Information Notice 2019-02, Emergency Diesel Generator Excitation System Diode Failures: Difference between revisions

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| issue date = 06/03/2019
| issue date = 06/03/2019
| title = Emergency Diesel Generator Excitation System Diode Failures
| title = Emergency Diesel Generator Excitation System Diode Failures
| author name = McGinty T, Miller C
| author name = Mcginty T, Miller C
| author affiliation = NRC/NRO, NRC/NRR
| author affiliation = NRC/NRO, NRC/NRR
| addressee name =  
| addressee name =  
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| page count = 5
| page count = 5
}}
}}
{{#Wiki_filter:ML18250A178 UNITED STATES
{{#Wiki_filter:UNITED STATES


NUCLEAR REGULATORY COMMISSION
NUCLEAR REGULATORY COMMISSION
Line 22: Line 22:
OFFICE OF NEW REACTORS
OFFICE OF NEW REACTORS


WASHINGTON, DC 20555 June 3, 2019 NRC INFORMATION NOTICE 201
WASHINGTON, DC 20555 June 3, 2019 NRC INFORMATION NOTICE 2019-02:               EMERGENCY DIESEL GENERATOR EXCITATION
9-02: EMERGENCY DIESEL GENERATOR EXCITATION SYSTEM DIODE FAILURES
 
SYSTEM DIODE FAILURES


==ADDRESSEES==
==ADDRESSEES==
All holders of an operating license or construction permit for a nuclear power reactor under Title
All holders of an operating license or construction permit for a nuclear power reactor under Title 10
of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of Production and


10 of the Code of Federal Regulations
Utilization Facilities, except those that have permanently ceased operations and have certified that


(10 CFR) Part 50, "Domestic Licensing of Production and Utilization Facilities," except those that have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel.
fuel has been permanently removed from the reactor vessel.


All holders of and applicants for a combined license under
All holders of and applicants for a combined license under 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants.


10 CFR Part 52, "Licenses, Certifications, and Approvals for Nuclear Power Plants."
==PURPOSE==
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform


==PURPOSE==
addressees of operating experience with regard to emergency diesel generator (EDG) excitation
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform addressees of operating experience with regard to emergency diesel generator (EDG) excitation system diode failures
 
system diode failures. The failure of EDG excitation system diodes may cause affected EDGs not
 
to be able to operate for their full mission times following a loss of offsite power event. The NRC


.  The failure of EDG excitation system diodes may cause affected EDGs not to be able to operate for their full mission times following a loss of offsite power event.  The NRC expects that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems.  INs may not impose new requirements, and nothing in this IN should be interpreted to require specific action.
expects that recipients will review the information for applicability to their facilities and consider


==DESCRIPTION OF CIRCUMSTANCES==
actions, as appropriate, to avoid similar problems. INs may not impose new requirements, and
Catawba Nuclear Station, Unit 2  On April 11, 2017, during monthly surveillance testing of


the 2A EDG, at Catawba Nuclear Station (Catawba), Unit 2, the EDG output breaker unexpectedly tripped open on an overcurrent relay actuation.
nothing in this IN should be interpreted to require specific action.


The breaker opened approximately three minutes after reaching full load (5750
==DESCRIPTION OF CIRCUMSTANCES==
kW).  The EDG was subsequently secured and declared inoperable.


During troubleshooting, the
===Catawba Nuclear Station, Unit 2===
On April 11, 2017, during monthly surveillance testing of the 2A EDG, at Catawba Nuclear Station


licensee discovered that diode CR4 in the EDG excitation circuit shorted, causing the
(Catawba), Unit 2, the EDG output breaker unexpectedly tripped open on an overcurrent relay


EDG output breaker to trip open on overcurrent.
actuation. The breaker opened approximately three minutes after reaching full load (5750 kW).


The 2A EDG was returned to service
The EDG was subsequently secured and declared inoperable. During troubleshooting, the licensee


on April 14, 2017.  The Catawba EDG excitation system, consisting of a Portec voltage regulator design, is an overcompensated design type.  This system supplies
discovered that diode CR4 in the EDG excitation circuit shorted, causing the EDG output breaker to


more power than required
trip open on overcurrent. The 2A EDG was returned to service on April 14, 2017.


for the generator field.  The voltage regulator uses shunt silicon
The Catawba EDG excitation system, consisting of a Portec voltage regulator design, is an


control rectifiers (SCR) to bypass the excess excitation current away from the generator
overcompensated design type. This system supplies more power than required for the generator


field to control generator output voltage.
field. The voltage regulator uses shunt silicon control rectifiers (SCR) to bypass the excess


These SCRs direct the excess current through
excitation current away from the generator field to control generator output voltage. These SCRs


diodes CR2, CR4 and CR6.
direct the excess current through diodes CR2, CR4 and CR6. Diodes CR2, CR4 and CR6 also


Diodes CR2, CR4 and CR6 also carry the generator field
carry the generator field current. As such, these diodes are in a conducting state for longer


current.  As such, these diodes are in a conducting state for longer intervals than other
intervals than other diodes in the bridge and, based on industry operating experience, internal


diodes in the bridge and, based on industry operating experience, internal temperatures
temperatures can be approximately 60 degrees Fahrenheit greater than diodes CR1, CR3 ML18250A178 and CR5. During a review, the NRC inspectors found additional failures of these diodes. In 2005, a similar failure occurred at Catawba with the CR4 diode on the 1A EDG. The licensee attributed


can be approximately 60 degrees Fahrenheit greater than diodes CR1, CR3 and CR5. During a review, the NRC inspectors found addition
the 2005 failure to operation of the diode at higher than optimum operating temperatures.


al failure s of these diodes.  In 2005, a similar failure occurred at Catawba with the CR4 diode on the 1A EDG.  The licensee attributed the 2005 failure to operation of the diode at higher than optimum operating temperatures.
The licensees corrective actions included: 1) modifying the EDG voltage regulator to address


The licensee's corrective actions included
design weaknesses for all four EDGs based on detailed simulation of the voltage regulator to


: 1) modifying
prevent diode damage, 2) implementing a diode and SCR replacement preventive maintenance


the EDG voltage regulator to address design weaknesses for all four EDGs based on detailed simulation of the voltage regulator to prevent diode damage, 2)
(PM) activity at a frequency of no greater than 18 months, and 3) issuing a revision to the Operating
implementing a diode and SCR replacement preventive maintenance


(PM) activity at a frequency of no greater than 18 months, and 3) issuing
Experience Program that addressed previously completed actions for operating experience insight, future engineering changes, and reviewing historical performance trends to ensure PM strategy


a revision to the Operating Experience Program
changes are identified.


that addressed previously
On August 22, 2017, the NRC issued a preliminary White finding (Agencywide Documents Access


completed actions for
and Management System (ADAMS) Accession No. ML17234A678) and an associated apparent


operating experience insight, future engineering changes, and reviewing historical
violation related to this event. The NRC issued the final white finding to Catawba on October 16,
2017 (ADAMS Accession No. ML17289A300). Additional details of this operating event are


performance trends to ensure PM strategy changes are identified.
provided in an NRC inspection report dated May 23, 2018 (ADAMS Accession No. ML18143B537).


On August 22, 2017, the NRC issued a preliminary
===Wolf Creek Generating Station, Unit 1===
On October 6, 2014, during a scheduled 24-hour surveillance testing, Wolf Creek Generating


White finding (Agencywide Documents Access and Management System (ADAMS) Accession No. ML17234A678) and an associated apparent violation related to this event. The NRC issued the final white finding to Catawba on October
Stations B EDG tripped, and a fire was observed in an associated excitation control cabinet. The


16, 2017 (ADAMS Accession No. ML17289A300).  Additional details of this operating event are provided in
fire was quickly suppressed, and, following post-maintenance surveillance, the EDG was restored


an NRC inspection report
to operable status on October 9, 2014. The failure of a diode in the power rectifier of the EDG


dated May 23, 2018 (ADAMS Accession No. ML18143B537
excitation system caused the failure of the power potential transformer (PPT). During a post- maintenance surveillance on June 11, 2014, the licensee noted a vapor coming from the cabinet
). Wolf Creek Generating Station, Unit 1  On October 6, 2014, during a scheduled 24
-hour surveillance


testing, Wolf Creek
housing the PPT. The PPT exhibited the same symptoms during subsequent surveillances. The


Generating Station's "B" EDG tripped
PPT was scheduled for replacement by the licensee in August 2014 during the next system outage.


, and a fire was observed
During the work management planning process, the PPT replacement was rescheduled for


in an associated
February 2015. The PPT failed on October 6, 2014.


excitation control cabinet.  The fire was quickly suppressed
The Wolf Creek Generating Station excitation system consists of a Westinghouse WNR voltage


, and , following post
regulator design. In the root cause analysis, the licensee identified the most probable direct cause


-maintenance
as thermal degradation of the Power Rectifier diodes. Due to the reduced contribution of field


surveillance, the EDG was restored to operable status on October 9, 2014.  The failure of a diode in the power rectifier of the EDG excitation system caused the failure of the power potential transformer (PPT).  During a post-maintenance surveillance on June 11, 2014, the licensee noted a vapor coming from the cabinet housing the PPT.  The PPT exhibited the same symptoms during subsequent surveillances.  The PPT was scheduled for replacement by the licensee in August 2014 during the next system outage.  During the work management planning process, the PPT replacement was rescheduled for February 2015.  The PPT failed on October
current and voltage from the PPT circuitry from a single diode failure, the voltage regulator would


6, 2014.   The Wolf Creek Generating Station excitation system consist
task the PPT to supply the remainder of the required current to the field. This increased current


s of a Westinghouse WNR voltage
would increase the internal temperatures of the PPT, leading to degraded windings within the PPT.


regulator design.  In the root cause analysis, the licensee identified the most probable direct cause as thermal degradation of the Power Rectifier diodes.
This condition resulted in the emission of vapor from the PPT, which was noticed by the licensee


Due to the reduced contribution of field current and voltage from the PPT circuitry from a single diode failure, the voltage regulator would task the PPT
prior to the event. The second diode eventually shorted, causing a short in the generator field. This


to supply the remainder of the required current to the field.
short caused a loss of excitation to the generator field and tripped the EDG. The root cause


This increase
analysis also stated that the station did not recognize the significance of aging or life cycle factors


d current would increase the internal temperatures of the PPT, leading to degraded windings within the PPT.  This condition resulted in the emission of vapor from the PPT, which was noticed by the licensee prior to the event.  The second diode eventually shorted, causing a short in the generator field.
associated with the EDG excitation system, resulting in an inadequate PM strategy of the excitation


This short cause
system. There were limited PM activities, limited knowledge of the exciter, and the design of the


d a loss of excitation to the generator
system lacked overcurrent protection/detection of the PPT.


field and trip
The licensees corrective actions included replacing the PPT and selecting the alternate rectifier


ped the EDG. The root cause analysis also stated that the station did not recognize the significance of aging or life cycle factors associated with the EDG excitation system
bank to restore the availability of the B EDG. In addition to immediate actions taken, the licensee


, resulting in an inadequate PM strategy of the excitation system.  There were limited PM activities, limited knowledge of the exciter, and the design of the system lacked overcurrent protection/detection of the PPT.
replaced all power diodes within all four rectifier bridges (two rectifier bridges for each EDG). On


The licensee's corrective actions included replacing the
October 27, 2015, the licensee implemented a corrective action to generate new preventive maintenance activities to periodically replace the diodes within the power rectifier and other


PPT and selecting the alternate rectifier bank to restore the availability of the B EDG.  In addition to immediate actions taken, the licensee replaced all power diodes within all four rectifier bridges (two rectifier bridges for each EDG).  On October 27, 2015, the licensee implemented a corrective action to generate new preventive maintenance activities to periodically replace the diodes within the power rectifier and other excitation system components as recommended by the operating experience
excitation system components as recommended by the operating experience.


.  Additional details of this operating event are provided in the NRC
Additional details of this operating event are provided in the NRC inspection report dated


inspection report dated August 19 , 2016 (ADAMS Accession No.
August 19, 2016 (ADAMS Accession No. ML16235A132).


ML16235A132
===Grand Gulf Nuclear Station, Unit 1===
).  Grand Gulf
On May 21, 2013, during the Division 2 standby diesel generator monthly surveillance test, the


Nuclear Station, Unit 1  On May 21, 2013, during the Division 2 standby diesel generator monthly surveillance test, the underfrequency alarm sounded
underfrequency alarm sounded, concurrent with a drop in indicated voltage from approximately


, concurrent with a drop in indicated voltage from approximately 4220 Volts to 2100 Volts.
4220 Volts to 2100 Volts.


The Grand Gulf
The Grand Gulf EDG excitation system consists of a Portec voltage regulator design. The licensee


EDG excitation
performed a failure modes analysis evaluation to determine the possible cause for the observed


system consists of a Portec voltage regulator design. The licensee performed a failure
conditions. During troubleshooting efforts, the licensee addressed the potential transformer (PT1),
the potential transformers fuses, inline fuses, and the voltage regulator circuit bridge diodes. The


modes analysis evaluation to determine the possible cause for the observed conditions.  During troubleshooting efforts, the licensee addressed the potential transformer (PT1), the potential transformer's fuses, inline fuses, and the voltage regulator circuit bridge diodes.  The failure modes analysis evaluation showed that all the listed components were in satisfactory condition, with the exception that one of the six diodes used in the voltage regulator circuit diode bridge (diode CR6) had shorted.
failure modes analysis evaluation showed that all the listed components were in satisfactory


The licensee replaced the shorted diode and returned the diesel generator to operational
condition, with the exception that one of the six diodes used in the voltage regulator circuit diode


status on May 24, 2013. Based on past similar issues with degraded diodes on the voltage regulator circuit
bridge (diode CR6) had shorted.


, the licensee
The licensee replaced the shorted diode and returned the diesel generator to operational status on


developed a preventive maintenance strategy to begin testing the diodes on all three
May 24, 2013. Based on past similar issues with degraded diodes on the voltage regulator circuit, the licensee developed a preventive maintenance strategy to begin testing the diodes on all three


divisions of diesel generators and to replace any suspect components before they caus ed a loss of excitation event.
divisions of diesel generators and to replace any suspect components before they caused a loss of


Additional details of this
excitation event.


operating event are provided in the NRC inspection report dated August 14, 2014 (ADAMS Accession No.
Additional details of this operating event are provided in the NRC inspection report dated


ML14226A998
August 14, 2014 (ADAMS Accession No. ML14226A998).
).


==DISCUSSION==
==DISCUSSION==
The failure of EDG excitation system diodes can adversely impact the ability of EDGs to perform their safety functions.  A review of operating experience indicates that events involving the failure of EDG excitation system diodes continue to occur.  Such failures may cause affected EDGs not to be able to operate for their full mission times following a loss of offsite power event.
The failure of EDG excitation system diodes can adversely impact the ability of EDGs to perform


Underlying causes have been attributed to marginal design, postulated age
their safety functions. A review of operating experience indicates that events involving the failure of


-related failure, manufacturing defects coupled with
EDG excitation system diodes continue to occur. Such failures may cause affected EDGs not to be


voltage overstress, operation at elevated temperatures, and inadequate preventive maintenance
able to operate for their full mission times following a loss of offsite power event.


.  Because these diodes appear to have a limited life, licensees should consider establishing an appropriate replacement schedule
Underlying causes have been attributed to marginal design, postulated age-related failure, manufacturing defects coupled with voltage overstress, operation at elevated temperatures, and


as part of effective preventativ
inadequate preventive maintenance. Because these diodes appear to have a limited life, licensees


e maintenance
should consider establishing an appropriate replacement schedule as part of effective preventative


in accordance with the Maintenance Rule.
maintenance in accordance with the Maintenance Rule.


Related NRC Generic Communications
===Related NRC Generic Communications===
Information Notice 2010-04, Diesel Generator Voltage Regulation System Component Due to


Information
Latent Manufacturing Defect, dated February 26, 2010, describes the failure of a linear power


Notice 2010-04 , "Diesel Generator Voltage Regulation System Component Due to Latent Manufacturing Defect," dated February 26, 2010 , describes the failure of a linear power reactor in an EDG voltage regulation system at
reactor in an EDG voltage regulation system at a plant where the licensees preventive


a plant where the licensee's preventive maintenance program did not address the EDG excitation system magnetic components
maintenance program did not address the EDG excitation system magnetic components that can


that can be subject to deterioration with age or time in service
be subject to deterioration with age or time in service. IN 2010-04 describes the corrective actions implemented by the licensee in response to the event, which included adding a preventive


.  IN 2010-04 describes the corrective actions implemented by the licensee in response to the event, which included adding a preventive maintenance task for thermography of EDG excitation system silicon controlled bridge rectifiers, power diode bridge rectifiers, current transformers, power transformers, and linear reactors.
maintenance task for thermography of EDG excitation system silicon controlled bridge rectifiers, power diode bridge rectifiers, current transformers, power transformers, and linear reactors.


Information
Information Notice 2007-36, Emergency Diesel Generator Voltage Regulator Problems, dated


Notice 2007-36 , "Emergency Diesel Generator
November 15, 2007, describes several events involving EDG voltage regulator problems, including


Voltage Regulator Problems," dated November 15, 2007, describes several events involving EDG voltage regulator problems, including an event that involved anomalies with the gate firing circuit output pulses to the silicon controlled rectifiers on the rectifier assembly.  The licensee's root cause evaluation for this event discussed that the EDG had a history of intermittent overvoltage problems because of
an event that involved anomalies with the gate firing circuit output pulses to the silicon controlled


a higher than expected impedance in the flyback diode portion of the rectifier chassis circuitry. IN
rectifiers on the rectifier assembly. The licensees root cause evaluation for this event discussed


2007-36 discusses various types of problems that are not limited to a single component or model of voltage regulator, and states that
that the EDG had a history of intermittent overvoltage problems because of a higher than expected


, "In general, the performance of a voltage regulator is very sensitive to any minor defects in any component of the voltage regulation system."
impedance in the flyback diode portion of the rectifier chassis circuitry. IN 2007-36 discusses


==CONTACT==
various types of problems that are not limited to a single component or model of voltage regulator, and states that, In general, the performance of a voltage regulator is very sensitive to any minor
This IN requires no specific action or written response.  Please direct any questions about this matter to the technical contact listed below.


/RA/      /RA/  Christopher Miller
defects in any component of the voltage regulation system.


Timothy J. McGinty, Director
==CONTACT==
This IN requires no specific action or written response. Please direct any questions about this


Division of Inspection and Regional Support
matter to the technical contact listed below.


Division of Construction Inspection
/RA/                                                  /RA/
Christopher Miller                                    Timothy J. McGinty, Director


Office of Nuclear Reactor Regulation
Division of Inspection and Regional Support          Division of Construction Inspection


and Operational Programs
Office of Nuclear Reactor Regulation                  and Operational Programs


Office of New Reactors
Office of New Reactors


Technical Contact
Technical Contacts:      Jason B. Carneal, NRR/DIRS


s: Jason B. Carneal, NRR/DIRS
301-415-1451 E-mail: Jason.Carneal@nrc.gov
 
301-415-1 451 E-mail: Jason.Carneal@nrc.gov


Vijay K. Goel, NRR/DE
Vijay K. Goel, NRR/DE


301-415-3730 E-mail: Vijay.Goel@nrc.gov
301-415-3730
 
                        E-mail: Vijay.Goel@nrc.gov
Note:  NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov , under "NRC Library," "Document Collections."
 
ML18250A178
*concurred via email
 
OFFICE  TECH EDITOR
 
NRR/DIRS/IOEB
 
NRR/DIRS/IOEB/BC
 
NRO/D C IP/QVIB//BC NRO/DSRA/SCP
 
B/BC NAME  JDougherty*
JCarneal* RElliot t* JJacobson* HWagage* DATE  9/24/18 2/6/19 2/2/19 2/5/19 2/15/19 OFFICE  NRR/DE/EEOB


NRR/DE/EEOB/BC
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library, Document Collections.


NRR/DIRS/IRGB/LA
ML18250A178                                        *concurred via email


NRR/DIRS/IRGB/PM
OFFICE TECH EDITOR        NRR/DIRS/IOEB    NRR/DIRS/IOEB/BC NRO/DCIP/QVIB//BC NRO/DSRA/SCPB/BC


NRR/DIRS/IRGB/BC
NAME  JDougherty*        JCarneal*        RElliott*        JJacobson*        HWagage*
DATE  9/24/18            2/6/19          2/2/19            2/5/19            2/15/19 OFFICE NRR/DE/EEOB        NRR/DE/EEOB/BC  NRR/DIRS/IRGB/LA  NRR/DIRS/IRGB/PM  NRR/DIRS/IRGB/BC


NAME VGoel* KNguyen* BCurran* MLintz* TInverso* DATE 2/27/19 1/31/19 1/31/19 2/28/19 2/28/19 OFFICE NRO/DCIP/D
NAME   VGoel*             KNguyen*         BCurran*         MLintz*           TInverso*
DATE   2/27/19             1/31/19         1/31/19           2/28/19             2/28/19 OFFICE NRO/DCIP/D          NRR/DIRS/D


NRR/DIRS/D
NAME  TMcGinty*          CMiller


NAME  TMcGinty* CMiller    DATE 3/12/19 06/03/19}}
DATE   3/12/19             06/03/19}}


{{Information notice-Nav}}
{{Information notice-Nav}}

Latest revision as of 15:58, 20 October 2019

Emergency Diesel Generator Excitation System Diode Failures
ML18250A178
Person / Time
Issue date: 06/03/2019
From: Mcginty T, Chris Miller
Office of New Reactors, Office of Nuclear Reactor Regulation
To:
Benney B
References
IN-19-002
Download: ML18250A178 (5)
v • d • e


UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NEW REACTORS

WASHINGTON, DC 20555 June 3, 2019 NRC INFORMATION NOTICE 2019-02: EMERGENCY DIESEL GENERATOR EXCITATION

SYSTEM DIODE FAILURES

ADDRESSEES

All holders of an operating license or construction permit for a nuclear power reactor under Title 10

of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of Production and

Utilization Facilities, except those that have permanently ceased operations and have certified that

fuel has been permanently removed from the reactor vessel.

All holders of and applicants for a combined license under 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform

addressees of operating experience with regard to emergency diesel generator (EDG) excitation

system diode failures. The failure of EDG excitation system diodes may cause affected EDGs not

to be able to operate for their full mission times following a loss of offsite power event. The NRC

expects that recipients will review the information for applicability to their facilities and consider

actions, as appropriate, to avoid similar problems. INs may not impose new requirements, and

nothing in this IN should be interpreted to require specific action.

DESCRIPTION OF CIRCUMSTANCES

Catawba Nuclear Station, Unit 2

On April 11, 2017, during monthly surveillance testing of the 2A EDG, at Catawba Nuclear Station

(Catawba), Unit 2, the EDG output breaker unexpectedly tripped open on an overcurrent relay

actuation. The breaker opened approximately three minutes after reaching full load (5750 kW).

The EDG was subsequently secured and declared inoperable. During troubleshooting, the licensee

discovered that diode CR4 in the EDG excitation circuit shorted, causing the EDG output breaker to

trip open on overcurrent. The 2A EDG was returned to service on April 14, 2017.

The Catawba EDG excitation system, consisting of a Portec voltage regulator design, is an

overcompensated design type. This system supplies more power than required for the generator

field. The voltage regulator uses shunt silicon control rectifiers (SCR) to bypass the excess

excitation current away from the generator field to control generator output voltage. These SCRs

direct the excess current through diodes CR2, CR4 and CR6. Diodes CR2, CR4 and CR6 also

carry the generator field current. As such, these diodes are in a conducting state for longer

intervals than other diodes in the bridge and, based on industry operating experience, internal

temperatures can be approximately 60 degrees Fahrenheit greater than diodes CR1, CR3 ML18250A178 and CR5. During a review, the NRC inspectors found additional failures of these diodes. In 2005, a similar failure occurred at Catawba with the CR4 diode on the 1A EDG. The licensee attributed

the 2005 failure to operation of the diode at higher than optimum operating temperatures.

The licensees corrective actions included: 1) modifying the EDG voltage regulator to address

design weaknesses for all four EDGs based on detailed simulation of the voltage regulator to

prevent diode damage, 2) implementing a diode and SCR replacement preventive maintenance

(PM) activity at a frequency of no greater than 18 months, and 3) issuing a revision to the Operating

Experience Program that addressed previously completed actions for operating experience insight, future engineering changes, and reviewing historical performance trends to ensure PM strategy

changes are identified.

On August 22, 2017, the NRC issued a preliminary White finding (Agencywide Documents Access

and Management System (ADAMS) Accession No. ML17234A678) and an associated apparent

violation related to this event. The NRC issued the final white finding to Catawba on October 16,

2017 (ADAMS Accession No. ML17289A300). Additional details of this operating event are

provided in an NRC inspection report dated May 23, 2018 (ADAMS Accession No. ML18143B537).

Wolf Creek Generating Station, Unit 1

On October 6, 2014, during a scheduled 24-hour surveillance testing, Wolf Creek Generating

Stations B EDG tripped, and a fire was observed in an associated excitation control cabinet. The

fire was quickly suppressed, and, following post-maintenance surveillance, the EDG was restored

to operable status on October 9, 2014. The failure of a diode in the power rectifier of the EDG

excitation system caused the failure of the power potential transformer (PPT). During a post- maintenance surveillance on June 11, 2014, the licensee noted a vapor coming from the cabinet

housing the PPT. The PPT exhibited the same symptoms during subsequent surveillances. The

PPT was scheduled for replacement by the licensee in August 2014 during the next system outage.

During the work management planning process, the PPT replacement was rescheduled for

February 2015. The PPT failed on October 6, 2014.

The Wolf Creek Generating Station excitation system consists of a Westinghouse WNR voltage

regulator design. In the root cause analysis, the licensee identified the most probable direct cause

as thermal degradation of the Power Rectifier diodes. Due to the reduced contribution of field

current and voltage from the PPT circuitry from a single diode failure, the voltage regulator would

task the PPT to supply the remainder of the required current to the field. This increased current

would increase the internal temperatures of the PPT, leading to degraded windings within the PPT.

This condition resulted in the emission of vapor from the PPT, which was noticed by the licensee

prior to the event. The second diode eventually shorted, causing a short in the generator field. This

short caused a loss of excitation to the generator field and tripped the EDG. The root cause

analysis also stated that the station did not recognize the significance of aging or life cycle factors

associated with the EDG excitation system, resulting in an inadequate PM strategy of the excitation

system. There were limited PM activities, limited knowledge of the exciter, and the design of the

system lacked overcurrent protection/detection of the PPT.

The licensees corrective actions included replacing the PPT and selecting the alternate rectifier

bank to restore the availability of the B EDG. In addition to immediate actions taken, the licensee

replaced all power diodes within all four rectifier bridges (two rectifier bridges for each EDG). On

October 27, 2015, the licensee implemented a corrective action to generate new preventive maintenance activities to periodically replace the diodes within the power rectifier and other

excitation system components as recommended by the operating experience.

Additional details of this operating event are provided in the NRC inspection report dated

August 19, 2016 (ADAMS Accession No. ML16235A132).

Grand Gulf Nuclear Station, Unit 1

On May 21, 2013, during the Division 2 standby diesel generator monthly surveillance test, the

underfrequency alarm sounded, concurrent with a drop in indicated voltage from approximately

4220 Volts to 2100 Volts.

The Grand Gulf EDG excitation system consists of a Portec voltage regulator design. The licensee

performed a failure modes analysis evaluation to determine the possible cause for the observed

conditions. During troubleshooting efforts, the licensee addressed the potential transformer (PT1),

the potential transformers fuses, inline fuses, and the voltage regulator circuit bridge diodes. The

failure modes analysis evaluation showed that all the listed components were in satisfactory

condition, with the exception that one of the six diodes used in the voltage regulator circuit diode

bridge (diode CR6) had shorted.

The licensee replaced the shorted diode and returned the diesel generator to operational status on

May 24, 2013. Based on past similar issues with degraded diodes on the voltage regulator circuit, the licensee developed a preventive maintenance strategy to begin testing the diodes on all three

divisions of diesel generators and to replace any suspect components before they caused a loss of

excitation event.

Additional details of this operating event are provided in the NRC inspection report dated

August 14, 2014 (ADAMS Accession No. ML14226A998).

DISCUSSION

The failure of EDG excitation system diodes can adversely impact the ability of EDGs to perform

their safety functions. A review of operating experience indicates that events involving the failure of

EDG excitation system diodes continue to occur. Such failures may cause affected EDGs not to be

able to operate for their full mission times following a loss of offsite power event.

Underlying causes have been attributed to marginal design, postulated age-related failure, manufacturing defects coupled with voltage overstress, operation at elevated temperatures, and

inadequate preventive maintenance. Because these diodes appear to have a limited life, licensees

should consider establishing an appropriate replacement schedule as part of effective preventative

maintenance in accordance with the Maintenance Rule.

Related NRC Generic Communications

Information Notice 2010-04, Diesel Generator Voltage Regulation System Component Due to

Latent Manufacturing Defect, dated February 26, 2010, describes the failure of a linear power

reactor in an EDG voltage regulation system at a plant where the licensees preventive

maintenance program did not address the EDG excitation system magnetic components that can

be subject to deterioration with age or time in service. IN 2010-04 describes the corrective actions implemented by the licensee in response to the event, which included adding a preventive

maintenance task for thermography of EDG excitation system silicon controlled bridge rectifiers, power diode bridge rectifiers, current transformers, power transformers, and linear reactors.

Information Notice 2007-36, Emergency Diesel Generator Voltage Regulator Problems, dated

November 15, 2007, describes several events involving EDG voltage regulator problems, including

an event that involved anomalies with the gate firing circuit output pulses to the silicon controlled

rectifiers on the rectifier assembly. The licensees root cause evaluation for this event discussed

that the EDG had a history of intermittent overvoltage problems because of a higher than expected

impedance in the flyback diode portion of the rectifier chassis circuitry. IN 2007-36 discusses

various types of problems that are not limited to a single component or model of voltage regulator, and states that, In general, the performance of a voltage regulator is very sensitive to any minor

defects in any component of the voltage regulation system.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contact listed below.

/RA/ /RA/

Christopher Miller Timothy J. McGinty, Director

Division of Inspection and Regional Support Division of Construction Inspection

Office of Nuclear Reactor Regulation and Operational Programs

Office of New Reactors

Technical Contacts: Jason B. Carneal, NRR/DIRS

301-415-1451 E-mail: Jason.Carneal@nrc.gov

Vijay K. Goel, NRR/DE

301-415-3730

E-mail: Vijay.Goel@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library, Document Collections.

ML18250A178 *concurred via email

OFFICE TECH EDITOR NRR/DIRS/IOEB NRR/DIRS/IOEB/BC NRO/DCIP/QVIB//BC NRO/DSRA/SCPB/BC

NAME JDougherty* JCarneal* RElliott* JJacobson* HWagage*

DATE 9/24/18 2/6/19 2/2/19 2/5/19 2/15/19 OFFICE NRR/DE/EEOB NRR/DE/EEOB/BC NRR/DIRS/IRGB/LA NRR/DIRS/IRGB/PM NRR/DIRS/IRGB/BC

NAME VGoel* KNguyen* BCurran* MLintz* TInverso*

DATE 2/27/19 1/31/19 1/31/19 2/28/19 2/28/19 OFFICE NRO/DCIP/D NRR/DIRS/D

NAME TMcGinty* CMiller

DATE 3/12/19 06/03/19


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