NRC FORM 366 US. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 03/31/2020(04-20 1 8)

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Regulatory Affairs, NEOB-10202, 13150-01041, Office of Management and Budget,:.; . .‘ ‘

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1. Facility Name 12 Docket Number f3. Page

Callaway Plant Unit I 05000 483 I OF

4. Title

Unplanned Loss of Switchyard Bus B Results in System Actuation

8. Other Facilities Involved

I Facility Name - Docket Number05000

Docket Number

05000I Facility Name

I I . This Report is Submitted Pursuant to the Requirements of I 0 CFR §: (Check all that apply)

LI 50.73(a)(2)(ii)(A) J 50.73(a)(2)(viii)(A)

D 50.73(a)(2)(ii)(B) 50.73(a)(2)(viii)(B)

D 50.73(a)(2)(iii) D 50.73(a)(2)(ix)(A)

50.73(a)(2)(iv)(A) 50.73(a)(2)(x)

D 50.73(a)(2)(v)(A) LI 73.71(a)(4)

El 20.2203(a)(2)(iii) 50.36(c)(2) 50.73(a)(2)(v)(B) 73.71(a)(5)

LI 20.2203(a)(2)(iv) 50.46(a)(3)(ii) 50.73(a)(2)(v)(C) J 73.77(a)(1)

000 D 20.2203(a)(2)(v) 50.73(a)(2)(i)(A) 50.73(a)(2)(v)(D) 73.77(a)(2)(i)

LI 20.2203(a)(2)(vi) 50.73(a)(2)(i)(B) 50.73(a)(2)(vii) J 73.77(a)(2)(ii)

El 50.73(a)(2)(i)(C) Other (Specify in Abstract below or in NRC Form 366A)

12. Licensee Contact for this LER

Licensee Contact Telephone Number (Include Area Code)TB. Elwood, Supervising Engineer, RegulatoryAffairs and Licensing 314-225-190513. Complete One Line for each Component Failure Described in this Report

Cause System Component Manufacturer Reportable to ICES Cause System Component Manufacturer Reportable to ICES

14. Supplemental Report Expected Month Day Year15. Expected Submission Date

Yes (Ifyes, complete 15. Expected Submission Date) No 08 30 2019Abstract (Limit to 1400 spaces, i.e., approximately 14 single-spaced typewritten lines)

On April 17, 2019 at approximately 0137 CDT, a valid actuation ofthe “A” emergency diesel generator (EDG) atthe Callawayplant occurred due to inadvertent tripping of the “B” switchyard bus. The plant was in No Mode (defueled) when theswitchyard 345-kV main generator output breaker (MDV53) was closed to backfeed the unit auxiliary transformer. MDV53immediately reopened due to a ground located on a set of current transformers that set up an unwanted path for current toflow between the protection circuits for switchyard breakers MDV53 and MDV55. De-energization of the “B” switchyard busresulted in de-energization oftwo transformers in the switchyard circuit connection to the 4.16-kV NBO1 safety bus. The lossof voltage on the NBOI bus triggered the “A” EDG actuation.

The investigation of the event is ongoing and additional information will be provided in a supplement to this LER. Correctiveaction to prevent recurrence will be generated as part of the root cause analysis process.

The automatic actuation of the emergency diesel generator was a valid actuation and is reportable in accordance with I 0 CFR50.73(a)(2)(iv)(A). There was no impact to the health and safety of the public as a result of this event. Systems responded asexpected (i.e. , per design) in response to de-energization of the “B” switchyard bus.

NRC FORM 366 (04-2018)

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 03/31/2020(04-2018)

L’ Estimated burden per response to comply with this mandatory collection request: 80 hours. ReportedI : LICENSEE EVENT REPORT (LER) lessons learned are incorporated into the licensing process and fed back to industry. Send commentsL)i’ regarding burden estimate to the Information Services Branch (T 2 F43) U S Nuclear RegulatoryCONTINUATION SHEET Commission Washington DC 20555 0001 or by e mail to tnfocollects Resourcenrcgov and tothe Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of

. . . . Management and Budget, Washington, DC 20503. If a means used to impose an information(See NUREG-1O22, R.3 for InstruCtion and guidance for completing this form collection does not display a currently valid 0MB control number, the NRC may not conduct orsponsor, and a person is not required to respond to, the information collection.

1. FACILITY NAME 2. DOCKET NUMBER 3 LER NUMBERYEAR SEQUENTIAL REV

05000- NUMBER NO.Callaway Plant Unit I 483- 001 - 00

NARRATIVE

I . DESCRIPTION OF STRUCTURE(S), SYSTEM(S), AND COMPONENT(S):

Switchyard System Description:As depicted in the figure provided on page 5 of this LER, the 345-kV Callaway switchyard consists of circuit breakers,disconnect switches, buses, transformers, and associated equipment. The offsite circuit connections to the onsiteswitchyard for Callaway are arranged such that there are four transmission lines incoming and connecting to theswitchyard via two physically separated rights of way (i.e. , each with two 345-kV circuits). Versatility for connecting theseoffsite circuits to the onsite plant power system is provided via a dual bus arrangement within the switchyard (i.e. 345-kVBus A and 345-kV Bus B) for which a breaker-and-a-half arrangement is utilized. A 345/13.8-ky safeguard transformer isconnected directly to each 345-kV bus through a disconnect switch which is capable of interrupting magnetizing current.Each transformer has two low-side breakers connected so that either transformer may supply (via underground duct) a13.814.16-kV engineered safety feature (ESF) transformer atthe plant. The safeguard transformers are sized so thateither transformer “A” or “B” has the capacity to handle the design shutdown or the design basis loss of coolant accidentload.

For the two offsite circuit connections required to satisfy the AC source requirements of Technical Specification (IS) 3.8.1,“AC Sources - Operating,” during plant operation (i.e., Modes I , 2, 3, and 4), two electrically and physically separatedcircuits provide AC power from the switchyard to the two 4.16-kV Class I E safety buses (NBOI and NBO2) via the ESFtransformers, XNBOI and XNBO2. Specifically, one required offsite circuit consists of either safeguard transformer A or B,which is supplied from switchyard Bus A or B and feeds through a breaker to ESF transformer XNBOI , which in turnpowers the NBOI ESF bus through its normal feeder breaker. The other required offsite circuit consists of the startuptransformerwhich is normally fed from the switchyard through breaker PA0201 and feeds power (from one ofthe startuptransformer’s two secondary windings) to ESF transformer XNBO2, which in turn powers the NBO2 ESF bus through itsnormal feeder breaker.

For shutdown conditions (i.e. , Modes 5 and 6 and during movement of irradiated fuel assemblies), either one of theabove-noted circuit connections is required to supply one ESF bus per the requirements of TS 3.8.2, “AC Sources -

Shutdown.”

The switchyard and circuit connection design for Callaway also provides for the supply of power to non-safety loads duringplant operation and shutdown conditions. During refueling outages, however, a backfeed alignment is employed toprovide powerto the non-safety 13.8-kV PAOI/PAO2 bus(es) via the unit auxiliary transformer. The unit auxiliarytransformer is backfed from the switchyard via the main power step-up transformers (used normally to couple the maingenerator output to the switchyard during plant operation).

2. INITIAL PLANT CONDITIONS:

At the time of the event, Refueling Outage 23 was ongoing and the plant was in No Mode with no movement of irradiatedfuel in the Fuel Building. Both trains of ESW and CCWwere in service and the “A” spent fuel pool cooling pump wasrunning. The “B” spent fuel pool cooling pump was available but not running. The protected train was the “A” train, andthe “B” emergency diesel generator was out of service. Both NBO1 and NBO2 buses were energized from their normal off-site power sources. That is, the NBO1 bus was being supplied from the “B” safeguard transformer (offthe “B” switchyardbus) via ESF transformer XNBOI , and the NBO2 bus was being supplied from the startup transformer via ESF transformerXNBO2.

NRC FORM 366A (04-2018)Page 2 of 5

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NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 03131/2020(04-2018)

Estimated burden per response to comply with this mandatory collection request: 80 hours. Reported,- .‘ LICENSEE EVENT REPORT (LER) lessons learned are incorporated into the licensing process and fed back to industry. Send comments1.’j regardmg burden estimate to the Information Services Branch (T 2 F43) U S Nuclear RegulatoryCONTINUATION SHEET Commission Washington DC 20555 0001 or by e mail to Infocollects Resource@nrc gov and tow,? the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of. . . . . Management and Budget, Washington, DC 20503. If a means used to impose an information(See NUREG-1022, R.3 for instruction and guidance for completing this form collection does not display a currently valid 0MB control number, the NRC may not conduct orh.ttQ;IIWYL_wnrcgovLrothng:rm/docrcoi[octionsLovr.ogsJstoffLs1O..22J3L) sponsor, and a person is not required to respond to, the information collection.

1. FACILITY NAME 2. DOCKET NUMBER 3 LER NUMBER.YEAR SEQUENTIAL REV

05000- NUMBER NO.Callaway Plant Unit I 483.- 001 - 00

NARRATIVE

6. CAUSE OF THE EVENT:

A ground wire was inadvertently added on circuit breaker MDV55 as part of a design change under modification package1 6-0021 that was implemented in the fall of 201 7 (during Refueling Outage 22) in which several switchyard breakers werereplaced with new and differenttype manufactured by Mitsubishi. When MDV53 was closed for the backfeed on April 17,201 9, this additional ground created a path for current to flow through the protective relays associated with MDV55, andthis caused the protection circuitry for MDV55 to detect an unexpected current surge due to the main transformer windingsdrawing current in order to charge the transformers’ field windings. The protective relay actuated on this inrush andinitiated a “B” Bus fault clearing of the other breakers. Specifically, the activation triggered circuit breakers MDV45,MDV75, 52-3, and 52-4 to open and clear the bus of load. The opening of 52-3 isolated the “B” safeguard transformerfrom the “B” switchyard bus, which in turn de-energized ESF transformer XNBOI which was powering the NBOI bus andcaused an actuation of Engineered Safety Features equipment.

7. CORRECTIVE ACTIONS:

The added ground wire that was part of the newly installed Mitsubishi breakers was removed from MDV55. In addition,drawings of the switchyard were reviewed for duplicate current transformer (CT) grounds of recently modified switchyardbreakers, and that review confirmed no similar situations exist that would lead to a similar plant event.

Investigation into the event is ongoing and additional information will be provided in a supplement to this LER. Correctiveaction to prevent recurrence will be generated as part of the root cause analysis process.

8. PREVIOUS SIMILAR EVENTS:

A review of Callaway LERs for the past three years did not identify any other examples of system actuations caused bythe same or similar reasons for the event. Upon completion of the root cause team findings, another search will becompleted using the new information to determine if previous similar events have occurred.

9. OTHER INFORMATION:

A figure of the Callaway switchyard is on the next page.

Page 4 of 5NRC FORM 366A 104-2018)

‘1 *

345KVSWITCHYARD I

—-.——l’-————--9’.J

NRC FORM 366A US. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 03/31/2020(04-2018)

%‘ Estimated burden per response to comply with this mandatory collection request: 80 hours. Reported/ LICENSEE EVENT REPORT (LER) lessons learned are incorporated into the licensing process and fed back to industry. Send comments

regarding burden estimate to the Information Services Branch (T 2 F43) U S Nuclear Regulatory‘F4i CONTIN UATION SHEET Commission, Washington, DC 20555-0001, or by e-mail to lnfocollects.Resourcenrc.gov, and to,3

1:4 the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of. . . . . Management and Budget, Washington, DC 20503. If a means used to impose an information(See NUREG-1022, R.3 for instruction and guidance for completing this form collection does not display a currently valid 0MB control number, the NRC may not conduct or

sponsor, and a person is not required to respond to, the information collection.

1. FACILITY NAME 2. DOCKET NUMBER • LER NUMBER__.

.

.

YEAR SEQUENTIAL REV05000- NUMBER NO.

Callaway Plant Unit I 483.

- 001 - 00

NARRATIVE

TO BLAND TO LOOSE TO MONTGOMERYSUB.

XFMR-A4:

U.G. CABLES SITE FEEDERSEE FIGURE 8.3-I

GEN-lREV. 151 0/07

UNION ELECTRIC COMPANYCALLAWAY PLANT

FINAL SAFETY ANALYSIS REPORT

FIGURE 8.2-5345KV ONE LINE AND

GENERAL ARRANGEMENTCALLAWAY PLANT

NRC FORM 366A 104-20181Page 5 of 5