ML20245B250
| ML20245B250 | |
| Person / Time | |
|---|---|
| Issue date: | 06/30/1989 |
| From: | Padovan L NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML20245B246 | List: |
| References | |
| TASK-AE, TASK-E905 AEOD-E905, NUDOCS 8906230099 | |
| Download: ML20245B250 (9) | |
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AE0D/E905 ENGINEERING EVALUATION REPORT ELECTRICAL BUS BAR FAILURES June 1989 Prepared by:
L. Mark Padovan i
Office for Analysis and Evaluation of Operational Data U.S. Nuclear Regulatory Connission 8906230099 890613 l
PDR ORG NEXD i
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SUMMARY
Failures of medium voltage electrical bus bars have been encountered at the Falo Verde, Kewaunee, Millstone, and Sequoyah nuclear facilities (Refs. I through 5). These failures, principally involving 4160 and 6900 volt AC buses, have resulted in bus bar electrical faults and fires, electrical power system undervoltage conditions, plant transients, and reactor trips.
Failure of the bus bars has been attributed te cracking of Noryl bus bar insulation (bus sleeving) combined with accumulation of moisture or debris in the bus bar housings. Cracked insulation, in the presence of moisture or debris, provide undesired phase-to-phase, or phase-to-ground electrical tracking paths, which have resulted in catastrophic failure of the buses.
Bus failures lead to loss of power to electrical loads connected to the buses, cause subsequent reactor trips, and initiate unnecessary challenges to plant safety systems.
DESCRIPTION OF OCCURRENCES As a result of the analysis of information obtained from the recent Palo Verde occurrence, the NRC's Office for Analysis and Evaluation of Operational Data (AE0D) initiated searches of available Licensee Event Reports (LERs) to deter-mine the generic applicability of the insulation failure experienced at Palo Verde. Several different data base search techniques were utilized to identify the specific electrical power system failures described in this report.
First, Nuclear Document System (NUDOCS) text searches were performed on LERs issued since 1984 to identify LERs containing the word "Noryl", which is the type of failed insulation encountered at Palo Verde. Secondly, Sequence Coding and Search System (SCSS) reviews were performed for an electrical bus manufacturer and the term Noryl. The SCSS search was then broadened to include all reports of electrical insulation problems. The following occurrences were found to involve cracking of Noryl bus bar insulation:
1.
Palo Verde Unit 1 On July 6, 1988, a phase B-to-ground fault occurred on 13.8 kV non-class 1E electrical bus E-NAN-S02 (Ref. 1). This fault ionized the air surround-ing the bus, and caused all three phases to short to ground.
Feeder breakers to non-class 1E buses E-NAN-502 and S03 did not immediately open, resulting in excessive currents being supplied by the unit auxiliary trans-j former and the subsequent rupture and ignition of the unit auxiliary l
transformer. This caused the supply breakers to buses E-NAN-501, S02, and the main generator output breaker to open on a unit auxiliary transformer
" sudden overpressure" signal. As the RCPs were powered from E-NAN-501 and S02, a reactor trip on low departure from nucleate boiling ratio ocurred due to low RCP speed, as expected.
The reactor was subsequently stabilized in hot standby on natural circulation.
Failure of bus E-NAN-502, initiated by the phase B-to-ground fault, was attributed to cracked and brittle Noryl insulation combined with dirt accumulation in the cubicles. These conditions allowed bus insulation breakdown and arc tracking to occur, causing the single phase-to-ground fault, which subsequently propagated into a three phase-to-ground fault.
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As corrective actions, the licensee identified inadequacies in the switchgear preventative maintenance program regarding cleaning of the switchgear, and also found that adequate environmental or housekeeping controls had not been implemented on the 13.8 kV and 4.16 kV switchgear.
2.
Kewaunee (two events)
On March 2,1988, with the plant at 93.3 percent power, a reactor trip and associated turbine trip were generated as a result of undervoltage conditions on 4160 volt electrical buses 1-1 and 1-2, which supply) power to the reactor coolant pump and main feedwater pump motors (Ref. 2. A main auxiliary transformer differential current alarm had been received in the control room, and smoke was reported in the basement of the turbine building. Differential current protection devices isolated the main auxiliary transformer from the electrical buses, the fire was extinguished, j
and the reactor was brought to the hot shutdown condition.
Investigations into the cause of the undervoltage condition on the buses revealed that an electrical fault had occurred on the bus bar from the "Y" winding of the main auxiliary transformer to buses 1-1 and 1-2 due to insulation failure. The bus bar was a Calvert Company 1/2 inch by 4 inch flat copper bar, with full round edge, tin finish at the joints, and rated at 4000 amperes per phase. The bus bar was encapsulated with Noryl flame retardant insulation, was supported on molded flame retardant glass polyester supports, and was enclosed in aluminum ducting with screened ventilation slots on the top and bottom.
The cause of the event was determined to be insulation failure on the bus bar, combined with accumulation of water and debris around the bus which provided a tracking path for the fault. The bus bar runs horizon-tally into the auxiliary building underneath areas where debris can fall into the bus work. Additionally, a plastic hose, installed above the faulted section of the bus to empty water into a floor drain, was empty-ing onto the floor. Water from this source was suspected to have dripped onto the bus work.
Damage to the bus bar extended along a ten foot section of the bus bar from the main auxiliary transformer to buses 1-1 and 1-2.
In addition, as a result of the force of the fault, insulation between the insulated bus bar supports experienced some cracking. Several non-safety related cables, located in a cable tray next to the bus, also experienced insula-tion failure dt to intense heat from the fire.
As corrective action, the utility performed the following:
The bus bar vendor and a consultant were utilized to examine the l
1 failed insulation and other portions of the bus to recommend cor-rective actions. Their recommendations included considering replacing the "open" duct enclosures with " closed" enclosures, l
The damaged section of the bar was replaced.
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All bus bar runs and ducting were cleaned and inspected. Annual inspection of bus bars are planned.
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A design change was implemented to suspend a protective cover over L
portions of the bus duct to prevent overhead debris and liquids from l
coming in contact with the bus bar insulation.
t Insulation on the affected bus bar, from the main auxiliary trans-former (MAT) to buses 1-1 and 1-2, was changed to 3M Company heat shrink sleeving (#BBPS 110/60), which was expected to provide better resistance to temperature and the effects of oil.
A previous similar event occurred at Kewaunee on July 10, 1987 (Ref. 3),
when a reactor trip and associated turbine trip occurred as a result of an UV transient on the same 4160 volt buses. Smoke and fire were also observed emanating from an electrical bus bar in the turbine bcilding, and the bus fire was similarly extinguished once the transformer was denergized.
However, in this case, a phase-to-ground fault occurred on the bus bar from the "X" winding of the main auxiliary transformer to 4160 volt buses 1-3, 1-4, 1-5, and 1-6.
This bus bar was a Calvert Company 1/2 inch by 4 inch flat aluminum bar, with full round edge, rated at 3000 amperes. The bus was also insulated with Noryl flame retardant insulation, supported on molded flame retardant glass polyester supports, and enclosed in an aluminum ventilated housing.
The phase-to-ground fault propagated into a phase-to-phase fault, creat-ing additional damage to the bus.
The cause of the event was also established to be insulation failure on the bus bar compounded by accumulation of particulate debris.
The bus bar was located perpendicular to the turbine building ventilation fans, which pulled dust filled air through a section of the bus bar. Dust and metallic powder debris were found to have collected on the bus bar insulation.
Insulation failure, combined with accumulation of debris, provided a tracking path from the phase to ground.
Corrective actions associated with this event included the following:
A 30 foot section of the bus bar from the main auxiliary transformer to buses 1-3,1-4,1-5, and 1-6 was replaced, utilizing new Noryl insulation.
The bus bar vendor and a consultant were contracted to provide an analysis of the insulation failure.
Recommendations included maintaining the bus bars in a clean condition, and developing a test and maintenance schedule.
Inspection and cleaning of the affected bus bar sections were conducted, and a procedure for triennial inspection and cleaning of all bus ducting was established. Seven months later the March 1988, event occurred. Annual inspections are now scheduled.
Millstone Units 1 and 2 During a visual inspection of the Unit 1 4160 volt AC load centers, while at 100 percent power on January 13, 1987, crack *ng was observed along j
horizontal Noryl bus bar insulation (Ref.4). The insulation deterioration
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. L problem was attributed to a manufacturing defect on General Electric (GE)
Company metalclad switchgear Type M-26 (4160 VAC) and Type M-36 (6900VAC).
, during the manufacturing process, " black" bus bar joint Apparently(GE # D50H47) contaminated the Noryl insulation, and over several compound years caused cracking of the insulation.
This visual inspection was prompted by similar bus bar insulation deterioration discovered in 1986 at Millstone Unit 2.
While performing normal maintenance on electrical breakers, utility personnel identified bus bar insulation cracking in certain 6900 volt switchgear. With GE's assist-ance, horizontal bus bar cracks were found in all the Unit 2 6900 volt AC buses, and in some of the 4160 volt buses.
As corrective action, the utility implemented GE's recommendations to replace the Noryl bus insulation with Bayblend insulation, and utilized a
" yellow" bus bar joint compound (GE #D50H109) in lieu of the " black" compound.
Sequoyah Units 1 and 2 On May 18, 1983, with Unit 1 at 100 percent power and Unit 2 at 98 percent power, scartup bus IB on Unit 1 failed due to a phase B-to-phase C fault, which propagated to ground.(Ref. 5). The failure was attributed to degradation of Noryl insulation on the Westinghouse Model EN-265 bus.
Further investigation revealed several degraded areas in the bus insulation at the. support blocks. Accordingly, startup bus 1B was declared inoperable on each unit.
As corrective action, all bus sections in the failed areas were replaced and the supports were relocated. Additionally, water drainage systems for the electrical buses were enhanced.
ANALYSIS AND EVALUATION Cracked insulation, resulting from high ambient temperatures and contamination from the use of " black" bus bar joint compound, has been observed on bus bars manufactured by Westinghouse, General Electric, and the Calvert Company, where Noryl flame retardant insulation was utilized.
Insulation failure, along with the presence of moisture or debris, provided undesired phase-to-phase, or phase-to-ground, electrical tracking paths, which resulted in catastrophic failure of the buses. Bus failures, in turn, created subsequent reactor trips, and caused unnecessary challenges to plant safety systems.
Corrective actions taken by the utilities included replacing damaged bus bar sections with bus bars utilizing insulation of a different type, substituting
" yellow" bus bar joint compound (GE #D50H109) for the " black" joint compound previously utilized, modifying bus bar enclosures to restrict ingress and accumulation of water and debris, and instituting enhanced inspection and cleaning of bus bar housings.
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E' J FINDINGS AND-CONCLUSIONS
-' The potentia 1' exists for medium voltme buses, utilizing Noryl insulttion, to experience catastrophic failure it not periodically-inspected and main-tained free of moisture and debris. ' Failures were experienced at Palo l
' Verde, Kewaunee, Millstone, and Sequoyah.
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- BusLfailures, in turn, cause subsequent reactor trips, and initiate unnecessary challenges to plant safety' systems.
Periodic inspections of Noryl insulated buses should be performed at nuclear power facilities to identify insulation cracking.
Enhanced maintenance programs for medium. voltage electrical buses should be developed and implemented by utilities to prevent buildup of water and' debris in the. bus housings.
Bus bar enclosures around Noryl insulated buses should be assessed to determine the need for modifications to the enclosures to prevent intrusion and accumulation of water or. debris.
REFERENCES 1.
Palo Verde Nuclear Plant Licensee Event Report 88-10. " Ground Fault in 13.8
' V! Sus Causes Fire in Unit Auxiliary Transformer and Reactor Trip", Docket 2+. 50-528, Arizona Public Service Company, August 4,1988.
2.
Kewaunee Nuclear Plant Licensee Event Report No.-88-01, " Insulation Failure and Dirt Accumulation Cause Electrical Bus Bar Failure and Reactor Trip",
' Docket No.'50-305, Wisconsin Public Service Corporation, April 4, 1988.
3.
Kewaunee Nuclear Plant I k ansee Event Report No. 87-09, " Electrical Bus Bar Failure Causes Undervo1 4 e on RXCP Buses and Reactor Trip", Docket No.
50-305, Wisconsin Public Service Corporation, August 10, 1987.
4 Millstone Nuclear Power Station Licensee Event Report No. 87-01-01, "4160 V Distribution Load Center Insulation", Docket No. 50-245, Northeast Utilities, September 9, 1987.
5.
Sequoyah Nuclear Plant Licensee Event Report No. 83-67, Docket No.
3 E
50-327, Tennessee Valley Authority, May 31, 1983.
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-UNITED STATES NUCLEAR REGULATORY COMMISSION
-0FFICE OF-NUCLEAR REACTOR REGULATION-
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WASHINGTON. D.C. 20555 May,.1989 NRC INFORMATION NOTICE N0. 89- : ELECTRICAL' BUS BAR FAILURES Addressees:
All. holders of operating licenses or construction permits for nuclear power reactors.
Purpose:
LThisinformationnoticeis.beingprovidedtoalertaddresseesoffailureof electrical bus bars due to the combination of Noryl insulation cracking.and moisture.or debris buildup in bus bar housings. Bus failures, in turn,
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created subsequent redctor trips, and caused unnecessary challenges to plant safety systems...It is expected that recipients will review the information for appli ability to their facilities and consider _ actions, as appropriate, to avoid similar problems. However, suggestions contained in this information
. notice do not' constitute NRC requirements; therefore, no specific action or written response.is required.
~ Description of Circumstances:
l '.
Palo Verde Unit 1 On July 6,-1988, a phase B to ground fault occurred on 13.8 kV non-class t
IE electrical bus E-NAN-S02. This fault. ionized the air surrounding the bus, and caused all three phases to short to ground. Feeder breakers to non-class 1E buses E-NAN-502 and S03 did not immediately open, resulting in excessive currents being supplied by the unit auxiliary ?ransformer (UAT), and the. subsequent rupture and ignition of the UAT. This caused the supply breakers to buses E-NAN-S01, S02, and the main generator output breaker to open on a UAT transformer " sudden overpressure" signal.
As the RCPs were powered from E-NAN-S01 ar 502, a r m tor trip on low departure from nucleate boiling ratio occarred, as expected. The reactor was subsequently stabilized in hot-standby on natural circulation.
Failure of bus E-NAN-S02, initiated by the phase B to ground fault, was attributed to cracked and brittle Noryl insulation combined with dirt accumulation in the cubicles. These conditions allowed bus insulation breakdown and arc tracking to occur, causing the single phase to ground fault, which subsequently propagated into a three phase to ground fault.
As corrective actions, the licensee identified inadequacies in the switchgear preventative maintenance program regarding cleaning of the switchgear, and also found that adequate environmental or housekeeping controls had not been implemented on the 13.8 kV and 4.16 kV switchgear.
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IN 83-May
, 1989
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Page 2 of 3 2.
Kewaunee (two events).
On March 2, 1988, with the plant at 93.3 percent power, a reactor trip and associated turbine trip were generated.as a result of undervoltage (UV) conditions on 4160 volt electrical buses 1-1-and 1-2, which supply power to.the reactor coolant pump-and main feedwater pump motors.
Investigations into the cause.of the UV condition on the buses revealed that an electrical fault had occurred on the bus bar from the "Y" winding of the to buses 1-1 and 1-2 due to insulation failure. The bus bar was a Calvert Company 1/2 inch by 4 inch flat copper bar, rated at 4000 amperes o
per phase. The bus bar was' encapsulated with Noryl flame retardant insula-tion, and was enclosed in aluminum ducting with screened ventilation slots on the top and bottom.
The cause of the event was determined to be insulation failure on the bus bar, combined with accumulation of water and debris around the bus which provided-a tracking path for the fault. The bus bar runs horizon-tally into the auxiliary building underneath areas where debris can fall into the bus work. Additionally, water from a plastic drain hose, located on the floor above the faulted section of the bus, was suspected' to have dripped onto the bus work.
A previous similar event occurred at Kewaunee on July 10, 1987,'when a reactor trip and associated turbine trip occurred as a result of an UV transient on the same 4160 volt buses. However, in this case, a phase to ground fault occurred on the bus bar from the "X" winding of the main auxiliary transformer to 4160 volt buses 1-3, 1-4, 1-5, and 1-6.
This bus bar was similar to the bus bar identified above, except that it was a flat aluminum bar,-rated at 3000 amperes.
The.cause of this event was also established to be insulation failure on
'i the bus bar compounded by accumulation of particulate debris. The bus bar,was located perpendicular to the turbine building ventilation fans, which pulled dust filled air through a section of the bus bar. Dust and metallic powder debris were found to have collected on the cracked bus bar insulation, provided a tracking path from the phase to ground.
Millstone Units 1 and 2 During a visual inspection of the Unit 1 4160 volt AC load centers, while at 100 percent power on January 13, 1987, cracking was observed along horizontal Noryl bus bar insulation. The insulation deterioration Company metalciad switchgear Type M-26 (g defect on General Electr problem was attributed to a manufacturing 4160 VAC) and Type M-36 (6900 VAC). Apparently, during the manufacturing process, " black" bus ar joint compound (GE # D50H47) contaminated the floryl insulation and over several years caused cracking of the insulation.
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IN 89-May 1989 Page 3 of 3 Sequoyah Units 1 and 2 On May 18, 1983, with Unit I at 100 percent power and Unit 2 at 98 percent power, startup bus IB on Unit I failed due to a phase B to phase C fault, which propagated to ground. The failure was attributed to degradation of Noryl Insulation on the Westinghouse Model EN-265 bus. Furtha-investiga-tion revealed several degraded areas in the bus insulation at the support blocks, and these buses were declared inoperable.
Discussion:
l Failures of medium voltage electrical bus bars, principally involving 4160 and 6900 volt AC buses, have resulted in bus bar electrical faults and fires, electrical power system undervoltage conditions, plant transients, and reactor trips.
Failure of the bus bars has been attributed to cracking of Noryl bus ba-insulation (bus sleeving) combined with accumulation of moisture or debris in the bus bar housings.
Insulation failure, along with the presence of moisture or debris, provide undesired phase to phase, or phase to ground, electrical tracking paths, which have resulted in catastrophic failure of the buses.
Cracked insulation, resulting from high ambient temperatures and/or contami-nation from the use of " black" bus bar joint compound, has been observed on bus. bars manufactured by Westinghouse, General Electric, and the Calvert Company, where Noryl flame retardant insulation was utilized.
Corrective actions taken by the involved utilities included replacing damaged bus bar sections with bus bars utilizing insulation of a different type, substituting " yellow" bus bar joint compound (GE #D50H109) for the " black" joint compound previously utilized, modifying bus bar enclosures to restrict ingress and accumulation of water and debris, and instituting enhanced periodic inspection and cleaning of bus bars and their housings.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical contact listed below or the Regional Administrator of the appropriate regional office.
Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical
Contact:
Mark Padovan, AE0D (301)492-4445
Attachment:
List of Recently Issued NRC Information Notices i
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