MARK M KIIGROUNDF FAULT CTIF TRE FICAT TION

Int troduct tionFauji Fertilizer Com mpany Limited has two Ur plants nea Sadiqabad, d rea ar ctRahimYarK KhanPakistan. Electricalpow wersourcetot thisAmmonia Distric Urea complex,aret twoGasTurbin nesTG701& TG702,servin ngrelentlessly as a b backbone of th plants for t past 30 ye he the ears. MARKII i installed on is TG70 TG701 tur 01. rbine has had Speedtronic and 125VDC power supply groun faults since 2003. These ground fault caused man mysterious nd e e ts ny outag gesontheturb bine.Historyda ataanalysisrev vealsatleasto onetrippingor emerg gency shutdow each year that can be attributed to these ground wn faults. Attempts for the detection and rectifica r n ation of these ground faults eral turnaroun with no s nds success. This year in plant were made in seve 09,thislongaw waitedtargetw wasaccomplish hed. turnaroundOct200 papergivesab briefoverview ofthesequen nceofthejobs conductedto Thisp find b both the groun faults one by one. It gives an insight t the hidden nd to proble ems that engi ineers came a across while dealing with M MarkII control system of General Electric, insta m alled on the G Turbine. It also unveils Gas some ingenious ideas that help ped us to tac ckle with the grounding

lem. probl

BackgroundTG701 MARKII control system has three possible sources of ground faults namely Speedtronic Ground Fault, 125VDC Ground Fault and Thermocouple Ground Fault. The Speedtronic Ground Fault and 125VDC Ground Fault were hampering the turbines smooth operation and functioning for the past seven years. The latest tripping of TG701 occurred in September just one month before the turnaround 2009 as a direct effect of 125VDCGroundFault. Ground faults are extremely dangerous to any typeofelectricalorelectronicssystems.Besides outagestheycancausesparkinganddamageto hardware. Groundfaultsareannunciatedwhenoneofthe two poles, positive or negative, is grounded. The fault should be addressed in first opportunity as grounding of second pole will resultinsparkingandtrippingofthemachine. Based on above, the overhaul of Speedtronic MARKII System and rectification of both groundfaultswereplannedintheTA2009.

Oncethegroundfaultwasremovedthevoltage shiftswerenormalized.

SNO1 2 3 4 5 6 7 8 9 10 11 12

SUPPLY TERMINALP28 P28,PCOM

REGULATOR VDC28.03

SGDD VDC28

P28,GND PCOM,GNDP12 P12,ACOM

23.68 4.311.99

23.6 4.5411.9

P12,GND ACOM,GNDN12 N12,ACOM

7.52 4.3912.04

7.5 4.4611.99

N12,GND ACOM,GNDP5 P5,DCOM

16.3 4.65.08

16.15 4.34.92

P5,GND DCOM,GND

0.53 4.56

0.62 4.44

Table:1SpeedtronicVoltageLevelswithGroundFault

Ground fault indication was consistently persisting on the Firstout Panel before turnaround.Itwasduetotheactuationofboth, Speedtronic SGDD card and the 64D relay for 125VDCgroundfaultdetectors. Ground fault rectification activity started with completeoverhaulingandcleaningofMARKII control system. Following modules were dismantledandcleaned: SpeedtronicPage:Speedtronicpageor1Lpage contains control and logic cards of the system, 126 in number. All cards were taken out checked,cleanedandinsertedback.

SystemMaintenance

Preliminary numeric and photographic sample dataofMARKIIcontrolsystemwascollectedat differentphasesofTG701shutdown.Thedata was later used for ground fault analysis and comparativestudy,oncethefaultwasremoved. This data helped us in determining the Speedtronic ground fault as permanent fault and the 125VDC as intermittent fault. Thermocouple ground fault was ruled out because the thermocouple ground fault detectorcardSGDEwasnotonalarm. The Speedtronic voltage levels under fault conditionareshowninthetablebelow.Aclear abnormalshiftof4.5VDCinallpowersupplies andCOMswithrespecttogroundishighlighted.

Fig1:SpeedtronicPageCards

Ontherearsideofthispagethousandsofwires areconnected.Alltheseconnectingwireswere checkedandcleaned.

Fig2:SpeedtronicPageRearView Fig4:BulkPowerSupplyModule

FinalRegulatorModule:Allpowersupplycards (+28VDC,+12VDC,12VDC,+5VDC,BusMonitor card) of Final Regulator were taken to instrument shop, checked and overhauled. The supplyconnectionsatthebackofregulatorrack were cleaned and lugs were changed where required.

Black Start Inverter: Black Start Inverter was dismantledfromthecontrolpanelandtakento electricalshopformaintenanceandcheckingof agingcomponents.

Fig3:BlackStartInverterInstallation

Fig5:FinalRegulatorSupplyCards

Power Supply Module: Bulk power supply module was overhauled in electrical shop. Its power switching cards EPSU and EPSV were found in bad condition covered with carbon dust. These cards were cleaned, checked and reinstalled. One capacitor was replaced as it wasshowingzeromicrofarads.

Source Selector Module: Source selector was removed from the cabinet, overhauled in the shopandreinstalled.

All cards and relays on this panel were also checkedcleanedthoroughly.

Fig6:SourceSelector

Speedtronic Relay Page:All70relaysonrelay page were taken out, cleaned and checked for contact & coil resistance. Relays with higher contact resistance were replaced. Cable connectionsatthebackofRelaypagewerealso cleaned. Mechanical actuation of relays was alsochecked.

Fig8:RemoteAnnunciatorModule

Remote Annunciator Panel: MarkII UCR panel was also cleaned and checked for any faulty instruments. Fused bulbs and lamp holders werereplaced. All above modules were extremely dirty and had carbon dust deposition. They were removed from their location for the first time andwereattendedinworkshop. Overhauling of 10 local annunciator cards and 17 receptacle cards was done at site. Terminal strips HCTB, LCTB, and RTB were also cleaned. Twelve Input Isolator cards were overhauled. Onecardwasalsoreplacedafterwards. All fused bulb and lamp holder of local and remotepanelswerealsochanged. Once the maintenance was completed system wasreinstalled,integratedandpoweredup.

MARKIICalibrationFig7:RelayPageandRemoteAnnunciator

RemoteAnnunciatorModule:TheModulewas taken to the instrument shop for overhauling.

Calibration of different controls and protection loopswasperformed.Checking&calibrationof power supply, Speed control loop, acceleration control loop, temperature control loop, PCD bias adjustment, overtemperature, speed protectionandstartupcontrolwasperformed.

therewasnogroundingonanyofthe125cards pulledoutof1Lpage. Inthenextstepallpowersupplycables28VDC, +12VDC, 12VDC, 5VDC, PCOM, ACOM and DCOM were disconnected on 1L page. These are130powersupplycablesdisconnectedfrom theverticalandhorizontalbusesof1Lpage. Disconnectionsequenceofpowersupplycables was from horizontal bus one by one from bottomtotop. Fig9:SpeedtronicCalibrationSetup

SpeedtronicGroundFaultSpeedtronic ground fault was persisting on SGDDgrounddetectorcardandonthefirstout panel. It was necessary to isolate wiring and hardware step by step while observing the statusofgroundfaultindication. All 126 cards on Speedtronic 1L page were pulled out one by one. Each card has 51 pins. Ground fault indication kept persisting till the lastcardwaspulledout.

There was a problem in disconnecting power supply of the first row of 1L page. This action would have disconnected the SGDD card from power supply whereas its ground fault indication was important to us in troubleshooting. A technique was adopted to overcome this problem. SGDD card was powered up outside the 1L page. External biasing was provided to the card through the wires directly from Main BusBar.Thisstephelpedusalotincatchingthe ground fault. 1L page was then completely disconnected from Main Bus Bar. Yet ground faultIndicationpersisted,rulingouttheground faultin1Lpage.

Fig10:1LPageCards Fig11:SGDDCardWired&PoweredUpOutsideits Slot

IntheendonlySGDDgrounddetectorcardwas leftonthe1Lpageandgroundfaultindication was there on the card. This confirmed that

Once the card was functioning all the power distributioncablesofregulatedsupplies28VDC, 12VDC, 12VDC, 5VDC, PCOM, ACOM, DCOM

were disconnected one by one from the main busbar.

traced and it led us to the fuel gas pressure transmitter96FG. To conclude the grounded part, 96FG was disconnected from the area JB while 12VDC still connected to main bus bar. SGDD card indication remained reset. The situation was verifiedseveraltimes.

Fig12:MainBusBarsof28VDC,+12,12,+5&COM

In the above step REMAN module, Local annunciator module, thermocouple processing module and synchronizer module were disconnected.SGDDgroundfaultindicationstill persisted.Atotalof235cableswereremoved while isolating all above modules from Main BusandtheirrespectiveTerminalBlocks.

Fig14:SGDDCardIndicationReset

Voltage level of complete system was rechecked for analysis. Voltage shift in all regulated supplies and COMs normalized as highlighted in the table. No effective voltage betweensupply,COMandgroundwasfound.It wasconcludedthatSpeedtronicgroundfaultis removed from MARKII system. Complete system was reassembled. All cables/modules wereconnectedonebyonetodetectanyother possible cause of Speedtronic ground fault. Finally1Lpagewasreconnectedtothebusbar. SNO1 2 3 4 5 6 7 8 9

SUPPLY TERMINALP28 P28,PCOM P28,GND

REGULATOR VDC28.03 28.08

SGDD VDC28 28.1

PCOM,GNDP12 P12,ACOM P12,GND

.7211.99 11.86

.7211.9 11.83

ACOM,GNDN12 N12,ACOM N12,GND

.7312.04 12.3

.7311.99 12.15

ACOM,GNDP5 P5,DCOM P5,GND

.765.08 4.9

.735.02 4.89

Fig13:ThermocoupleModule&Annunciator

10 11 12

DCOM,GND

.75

.71

After disconnecting about 50 more cables we found the suspected cable on 12VDC distribution circuit. The cable was physically

Table2:SpeedtronicVoltageLevelswithoutGround Fault

No ground fault appeared in reassembling the system.SGDDcardwasreinstalled.

125VDCGroundFault125VDC Ground fault was categorized as an intermittent ground. It appeared occasionally on 64D ground detector relay and would reset immediately. The data pertaining to 125VDC powersupplywascollectedforanalysis. Therewasanabnormalshiftin125DCvoltage levels indicating a Ground Fault. This type of Ground Fault is hard to find as it is subject to circumstances. SNO FUSEFU12 FU7 FU6 FU5 FU4

TERMINALFU12+,GND FU12,GND FU7+,GND FU7,GND FU6+,GND FU6,GND FU5+,GND FU5,GND FU4+,GND FU4,GND

ASFOUND

Fig14:FaultyFuelGasTransmitter96FG

1 2 3 4 5 6 7 8 9 10

New transmitter 96FG was installed in the area. During a thorough examination of faulty 96FG in the workshop the black signal cable of 96FG was found showing only 1.7 resistance with ground. The Ground Fault persisted even when the suspected cable was desoldered from the PCB board. Finally, it was concluded thatthesignalterminalandgroundterminalof PCBboardof96FGwereshort.

VDC 128 12 132 1.2 132.5 1.55 133 .09 132.6 .07

NORMAL VDC70TO60 70TO60 70TO60 70TO60 70 TO60 70TO60 70TO60 70TO60 70TO60 70TO60

Table3:125VDCVoltageLevelwithGroundFault

During one of its random appearances the Ground Detector Relay 64D remained actuated for some time. Immediately, all the fuses were pulled out one by one starting from FU7. The ground fault normalized as soon as FU4 negative supply fuse was pulled out. The grounding appeared again immediately after thereinstallationofthefuseFU4. FU4 fuse supplies power to all of the input isolator cards ISOL. All these cards and their respective circuits were checked. Some variation in the voltages with ground was observedonISOLSIIE0Gcard.

Fig15:New96FGInstalled

The 125VDC ground fault was finally found in the cables entering the MARKII cabinet from bottom. These cables were routed over a channelsupport.Onthetopofthesecablewas a horizontally bolted plate. These cables were pinched in between the support and the top metal cover plate. Any weight / load on the upper plate would press these cables against thesharpedgesofsupport. Fig16:125VDCFuses

The voltage level on SIIE0G card was found between 56 to 48 VDC. These inputs were related to Boiler damper and Bypass damper logic and were coming from AIU4 cabinet. Some pairs of this multicore cable were also terminatedonRTBandHCTB1. This multicore cable was disconnected from both ends the turbine and AIU4 side in UCR rackroom and cable was meggered and were found healthy. Furthermore, AIU4 logic was alsoanalyzedandcableswerephysicallytraced. MeggeringofareacablesofBoilerDamperand Fig17:Topplatetakenasidetoseethecablesoverthe Bypass Damper was also performed. Although verticallymountedsupport the variation was noticed in voltage levels but evenafterdisconnectingtheAIU4cabinetfrom Twooutofsevenpairsofthiscablewereinuse. Speedtronic system 125VDC ground fault One of the cores had a major cut while others appeared regularly. Hence it was obvious that hadminorcuts. AIU4didnotparticipateinthegroundfault. Ground fault checking procedure was also incorporatedinroutinechecking/calibrationof switches, limit switches, fire detectors, probes, FDs,SparkPlugetc.Specialformatsweremade foreachinstrument.Grounddetectorrelaywas still energized after all these checks were performed. In subsequent step SOVs were checked for the ground fault. 20FD liquid fuel SOV was found grounded and rectified immediately. Same procedure was repeated for liquid fuel purge SOV 20PL and fuel clutch SOV 20CF. Still 125VDC Ground Fault persisted in the system indicatingmultipleGroundFaults. Fig18:CutontheGroundedCable

The cables were then insulated, protected and properly routed to their respective locations. The cables which were not being used in the

systemwereseparatedandadequatelysecured forfutureuse. The Ground Fault indication on Firstout panel reset immediately as soon as 125VDC ground fault was rectified. Voltage levels normalized aftertheremovalof125VDCgroundfault. SNO1 2 3 4 5 6 7 8 9 10

Power supply switching protection system is available on bulk power supply hence it is at lesser risk than the Speedtronic. Still the 1L page, 1R Page, Local Annunciator, and Remote Annunciator,ThermocoupleProcessingmodule, Flame Detector Module, Spark Plug ignition Module, synchronizer and Relays could have directlyaffectedbygroundingoftheotherbus.

FUSEFU12 FU7 FU6 FU5 FU4

TERMINALFU12+,GND FU12,GND FU7+,GND FU7,GND FU6+,GND FU6,GND FU5+,GND FU5,GND FU4+,GND FU4,GND

ASLEFT VDC67.2 64.3 67.3 64.7 67.2 63..93 67.0 64.7 67.13 64.77

NORMAL VDC70TO60 70TO60 70TO60 70TO60 70TO60 70TO60 70TO60 70TO60 70TO60 70TO60

AcknowledgementWe whole heartedly thank the guidance and directions of our seniors who paved the way towardsasuccessfulcompletionoftheproject. Tanvir Vaqar is senior department managerElectrical & Instrument at Fauji Fertilizer Company Limited (FFC), Sadiqabad Pakistan. He holds BE degree in electrical engineering. Over the past 27 years he has been working in the area of electrical powergenerationanddistributionnetworks,control & protection systems design, modification and maintenance.

Table4:125VDCVoltageLevelwithoutGroundFault

RecommendationsOnce the Ground Fault is removed from the system, more responsibility comes on our shoulders.Followingareoursuggestions. 1. WhiledetectingaSpeedtronicgroundfault theSGDDcardcanberemovedfromitsslot andbiasedfromMainBus 2. While biasing SGDD card externally never insertanotherSGDDcardinslot`1L0Tasit wouldgiveambiguousresults.Useonlyone SGDDcardtodetectGroundFaults 3. 125VDC ground detector relay should be cleaned and checked before starting to searchforthegroundfault.

AsgharBhuttaisSectionHeadofInstrumentIatFFC.HeholdsBEdegreeinElectronics.Hisareaof expertiseisutilitiescomplexcontrolsystems.He

wasinvolvedintheerection&commissioning ofUreaplantexpansionunitin1993.

AuthorsAdeel Imtiaz is an Instrument Engineer at FFC, Pakistan. He holds M.phil and M.Sc degree in electronics. His current assignment is Waste Heatboilersandturbinecontrolsystems. Usman Ghani is a Trainee Instrument Engineer at FFC, Pakistan. He holds BE degree in Electronics. Currently he is working on DistributedControlSystem.

ConclusionGround fault has existed in the TG701 in differentformsconsistentlyfrom2003to2009. Negative12VDCsupplyof96FGwasgrounded. If the positive 12VDC supply cable was grounded for even a moment, it could have caused severe results. By design there are no protection fuses in Speedtronic as in 125VDC circuit.