SNS Modulator Fires; Causes, Mitigation, and Long-Term Plans

David E. Anderson

2 Managed by UT-Battellefor the U.S. Department of Energy

HVCM Simplified Block Diagram

IGBT SWITCH PLATE ASS'Y. (3X)

SAFETY ENCLOSURE

B PHASE SHOWN

CRes

CRes

CRes

A

B

C

5th Harmonic Trap 7th Harmonic Trap

13.8 kV : 2100 Vdelta wye

50 mH

OUTSIDE FILTERS AND TRANSFORMER

1.5 MVA

FUSEDDISCONNECT

BUILDINGINTERFACE

Fuse / Contactor

4 mH450 A

4 mH450 A

SCR CONTROLLER

+/-1300 V, 450 A

DC+

DC-

DC+

DC-

0.112 F

0.112 F

40X RG-8

40X RG-8

RdeQ

2X 0.03 uF500 pF

HV divider

CT

OIL-FILLED MODULATOR TANK

CONTROLRACK &

CONTROLLER

CAP RACK

3 Managed by UT-Battellefor the U.S. Department of Energy

HVCM Major Subsystems

4 Managed by UT-Battellefor the U.S. Department of Energy

Cavity/Klystron/Modulator Layout

• Multiple HVCM/Klystron Configurations

• Peak Power 11 MW, Average Power 1 MW design

115 kV125 kV

≤135 kV ≤75 kV 75 kV

5 Managed by UT-Battellefor the U.S. Department of Energy

HVCM “Smoke Generating” EventsHVCM Events by Year

9

41

20

2007

2008

2009

HVCM Events by Unit

0123456789

10111213

CC

L-1

CC

L-2

CC

L-3

CC

L-4

RFQ

-DTL

-1

DTL

-3

DTL

-5

SC

L-1

SC

L-5

SC

L-9

SC

L-12

SC

L-14

SC

L-15

SC

L-18

SC

L-21

RF-

Test

Unit

Num

ber o

f Eve

nts

70 Total Events1 during last production run

Most do not result in fires but response is consistent until incident evaluated

6 Managed by UT-Battellefor the U.S. Department of Energy

HVCM Fire Causes – Bus Arcing• 1st fire, none since, in RF Test Facility• Workmanship or residual dirt believed responsible• Repeated arcing acted as ignition source for combustibles• Corrected with improved training of assemblers, no faults w/ same root

cause since (Jan 07)

7 Managed by UT-Battellefor the U.S. Department of Energy

HVCM Fire Causes – Insulation Degradation• Cause of 2 fires and likely many of the IGBT failures• Original design relies on single layer of DMD to insulate cooling tubes from

different polarity bus• Interference fit between tube and bus compresses DMD and can cut material

if sharp edges present• Corona degrades insulation over time, resulting in arc event• Insulation double, short-term sol’n., cutout long-term

8 Managed by UT-Battellefor the U.S. Department of Energy

HVCM Fire Causes – Capacitors• Mfgr.’s cap lifetime ratings 100,000 hours @ 3000 V, Expanded the lifetime on the

spec. to 1 million hours

• Experience indicates 10-15 khours @ ≤ 2300 V

• Replaced all warm linac caps w/ higher lifetime spec caps

• Replacing all other caps with new caps, starting to see failures of RSO units

9 Managed by UT-Battellefor the U.S. Department of Energy

HVCM Capacitor ComparisonDielectric Fluid Dissipation Factor

0

0.001

0.002

0.003

0.004

0.005

0.006

10 100 1000 10000 100000

frequency, Hz

Diss

ipat

ion

Fact

or

DF IPB1DF IPB2DF RSODF SODF BTDP

Capacitor Type

Oper. Temp. Range

IPB 100-120°FBTDP small 140-150°FBTDP large 135-160°FRSO 100-120°FSolid Potted <100°F

• IPB no longer available, original batch of capacitors as delivered• Others tried

– Reconstituted mica, failed in tens of minutes– Another manufacturer’s RSO, failed in 18 hours

IPB Isopropyl BiphenylBTDP Benzyltoluene DiphenylethaneRSO Rapeseed Oil

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HVCM Fire Causes – IGBTs

• Usually less severe, lower collateral damage

• At transition to 60 Hz operation, incidents increased significantly

• Improved thermal bonding procedures implemented

• Overvoltage problem solved, minimal problems since

11 Managed by UT-Battellefor the U.S. Department of Energy

HVCM Fire Mitigation – CO2 Suppression• Dedicated CO2 system installed• Smoke detector installed• EPICS screens updated• Manual discharge from CCR if smoke

detector trips• Prevent or minimize system damage

12 Managed by UT-Battellefor the U.S. Department of Energy

HVCM Fire Mitigation – Additional

• Over Current detection on energy storage capacitor bank to limit fault energy

• Retraining workers / rewriting procedures for operators• Implement Alarm Handler for smoke detector events• Installing shrapnel shields around capacitors to minimize collateral

damage• Emergency Off modified to remove 2100 V primary energy source

automatically• Replace combustible materials inside Enclosure with UL-94 V0 rated

plastics (G9 Phenolic)• Replace cables with plenum-rated cables• Design new IGBT drive circuitry to shut IGBTs off if over current detected

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Future Capacitor Replacement

IC

VCE

VC

RSO Plastic Case Self-Clearing Metallized Polypropylene

• Ran capacitors until July shutdown• Assess self-clearing capacitor degradation• Simulate 10 years of faults on preferred capacitor• Choose and order capacitor selected• Install when delivered during 1st FY10 Extended Shutdown Period

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Future “Series Switch”

IC

VCE

VC

• Decouples primary energy storage when fault detected• Minimizes energy delivered to fault, prevents collateral damage• Can be used to add a future redundant H-bridge

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Capacitor / IGBT Preventative Maintenance

IC

VCE

VC

• Liquid dielectric– Monitor case temperature– Inspect periodically for leakage– Inspect for case dimensions out of tolerance– Return suspect units to manufacturer for analysis and design improvements

• Solid potted self-clearing– All of above– Periodically measure capacitance value and replace when 5% degradation occurs

• IGBTs– Monitor for changes in

• Turn on time• Turn on delay• Turn off time• Turn off delay• Gate characteristics• Monitor substrate temperature with thermal interlocks

All monitored via transformer flux monitoring system

Periodically monitored during shutdowns

16 Managed by UT-Battellefor the U.S. Department of Energy

ORPS Reporting• ORPS reporting threshold is the activation of an automatic fire suppression

system but NOT a manually-actuated or detection system– No automatic suppression employed

– Building central detection system (VESDA) rarely detects HVCM events

– ORPS considers burn times > 5 minutes significant

– No HVCM event has passed the reporting threshold

– All events have been contained inside the aluminum / stainless steel Safety Enclosure

• UT/Battelle and DOE ORO periodically review the HVCM smoke event history through self-assessment and oversight activities

17 Managed by UT-Battellefor the U.S. Department of Energy

Conclusion• Many “smoke generating” events to date, none severe

• Engineering and procedural controls and protection systems put in place

• Nature of component failures believed to be well understood, PM and component replacement underway to address

• Additional protection systems in design to further enhance system availability

• Hopeful that these event can be significantly reduced in the future

QUESTIONS?