoastguardAgencyhasbeenrecommendedtoproducespecificguidanceregardingtheharmfuleffectsofexcessiveharmonicdistortioninelectricalnetworksandtoupdatetheCodeofSafeWorkingPracticesforMerchantSeamentoraiseawarenessaboutthehazardsofarc-flashinhighvoltageequipment.
QM2’smanager,CarnivalUKhavealsobeenrecommendedto:improvethestandardsofprotectionagainsttheeffectofharmonicdistortionandcomponentfailure;and,toreviewthemachineryalarmsystemsfittedtoQM2 inordertoidentifyandprioritisethosealarmswhichindicatefailureconditionsthatcouldsignificantlyaffectthesafetyofthevessel.
PhotographcourtesyofJörnPrestien
Queen Mary 2
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SECTION 1 - FACTUAL INFORMATION
1.1 PARTICULARS OF RMS QUEEN MARY 2 AND ACCIDENT
SHIP PARTICULARS
Flag UnitedKingdom
Classificationsociety Lloyd’sRegister
IMOnumber 9241061
Type Passengercruiseliner
Registeredowner CarnivalPLC
Manager(s) CarnivalPLC
Construction Steel
Lengthoverall 344.3m
Grosstonnage 148528
Propulsion(power) Dieselelectric(4x21.5MW)
VOYAGE PARTICULARS
Portofdeparture Southampton
Portofarrival Barcelona
Typeofvoyage Internationalvoyage
MARINE CASUALTY INFORMATIONDateandtime 23September2010at0425
Typeofmarinecasualtyorincident
LessSeriousMarineCasualty
Locationofincident 40º44.36N,001º49.42E,36nmSWofBarcelona
Placeonboard Aftharmonicfilterroom
Injuries/fatalities None
Damage/environmentalimpact
Twocapacitorsdamaged,busbarsandinsulatorsonseveralothersdamaged,bulkheadstiffenersbuckled,enclosurepaneldoorsblownout,steeldoorsdamaged.
Shipoperation Onpassage
Voyagesegment Mid-water
Personsonboard 3823attimeofaccident
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1.2 BACKGROUND
QM2waspropelledbyfourvariablespeedelectricmotorsknownas‘poddeddrives’.Thevariationofspeedwasachievedbychangingtheelectricalfrequencyinthepowersupplytothemotors.Anunwantedeffectofvaryingthefrequencywasdistortionofthesupplyvoltagewaveform.Thedegreeofdistortionwasquantifiedastotalharmonicdistortionandhadtobemaintainedwithinlimitsdefinedbythevessel’sclassificationsociety.OnQM2,thiswasachievedbyusingharmonicfilters(HF);HFunits,consistingofatunedloadofcapacitorsandinductors,wereconnectedtotheforwardandaftmainswitchboards(MSB).EachHFconsistedoftwosections,knownasrank11.3andrank4(Figure1).
TheMSBswereconnectedtoeachotherthroughcircuitbreakers(Figure 2).EachHFwaslocatedinsideaseparateroomwithintheMSBcompartment.BothMSBandHFcompartmentswereprotectedbyawater-mistbasedlocalapplicationfire-fightingsystemknownbyitstradenameasHi-Fog.
1.3 NARRATIVE
At0415on23September2010,thewatchkeepingofficeronthebridgeofQM2gave2hours’noticetotheengineroomwatchkeeperinpreparationforarrivingatBarcelona(Figure 3).ThevesselhaddepartedfromSouthamptonon19Septemberandwasonacourseof026ºataspeedof18.6knotswithallfourpoddeddrivesrunning.Therewerefourengineersintheenginecontrolroom(ECR)includingtwofromtheprevious,offgoingwatch.Justafter0425theyheardaloudbang,whichwasfollowedbycompletefailureoftheengineroomlightingexceptthetransitionallighting.Almostimmediately,themainpropulsionmotors’outputpowerdroppeddowntounder5MWandthemotorsstoppedapproximately16secondslaterwhenallthegeneratorsshutdown.
Theemergencygeneratorstartedautomaticallyandrestoredlightingandotheressentialsupplies.ThethirdengineerfromtheoffgoingwatchrantoinvestigateandsoondiscoveredthickblacksmokecomingfromtheaftMSBroom.AlertingtheotherengineersintheECRwithhisportableultrahighfrequencyradio,heclosedthewatertightdoorbetweentheengineroomandthespacewhichledtotheaftmainswitchboardroom.Healsopreparedafirehoseandlaiditattheentrancetothewatertightdoor.SeveralsmokeandheatdetectionsensorsfromtheaftHFandtheaftMSBroomswereactivated.
Ataround0430,thebridgeteammobilisedtheassessmentparty,whichcomprisedsixseniorrepresentativesfromthedeck,engineandhoteldepartments.Thedeckandenginefireparties,togetherwiththeboundarycoolingparty(atotalof26personnel)stoodby,alongwithallthehotelmanagerswhowerespecificallytaskedtodealwithpassengerqueries.Nopassengerorgeneralcrewannouncementwasmade.The‘notundercommand’lightswereswitchedonandtheSpanishcoastguardwasinformed.
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Figure2
Power converter
Power converter
Tf11kv 2 x 1.5kv
HF Aft
GTG1 GTG2 DG1 DG2 DG3 DG4
HF Fwd
Pod 1 Pod 3 Pod 2 Pod 4
Power converter
Power converter
Bus tie interconnecting
breaker
Tf11kv 2 x 1.5kv
Tf11kv 2 x 1.5kv
Tf11kv 2 x 1.5kv
Highvoltageelectricalnetwork
Figure1
Propulsionsystemandharmonicfilter
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6
By0439,thesecondelectro-technicalofficerandathirdengineer,bothwearingbreathingapparatusandcarryingcarbondioxideportablefireextinguishersandathermalimagingcamera,werereadytocarryoutaninspectionofthescene.TheyenteredtheaftMSBroomandmadetheirwayintotheHFroom,stoppingjustafterenteringthedoorway(Figure 4).Theyconfirmedthatthespacewasfilledwiththickblacksmoke;buttherewasnofireand,usingtheirthermalimagingcameras,theydidnotdetectanyhotspots.TwoHi-Fogglassbulbs(Figure 5),oneintheaftHFroomandanotherintheaftMSBroomhadrupturedandtherewasapproximately10mmofwateronthedecksofbothcompartments(Figure 6).
ThecrewattemptedtoshutofftheHi-Fogwatersupplyusingthemanualshut-offcocklocatedoutsidetheaftMSBroom;buttheywereunsuccessfulasthehandleshearedwhentheytriedtoforceit.TheHi-Fogsystemwasthenswitchedoffatthecentralcontrolstation,andat0452extractionfanswerestartedtoclearthesmoke.
By0455threedieselgeneratorshadbeenstartedandconnectedtotheforwardMSB;theaftMSBwaskeptisolatedbydisconnectingthebustiebreakersbetweenthetwoswitchboards(Figure 2).By0523QM2wasunderwayusingpods2and4,andthevesselberthedatBarcelonajustbefore0900.RepresentativesofConverteam,themanufactureroftheHFandpropulsiondrive,andLloyd’sRegister(Europe,MiddleEastandAsia)(LR)theclassificationsocietyforthevessel,carriedoutadetailedassessmentoncethevesselwasalongside.Theyconcludedthat,afterphysicallyremovingthecircuitbreakerbetweentheMSBandtheHF,itwassafetoputtheaftMSBbackon-line.LRimposedaconditionofclassonthevessel,toremainuntiltheaftHFissuewasrepaired.Ataroundmidnight,thevesselsailedfromBarcelona.
Figure3
PositionofQM2atthetimeoftheaccident
ReproducedfromAdmiraltyChart 1704bypermissionoftheControllerofHMSOandtheUKHydrographicOffice
Barcelona
Positionofaccident
QM
2’strack
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Locationofharmonicfilter
Figure4
Doors
Aftharmonicfilter
Switchboardroom
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Figure5
Hi-fogglassbulbintheaftmainswitchboardroom
Figure6
Waterintheaftharmonicfilterandmainswitchboardrooms
Crossfloodingduct
DamageddoorwithA-60insultation
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1.4 DAMAGE
ThedamageintheaftHFroomwasextensive.MostoftheoccupantsofthesixcrewcabinslocateddirectlyabovetheaftHFroomheardthenoiseoftheblastanddescribedthesoundasa‘loudbang’.Therewasnodamageinsidetheircabins.Panelsoneithersidesoftherank11.3cubiclewerebadlydistortedandcoveredwithsoot.Panelsfortherank4cubiclewerealsobulgedout.Theflooranddeckheadwerecoveredwithsootanddielectricoilwasfoundallovertherank11.3HFsection,includingtheareaunderthecapacitorsandontheinsideofthetoppaneloftheenclosure.Onecapacitorwasfoundtohavelifteduparound60cmfromitsnormalposition.Itsbottomcoverhadbentoutwardsandblownoutatitsweldedjoint.Ithadwrencheditselffreefromitsfoundationbolts,breakingtheconnectiontothecapacitorelementsinside.Allthecapacitorsfittedinthesamerowasthefailedonehadsuffereddamageontheirterminals.Thecasingofanothercapacitorwasfoundtohavebulgedseverely(Figure 7 and Figure 8).
The‘A-60’ratedsteeldoortothecompartmentwasfoundforcedoutthroughitsframeandtheadjoiningsteeldoorfortheMSBroomwasalsobuckledandsplit.AsteeldoorattheinboardentrancetotheaftMSBroomandlocatedapproximately20mawayfromthesiteoftheexplosionwasforcedoffitshinges.Thestiffenersonthebulkheadofthecompartmentwerebuckledandthesteelcoverplateonthecross-floodingductbetweentheHFandMSBroomswasblownoutintotheMSBroom.Thepolythenecoveringonafireextinguisher,whichwaslocatedapproximately2mawayfromthefailedcapacitor,wasfoundsinged.ThecornersofallthreehighvoltagebusbarsthatfedintotheaftHFhadbeenmelted(Figure 9).
Figure7
Failedcapacitor
Capacitorterminals
Capacitorconnectionbusbars
Failedcapacitor
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Figure8
Topviewofbulgedcapacitor
120m
m
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Figure9
Evidenceofarcingontheharmonicfilterbusbars
10mm
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1.5 VESSEL MANAGEMENT
QM2conductedseatrialsinSeptemberandagaininNovember2003.ThevesselwasformallyhandedovertoCunardLine(adivisionoftheCarnivalCorporation)inDecember2003,commencinghermaidenvoyageon12January2004.From2004to2007technicalmanagementwascarriedoutbyPrincessCruisesInternational,LosAngeles,whichwasalsoadivisionoftheCarnivalCorporation.InNovember2007technicalmanagementwastransferredtoCarnivalUKbasedinSouthampton.
Twotechnicalsuperintendentslookedafterthevessel’sday-to-daymechanicalandelectricalrequirementsrespectively.Allroutinerequestsfromthevesselforsparepartswereco-ordinatedandapprovedbytheappropriatesuperintendent.Thevessel’sAmosComputerisedMaintenanceManagementSystemkeptarunningrecordofsparepartsusageandalsohadaminimumbalancefunctiontomaintainastockofcriticalspareparts.Itdidnothavethefunctionalitytoidentifytrendsforabnormalconsumptionofspareparts.AminimumofonesparecapacitorforeachofthetwoHFrankswasrequiredtobekeptonboard.
1.6 ELECTRICAL SYSTEM
1.6.1 Power generation and distribution
QM2hadatotalpowerproductioncapacityof135MVAat11kV(60Hz)alternatingcurrent(AC).Twogasturbinedrivengenerators(GTG)suppliedtheaftMSBandfourdieselenginedrivengenerators(DGs)suppliedtheforwardMSB.AtthetimeoftheaccidentthreeDGsweresharingthepowerrequirement.OneGTGwasoutoforder.
Innormaloperation,theMSBswerealwaysconnectedtoeachotherthroughthetwobustiecircuitbreakers.Theauxiliarymachinerywassuppliedat690V,withtheexceptionoftheairconditioningcompressorsandbowthrusters,whichwereratedat11kV.Themainpropulsionmotorswereofsynchronoustype,suppliedthroughstepdowntransformersandpowerconverters.Eachmotorconsistedoftwoindependentsetsofwindings(halfmotors)operatingwithinafrequencyrangeof0to15.8Hz(0–137RPM)andvoltagerangeof0to2830V,whichvariedinlinearproportiontothefrequency.
1.6.2 Propulsion transformers and power converters
Thepropulsiontransformerssteppeddownthevoltagefrom11kVto1510V.Eachhadadeltaconnectedprimarywindingandtwosecondarywindings:onestarwoundandtheotherdeltawound.Thedeltaandstarconnectionsgeneratedaphasedifferencebetweentheirvoltageoutputsof30electricaldegrees.
Therewereeightpowerconverters,oneforeachhalfmotor,withoneconnectedtotheoutputofeachsecondarycoilofthetransformer.Theconverterconsistedofanetworkbridge,whichrectifiedthe1510VACtoadirectcurrent(DC);aDClink,whichincludedaninductorcoilof5.5mHdesignedtodampsurgesandeliminatecurrenttransients;andamotorbridgewhichinvertedtheDCsupplyintoanACwaveformofvariablefrequencyandvoltage.Boththenetworkandmotorbridgeswerethyristorcontrolledandsuppliedthesynchronousmotorsatafrequencytoprovidetherequiredpropellerspeed.Theelectricalphaseshiftingachievedbythedelta-startransformersecondarywindingoutput,combinedwiththefullwave
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rectificationprocessresultedin4pulsesperphaseand12pulsesforthe3phasesupplytothemotor.Thepowerconverterwasthereforecalleda12pulseconverter(Figure 10)andwasselectedbecauseofitslowerlevelofharmonicdistortioncomparedwithothertypesofconverters.
A‘Buchholz’gasactuatedpressuredetectionrelay,withaninversetimecharacteristic,monitoredchangesinthevapourpressureinsidethetransformeroiltank.Itwassettoraiseanalarmanddisconnectthetransformerelectricallyifafaultoccurredthatcausedtheoilvapourpressuretorise.Theoilinthepropulsiontransformerswastestedregularlyforthepresenceofdissolvedgasestodetectarcingorpartialdischargeandprovideanindicationoftheconditionofthetransformer.
1.6.3 Main circuit breakers and protection devices
AllthecircuitbreakersandtheirprotectionrelaysweresuppliedbyAseaBrownBoveri(ABB)Sace.EachHFhaditsowncircuitbreakeroftypeHD4/Cwhichhadavoltageratingof12kVandacurrentratingof1600Awithabreakingcapacityof
Figure10
Powerconvertersforpropulsionmotors
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50kA.Inadditiontoovercurrentandshortcircuitprotection,thebreakerhadanundervoltageprotectiontrip.ThecircuitbreakercouldalsobetrippedbyoverloadcurrentsensorsintheHFitself.TheseweresuppliedandfittedtotheABBbreakersbyConverteam.DeteriorationofthecapacitorsintheHFwasmonitoredbymeasuringthecurrentimbalancebetweenthecapacitorsineachofthethreephases.Discussedinmoredetaillaterinthereport,detectionofanunbalancedcurrentbetweenthecapacitorswasalsodesignedtotriptherelevantHFcircuitbreaker.
Thereweretwobustieinterconnectingbreakers,oneoneachMSBandratedat12kV,3600Awithabreakingcapacityof50kA.Thetiebreakershadshortcircuit,overcurrent,andovervoltageprotectiontrips,butdidnothaveanundervoltagetripfunction.Immediatelyaftertheaccident,thebustiebreakerintheforwardMSBwasfoundtripped.Thesecondbustiebreakerwasfoundintheclosedposition.
AlltheDGsupplybreakerswereratedat11kVand1000A.Thegeneratorswereprotectedbytworelaysknownas‘SynpholD’and‘Ref542’withtheovercurrenttripfunctionbeingduplicatedinbothrelays.Inaddition,therewereseveralinstanttripfunctionstoprotectgeneratorsagainstinternalfaults.
Incaseofseriousfaultsintheconnectedloads,theprotectionrelayswereconfiguredtotripthebustieinterconnectorclosesttothegenerators.Ifthefaultdidnotclearbytrippingthebustie,thegeneratorbreakersthemselveswouldtrip.Theprotectionsystemalsomonitoredovercurrent,negativesequence(phaseimbalance),under/overfrequencyandovervoltageparameters.Imbalanceornegativesequencingwasdetectedwhenanyconnectedloadwasopencircuitedinonephasecausinganasymmetricphasefault(knownassinglephasing).Thealarmindicatingnegativesequencingwasprovidedbytherelay‘SynpholD’.
Theprotectionrelayswerecapableofstoringarecordofwhyeachtriphadoccurred.However,theoptionforstoringhistoricaldataconcerningblackoutswasnotchosenatbuildandtheinformationwhichcouldhaveidentifiedwhythegeneratorsshutdownwasnotrecorded.
1.6.4 Selectivity study
Aselectivitystudy,alsoknownasadiscriminationstudy,wascarriedoutduringthedesignphaseforQM2aspartofLR’srequirementsforplanapproval.Itspurposewastodemonstratethattheprotectiverelaysforcircuitbreakersthroughoutthenetworkhadbeensetsuchthatifafaultoccurredinonepartofthenetwork,thecircuitbreakerclosesttothefaultwouldopenfirsttoisolatethefaultandprotecttherestofthenetwork.TheselectivitystudyforQM2wasapprovedbyLRinApril2003.Theprotectionsettingsofallthe11kVcircuitbreakersweretestedbythesecondarycurrentinjectionmethodduringadrydockingperiodinOctober2008.
Immediatelyaftertheaccident,theMAIBrequestedthatCarnivalUKcheckthesettingsontheprotectiverelaysoftheHF,interconnectingbustieandgeneratorbreakerswiththeassistanceofthesystemmanufacturers,ABB.AlthoughABBwastaskedtocarryoutthisjob,ithadnotbeencarriedoutbythetimethisreportwasbeingfinalised.
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1.7 AUTOMATION AND POWER MANAGEMENT
1.7.1 Integrated automation system
TheIntegratedAutomationSystem(IAS)wasprovidedbyValmarineAS,NorwayandwasbasedonitsDamaticXDsystem.Alocalareanetworkofcomputerspermittedtheship’screwtomonitorthemajorityofship’smachineryfromstationslocatedaroundtheshipandtocontrolthemfromcertainlocations,suchastheECRandwheelhouseworkstations.Italsoprovidedthemachineryalarmsystemforthevesselthroughfouralarmstations.
Thefiredetectionandalarmsystemwasindependent;howeveritprovidedaninputtotheIASindicatingthedeckonwhichafiredetectionsensorhadbeenactivated.Twofirealarms,onefromtheaccommodationandtheotherfromtheengineroom,appearedontheIASapproximately30minutesbeforetheaccident.Therewerenocorrespondingalarmsonthefiredetectionsystempanel.AttherequestoftheMAIBtheship’screwinvestigatedthesourceofthisfirealarmandestablishedittobespurious,duetoawiringdefectontheIAS.
1.7.2 Alarm sampling sequence and history
Theexecutioncyclesforthealarmhandlingstationswereat400msintervalsforeverythingrelatedto11kVsystemsandforthosewhichwerepartofthepowermanagementsystem(PMS).Lesscriticalalarmsweremonitoredat1100msintervalsandallotherbinaryalarms,notrelatedtothePMS,weresampledat900msintervals.Thealarminput-output(IO)cardatthealarmprocessingstationwouldcyclethroughthealarmchannelsandholdeachalarminabuffer,allocatingthemallwiththesametimestamp.Thereforethetimestampindicatedonthealarmprintoutwasnotalwaystheexacttimethatthealarmwastriggered.
Inthe10minutesduringandimmediatelyaftertheexplosiontherewerenearly500alarmsregisteredontheIAS.Inthemidnightto0400hourswatchon23September,atotalof468lineswereprintedontheECRalarmprinter,ofwhich235werecriticalornon-criticalalarms.Theremainderwererecordsofthealarmsbeingacknowledged.
ItwasnotpossibletoestablishwhethertheaftHFcircuitbreakertrippedduetoanoverloadineitherrank,orduetoanimbalanceinitsrank4sectionastheAISdidnotdifferentiatebetweenthetwotypesofalarmsandrecordeditasan‘overload/unbalance’.Itisknown,however,thatwithin300msto500msofthealarm,thecircuitbreakeropened.Almostsimultaneouslyorjustbeforethecircuitbreakeropened,therewereseveral‘IO-FAULT’alarms(indicationoflossofreactivepower)atbothhighvoltageswitchboardsandgenerators.Therewerealsoseveral‘DISCREPANCY’alarms(indicationthattheswitchboardhaddetectedalossofonephase)atseveralbusbarsinthe690Vcircuit.NoundervoltagealarmsineitherthehighorlowvoltagecircuitswereregisteredbytheIAS.
1.7.3 Propulsion system alarm log
Thepropulsionsystemhaditsownmonitoringsystem,providedbyConverteam,calledP1200,whichcommunicatedimportantalarmstotheIAS.Atapproximately0348thealarm‘halfdrivealarmlamp’forpropulsionmotorno.3,halfdriveno.2cameupontheP1200system.Thesamealarmreappearedataround0356,
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0405and0406.Thisalarmindicatedthatafaultwasdetectedinthisparticularhalfdriveandwasintendedtobeaprecursortothemorespecificalarmswhichwouldfollow.WhileallthealarmsontheP1200systemwereavailabletoengineroomwatchkeepers,theIASwasconsideredtobetheprimaryalarmsystem.
ImmediatelyaftertheHFcircuitbreakertrippedataround0425,alarmsactivatedindicatingproblemswithallofthepropulsionmotorconvertersandexcitationsystems.ThepropulsionsystemP1200alarmlogindicatedthatallthefourpropulsionmotorsregisteredundervoltagealarmsimmediatelyaftertheHFbreakertripped.However,thesewereveryshorttermandtheactualvoltagelevelswerenotrecordedbytheP1200systemortransferredtotheIAS.Themotorsthemselvescontinuedtorununderreducedpowerforapproximately16secondsaftertheundervoltagealarmsuntilthegeneratorsthemselvesshutdown(Figures 11aand11b).AnannotatedrecordofalarmsfromtheIASandfromtheP1200systemforpropulsionisavailableatAnnex A.
1.7.4 Power management system
Thepowermanagementsystem(PMS)wasafunctionoftheIAS.Itcontrolledthestart,stop,autosynchronisationandloadsharingfunctionsofthegenerators.However,itwasnotdesignedtohaveanyinputfromtheprotectionrelaysofgeneratormaincircuitbreakers,anddidnotreducepropulsionpowerorreactinanywaytothefailureofharmonicfilters.
1.8 HARMONICS
1.8.1 Harmonic distortion
WhenanACelectricalloaddrawsnon-sinusoidalornon-linearcurrents,ittendstodistortthewaveformofthesupplyvoltage.Theswitchingactionofthyristorsinthepowerconvertersresultsinnon-sinusoidalcurrentsbeingdrawnfromthegenerators.Whilethesupplyoriginallydeliversa‘clean’sinusoidalvoltageatthefundamentalfrequencyof60Hzthepowerconverters,indrawingadistortedcurrent,causethesupplyvoltagefromthegeneratorstodistort.Thisgeneratesvoltagesattheharmonicsofthefundamentalfrequency,whichinturnaffectalltheconnectedloadsregardlessofwhethertheyarelinearornon-linear.Inmarineandoffshoreinstallations,electricvariablespeeddrivesarethemainsourceofharmonicdistortiontocurrentandvoltagewaveforms.
1.8.2 Harmonic distortion in marine systems
Powerconvertersproduceharmonicsatfrequenciesaccordingtotherelationshipnp±1wherenisapositiveintegerandprepresentsthenumberofpulsesinthepowerconverter.Therefore,forthe12-pulseconvertersfittedonQM2thepredominantharmonicswereatthemultiples11,13,23,25,35,37andsoonofthefundamentalfrequency(60Hz).Inaland-basedgenerationsystem,thedistortionofcurrentduetonon-linearloadsdoesnotresultinsignificantvoltagedistortionatsourcebecausetheseloadsareaminorfractionofthetotalpowergenerationcapacity.Consequently,theeffectsremainlocaltothenon-linearload.Onanelectricallypropelledvessel,suchasQM2,thepropulsionmotorsconsumemorethan70%ofthetotalgeneratedpowerandtheresultingvoltagedistortionismoresignificant.Theimpedanceofamarinegeneratorisalsonormallyquitehighcomparedtothatoftheutility’stransformer.Thisresultsinalargervoltagedistortionatthesource
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Figure11a
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126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144
Variationofvoltageatpropulsionmotorno.1halfdrivemotorno.1immediatelyaftercapacitorexplosion
Figure11b
90voltsrise
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whenharmoniccurrentsaredrawnbysubstantialnon-linearloadssuchasthemainpropulsionmotorpowerconverters.Asmoregeneratorsareaddedtoincreasethepowergenerated,theyalsohavetheeffectofloweringtheoverallimpedance,reducingtheamountofharmonicdistortion.
1.8.3 The harmful effects of harmonics
Excessiveharmonicdistortionofcurrentandvoltagecancauseheatinginthewindingsoftransformers,generatorsandinductionmotors,whichcouldpotentiallyresultinfire.Someoftheothermorecommonandunpredictableeffectsofexcessiveharmonicdistortionare:
• Disruptionintheoperationofuninterruptiblepowersupplies(UPS)
• Spurioustrippingorfailureofsensitiveelectronicandcomputerequipment,measurementandprotectionrelays
• Voltageresonancesleadingtotransientovervoltageandovercurrentfailuresintheelectricalnetwork
• Electromagneticinterference(EMI)resultingindisruptiontocommunicationequipment
• Malfunctionofcircuitbreakersandfuses.
1.8.4 Total harmonic distortion
Totalharmonicdistortion(THD)ofvoltageandcurrentistheratiousedtodescribethedistortionintheelectricalpowergenerationanddistributionsystem.Itiscalculatedbytheratiooftherootmeansquare(RMS)valueoftheharmoniccontenttotheRMSvalueofthefundamental.Itisnormallyexpressedasapercentage,calculatedusingtheexpression:
where
Vh=RMSamplitudeofvoltageatharmonicorderh
V1=RMSamplitudeofthefundamentalvoltage.
LR’srulesonharmonicdistortionofvoltagestate:
Unless specified otherwise, the total harmonic distortion (THD) of the voltage waveform at any a.c switchboard or section-board is not to exceed 8 per cent of the fundamental for all frequencies up to 50 times the supply frequency and no voltage at a frequency above 25 times supply frequency is to exceed 1.5 per cent of the fundamental of the supply voltage.
Allotherclassificationsocietiesplacealimitof5%onTHDofvoltage(THDv).
T H D = h=2
2
h
8
1001
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TheInstitutionofElectricalandElectronicEngineers’(IEEE)RecommendedPracticeforElectricalInstallationsonShipboard(IEEEStandard45-2002),states:
A dedicated propulsion bus should normally have a voltage total harmonic distortion of no more than 8%. If this limit is exceeded in the dedicated propulsion bus, it should be verified by documentation or testing that malfunction or overheating of components does not occur. A non-dedicated main generation/distribution bus should not exceed a voltage total harmonic distortion of 5%, and no single voltage harmonic should exceed 3%.
IEC60034-1,2004,RotatingElectricalMachines-Part1:RatingandPerformance,requiresthattheTHDvforsynchronousmotorsabove300kWoutputshouldnotexceed5%.Itdoesnotspecifydistortionlevelsforindividualharmonics.
1.8.5 QM2 sea trials and system modelling
ConverteamcarriedoutsimulationsofQM2’selectricalsystemin2002andcalculatedtheTHDvwithoneandtwoHFsinthecircuitanddifferentnumbersofgeneratorsonload.Thesimulationpredictedtheworstcasewouldbe7.6%THDv.Seatrialswereperformedin2003toensurethatTHDvwaswithin8%inbothhighandlowvoltagesectionsoftheelectricnetworkonboard.THDvwasmeasureduptothe49thharmonicunderseveraldifferentpowerconditionsrangingfromtwotosixgeneratorson-lineandthemaximumspeedpossibleineachconfiguration.TheworstcasewasrecordedwiththecombinationofoneHFwithtwoDGs;THDvatthe11kVbusbarwasrecordedas3.4%,andatthe690Vbusbaras6.5%.Notrialsorsimulationswerecarriedoutwithouttheharmonicfiltersconnectedtothesystem.NotrialswerecarriedoutwiththreeDGsandoneHF,thecombinationbeingusedatthetimeoftheaccident.
Harmonicdistortionwasnotroutinelymeasuredinserviceasthecrewwerenotawarethattheyhadtheappropriatemeasuringequipmentonboard.Aftertheaccident,thecrewfoundtheequipmentandusedittomeasuretheTHDvonthevessel.TheirreportconcludedthatTHDvwasgenerallylessthanthatcalculatedbyConverteamduringtheseatrials,althoughtheyreportedthatthe35thand37thharmonicsexceededLR’srequirementsof1.5%forthoseharmonicsabovethe25thharmonicorder.
TheMAIBcommissionedaspecialistpowerqualitytestingandmonitoringcompany,HarmonicSolutionsCo.UK,tocarryoutTHDvmeasurementswhileQM2wasonpassageduringtheperiod8to11December2010.
Table 1 showsthecomparisonofthemeasurementsrecordedinthelatertestswiththoserecordedduringtheseatrialsin2003whenasingleharmonicfilterwasincircuitwithfourDGsandoneGTGgeneratorsupplyingtheload.
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Condition MAIN CIRCUIT BREAKERS Comments
4 x DG
+
1 x GTG
(one HF in use)
TP1HT
11Kv
TP1F
690V
TP5F
690V
PE80
400V
PE81
400V
PEL8
208V
2003
THDv2.2% 1.8% 6.1% 2.7% 2.3% 2.2% Exact loading
unknown.
2010 THDv 5.2% 2.9% 8.44% 3.26% 3.71% 2.7%
2010 11kV measurement at
forward HF circuit breaker.
Table 1:ComparisonofthelevelsoftotalharmonicdistortionmeasuredduringseatrialsandMAIBtests.(TP1HT,TP1F,TP5F,PE80,PE81andPEL8arenamesofmaincircuitbreakerssupplyingspecificlocationsintheelectricalnetwork.)
Initsreportofthepowerqualitytests,HarmonicSolutionsstated:
In conclusion, no evidence of direct or contributory factors in the failure of the aft harmonic filter capacitor(s) due to excessive voltage distortion, voltage spikes or other mains disturbances was found during the voyage.
Amonthaftertheaccident,ConverteamcarriedoutasimulatedstudyonharmonicdistortionlevelsexpectedwithoutanyHFinuse.Byextrapolatingthegraphforthecasewithnofiltersinuse,theTHDvwaspredictedtoexceed22%at70%speed.ThisestimationwassubsequentlyconfirmedbyConverteamwhentheycalculatedtheamountofTHDvinthisconfiguration(Figures 12a, 12b and Figure 13).Initsconclusion,theConverteamstudystated:
This study highlights the necessity to use one filter or to operate at slow speed with no filter to keep the harmonic distortion level under 8%. Additional measurements on board should be done on board to check the harmonic level with the LV [low voltage] pollution. [sic]
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Simulationofexpectedtotalharmonicdistortionofvoltagewithoneharmonicfilterinuse
Figure12a
Figure12b
Simulationofexpectedtotalharmonicdistortionofvoltagewithoutharmonicfilters
3DG,0GTG
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1.9 HARMONIC FILTERS
1.9.1 Harmonic mitigation
Thereareseveralmethodsusedtocountertheeffectsofharmonicdistortioninmarinepowersystems,including:
• activeorpassivefilters
• increasingthenumberofpulsesinpowerconvertersbyusingmultiplephaseshiftedsecondarywindingsinpropulsionmotorsupplytransformers
• installinggeneratorswithalargesub-transientreactance.
Thepredominantharmonicsthatareexpectedtooccurintheelectricalpowerconversionsystemarecalculatedatthedesignstage.TheHFsinQM2wereofthepassivefiltertype,consistingofatunedcircuitofcapacitorsandinductorsprovidingalowimpedancepathtothepredominantharmonics,intheelectricalnetwork.Thefrequencytowhichthefilterwastunediscalledtheresonantfrequencyofthesystemandisgivenbytherelationship:
fr=1/(2π(LC))
(wherefrisresonantfrequencyinhertz;LisinductanceinhenriesandCisthecapacitanceinfarads)
0 10 20 30 40 500
5
10
15THDu = 22.0943 %
%
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Figure13
Simulationofexpectedtotalharmonicdistortionofvoltagewiththreedieselgeneratorsandat70%propulsionpoweroutput
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TheharmonicfiltersonQM2weresuppliedbyConverteam,formerlypartofAlstomPowerConversion,France.EachHFconsistedoftwosub-sectionstunedtoresonantfrequenciesequivalenttothe11.3rdand4thharmonics,andreferredtoonboardthevesselasrank11.3andrank4respectively.Therewere15capacitorsinrank4and12inrank11.3.Thecapacitorsforeachrankwereconnecteddirectlytothethree-phase11kVmainssupplywiththeirterminalsconnectedinacircuitconfigurationknownasadoublestartermination(Figure14).The11.3rankincludedagroupofinductors,withatotalinductanceof0.507mHandabankofcapacitorswithatotalcapacitanceof108.8µF.Aresistorbankwasalsoconnectedinparalleltotheinductorstolimittheamountofin-rushcurrentintocapacitorswhentheHFwasstartedup.
Figure14
Rank11.3ofharmonicfilters
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Althoughthe4thharmonicisnonexistentina12pulseconverter,rank4wasincludedintheHFonQM2tocountertheexcitationoflowerorderresonantfrequenciesandalsotomitigateanyharmonicsfromnon-linearlowvoltageloads.TheexactdetailsofallthelowvoltageloadswerenotavailabletoConverteamwhenthefilterwasbeingdesigned,andadditionalmarginswereincludedinrank4tomitigateanyunexpectedharmonicdistortionfromthelowvoltagenetwork.Implementingfiltersattworanksalsodistributedthereactivepowerrequirementofthenetworkacrossthetworanks.Asthecapacitorsrepresentedalargereactivecomponentoftheelectricalload,theship’screwhadtoensurethattheharmonicfilterswerealwaysswitchedonafterstartingthepropulsionmotorsandswitchedoffbeforestoppingthem.
OneHFcouldbenominatedasthepriorityfilterandtheothercouldbesettoswitchinoroutautomaticallyasrequired.Theswitching-inthresholdwasbasedonthepowerdemandofthepropellermotors,andthereforewhiletherunningfilterwouldremainon-lineuntilitwasmanuallyswitchedoff,theotherfilterwouldcutinandoutdependingonthenumberofgeneratorsinuseandhencethepowerbeingdemanded.TheswitchinglogicwasdesignedtokeepbothHFsrunningifmorethanfourgeneratorswererequiredtosupplytheelectricalload.
TheaftHFwasselectedastherunningfilterwhenthevesseldepartedSouthamptonon19September.TheforwardHFremainedonstandbyandwasonlyputonloadtwice,forbriefperiods,during21September.Twodaysaftertheaccident,thetotalrunninghoursoftheforwardHFwasnotedas34419.TheaftHFhadbeenusedfor24282hours.
1.9.2 Acceptance tests
FactoryacceptancetestsfortheharmonicfilterswerecompletedinJuly2002inthepresenceofLR.Aspartofthesetests,theimbalancealarmandtripsettings,alongwiththeirrespectivetimedelays,wereverifiedbyinjectingacurrentintotherelaysconnectedtothesecondarywindingoftheimbalancecurrenttransformer.ProceduresforcarryingoutharbouracceptancetestsontheharmonicfiltersweredevelopedbytheshipbuildingsectionofCarnival;buttherewerenoinstallationandcommissioningrecordsavailable.
1.9.3 Maintenance manual and safety management system
Converteam’smanualfortheHFcontainedamaintenanceschedulewhichrequiredtheprotectiondevicestobecheckedevery12months.Itdidnotspecifyhowthechecksweretobecarriedoutandcontainedonlythefollowing:
Each 12 months
Check the protections (see main switchboards)
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ThemanualalsocontainedatableentitledFailure modes analysis(Figure15).ThisThisanalysisdidnotincludethefailureoftheimbalancecurrentprotectionsystem.ItalsocontainedareferencetooverpressuredetectionintheHFs,afeaturewhichwasnotpresentintheHFsonQM2.
Thevessel’ssafetymanagementsystemhadnotidentifiedtheHFasacriticalpieceofequipmentunderthedefinition1providedundersection10.3oftheISMCode.
1 The Company should identify equipment and technical systems the sudden operational failure of which may result in hazardous situations.
Figure15
Failuremodesanalysissectioninmanufacturer’smaintenancemanualforharmonicfilters
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1.10 CAPACITORS
1.10.1 Design, construction and manufacturing
ThecapacitorsweremanufacturedbyVishayElectronic(Vishay)atBlatnaintheCzechRepublic.Eachcapacitorcan2measuredapproximately93x35x18cmandconsistedof36individualelements(Figures 16a, b, c and d).Anindividualelementhadacapacitanceofapproximately12µFandagroupofninesuchelementsconnectedinparallelhadacapacitanceof108.8µF.Foursuchgroupsofninewereconnectedinseriesachievingatotalcapacitanceof27.2µFforeachcan3.Thecapacitorelementsweremadeupoftwoaluminiumfoilsseparatedbyaninsulatingpolypropylenelayer.TheentireassemblywaswrappedininsulatingpaperandimmersedindielectricoilJarylecC101,whichfilledthecan(AnnexB);theoilandthepolypropylenemadeupthedielectricmediumforthecapacitor.Thecapacitorassemblywasratedat8242V.Itwasnotfittedwithanyinternalfuses,pressuremonitoringorreliefdevices.
TheconstructionandtestingstandardsthatwereusedbyVishaywere:
• IEC60871-1ShuntcapacitorsforACpowersystemshavingaratedvoltageabove1000V–Part1:General;and,
• IEC60871-2ShuntcapacitorsforACpowersystemshavingaratedvoltageabove1000V–Part2:Endurancetesting.
Undernormaloperatingconditions,Vishayestimatedthateachcapacitorshouldhaveaworkinglifeofaround20years.
Thetemperatureratingofthecapacitorswas‘-15B’4whichmeantthattheminimumtemperatureatwhichthecapacitorcouldoperatesatisfactorilywas-15ºC;Bindicatedthatthemaximumambientoperatingtemperaturewaslimitedto45ºC,withthehighestmeanoveranyperiodof24hbeing35ºC.InDecember2010,whilethevesselwasinCaribbeanwaters,surfacetemperaturemeasurements,usingwax-filledstickers,werecarriedoutoncapacitorsinbothranksoftheforwardHF.Themaximumrecordedsurfacetemperaturewas49ºConarank4capacitorand46ºConrank11.3.Ameasurementofthetemperatureonacapacitorinrank11.3inthesamepositionastheonewhichfailedon23September,wasrecordedaslessthan44ºC,whichwastheminimumthewax-basedsensorwascapableofmeasuring.ThehightemperaturealarmforcoolingairinsidetheHFenclosurewassetat47ºCwitha‘high High’alarmsettingof50ºC.ThemaximumtemperatureoftheairintheforwardHFduringthetestperiodwasmeasuredat36ºC.
2 Can–thetermusedtodescribetheexternalcasingofacapacitor.3 Whencapacitorsareconnectedinparallel,thetotalcapacitanceofthegroupisgivenbytherelationshipC=c1+c2+…+cnwherec1…cnarecapacitancesofindividualcapacitorsandCisthecombinedcapacitance.Whenconnectedinseriestherelationshipis1/C=1/c1+1/c2+…+1/cn
4 InaccordancewithIEC60871-1clause4.1c
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Capacitorusedinharmonicfilters
Figure16a Figure16b
Internalelements
Blockdiagram Singlecapacitorelement
Figure16c Figure16d
Exploded
Bulged
93cm
18cm
35cm
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IEC60871-1statesthatwheninductiveelementsareincludedinserieswithcapacitorstomitigatetheeffectsofharmonics,thevoltageatthecapacitorterminalswouldexceedtheexpectedvoltageatthecapacitor.Therefore,suchcapacitorsshouldberatedtowithstandtheincreasedvoltage.Converteam’sspecificationtoVishayforthecapacitorsdidnotmentionthisfactorspecifically,butachievedasimilareffectbyrequiringtheratedvoltageofthecapacitorstobeanamountequaltothesumofthefundamentalvoltageatthecapacitorbankandalltheharmonicvoltagesuptothe30thharmonicorder.
ThecapacitormanufacturingprocesswasnotsubjecttoanyLRapprovalprocedures,andthesociety’srulesatthetimedidnothaveanyrulesregardingtheconstructionorfittingofcapacitors.TherewerenorequirementsinLR’srulesforcapacitorstobefittedwithprotectionormonitoringdevicesandConverteamdidnotincludeanyinitsspecification.However,Vishayprovidedthecurrentimbalancedetectionsystemasstandard.
Shortlyaftertheaccident,Converteam’ssupplierqualityassurancedepartmentcarriedoutanauditofVishay’sconstructionprocessagainstIEC60871-1.Nodeficienciesinthecapacitordesignandconstructionprocesswererecorded.
1.10.2 Test of failed capacitors by Vishay
AllthecapacitorsfromRank11.3thatwereinuseatthetimeoftheaccident,includingtheexplodedandbulgedones,werereturnedtoVishay’sBlatnafacility.BoththefailedandbulgedcapacitorelementswereexaminedbyVishaystaff.Intheirsubsequentreport,theyidentifiedthatthetwomostcommonreasonsforseverereductionincapacitorlife,wereexposuretofrequenttransientvoltagesandincreasedoperatingtemperatures.Vishaywasunabletoformanyconclusionsastothecauseforfailureduetotheseveredisintegrationandfusingtogetheroftheelementsintheexplodedcapacitor.Vishayconcludedfromitsexaminationofthebulgedcapacitoranditselements,thatthedefectsobservedontheelementsmusthaveoccurredoveralongerperiodoftime.Intryingtoestablishtheprobablecauseoffailure,thereportstated:
‘Another theory could be a defect on parts of the safety circuit, the failing of an element in a capacitor cannot be recognized. Due to the increased voltage on the sound groups flashover will happen in these groups after some time of operation. So this capacitor will end with a shortage.’[sic]
Thereportwentontostatethatifadefectivecapacitorwentundetectedbyafailedsafetycircuit,itwouldsufferfrominternalheatingasitdrewcurrent,resultinginthepolypropylenefilmswellingupandeventuallyrippingoffthecontactsontheelements.Thiswouldcauseanelectricarctodevelopatthebrokenconnections,whichinturnwouldcausethedielectricoiltovaporise,increasingtheinternalpressureand,ultimately,thecatastrophicfailureofthecapacitor.ThereportofVishay’sexaminationisatAnnex C.
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1.10.3 Test of failed capacitors commissioned by MAIB
TheMAIBcommissionedERATechnologyLtdtocarryoutanindependentexaminationofthetwodamagedcapacitorsandoneintactcapacitorfromRank11.3.Itsmajorfindingswereasfollows:
• Excessive currents leave characteristic signs of overheating damage. No such signs were observed in any of the elements inspected.
• When a capacitive element fails, it almost inevitably fails short circuit.
• One of the roots of an element failure was traced to the junction of the edge of the foil and a crease in the electrode film. Such creases are natural stress raisers and should be avoided as much as is practical(Figure17).
• The fact that the unit did fail catastrophically indicates that either there was a system side problem overstressing the capacitor or that the capacitive elements could not perform to their stated rating.[sic]
Crease
Foiledge
Figure17
Creaseoncapacitorelement
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Anelementfromthebulgedcapacitorwasinspectedandverysmallroundholes,thelargestofwhichwasapproximately0.5mmdiameter,werefoundbetweentwolayers(Figure18).Withreferencetotheseholes,thereportstated:
‘This type of damage is typically associated with transient overvoltage. However, it is not possible to say, from the physical evidence whether or not the transient over voltage was generated by disturbances [in the supply] or as a result of the prior failure of other elements in the capacitor.’
Thereportconcluded:
‘… it is probable that the incident was initiated by the failure of a single capacitor element and that the failure initiated the progressive failure of the whole unit …’
Holesburntbetweenlayers
Figure18
Holesincapacitorelementfrombulgedcapacitor
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1.10.4 Overvoltage in the electrical network
Inthefirstyearofservice,QM2hadseveralissuesrelatedtoovervoltagesintheelectricalnetwork.Voltagefluctuationwasrecordedtentimesinoneparticularday,withthehighestvoltagelevelreaching13.4kV,nearly22%morethantheratedvoltage.On9September2010,bothanchorwindlassmotordrivesweredamagedduetoovervoltageinthesupplyfromthemainswitchboard.Thevesselalsoblackedoutduringthisevent.
IEC60871-1statesthatacapacitorwouldexperiencethemaximumovervoltagewhenitisswitchedon.Thefirstpeakofthistransientovervoltageisexpectedtobeashighas2√2timestheappliedRMSvalueofthevoltage,foramaximumdurationofhalfacycle.Thestandardalsorequiresthatthecapacitorbecapableof1000switchingoperationsinayear.
LR’srulesallowpermanentvariationofvoltageupto+6%and-10%intheelectricalnetwork.Transientvariationsofupto20%oftheratedsupplyvoltage,witharecoverytimeof1.5sarealsopermitted.
1.10.5 Dielectric oil test
Sixdaysaftertheaccident,approximately30mlofdielectricoilwascollectedfromthedeckoftheaftHFenclosure.Forcomparison,afurther190mlwascollectedfromasealedcapacitorwhenitwasopenedupforexaminationbyERATechnology.Thesampleswereanalysedbyaspecialistlaboratory,NynasNaphthenics,anditsfindingsaresummarisedasfollows:
• Thermaldegradation,orageingofJarylecC101andpaperinsulationleadstotheformationofgases.
• Localisedoverheatingcancausethegasformationtobecomecapableofcarryingacharge.Thisprocesscanaccelerateintheresultantelectricfieldcausingfurtherionization.Duetotheself-healingpropertiesoftheoil,theionisationprocessissometimeshalted,thuspreventingfurtherdamage.However,solidssuchaspolypropyleneandpaperwhichmakeuptheinsulationinthedielectricmediumdonothaveself-healingproperties.
• Moistureconcentrationinbothoilsamples(fromexposedandsealedcapacitors)wasfoundtobewellabovethetypicalvalues(40mg/g)reportedbythesupplierofdielectricfluid.Asthemoistureincreases,voltagebreakdownthresholdsarelowered.
• Furtherwatermoleculesareproducedwhenpaperdegrades,increasingtheoverallmoisturecontent.
• AlthoughtheflashpointofJarlylecC101isnormally144oC,thegasesformedduringthebreakdownoftheliquidandsolidpartsofthedielectricmediumlowertheoverallflashpoint,makingthemixtureflammableatroomtemperature.
• Acetylenewaspresentintheoilsamplefromtheexplodedcapacitor,indicatingahighprobabilityofinternalarcingthroughthedielectricmedium.
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Thereportstated:
The generation of gases during a partial discharge, sparking or arcing, is very rapid and if the system is a closed system, the high pressures generated may compromise the containment by rupturing or even exploding. Further those gases have a high content of hydrogen and light hydrocarbon gases which themselves are flammable and explosive in the presence of oxygen or air.
1.10.6 Supply and returns history
ThefirstrecordedfailureofacapacitoronQM2wasinJanuary2006.Sincethen,Vishayhassupplied11replacementcapacitorstothevessel:2forrank4and9forrank11.3;nosparesweresuppliedatdeliveryin2003.SevencapacitorswerereturnedfromthevesseltoVishay,ofwhichonewasfoundtohaveadefectiveinternalelement,fivehadsustainedmechanicaldamageandonehadanoverheatedcontact.Atthetimeoftheaccidenttherewasonesparecapacitoronboard,whichwasforrank4.
1.10.7 Maintenance and failure history
Whilethevesselwasinservice,capacitorreplacementwasgenerallytriggeredbyanimbalancealarmorastheresultofavisualinspectionidentifyingthatthecasinghadbulgedordielectricfluidwasleaking.Theusualpracticeuponreceivinganimbalancealarmwasforthedutyengineertoalertthechiefelectricalofficer,whowouldthenvisittheappropriateHFcompartmentandcarryoutavisualexaminationandcheckforanysmellofdielectricoil.Ifthecausecouldnotbeestablishedbyvisualexamination,thecrewwouldisolatetheHFconcernedandidentifythecapacitorthathadcausedthealarmbydisconnectingthecapacitorsineachphaseandmeasuringthetotalcapacitance.Oncethedefectivephasehadbeenidentified,eachcapacitorinthatphasewouldbedisconnectedandmeasuredindividuallyuntilthefailedunitwasfound.Itwasdifficulttogainaccesstoeachcapacitorandthecrewwouldoftenhavetoshiftallthecapacitorsalongthefoundationrailsandinsertthereplacementoneattheend.Theserialnumbersofdamagedcapacitorswerenotrecorded.
AtotaloffivecapacitorswerereplacedintheforwardHF.Ofthese,itwasrecordedthatthreewerefromrank11.3;butitwasnotknownwhichranktheothertwocapacitorscamefrom.Whenthefailuresfirstbegantooccurinearly2006,crewhadtotakecapacitorsfromtheaftHFtouseintheforwardHFastheonlyspareoneonboardwasdamagedduringfitting.ThevesselthenoperatedwithouttheaftHFforseveralweeksuntilthesparecapacitorsweredelivered.
SixcapacitorswerereplacedintheaftHF,withthefirstonefailinginrank4duringJuly2006.Theremainingfivewereallfromrank11.3.Themaintenancemanagementsystemdidnotrecordthereasonswhythesecapacitorswerereplaced.However,ofthefivecapacitorsreplacedinrank11.3oftheaftHF,itwaslaterfoundthatfourwerechangedafterroutinechecksconductedduringdrydockingperiods.Duringthedrydockingperiodin2006Converteam,workingwithVishay,replacedtwocapacitorsinrank11.3oftheaftHF.Onewasfoundtobeleakingandtheotherhadbentconnections.ThecapacitancevaluesofthedamagedcapacitorswerenotrecordedandtheywerenotreturnedtoVishayforfurtheranalysis.
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ConverteamcheckedallthecapacitorsinbothharmonicfiltersinOctober2008whilethevesselwasindrydock.Twocapacitorswerefoundtobedefective,withcapacitancereadingsof36.8µFand36.9µF.Thesereadingsshouldhaveactivatedtheimbalancealarm,butthecorrespondingalarmwasnottriggered.ConverteamreporteditsfindingsaboutthefailedcapacitorstoCarnivalinadocumentwhichsummarisedallofthemanytasksthathadbeencarriedoutindrydock.Thedocument(Figure 19)alsoindicatedthattheimbalancedetectionsystemhadnotbeentested.
1.10.8 Classification society survey records
TheaftHFwaslastsurveyedinNovember2006andtheforwardHFinJuly2010underthe5-yearlycycleofcontinuoussurveyofmachinery(CSM).TheCSMsurveyconsistedofavisualexaminationoftheharmonicfilterswhilethevesselwasinport.Imbalancealarmsandtripswerenottested.
Althoughthecapacitorswereinsealedcans,thesurveylistedarequirementfora‘Converter and harmonic filter insulation fluid test’.Thecapacitorswerecreditedaspassingthistestinbothsurveys,despiteitbeingimpossibletowithdrawfluidwithoutdamagingthecasing.
1.10.9 Polypropylene vapour
In2007,theclassificationsocietyDetNorskeVeritasAS(DNV)sentacircularaimedat‘manufacturers of frequency convertors for propulsion and thrusters’(Annex D).InthiscircularDNVraisedtheissueofflammablegasesbeingreleasedfrompolypropylenefilmusedincapacitors.Thecircularstated:
Recent knowledge has shown that there is a risk that the film material may release flammable gases to the environment because of overheating or melting. In enclosed cubicles these gases may lead to a hazardous environment that can ignite and cause explosion. Resent experience shows that this may cause danger to personnel and risk for damage to equipment located in the vicinity [sic].
ExtractfromConverteam’sdrydockattendancereporttoCarnivalUKin2008
Figure19
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1.10.10 Improvements made to capacitor design
Duringthecourseofthisinvestigation,allthecapacitorsintheaftHFonboardQM2werereplaced.AnumberofchangestothedesignofthecapacitorsweremadebyVishay,whichimprovedtherobustnessofthecapacitors.Voltageofeachelementwasreducedby20%andbyusingadifferentgradeofpolypropylenefilmtheelectricalstresswasreducedby5%.Eachcapacitorelementwasindividuallyfusedandpressuresensorswerefittedtodetectinternaloverpressurisation.
1.11 CURRENT IMBALANCE DETECTION SYSTEM
1.11.1 Construction
Thetwoneutralpointsatthedoublestarterminationofthecapacitorcircuitwereconnectedtotheprimarycoilofacurrenttransformer(Figure 20)whichhadacurrentratio10:1(50Aattheprimarywindingand5Aatthesecondarywinding)andwaspartofthemonitoringandprotectionsystem‘EstaSym 3C’suppliedbyVishayElectronicsGmbH,Germany.Therewerenorecordsavailabletoverifythetransformer’sservicehistory.
ThesecondarywindingofthetransformerwasconnectedtoacurrentimbalancedetectionrelayunitwithadisplayfittedonthepaneloftheHFenclosure.Thecurrentimbalancedetectionsystemworkedontheprinciplethatwhenallthecapacitorswereingoodorder,thetwoneutralpointsofthedoublestarterminationwouldbeatasimilarpotential.Consequently,therewouldbeverylittlecurrentflowingacrosstheprimarywindingofthetransformerandthereforeverylittleinducedsecondarycurrentwouldberecordedonthedisplay.Ifacapacitordegraded,thechangeintheoverallcapacitancewouldunbalancethesystemandcauseacurrenttoflowthroughthetransformer.Animbalancealarmwouldbetriggeredifthiscurrentexceeded400mA,andatorabove800mAthemaincircuitbreakeroftheHFwouldtrip,isolatingtheHFfromtheelectricalnetwork.Afullshortcircuitinonegroupofelementsinthecapacitorwascalculatedtodevelopanunbalancedcurrentof1300mA.Bothalarmandtriprelayshadatimedelayof300mstoreducethenumberofspuriousalarmsortripsduetotransienteffects.
Immediatelyaftertheaccident,theimbalancedetectionunitforrank4oftheaftHFindicated800mAandtheequivalentunitonrank11.3(wherethefailuretookplace)showed‘000’(Figure 21).Coolingwaterleakageandcoolingairalarmswerealsoprovidedforeachrank.Thesealarmsdidnotactivateduringtheaccident.Anoverloadalarmwasalsoprovidedbutitwasnotpossibletodeterminefromthealarmrecordsifthisactivatedduringtheaccident.
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Figure20
Currenttransformerfortheimbalancedetectionsystem
Primary
Secondary
Figure21
Currentimbalanceindicatorsintheaftharmonicfilterroom
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1.11.2 Test by Converteam
InJanuary2011theprotectionsystemforRank11.3wastestedbyConverteamandwitnessedbyMAIBinspectors.Currentwasinjecteddirectlyintotherelaystosimulateimbalancecurrents.Theywerefoundtobefunctioningcorrectlyandsettingswereverifiedasbeing400mAforthealarmand800mAforthetripfunctions.Theprotectiontransformerwindingswerethencheckedforelectricalcontinuity,andboththeprimaryandsecondarywindingswerefoundtohavefailedinopencircuit.Thetransformer,whichwasasealedunit,wassenttoVishayforanintrusiveexamination.
1.11.3 Examination by Vishay
ThecasingoftheimbalancecurrenttransformerwascutopenbyVishayandwitnessedbyrepresentativesfromMAIBandConverteam.Thetransformeroilwasverydarkinappearanceandcontainedvisiblesoliddebris.Theprimarywindingwasfoundseverelydamaged,withmostofitsinsulationmissing.Thecopperwireinthesecondarywindingwasbrokenatoneplaceandheldtogetherbydamagedinsulation.Thewirehadflattenedandinsulationbrokenwhereithadbeenwoundaroundtheedgesofthetransformer’sironcore(Figure 22).ThecompletereportisatAnnexE.
Figure22
Currenttransformerfortheimbalancedetectionsystemwithitscasingcutopen(inset:secondarycoil)
Primarycoil
Secondarycoil
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1.11.4 International standard for current transformers
IEC60044-1Instrumenttransformers–Part1:Currenttransformers,statesCurrent transformersintended for both measurement and protection shall comply with all the clauses of this standard.Thestandarddefinesindetailseveralrequirementscoveringtheareasofdesign,testing,accuracyandinsulation.ThecurrenttransformerusedintheEstaSym3CprotectionsystemforharmonicfiltersonQM2wasbuiltin-housebyVishayandwasnotintendedtomeetanyspecificstandard.
Thestandardstated:
Current transformers should not be operated with the secondary winding open-circuited because of the potentially dangerous overvoltages and overheating which can occur.
InFebruary2011,attheMAIB’srequest,ConverteamcarriedoutaverificationauditofVishay’smanufacturingprocessforthecurrentimbalancetransformeragainstIEC60044-1,eventhoughconstructiontothisstandardwasnotinthescopeoftheoriginalrequirementpresentedbyConverteamtoVishay.Ofthe23requirementsthatwerechecked,15didnotcomplywiththestandard.Someofthemostsignificantitemswhichdidnotcomplywere:
5.1.4 Insulation requirement for secondary windings
5.1.5 Inter-turn insulation requirement
8.4 Inter-turn overvoltage test
Converteamalsocarriedoutananalysisofinitialsub-componentinspectionsthatwerecarriedoutbyVishayduringthemanufactureofthecurrentimbalancetransformersforQM2.Fivetransformersub-componentswereidentifiedandthechecksthatwerecarriedoutincludedvisualinspection,mechanicalchecks,electricaltests,chemicalanalysisandverificationofproductdatafromthecomponentsupplier.Postproductionelectricaltestsweresuccessfullycarriedoutonbothwindings.
However,despitetheseshortcomings,Converteam’sconformitycheckssummarised:
In conclusion, we can say that process, inspection and test done by VISHAY is suffisant compared to the use and function of this current transformer [sic].
1.11.5 Monitoring and protection
TheSOLAS5regulationregardingthemonitoringandprotectionofmachinerystates:
‘Where main or auxiliary machinery including pressure vessels or any parts of such machinery are subject to internal pressure and may be subject to dangerous overpressure, means shall be provided where practicable to protect against such excessive pressure.’
5 SOLASconsolidatededition2009,ChapterII-1,PartCRegulation27.2
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AtechnicalbrochurepublishedbyVishay‘MountingandMaintenanceInstructions,StaticPowerCapacitors,NaturallyAir-Cooled’(Annex F)stated:
Explosion or Fire hazard: Even if monitoring and safety devices exist, the incidence of overloading or an important electrical defect may cause the destruction of the casing and/or the bushings. Another consequence of this may be that the capacitor’s combustible component parts catch fire. This aspect shall be taken into account at the site of erection of the capacitor.
InatechnicalpaperpublishedbyTheInstituteofMarineEngineersin1995titled‘Electricpropulsion–aviewfromaclassificationsociety’6,theauthorsinadiscussiononharmonicfiltersstated:
‘With regard to filters, it is perhaps worth noting that overpressure in capacitors cans may have to be included in the protection arrangements…’
Whiletheauthorsdiscussedthismatterinthepaper,therewasnospecificregulationorclassificationsocietyrulethatrequiredsuchprotectionofcapacitorsinhighvoltageequipment.ThegeneralrequirementinSOLASwasnotinterpretedasbeingapplicabletothecapacitors.
WhentheIASreceivedanimbalanceoroverloadalarmfromanHF,thedutyengineercouldinterrogatethegraphicaluserinterface(GUI)intheECR,whichindicatedtherankoftheHFwhichwasinalarm(Figure23).TheGUIdisplayforboththeoverloadandtheimbalancealarmwascombined,andread‘OVERLOAD/UNBALANCE’.IfthecircuitbreakertotheHFwastripped,itwouldbeindicatedasanadditionalalarmintheIAS,asanelectricalfaultonthebreaker.Therewerenorecordsoftheimbalancealarmsortripsbeingtestedeitheratcommissioning,oratanyotherpointinthelifeofthevessel.
6 J.B.BormanandB.P.SharmanElectricpropulsion–aviewfromaclassificationsociety.InElectric Propulsion: The Effective Solution,TheInstituteofMarineEngineering,October1995
Figure23
Enginecontrolroomdisplayofharmonicfilteroperatingparameters
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Theimbalanceprotectionrelaysweresettoalarmat400mAandtripat800mA.Thevalueofanyimbalancecurrentthatexceededthesesettingswasshownonthedisplayuntilmanuallyreset.StaffatVishayconsideredittobeextremelyunlikelythatthedisplaywouldindicate000mAinnormalserviceasitwouldindicatethatallthecapacitorshadexactlythesameperformance.Theinstructionmanualdidnotmentionthispoint.
1.11.6 History of imbalance alarms
TherewasnoevidencetoindicatethattheHFbreakershadevertrippedduetoimbalancecurrentsinQM2’sservicehistory.TheimbalancealarmcurrentsdisplayedontheEstaSymdisplayswerenotusuallyrecorded,exceptonrareoccasionswhentheship’screwdiscussedtheseissueswithConverteam.Ahistoricalanalysis(from2004to2010)ofharmonicfilteroverload/imbalancealarmsrevealedthattheaftHFalarmhadbeentriggeredonnineseparateoccasions.TheIASsystemdidnotdisplayorstoreinformationonwhichrankofthefilterhadregisteredanalarmcondition.
Otherrecordsshowedthatanalarm,indicatingproblemswithrank4oftheaftHF,occurredinJuly2006whilethevesselwasen-routetoNewYork.Theship’sstaffisolatedtheHF,carriedoutavisualexaminationand,findingnothinguntowardtheyswitcheditbackon-line.AfterarrivingatNewYork,theymeasuredthecapacitanceofeachindividualcapacitorintherank.Theyfoundthatoneofthemmeasured38.9µF,indicatingthatagroupofnineelementshadstoppedworkingduetothefailureofoneormoreelementsinthatgroup.
InDecember2010,whilethevesselwassailingatnormalservicespeed,thealarmforrank4oftheforwardHFactivated.Oninspection,theimbalancecurrentwas1250mA.Althoughwellinexcessofthetripsettingof800mA,theHFdidnottripduetoawiringfaultintheprotectionsystem.Thecrewstoppedthepropulsionmotors,switchedoffthecircuitbreakerfortheharmonicfilterandincreasedthespeedbackupto50rpmwhilemonitoringtheTHDvusingaportableinstrumentthatwasheldonboard.InthiswaytheywereabletorestorelimitedpropulsionpowerwhilemaintaininganacceptableTHDvof6.2%.Theysubsequentlyreplacedacapacitorinrank4whosecapacitancemeasured38.4µF.Thevesselwasthenbroughtbackuptofullspeed.Theimbalancereadingofrank4returnedto120mA.Theimbalancecurrentinrank11.3remainedat65mAthroughouttheincident.
1.12 ARC-FLASH
1.12.1 Phenomenon of arc-flash
Whenanelectriccurrentflowsthroughairgapsbetweenconductors,anarc-flashissaidtooccur.Anarccanformbetweenphase-to-ground(orneutral),orphase-to-phase,andisaccompaniedbyionisationofthesurroundingair.Whentheairqualityisdegradedwithmoistureorotherimpurities,thepossibilityofanarcstrikingisincreased.Thearccolumntemperaturecanvaryfrom5000Kto20,000Kandtheintenseheatcanvaporisetheconductorsandsurroundingmaterials.Copperinsolidformsublimatesto64,000timesitsvolume,causingfurtherdeteriorationoftheair’sinsulationquality;aphase-to-phaseorphase-to-groundarcingfaultcanescalateintoathree-phasearcingfaultinlessthan1/1000ofa
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second.Theheatofthearccolumnalsoheatsupthesurroundingair,whichthenmovesintothesurroundingcoolerairwithaspeedexceedingthatofsound,causingashockwaveandexplosivenoise.
Anarc-flashisnotnormallyestablishedbelowaphase-to-phasevoltageof208Vorphase-to-groundvoltageof120V.Athighervoltages,itcancausesevereshockwaves,splatteringofmoltendebris,loudexplosionsduetotherapidlyreleasedvapouraswellasseriousburninjurytoanyoneinthevicinity.Thearchasatendencytomoveawayfromitssource.
TheMaritimeandCoastguardAgency’s(MCA)CodeofSafeWorkingPracticesforMerchantSeamen(COSWP)containssomeguidanceonworkingwithhighvoltagesanddiscussestheappropriateuseofpermitandsanctiontowork.Itdoesnotcontainanyadviceoncontrolmeasurestomitigatethehazardsofarc-flash.
1.12.2 High voltage regulations for enclosures
LR’srulesonswitchgearandcontrolgearassemblieswereonlyappliedtothemainswitchboardsonQM2andwerenotapplicabletotheHFenclosures.Undersection7.16.5.,LR’srulesstate:
For switchgear and control gear assemblies, for rated voltages above 1 kV, arrangements are to be made to protect personnel in the event of gases or vapours escaping under pressure as the result of arcing due to an internal fault. Where personnel may be in the vicinity of the equipment when it is energised, this may be achieved by an assembly that has been tested in accordance with Annex A of IEC 62271-200 and qualified for classification IAC (internal arc classification)
ThemainswitchboardenclosuresweresuppliedbyABBandweretheUnigearC/G12type.Theconstructionconsistedofanextrachamberabovethemaincircuitbreakers,whichwasopentotheatmosphere.Eachswitchgearcompartmentwasfittedwithanarcflapontop,designedtoreleasethepressureincaseofanarc-flash.Thebusbar,circuit-breakerandcablecompartmentswerephysicallyandelectricallysegregatedinthisdesign.
TheenclosurefortheharmonicfilterwasprovidedbyConverteamandwasconstructedtoprovideaningressprotection(IP)of44;thislevelofprotectionwasselectedbyConverteamasanappropriatestandardinordertopreventthecoolingairinsidetheenclosurefromescapingandmeettheshipyard’sspecification.
1.12.3 Test of soot
InDecember2010,asampleofthesootfromthedeckheadandapieceofinsulatingfoamfromacopperpipepassingunderthedeckheadoftheaftharmonicfilterroom,weresentforanalysistoaspecialistlaboratory.Thepurposeofthetestwastoinvestigateifsubstantialamountsofcopperoxidewerepresentinthesoot,whichwouldestablishthatanarc-flashhadtakenplace.Thereportsummarisedthatthesootcomprisedmostlyofcarbon,presumably from burnt organic material which might include oil and polymeric material. Theanalysisrevealediron;zincoraluminium;andafewcopperparticles.Nocopperoxidewasidentified.Agreendepositontheinsidesurfaceoftheinsulationwasreportedasappeared to be copper chloride, possibly the result of a reaction of copper in a marine environment.
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1.12.4 Expert opinion
TheMinistryofDefence(MOD)hasprocuredanumberofvesselswithhighvoltagepropulsionsystemsandhasstudiedthepotentialrisksfromarc-flashincidentsandhowbesttomitigatethehazards.SpecialistMODstaffreviewedphotographsofdamagetotheaftHFonQM2,andconcludedthatitwashighlylikelythatanarc-flashoccurredduringtheaccident.TheirobservationsarereproducedatAnnex G.
1.13 HI-FOG FIRE SUPPRESSION SYSTEM
1.13.1 Manufacture and design
QM2wasfittedwithawater-mistsystem,knownbyitstradename‘Hi-Fog’, thatwasdesignatedasawater-mistsystemforlocalapplicationfire-fightinginmachineryspacesofcategoryA7.ItwasmanufacturedbyMarioffOy,Finlandandwasinstalledwhenthevesselwasbuilt.
Partsofthesystemwerefittedwithdifferenttypesofactivationmethods,intendedtolimitunwantedactivationandconsequentdamage.Themainmachineryspaceswerefittedwithsprayheadswhichrequiredmanualactivation,achievedbyoperatingasinglebutton.Thecommunicationrooms,wheelhouse,safetycentre,ECR,batteryroom,chartrooms,andpoolroomshadapre-actionsystem.Thisrequiredaflameorsmokesensortotrigger,activatingasolenoidcontrolledvalvewhichallowedwatertoenterthepipesandthenflowthroughthesprayheads.Theaccommodationspaces,includingDeckBwherethemainswitchboardroomsandharmonicfilterroomswerelocated,allhadsprayheadswithglassbulbsthatweredesignedtobreakatanambienttemperatureexceeding57ºC.
TheHi-FogpipingsystemintheMSBandHFroomswasofthe‘wet’type,filledwithfreshwaterat25barspressure.Ifasprayheadwasactivatedthesystempressurewoulddropto15bars,activatingthesprinklerpumpaccumulatorunit.Thisdeliveredapressureof140bars,whichresultedinwaterbeingappliedintheformofafinemist.Water-mistwasnormallyformedabove45barspressure.Dependingonthenumberandlocationofthesprayheadsthathadbeenactivated,andwhethertherewastrappedairintheline,designassumptionsestimatedthatitcouldtakeupto2minutesforthehighpressurewaterpumptocutin.ItwaspossibletodeterminefrommachineryrecordsonQM2thatthehighpressurewaterpumpsstarted6minutesaftertheexplosionoccurred.
7 MachineryspacesofcategoryAarethosespacesandtrunkstosuchspaceswhichcontaineither:
1. internalcombustionmachineryusedformainpropulsion;
2. internalcombustionmachineryusedforpurposesotherthanmainpropulsionwheresuchmachineryhasintheaggregateatotalpoweroutputofnotlessthan375kW;or
3. anyoil-firedboileroroilfuelunit,oranyoil-firedequipmentotherthanboilers,suchasinertgasgenerators,incinerators,etc.
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1.13.2 Design appraisal
ThedesignappraisalwascarriedoutbyLRandwasconsideredtocomplywiththefollowingrequirements:
• SOLAS 1974, as amended, Chapter II-2/10;12.
• IMO Res. A800(19):IMOResolutionA.800(19)RevisedGuidelinesforapprovalofsprinklersystemsequivalenttothatreferredtoinSOLASRegulationII-2/12.
• MSC/Circ. 913:MaritimeSafetyCommitteecircularMSC/Circ.913.Guidelinesfortheapprovaloffixedwater-basedlocalapplicationfire-fightingsystemsforuseincategoryAmachineryspaces.
• ISO 15371:2000:Shipsandmarinetechnology-Fire-extinguishingsystemsforprotectionofgalleycookingequipment.
• IMO Res.[sic]668:MaritimeSafetyCommitteecircularMSC/Circ.668.AlternativearrangementsforHalonfire-extinguishingsystemsinmachineryspacesandpumproomsCargopumpRooms.
• TheRulesandRegulationsfortheClassificationofShips,Part6,Chapters1and2.
ThedesignappraisaldocumentlistedallthelocationswhereHi-Fog sprayheadsweretobeinstalled,beginningwithdecks13and14downtodeckBunderthesectionheaded‘ACCOMMODATION SPACE Comments’.TheitemisedlistofcompartmentsindeckBdidnotincludetheMSBorHFrooms(Figure 24)whichwerewithinthispartoftheship.Therewasnorecordofthesecompartmentsbeingconsideredseparatelyorinanyotherpartofthedesignappraisal.
Figure24
ItemisedlistingofareasfittedwithHi-fog outlets
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In2004,MarioffOyconductedteststodemonstratethatitwassafetousetheHi-Fogsysteminhighvoltagecompartments(AnnexH).AtthetimethatQM2wasbuilt,noneoftheclassificationsocieties,includingLR,hadanyexplicitreferenceintheirrulesregardingtheuseoffixedwater-basedlocalapplicationfire-fightingsystemsincompartmentscontaininghighvoltageequipment.
1.13.3 Current regulations
ThecurrentLRRulesunderChapter2(ElectricalEngineering),Section16.3Fixedwater-basedlocalapplicationfire-fightingsystem(FWBLAFF)state:
High voltage equipment and their enclosures are not to be installed in protected areas or adjacent areas.
MostclassificationsocietiesrequirethatelectricalequipmentinareasprotectedbyFWBLAFFaretobecontainedinenclosureswithingressprotection44(IP44).TheFSSCode8referstoMSCCircular1165,whichinturndiscussestheuseofwater-mistsystemsonlyinrelationtocategoryAmachineryspaces.TheonlyguidanceavailableregardingtheuseofFWBLAFFincompartmentswithhighvoltageequipmentisafootnoteinIACSrequirementE20.Itstates:
‘Additional precautions may be required to be taken in respect of high voltage installations.’
TheIMOMaritimeSafetyCommittee’ssub-committeeonfireprotectionagreedseveralrevisionstoMSCCircular913atits54thsessionheldinApril2010.However,therevisionsdidnotincludeanyreferencetotheuseofFWBLAFFinhighvoltagecompartmentsorenclosures.
1.14 POPULARITY OF ELECTRIC PROPULSION AND VARIABLE FREQUENCY DRIVES
AstudycarriedoutbytheMAIBindicatedthatofalloperationalvesselsover100grosstons,1.85%havehighvoltageelectricpropulsionsystems.Ofthevesselswhicharebeingconstructed,4.10%areelectricallypropelled.Variablespeedmotorsarebecomingcommonplaceinnewshipswithelectrically-drivenpropulsion,cargopumpsorotherapplications,benefitingfromthistechnology.Modernliquefiednaturalgasshipspredominantlyusevariablespeedmotorsintheirre-liquefactionplants.Thereare545vesselsinvolvedinoffshoreactivitiesfittedwithelectricpropulsion.
Themajorclassificationsocietieswereaskedtoreporthowmanyvesselstheyhadontheirregistersthatusedhighvoltagepowersystems.DNVreportedthat319vesselswereregisteredwhichusedHVelectricpropulsion,themajorityusingcapacitor-basedharmonicfilters.NipponKijiKyokai(ClassNK)had14vesselsregisteredasbeinginserviceandafurthernineunderconstruction.LRwasunabletoprovidesimilarinformationregardingthenumberofvesselsthatwerepotentiallyatriskfromsimilarproblems.
8 TheInternationalCodeforFireSafetySystems(FSSCode)
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Atthetimeofpublication,Converteamhadsupplied65vesselswithsynchro-convertersandthemajorityofthemwerefittedwithpassivetunedfiltersusingsimilarcapacitors.CarnivalUKandP&OAustraliaoperatefourvesselswithHFsystemsidenticaltothatofQM2.
1.15 SIMILAR ACCIDENTS
1.15.1 Capacitor failures
ThecircularonthehazardsofpolypropylenevapourwhichDNVdistributedin2007tomanufacturersofpowerconverterswaspromptedbythreeaccidentsonboardoffshoresupportvessels.Inthefirstcaseasmoothingcapacitor,approximately20to30kginweight,fittedintherectifiercircuitofa690Velectricpropulsionsystemexploded,blowingoutthesteeldoorsofitsenclosures.Inasecondincident,acapacitorinapassiveharmonicfilterfailed,resultinginthedisruptionofvariablespeedseawatercoolingpumpsforthelowtemperaturecoolingsystem.Detailedanalysisbythemanufacturersconcludedthatwhenthepassiveharmonicfilterwaslostduetocapacitorfailure,theharmonicdistortionincreasedtoapproximately20%which,inturn,affectedthespeedregulationsystemoftheseawaterpumpmotors,causingthemtoshutdown.Inathirdincident,theharmonicfilterenclosurepanelwasblownoutwhenacapacitorexploded.
1.15.2 Arc-flash accidents
TheIEEEGuideforPerformingArc-FlashHazardCalculations,IEEE1584-2002(©2002IEEE9)containsatableof49arc-flashaccidentsinland-basedinstallations.Itcontainsdetailsofvoltageofequipment,activityundertakenatthetimeoftheaccident,andinjuriessustained.ThistableisreproducedinAnnexI.
InastudycarriedoutbytheMOD,17fatalitieshavebeenreportedonnon-UKsubmarinesasadirectresultofarc-flashincidents.Thestudyalsoexaminedaccidentsonmerchantshipsthatwerelikelytobeattributabletoanarc-flashfault.Thefollowingarc-flashaccidentswereidentified:
• In1990Regent Star,apassengervessel,sufferedanMSBfiredisablingthevesselmid-river.
• In1993,thero-rovesselUnion Rotoruaexperiencedafireinher6.6kVMSBandhadtobetowedtoport.
• In1995thepassengervesselCelebrationhadamajorelectricalfireintheECR.Sun Vista,apassengervessel,sankinMalaccaStraitwhenherswitchboardcaughtfire.
• In2000,thepassengerferryColumbia wasdisabledfollowingafireinherMSB.
• In2002,thepassengervesselStatendamhadanarc-flasheventinamaincircuitbreaker.Thiscausedafirewhichspreadtoothercompartments,requiringthevesseltobetowedtosafety.
9 http://standards.ieee.org/
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SECTION 1 SECTION 2 - ANALYSIS
2.1 AIM
Thepurposeoftheanalysisistodeterminethecontributorycausesandcircumstancesoftheaccidentasabasisformakingrecommendationstopreventsimilaraccidentsoccurringinthefuture.
2.2 THE ACCIDENT
2.2.1 Explosion
Although‘halfdrivealarmlamp’indicationsappearedontheP1200alarmsystem,thesewerenotreadilyapparenttoQM2engineroomwatchkeepers.Eveniftheyhadbeen,theirsignificancewasnotobvious,anditisconsideredunlikelythatanyofthewatchkeeperswouldhaveinterpretedthemasearlywarningthatacapacitormightfailcatastrophically.Theyonlybecameawareoftheproblemwhentheyheardtheexplosionandsawthevesselblackout.Theyestablishedthelocationoftheexplosionbythethickblackfumesemanatingfromthemainswitchboardroom.TheaftHF‘overload/unbalance’alarmindicationintheECR;thealarmindicatingtheopeningoftheaftHFcircuitbreaker;theexplosion;lowvoltageblackoutfollowedbythestoppingofmainpropulsionmotors;andfinallytheshutdownofthegenerators,happenedwithinafewsecondsofeachother.Therewasnoopportunityforthecrewtointervene,eithertopreventtheexplosionorthesubsequentlossofpower.
Ignitionofdielectricvapour
Evaluationofthefailedcapacitorsestablishedthatconsiderablearcinghadtakenplacebetweenthefoillayers.Thepresenceofacetyleneinthedielectricoilsample,evenaftertheoilwasexposedtoatmosphereforseveraldays,furthercorroboratesthis.Bulgingofthecapacitorcanscouldonlyhavebeenasaresultofinternalpressurisationbyvapourcreatedbythebreakdownofthedielectricinsulationmaterialsbyinternalarcing.Thereductioninflashpointcausedbythemixingofthisflammablevapourwithexplosivegaseslikehydrogenandacetylenewouldhaveincreasedthelikelihoodofthemixturebeingignitedbyoverheatedorsmoulderinginsulationmaterial.Similarly,anarcbetweentheopposingplatesofthecapacitorcouldalsohavecausedignition.ThepredominanceofcarbonfoundinthesootsamplefromtheaftHFroomconfirmsthatburningofthevapourmixturehadtakenplace.
Electricalarc-flash
Theprobableconditionsthatexistedjustbeforetheaccident:oilweepingorsprayingoutfromthefailingcapacitorcasing;11kVvoltageacrossthethreephases;andthesubsequentreleaseofflammablegasesfromthecapacitors,wereidealforanarc-flasheventtotakeplace.Themeltedcornersofthecopperbusbarsindicatethatarcingdidtakeplacebetweenthephases,anditisthereforepossiblethattheexplosivenoiseheardbythecrewmemberswasduetotheshockwavesproducedbytherapidlymovinghotairemanatingfromthearccolumn.However,themajorityofcopperappearedtohavemeltedandre-solidifiedinsteadofsublimatingasinaclassicarc-flashevent,possiblyexplainingwhycopperoxidewasnotdetectedinthesootsamplethatwascollectedfromthedeckhead.Itisalsopossiblethatmuchof
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thecopperoxidewascontainedwithintheenclosureandwascleanedawaybeforethesamplewastaken.Oneormoresmallerarc-flasheventscouldalsohavetakenplaceinthepastandbeeneitherundetectedorunreported.
Cause
Itismostlikelythattheexplosionwasduetoacombinationofbothevents:ignitionofthevolatiledielectricvapourreleasedfromthecapacitorandanelectricalarc-flash.Whileitiscertainthattheseeventswereinterdependentandoccurredclosetogether,ithasnotbeenpossibletodeterminewhichwasthetrigger.
Irrespectiveofthecauseoftheexplosion,thedamagefollowingitclearlydemonstratedthattheenergyreleasedwassignificant.Itwasextremelyfortunatethattherewasnooneinthevicinitywhentheexplosiontookplaceastheamountofenergyreleasedwaseasilycapableofcausingfatalinjuries.Thisemphasisesthepotentialdangertocrewenteringtheharmonicfilterroom,especiallytoinvestigateanalarm.IftheHFenclosurehadbeenbuilttothesamestandardashighvoltageswitchgear,itwouldhavehadabuilt-inmechanismtoreleasetheexplosivegases,thedamagecausedbytheexplosioncouldhavebeenmitigatedmoreeffectively,andanycrewnearbybetterprotected.Thecaseforuniformapplicationofhighvoltageswitchgearprotectionstandardstoallotherhighvoltageequipmentwherecrewinterventionmayberequiredduringoperationisthereforecompelling.
2.2.2 Sequential blackout
Sequence
Itwasevidentthattheblackoutwassequentialratherthaninstantaneous.Whenthecapacitorliftedupfromthemountingitbrokeitsconnectiontothe11kVlinesupply,therebycausingalossofreactivepowerasindicatedbythe‘IO-FAULT’alarmsatthe11kVbusbarsandgenerators.Thecapacitorbreakingitsconnectionstothebusbarswouldhaveresultedinanopencircuitinoneofthethreephases,andwasmostlikelytohavebeenthecauseofthediscrepancyalarmswhichindicatedthattheswitchboardhaddetectedalossofonephase.Thesingle phasingislikelytohavetriggeredthenegativesequencedetectionrelayofthegeneratorsandwouldexplainthesubsequenttrippingoftheforwardMSBbustiebreakerinaccordancewiththediscriminationsetting.ThiswouldalsoexplainwhythecorrespondingbreakerontheaftMSBremainedclosedevenafterallthegeneratorsshutdown.Whilethisscenarioisconsideredthemostlikelygiventheavailableevidence,itcannotbestatedwithcertaintybecausethebreakershadnotbeenconfiguredtorecordwhyeachofthemtripped.
Itisalsopossiblethatduringtheaccident,therewasashortcircuitorarcflashbetweentwophaseswhichcouldexplainthevoltagedipcausedatthepropulsionnetworkbridgeshownbythealarmsontheP1200system.However,itisnotclearwhytheundervoltagealarmdidnotappearattheIAS.Although,theexplosionandthefailureofelectriclightsoccurredalmostsimultaneously,thegeneratorscontinuedtomaintainmainvoltageforapproximately16secondsaftertheevent,asseeninFigures 11aand11b.
Thesediscreteevents,occurringoverseveralseconds,establishthattheblackoutwassequential,startingatthelowvoltagesideofthenetwork,subsequentlyaffectingthehighvoltagesideandfinallyresultinginalltheDGsshuttingdown.
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Causeofblackout
Withouttheinformationfromeachofthebreakers,itwasnotpossibletoestablishexactlywhythegeneratorsshutdown.ThealarmsequenceduringtheincidentindicatesthattheHFcircuitbreakeropenedfirstinresponsetothecatastrophiceventwithinrank11.3,suggestingthatitsdiscriminationsettingwascorrect.However,astheyhaveyettobeverified,thiscannotbeconfirmed.
Acommoncauseforblackoutisalargevariationofvoltagewithinthenetworkwhenasignificantproportionoftheelectricalloadissuddenlyappliedorabruptlydisconnected.Fromthepropulsionsystemalarmlog,itisevidentthatallthepropulsionmotors’networkbridgesregisteredlowvoltagealarms,althougheachtimetheyappeartohaverecoveredtonormaloperatingvoltage.Surprisingly,therewasnoindicationoflowvoltageontheIASanditisthereforenotclearwhythegeneratorsshutdown.ItislikelythatthehighvoltagenetworkwasunabletorecoverfromtheinstabilitycausedduetothedisruptionswithintheaftHF.
Itishighlylikelythatjustbeforetheaccident,thetwocapacitorsintheaftHFhaddegradedtosuchanextentthattheabilityofthe11.3rankHFtoabsorbharmonicswouldhavebeenseverelycompromised(seeTable 2 showninsection2.3.3).ItfollowsthattheTHDvjustbeforetheHFfailedwouldhavebeensignificantlyhigh(Figures 12 and13),perhapscausingthestartoftheelectricalinstabilityasindicatedbythefirstalarmononeofthehalfdrivesofpropulsionmotorno.3,around36minutesbeforetheaccident.
Asthevesselwaswellawayfromtrafficandnotincongestedorshallowwaters,thelossofpowerfor30minutesdidnotcauseanynavigationaldifficulties.However,losingcontrolofalargecruiselinerduetoanelectricalblackout,with3823peopleonboard,isaseriousconcern.Thisaccidentdemonstrateshowelectricalinstabilitycancauseunpredictableandpotentiallydisastrousconsequencesinmarinehighvoltageelectricalnetworks.Itisthereforenecessarytoconsiderhowsuchtransienteventscanbemonitoredandrecordedtounderstandtheexactnatureandcauseofelectricalinstabilitiesandthebestwaytomitigatethem.
2.3 HARMONIC DISTORTION OF CURRENT AND VOLTAGE
2.3.1 Awareness
Onconventionalvesselswithveryfewnon-linearloads,harmonicdistortionofcurrentandvoltagehasnottraditionallybeenanissueofconcern.However,electricpropulsionwithvariablespeedACmotorsisrapidlybecomingthepreferredmethodofpropulsiononseveraltypesofmarinevessels.VariablespeedACmotorsarealsobecomingmorecommonasprime-moversinvariousauxiliarymachines.Theassociatedproblemswithharmonicdistortionarethereforeincreasing.Itisimportantthatships’crewsgainathoroughunderstandingoftheissueofharmonicdistortion,sothattheyarebetterabletoappreciatetheimportanceoftheharmonicmitigationequipmentonboardandtaketimelyactionifsuchequipmentfailsordeteriorates.
2.3.2 Simulations and trials
AlthoughthelikelyTHDvwascalculatedatQM2’sdesignstagein2002andmeasuredduringthevessel’sseatrialsin2003,theeffectoflosingbothharmonicfilterswasnotconsidered.ItmaybearguedthatQM2wasneverintendedtooperate
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withoutanHFandcouldmaintainherharmonicdistortionlevelswithinacceptablelimitswhilemaintainingservicespeedwithoneHF;thereforeitwasnotnecessarytomodelthecaseofthevesseloperatingwithouteitherHF.Nevertheless,duetothedelayinthesupplyofspares,thereweremanyoccasionswhenthevesselhadonlyoneHF.AswasdemonstratedinDecember2010,theoneremainingHFcouldhavefailedatanytime,leavingthevesselwithnomitigationagainstharmonicdistortion.Moreover,astherearenorequirementsfromclassificationsocietiesregardingredundancyinharmonicfilters,manysmallervesselsmayonlybefittedwithoneHF,andbeoperatinginservicewithnoguidanceontheeffectonships’electricalsystemsshoulditfail.
ThesimulationstudyconductedbyConverteamestablishedthattheTHDvwouldhavereached22%hadthepropulsionplantcontinuedtooperateat70%poweroutputaftertheaccident.Itwasfortunatethattheundervoltageinthenetworkbridgeofthepropulsionconverterscausedthepoweroutputtoreducetolowlevels;otherwisetheconsequencesofoperatingathighlevelsofTHDvcouldhavebeensevere.
InDecember2010,whenQM2developedadefectontheforwardfilter,itwasonlytheincorrectwiringintheprotectionsystemthatpreventedthecircuitbreakerfromtripping,thusavoidinganotherpotentialblackout.Thevesselwasapproachingportatthetimeandthepotentialconsequencescouldhavebeenveryserious.However,thehighimbalancecurrentof1250mAthatwasrecordedatrank4oftheforwardHFwasindicativeofacapacitorinanadvancedstageofdegradation,anditwasalsoveryfortunatethatthedegradedcapacitordidnotfailcatastrophically.Asthecrewhadaccesstotherecentlycompletedtheoreticalmodellingdata,aswellasaninstrumentwithwhichtomeasuretheharmonicdistortion,theywereabletomanagethesituationwellandmakethenecessaryrepairswithoutcompromisingthesafetyofthevessel,thepassengersorcrew.
2.3.3 Monitoring and in-service verification
ThemeasurementsofTHDvthatweremadeduringtheseatrialsin2003demonstratedthatitwaspossibletomaintainthemwithinthe8%marginsthatwererequiredbyLR.However,nomeasurementswerecarriedoutwiththreeDGsandoneHFinuse,theoperatingconfigurationjustbeforetheaccident.Whileitwouldbeimpracticaltoexpecttheseatrialstocoverallthepossiblecombinationsofgenerators,harmonicfilters,poweroutput,networkconfigurationandmeasurementpoints,itwouldbesensibletoverifytheharmonicdistortionlevelsinthehighestriskandoperatingconfigurationsusedinservice.
ThecomparisonmadeinTable 1betweentheTHDvmeasurementsof2003and2010demonstratesanoverallincreaseinTHDvlevelsatallthemeasuredpoints.AsTHDvinthenetworkshouldnotincreaseunlessthenetworkimpedanceshavechanged,thereasonforthisincreaseinharmonicdistortionmustbeattributedtothedifferenceinelectricalloadingconditionsbetweenthetwomeasurements.ThisillustratesthatTHDvisnotstaticandwillchangeinoperation,furtherreinforcingthebenefitofcontinuous,oratleastperiodicchecks.
DuetotheinternalcircuitryofQM2’sHFcapacitors,ashortcircuitinoneelementinagroupofninewouldresultintheshortcircuitoftheentiregroup,whichwouldcausethecapacitanceoftheremainingthreegroupstoriseby33%.Ifallfourcapacitorcansinonephasesufferedthesamedegradation,theoverallcapacitance
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ofthatphasewouldhavebeen145µF,resultinginthede-tuningoftheHFinonephaseandresultantharmonicpollution(Table 2).Iftheharmonicdistortionhadbeencontinuously,orevenonlyperiodicallymonitoredonboard,thechangesinTHDvcouldhavebeeneasilydetectedandpreventiveactionstakenbeforethecapacitorfailedcatastrophically.
Capacitance(µF) Harmonic order Possible conditions
108.8 11.3 Allelementsingoodcondition
127.0 10.4 Oneelementeachshortedintwocans;twogoodcans.
136.0 10.1 Oneelementeachshortedinthethreecans;onegoodcan.
145.0 9.7 Oneelementshortedineachofthefourcans.
190.0 8.5 Threegroupsshortedinonecan;threegoodcans.
Table 2:Illustrationshowingthevariationoftunedharmonicorderwithinternaldegradationofcapacitorsinrank11.3oftheharmonicfilter.ThevaluesofharmonicorderarecalculatedbyMAIBusingtherelationshipdiscussedinSection1.8.1assumingthecapacitancechangesinonephaseonly.
Duringthe3dayswhenMAIBevaluatedthepowerqualityonQM2,theaftHFwasnotavailableandonlytheforwardHFwasinuse.Ithadafunctionalcurrentimbalancedetectorand,astherewerenoalarms,itcanbeassumedthatthecapacitorsinbothranksoftheforwardHFwereinsatisfactorycondition,andtheTHDvlevelswouldhavebeenmaintainedwithinacceptablelimits.However,thepowerqualityofthevesselduringthetimepriortotheaccidentwhentheaftharmonicfilterwasinuse,wouldhavebeendifferent.Astheimbalancedetectorforrank11.3wasdefective,andthecapacitorshaddegradedwithoutbeingnoticed,theunitwouldhavebecomede-tunedanditiscertainthattheaftHFwouldnothavebeenaseffectiveastheforwardone.
Theovervoltageproblemsthatweredocumentedduringthefirstyearofthevessel’sservice,andagainasrecentlyas2weeksbeforetheaccident,wereindicativethatovervoltageconditionsexistedintheelectricalnetwork.Theholesfoundononeofthecapacitorelementsweretypicalsymptomsofovervoltagedamage.Thiscouldhaveoccurredduringtheaccident,orevenearlierinservice;withoutcontinuousmonitoring,transientovervoltageishardtorecord,quantifyoranalyse.
ThelessfrequentusageoftheaftHF,reflectedbyitssignificantlylowerrunninghourscomparedtotheforwardone,couldperhapsbeattributedtoitsunsatisfactoryperformance.Itiscommon,whentwomachinesareprovidedforthesamepurpose,thateventhoughbothworkacceptably,oneismoreeffectivethantheotherandispreferredbytheoperators.ItisquitepossiblethatthisoccurredwiththetwoHFs,andtheaftunitwasincreasinglyleftasthestand-by,cuttinginonlytoassisttheforwardHFandrarelyusedonitsown.Hadtheharmonicdistortionmeasurement
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equipmentthatwasprovidedtothevesselbeenused,itwouldhavegiventhecrewsomeinsightintothepowerqualityandthecomparativeeffectivenessofthetwoHFsonboard.
Regularmonitoringofpowerqualityusingapre-determinedpatternofpropulsionmotorloadingwithacompleterecordofoperationalparameterswouldhelpensurethattheharmonicdistortionlevelsonboardarecloselymonitoredasthevesselanditsequipmentageandoperatingconfigurationschange.Anon-linemonitoringsystemthatrecordsalltheparametersandcanbetriggeredtomakespecificrecordingsoftransientvoltagespikesorresonances,wouldbeinvaluableinassessingtheongoingqualityofpower.Itwouldalsobeaveryusefultooltoinvestigatetherootcauseofaccidentscausedbyanomaliesintheelectricalnetworkandtoidentifyincipientfaultsinthesesystems.Land-basedutilitiesmonitortheirpowerqualityasamatterofroutine.Inamarinevesselwhereharmonicdistortionhasthepotentialtodisruptitselectricalnetwork,theneedforpowerqualitysurveillanceisevenmoresignificant.
2.4 CAPACITOR FAILURES
2.4.1 Initiation of failure
Construction,designandrating
QM2’sHFcapacitorsweremanufacturedbyVishay,oneoftheleadingsuppliersofthisproductwithanestablishedglobalsupplychain.Converteam’sauditsofVishay’sprocesseshadnotrevealedanydeficiencies,andestablishedthatthemanufacturingandtestingprocessadheredcloselytoIEC60871parts1and2.TheindependenttestscarriedoutonbehalfofMAIBdidnotrevealanymajorflawsinconstruction,exceptthecreasingonthecapacitorelementfoilinonecapacitorelement.Nevertheless,theoveralldesignandconstructionofthecapacitorwasacceptableandmetConverteam’sspecification.
Thecapacitors’voltageratingof8242Vwas30%inexcessoftheexpectedvoltageacrossthecapacitorsconnectedintheirstarconfiguration.FromexaminationofthespecificationitisconsideredthatVishayhadexceededtherequirementfromConverteamtoconsidertheharmonicvoltagesuptothethirtiethharmonicinadditiontothefundamentalfrequency.Therefore,itishighlyunlikelythatthecapacitorwasunder-ratedbydesign,evenwhenconsideringtheincreasedvoltagesduetoinductiveeffects.Whereswitchingtransientovervoltageisconcerned,LRrules,whichallowa20%increaseforaperiodof6to7secondsdonotmatchtheequivalentIECrequirementwhichallowsforamuchlargertransientvoltagelastinghalfacycleor1/120ofasecond(considering60Hzfundamentalfrequency).AstherequirementsofIEC6087pertainspecificallytohighvoltagecapacitorsandtheLRrulesaremoregeneral,itwouldbelogicalinthelongertermforLRtoreconsidertheirrulesontransientovervoltageswhentheyspecificallyapplytocapacitors.
ThemaximumtemperaturerecordedonthesurfaceofoneoftheforwardHFcapacitorcanswas49ºC,duringwinterconditions;therefore,highertemperatureswerelikelytooccurduringtropicalweather.However,thecirculatingairtemperatureintheforwardfilterasindicatedinFigure23 waswellwithinthepermittedvalue.Assumingthatthecoolingsystemhadsufficientcapacitytomaintaintheambienttemperaturebelow45ºCevenundertropicalconditions,itisunlikelythatthedegradationwascausedbyhighambienttemperatures.Theeffectofthealarm
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thresholdsbeingsettoohighat47ºCwasnotthoughttoaddsignificantlytotheriskofahighambienttemperature.Thelong-termeffectsofthehighersurfacetemperaturesonthecapacitorcanswaslessclearandwouldmeritfurthermonitoringandconsideration.
Switchingfrequency
TheIECstandard’srequirementforcapacitorstowithstand1000switchingcyclesinayear(anaverageof2.7aday),isbasedonanexampleofaland-basedinstallationwheretheoperationpatternexhibitsminimalvariationonadailybasis.OnavessellikeQM2,thathasawidelyvaryingoperatingpattern,itcouldbenecessarytovarypropulsionspeedquitefrequently,requiringthestandbyharmonicfiltertobeswitchedinandoutseveraltimesduringtheday.Eventhoughitwasoutofserviceforsometime,the10,000hoursdifferenceinrunninghoursbetweenthefiltersindicatesthattheaftHFwaspredominantlyusedasthesecondfilterandthereforesubjecttomoreswitchingoperationsthantheforwardone.ItisthereforeconsideredthatbothHFscouldhaveexperiencedmoreswitchingcyclesthanexpectedinaland-basedinstallation,withtheaftHFbeingmoreatrisk.
Whilethestandardsandspecificationsforland-basedHVequipmentandinstallationsprovidevaluableguidance,itmustbeacceptedthattheconditionsandoperatingpatternsinmarineinstallationscandiffersubstantially.Itisthereforeessentialthattheeffectsofthesedifferencesareunderstoodandthatdesignmarginsareincreasedaccordingly.
Summary
ThecapacitormanufacturingprocesssatisfiedthecriteriaoftheIECstandardandmetConverteam’sspecification.Howeveritislikelythatthecapacitorsstartedtodeterioratefarsoonerthantheirexpectedlifetimeof20yearsduetotheoperatingconditionsbeingharsherthanexpected.Theinitialdegradationofthecapacitorwaslikelytohavebeencausedbyoneorbothofthefollowing:
• beingsubjectedtovoltagesinexcessoftheirdesignrating
• beingexposedtofrequentvoltagetransientsduetoincreasednumberofswitchingcycles
Failureinitiationduetominormanufacturinganomaliessuchastheexistenceofstressraisersonthecapacitorelements,thoughlesslikely,cannotbecompletelyruledout.
2.4.2 Progression to catastrophic failure
Onceinitiated,thedamageonthesolidcomponentsinthedielectricmediumwouldhavebecomeaweakspotandthemostlikelylocationtosufferfurtherarcing.Althoughthedielectricoilhadself-healingpropertiesandcouldabsorbsomeofthedamagedonebypartialdischargeorarcing,thedeteriorationofthesolidinsulationwouldhavebeenpermanent.Theprocesswouldhavecontinueduntiloneoftheelementsinagroupofeightsufferedashortcircuit,therebyshortingthegroupandincreasingthevoltageacrosstheremaininggroupsby33%.Acompleteshortcircuitofonegroupwouldhavedevelopedanimbalancecurrentof1300mA,farinexcessofthetripcurrentofthecurrentimbalancedetector.Iftheimbalancedetectorhad
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beenfunctional,theaftHFbreakershouldhavetrippedandtakentheHFoff-line;thecapacitordamagewouldhavebeendetectedlongbeforeitreachedthecriticalpointwhereanexplosionwouldoccur.
Asthedeteriorationprogressed,thevoltageacrossthelastgroupofremainingcapacitorelementswouldhaveincreasedtoamaximumof400%oftheratedvoltageleadingtoheavyarcingandrapidvaporisationofthedielectricoilandthepolypropylenefilm.Therewasnowaytoreleasethevapour,andtheinternalpressurewouldhaveincreaseduntilthecapacitorcasingrupturedattheweldedjointatthebase.Thepressurisedoilandvapourwouldhaveescapeddownwardswithsubstantialvelocity,resultingintheentirecanliftingup,breakingfreefromitsfoundation,disconnectingitselffromitsbusbaranddamagingneighbouringcapacitorsinthephasegroup.
TheSOLASrequirementformachinerysystemsthatarenormallyunderpressuretohavepressurerelievingarrangementscouldbeinterpretedtoincludeonlythosesystemswhichcontainafluidunderpressure.However,somesystemscouldbepressurisedunderabnormaloperatingconditions,asillustratedbythisaccident.Ananalogyisthewaterjacketofahighpressureaircoolerinanaircompressor,whichisrequiredtobeprovidedwithaburstingdisk,incasepressurisedairleaksintothewaterside.ItisimperativethatequivalentmeansareprovidedinHVcomponents,eithertorelievetheexcesspressure;warntheoperatorwithalarms;orshutdownthesystemtopreventfurtherpressurisation.Theinherenthazardofsuddenanduncontrolledreleaseofpressureenergyfromelectricaldevicessuchascapacitorsneedstoberecognisedandaddressedasamatterofpriority.
2.4.3 Design changes in new capacitors
Someofthechangesmadetothedesignofthecapacitorsthatwerefittedaftertheaccident-especiallythechangeintheconfigurationofelementgroupingsresultinginthedecreaseofvoltageacrossindividualelementsby20%,andthethickerpolypropylenefilmwithabettergradientofelectricalstrength-appeartobeanaturalreactionofanymanufacturertomaketheirproductmorerobustwhiletherootcauseoffailureremaineduncertain.Thefittingofinternalfusesandpressuresensorsrecognisesthenonfailsafenatureofthecurrentimbalancetransformerandaugmentstheprotectionprovidedbytheimbalancedetectionsystem.
Ifthecapacitorelementswereindividuallyfused,thedegradedelementswouldhavebeenisolatedandthewholeunitwouldhavecontinuedtofunctionforalongerperiod.Nevertheless,similarfailuresonotherunitscouldeventuallyleadtoprogressivelyincreasingvoltagesacrossthehealthyunits,eventuallyleadingtoacatastrophicfailurewithsimilarconsequences.Thedecisiontofitpressuresensorsonthetransformeroiltankrecognisedtheriskofarcingandinternalover-pressurisationbytheoilvapour.Itwasunfortunatethatthesamereasoningdidnotextendtotheprotectionofcapacitors,eventhoughthecapacitorinformationbrochurebyVishay,aswellasacademicpapersonthesubject,clearlyidentifiedtheserisks.ThisaccidentdemonstratestheimportanceofdesigninginsafetyfeaturesinHVelectricaldevices.
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2.4.4 Maintenance and replacement history
ThemaintenancehistoryfortheHFswassparseandcontainedinsufficientdetailsofthefailuremode,serialnumbersoffailedunits,ortherankstheywerefittedin.However,itwaspossibletodeterminethatatleastsixcapacitorshadbeenreplacedduetointernalfailureofcapacitorelements.Despitesucharelativelylargenumberoffailures,therewerenoattemptstoanalysethecause.Thecrewacceptedtherateoffailureasoneoftheunavoidablefeaturesofthenewtechnology,anddidnotquestionitfurther.Thefrequentmechanicaldamagesufferedbythecapacitorsduringtransitpossiblymaskedthefactthat,onaverage,onecapacitorwasbeingreplacedeachyearduetointernaldegradation.
Technicalmanagersdidnotrecognisethistrendasthecapacitorswerejustafewcomponentsamongmanythousands,andsowereunabletotakeanypreventativeaction.Theywerenotaidedbytheirmaintenancesystem,whichdidnotanalysetherateoffailureoralertthemtotheunusuallyhighconsumptionofreplacementparts.Incontrasttothetransformers,therewasnofacilitytotesttheconditionofthecapacitor’sdielectricfluidand,despitethisbeingaconditionoftheCSMsurvey,theimpossibilityofthistaskwasnotrecognisedbyeitherthecreworLR.AsHFshadnotbeenincludedinthelistofcriticalequipmentonQM2,anotheropportunitytorecogniseandactuponthefrequentfailureswaslost.NeitherConverteam,theoriginalequipmentsupplier,norVishay,thecapacitormanufacturer,identifiedthehighconsumptionofcapacitorsonboardQM2.
Componentfailurecanoftenbeasymptomofanunderlyingproblemwiththeequipmentorsystem.Shipmanagersandcrewshouldbemorealerttothis,particularlywithnewtechnology.Maintenancemanagementandassociatedparts’requisitionsystemsshouldbeusedtorecordcomponentfailuresinsufficientdetailtoallowmoremeaningfulanalysisandgiveanearlywarningtopreventmoreseriousproblems.
2.4.5 Current imbalance detection system
Thecapacitorexplodedbecausetheinoperativeimbalancedetectionsystemcouldnotidentifythedeteriorationofthecapacitors.Thefailureoftheimbalancesystem’scurrenttransformerremainedundetectedforwhatcouldhavebeenseveralyears.Itiscertainthatthissystemwasnotworkingin2008whenConverteamdetectedtwodefectivecapacitorsduringaroutinecheckindrydockbecausenoimbalancealarmswererecordedbytheIASsystematthecorrespondingtime.Furtherexaminationofthemaintenancehistoryestablishedthat,ofthefivecapacitorsreplacedinrank11.3,fourtookplaceindrydockasaresultofroutinechecks,ratherthanbecauseofanimbalancecurrentalarm.Althoughthewindingsofthecurrentimbalancetransformerweretestedduringmanufacture,subsequenttestsofthesystemwerebysecondarycurrentinjection,bypassingthewindings.AstheIASdidnotspecificallyregisterwhichrankoftheHFhaddevelopedanimbalancecurrent,itwasnotpossibletoconfirmifthecurrentimbalancetransformersonrank11.3hadeverworkedinservice.
ItisconcerningthatnoneofConverteam,theship’sengineersandCarnival’stechnicalmanagementteamquestionedwhythecurrentimbalancedetectionsystemhadnotregisteredanalarmwhentwocapacitorswerefounddegradedduringroutinechecksinOctober2008.AlthoughConverteam’sinspectionreportindicatedthattheprotectionsystemshadnotbeentested,noquestionswereraised.
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Eveniftheprotectionsystemshadbeentested,thiswouldprobablyhavebeenbysecondarycurrentinjection,asdoneduringthefactoryacceptancetest.Thismethodwouldhavebypassedthecurrenttransformerandanopencircuitfaultinatransformerwindingwouldhavegoneunnoticed.EventhoughanannualtestoftheprotectionsystemwasrequiredbyConverteam,itisdisconcertingthattherewasnorecordofthishavingbeendone.
AlthoughtherewasnorequirementonVishayforthecurrentimbalancetransformertoconformtoIEC60044-1asithadnotbeenspecificallyrequestedbyConverteam,itisneverthelessconsideredtheappropriaterecognisedstandardformeasurementandprotectiontransformers.Itisunfortunatethat,whenauditedagainstIEC60044-1aftertheaccident,15of23requirementswerenotmet.Theseshortcomingswerelikelytohavecontributedtotheprematurefailureofthetransformer.Inaddition,therewasnoverificationoftheHFprotectionsystemduringthefactoryacceptancetests.Whiletheproductionofcapacitorswasstrictlycontrolledandcarriedoutinaccordancewiththeapplicablestandards,thetransformer,attheheartofthemonitoringandprotectionsystem,wasconstructedandputintoservicewithoutthesamediligence.LR,whoserepresentativeswerepresentatthefactoryacceptancetest,acceptedasuperficialtestoftheprotectionsystem,whichdidnotincludeacheckofthetransformer.
AlthoughthecurrentimbalancedetectionwastheonlyprotectionsystemfortheHFs,ithadnoback-upanddidnotfailsafe.Theonlypossibleindicationofafailedprotectionsystemwouldhavebeena000mAreadingontheimbalancecurrentdisplay.However,sinceneithertheinstructionmanualnormaintenancesystemmentionedthis,andthata‘good’readingwouldhavebeenafew10sofmA,itwashighlyimprobablethatthecrewwouldhaveappreciatedthesignificanceofthissubtledistinction.Protectionsystemsforcriticalequipment,especiallywhentherearenoalternativeorback-upsystems,mustbefailsafe.Theyshouldalsobetestedatregularintervalstoverifythatallthesub-componentsinthesystemarefunctional.Greater,andmorecareful,considerationoftheprotectionsystemsprovidedformitigatingthefailureofHVequipment,isrequired.
2.5 HIGH VOLTAGE ENCLOSURES
2.5.1 Protection
Therewasevidencetosupportthatanarc-flashhadtakenplaceduringthisaccidentonboardQM2.Consideringtherecurrentproblemswiththevessel’sdamagedandleakingcapacitors,itisalsopossiblethatoneormoresucheventsmightalsohavetakenplaceinthepast.
Inthedesignofelectricalequipmentforthemarineindustry,insufficientconsiderationisgiventoitsabilitytowithstandanarc-flashevent.Therequirementofclassificationsocietiesforprotectionagainstarc-flashislimitedtohighvoltagemainswitchboardsonthepremisethattheriskofanarc-flasheventinjuringacrewmemberismoreprevalentinaswitchboard,wheremanualoperationofcircuitbreakersmaysometimesbenecessary.However,aswasthepracticeonQM2,theHFroomwasenteredroutinelybythecrewtoinspecttheHFcomponents.TheyalsotendedtoentertheroomwhentheHFalarmhadactivated,andtheHFequipmentwasthereforeinitsmostdangerouscondition.Thevesseloperatedwithadefectiveprotectionsystemandincorrectlywiredtripcircuits;itisthereforevery
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fortunatethatanexplosiondidnottakeplacewhilesomeonewasinsidetheHFroom.Thisaccidentdemonstratesthatitisnolongertenabletorestrictarcresistantmeasurestomainswitchboardsalone.
2.5.2 Awareness of arc-flash
AlthoughtheCOSWPcovershighvoltageworkpermitandsanction-to-testprocedures,thereisnomentionofthehazardsofarc-flash.Awarenessofarc-flashhazardsneedstoimprovethroughoutthemarineindustrysothatdesigners,builders,owners,operatorsandengineersunderstandtheproblemsandthepotentialriskreductionmeasuresavailabletoreducethehazardtoaslowasisreasonablypracticable.Itisimportantthatthehazardsofarc-flashandthepersonalprotectiveequipmentwhichcouldhelppreventinjurywhenworkingnearliveelectricalequipmentbeincorporatedintotheCOSWPattheearliestopportunity.
2.6 ALARM MANAGEMENT
Duringthewatchbeforetheaccident,thedutyengineeracceptedapproximatelyonealarmeveryminute.Itishighlylikelythatthenumberofalarmsduringthebusyhoursofthedaywouldhavebeenevenhigher.Thepurposeofanalarmistoalertthewatchkeepertoananomalysothatappropriatecorrectiveactionsmaybetaken.However,ifthealarmsappearasfrequentlyasoneeveryminute,itwouldbealmostimpossibleforthewatchkeepertodealwiththemeffectively.Halfanhourbeforetheaccident,thedutyengineerhadacceptedtwofirealarmswithouttakinganyfurtheractionandwithoutactuallyknowingatthetimethatthesewerefalsealarms.
AlthoughduringthisaccidenttherewerenoalarmsontheIAStowarnthewatchkeeperoftheimpendingexplosionandblackout,aseriesof‘halfdrivelampalarms’begantoappearontheP1200systemstarting36minutesbeforetheaccident.ThefrequencyofalarmsontheIASataroundoneeveryminute,inadditiontoalarmsfromtheP1200systemismostlikelytohaveoverwhelmedthewatchkeeper,anditisnotsurprisingthatthepropulsionmotoralarmswerenotactedupon.Therefore,itisimperativethatshipmanagers,inconsultationwiththeclasssocietyconcerned,carefullyreviewmachineryalarmstomakesurethatcrewarewarnedaboutmajorequipmentfailuresandthatalarmsareprioritisedtofocusontheareasmostcriticaltomaintainingthesafetyoftheship.
2.7 WATER-MIST IN HIGH VOLTAGE COMPARTMENTS
ItwasapparentthatthedesignappraisalprocessfortheHi-FogsystemdidnotincludetheMSBandHFrooms;thesecompartmentswerenotincludedintheappraisaldocument.Whereasseveralcriticallocationssuchaswheelhouse,communicationroomandECRwerefittedwithapre-activationsystem,andthemachineryspacerequiredmanualinterventiontoreleasewater,theMSBandHFenclosureswerefittedwiththemostbasic‘wet’pipesystemasusedintheaccommodation,whichreleasedwaterassoonasthesprinklerbulbruptured.Itisunlikelythatthistypeofsystem,withtheriskofapressurisedpipeleakinginserviceandsprayingwaterontothehighvoltageenclosuresandequipment,wouldhavebeenselectedifthesecompartmentshadbeenincludedintheappraisalprocess.
AlthoughtheHFenclosuressatisfiedLR’srequirementtomeetIP44standards,thishadbeendoneinordertoprovideaneffectivemeansofcontainingthecoolingairandmeettheshipyard’sspecificationratherthanasanovertmeansof
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protectionagainstwatermist.TheHFenclosurewasseverelydisruptedassoonastheexplosionoccurredandwasunabletomaintainprotectiontoIP44standardthereafter.
AlltheIMOreferencedocumentsagainstwhichtheappraisalwascarriedout,pertainedtomachineryspacesofcategoryA;itisunlikelythatthesuitabilityofusingFWBLAFFsystemsinhighvoltageareaswasthoroughlyconsideredordiscussed.
Therewasnoevidencetosuggestthatthewaterspray,whichwassustainedfor6minutes,orthesubsequentwater-mist,interferedwiththehighvoltageequipment.Theshiphadsufferedacompleteblackoutofallsystemswithinlessthan20secondsoftheexplosion,andtheelectricalnetworkwasdeadwiththepossibleexceptionofstoredenergyincomponentsduetocapacitiveeffect.Itwasfortunatethatthetwoengineerswhoenteredthecompartmentimmediatelyaftertheblackout,whileconsiderablewaterwassloshingaboutonthedeck,didnotcomeintocontactwithanychargedcomponents.Theconsequenceswouldalmostcertainlyhavebeenfatal.
AlthoughtestsconductedbyMarioffafterQM2wasdelivered,demonstratedthattheHi-Fogsystemwassafeforuseinahighvoltageenvironment,thereisnorecordthatthesetestresultshavebeenendorsedbytheIMOorIACS.ThereislittleornoguidanceonthissubjectintheFSSCodeorclassrules.LR’scurrentrulesareconfusing,explicitlyprohibitingtheinstallationofhighvoltageequipmentorenclosuresinareasprotectedbyFWBLAFFsystemsinonepartyetallowingFWBLAFFsystemsinareascontainingelectricalequipmentwithIP44ratedenclosures.Itisthereforenecessaryfortheclassrulestobereviewed,withtheaimofremovingsuchinconsistenciesandensuringthatinstallationsareproperlyconsidered.TheFSSCodemayalsoneedtobeamendedinduecoursetoincludeappropriateguidelinesfortheuseofFWBLAFFsystemsonhighvoltageequipmentandenclosures.
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SECTION 3 - CONCLUSIONS
3.1 SAFETY ISSUES DIRECTLY CONTRIBUTING TO THE ACCIDENT WHICH HAVE RESULTED IN RECOMMENDATIONS
1. Itislikelythattheinitialdegradationofthecapacitorwasduetooneorbothoftwoanomalies:beingsubjectedtovoltagesinexcessoftheirdesignrating,orbeingexposedtofrequentvoltagetransientsduetoincreasednumberofswitchingcycles.Instancesoffailureinitiationduetominormanufacturinganomalies,thoughlesslikely,cannotbecompletelyruledout.[2.4.1]
2. Certainengineeringsystemscouldbepressurisedunderabnormaloperatingconditions.Theinherenthazardofinternalpressurisationofharmonicfiltercapacitors,andthesuddenanduncontrolledreleaseofpressureenergyfromthemwasnotconsidered.[2.4.2]
3. Thecapacitorexplosioncouldhavebeenpreventedifthecurrentimbalance-basedprotectionsystemwasfunctional.Theprotectionsystemwasnotdesignedtofailsafely;itispossiblethatthefailureofitstransformerremainedundetectedforseveralyears.Thereisnoevidenceoftheprotectionsystembeingtestedduringtheservicelifeofthevessel.[2.4.5]
4. ThefirstpossibleindicationofthedevelopingaccidentwasavailableattheP1200propulsionmonitoringsystemaround36minutesbeforetheaccident.ThesheervolumeofalarmsfromtheIASataroundoneeveryminute,inadditiontoalarmsfromtheP1200system,ismostlikelytohaveoverwhelmedthewatchkeeperanditisnotsurprisingthatthepropulsionmotoralarmswerenotactedupon.[2.6]
3.2 OTHER SAFETY ISSUES IDENTIFIED DURING THE INVESTIGATION ALSO LEADING TO RECOMMENDATIONS
1. Thisaccidenthighlightsthedangerstothecrewwhenenteringtheharmonicfilterroom,especiallytoinvestigateanalarm.Iftheharmonicfilterenclosurewasbuilttothesamestandardsashighvoltageswitchgear,thedamagecausedbytheexplosioncouldhavebeenmitigatedmoreeffectively.Thecaseforuniformapplicationofhighvoltageswitchgearprotectionstandardstoallotherhighvoltageequipment,wherecrewinterventionmayberequiredduringoperation,isthereforecompelling.[2.2.1]
2. Losingcontrolofalargecruiselinerduetoanelectricalblackout,with3823peopleonboard,isaseriousconcern.Thisaccidentdemonstrateshowharmonicdistortioncanleadtoelectricalinstabilityandcauseunpredictableandpotentiallydisastrousconsequencesinmarinehighvoltageelectricalnetworks[2.2.2]
3. AsvariablespeedACmotorsarebecomingmorecommonforpropulsionaswellasforauxiliarymachineryprimemovers,ships’crewwillbeexposedtovarioustypesofharmonicmitigationequipment.Itisimportantthatships’crewsgainathoroughunderstandingoftheissueofharmonicdistortionandharmonicmitigationequipment,sothattheyarebetterabletoappreciatetheimportanceoftheequipmentonboardandtaketimelyactionifsuchequipmentfailsordeteriorates.[2.3.1]
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4. InDecember2010,whenQM2’sforwardHFalsofailed,thecrewhadaccesstomodellingdataontheexpectedleveloftotalharmonicdistortionofvoltagewhenallharmonicfiltersfail,andaninstrumentwithwhichtomeasuretheharmonicdistortionunderthesecircumstances.Theywerethereforeabletomanagethesituationwellwithoutcompromisingthesafetyofthevesselorherpassengersandcrew.[2.3.2]
5. Inavesselwhereelectricalinstabilityhasthepotentialtodisruptitselectricalnetwork,possiblyleadingtoablackoutandlossofcontrolinrestrictedwaters,theneedforpowerqualitysurveillanceissignificant.Regularmonitoringofpowerquality,usingapre-determinedpatternofpropulsionmotorloadingwithassociatedmotoroperatingparameters,wouldhelpensurethatthehealthofelectricalequipmentonboardiscloselymonitoredasthevesselanditsequipmentagesandusagepatternschange.[2.3.3]
6. AlthoughQM2’scurrentimbalancedetectionwastheonlyprotectionsystemfortheharmonicfilters,ithadnoback-upandwasnotfailsafe.Protectionsystemsofcriticalequipment,especiallywhentherearenoalternativeorback-upsystems,mustfailsafe.Theyshouldalsobetestedatregularintervalstoverifythatallthesub-componentsinthesystemarefunctional.[2.4.5]
7. Awarenessofarc-flashhazardsneedstobesignificantlyincreasedthroughoutthemarineindustry,sothatdesigners,builders,owners,operatorsandengineersunderstandthehazard,risksandthepotentialriskreductionmeasuresavailabletoreducethehazardtoaslowasisreasonablypracticable.AlthoughtheCOSWPcovershighvoltageworkpermitandsanction-to-testprocedures,itdoesnotmentionthehazardsofarc-flash.[2.5.2]
8. Thereislittleornoguidanceregardingtheuseoffixedwater-basedlocalapplicationfire-fightingsystemsincompartmentswithhighvoltageequipment.LR’srulesonthesubjectprovideconflictingguidance.[2.7]
9. ThefailuretoidentifythetrendofhighconsumptionofcapacitorsonboardQM2wasamajorcontributoryfactorinthisaccident.Componentfailurecanoftenbeasymptomofanunderlyingproblemwithanequipmentorsystem.Shipmanagersandcrewshouldbemorealerttothis,particularlywithnewtechnology.[2.4.4]
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SECTION 4 - ACTION TAKEN
TheMarine Accident Investigation Branchhas:
• InDecember2010publishedasafetybulletin,informingtheindustryoftheaccidentandprovidingguidanceoncheckstodetectimpendingcapacitorfailures(Annex J).
• Issuedaflyertotheshippingindustrywiththisreport,whichdetailsthelessonslearntfromtheaccident(Annex K).
Carnival PLChas:
• IncludedharmonicfiltersinitslistofcriticalequipmentforQM2,andhasensuredthatproceduresexistforregulartestingoftheirprotectionsystems.
• Developedandimplementedproceduresfordealingwithharmonicfilterimbalancealarmsandenteringharmonicfilterroomsafteranalarmhasactivated.
• ReplacedallthecapacitorsinQM2’saftharmonicfilter.
Converteamhas:
• Issuedaservicebulletinalertingitscustomerstothepotentialforcatastrophicfailureofitscapacitorsandthecircumstancesthatcouldleadtothis.
• ReplacedthecapacitorsinQM2’saftharmonicfilterwithanimprovedandsaferdesign.
• Carriedoutaqualityauditofthecapacitormanufacturer.
Lloyd’s Register (Europe, Middle East and Asia)has:
• Progressedwithexistingworktoreviewitsrulestorecogniseandmitigatethehazardofarc-flashassociatedwithharmonicfiltersfittedinhighvoltageelectricnetworks.
• ChangedtheContinuousSurveyofMachineryrequirementstoremovetheneedtotestsamplesofdielectricfluidfromcapacitorcans.
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SECTION 5 - RECOMMENDATIONS
Lloyd’s Register (Europe, Middle East and Asia)isrecommendedto makeasubmissiontoIACStodevelopaunifiedrequirementto:
2011/149 Improvethestandardsofprotectionthatarerequiredagainstharmonicdistortionandcomponentfailureinvesselsoperatinghighvoltagenetworks,toensure:
• thereisarequirementinallnew-buildvesselsthatmaybeaffectedbyharmonicdistortionofcurrentandvoltagethat:
Intheeventthatallharmonicmitigationsystemsfail,informationisprovidedonboardtodescribethemaximumextentofharmonicdistortionthatcanbeexpected.
Guidanceisprovidedsothatcrewcantakeeffectiveactiontokeeppowerandpropulsionequipmentoperating(atanappropriatepoweroutput)ifharmonicmitigationequipmentdegradesorfails.
• On-linemonitoringofharmonicdistortionofvoltageisrequiredfornewbuildvesselsand,forexistingvessels,thereisperiodicmonitoringtodetectchangeordegradationofharmonicdistortionlevels.
• Specificrequirementsaredevelopedtodetectandmitigateagainstthefailureofhigh-energystoragedevicessuchascapacitors.
2011/150 Reviewtherequirementsfortheenclosureofhighvoltagesystemstoensurethatthedegreeofprotectionisconsistentforallequipmentwherecrewinterventioncouldberequiredandthehazardfromarc-flashexists.
2011/151 Introduceaspecificrequirementensuringthatwherethefailureofanequipmentormachinerymayleadtoseriousdamagetothevessel,orinjurytopersonnel,itsprotectionsystemmustbeofa‘failsafe’type.
Lloyd’s Register (Europe, Middle East and Asia)isalsorecommendedto:
2011/152 Reviewandclarifyitsrulesontheinstallationoffixedwater-basedlocalapplicationfire-fightingsystemsincompartmentscontaininghighvoltagesystemsand,throughIACS,proposetheappropriateamendmentstoincorporatethisguidanceintheFSSCode.
The Maritime and Coastguard Agencyisrecommendedto:
2011/153 Usingthisreportandtheaccompanyingsafetyflyerasabasis,publishamarineguidancenoticetoraiseawarenessofthepotentialhazardsofexcessiveharmonicdistortionofcurrentandvoltage.
2011/154 ReviewandupdatetheCodeofSafeWorkingPracticesforMerchantSeamen(COSWP)toprovidemoredetailedinformationonthehazardsassociatedwithhighvoltageequipment,includingarc-flash.
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Carnival UKisrecommendedto:
2011/155 Improvethestandardsofprotectionagainsttheeffectsofharmonicdistortionandcomponentfailureby:
• Instigatingaprogrammeofmodellingorotherappropriatemeanstodevelopsafevesseloperatingparametersandprocedurestobeusedintheeventofharmonicfilterfailure.
• EnsuringthatRMS Queen Mary 2’smaintenancesystemidentifiesallcriticalhighvoltagesystemprotectiondevices,andcontainsproceduresforperiodiccheckstoconfirmthattheyfunctioncorrectly.
• Implementingamethodofidentifyingandanalysingunexpectedlyhighratesofcomponentfailuresinharmonicfilterequipmentandotherhighvoltagesystems.
2011/156 ReviewthemachineryalarmsystemsfittedtoRMS Queen Mary 2inordertoidentifythosealarmswhichindicatefailureconditionsthatcouldsignificantlyaffectthesafetyofthevessel.Indoingso,actionshouldbetakentoprioritisesuchalarmsaboveothersthatrelatetothemoregeneraloperationoftheship,sothatoperatorscanmorereadilyrecognisecomplexsystemfailuresandrespondappropriately.
Marine Accident Investigation BranchDecember 2011
Safetyrecommendationsshallinnocasecreateapresumptionofblameorliability
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