05000483/LER-2019-001-01, Unplanned Loss of Switchyard Bus B Results in System Actuation

Unplanned Loss of Switchyard Bus B Results in System Actuation

ML19312B669
Person / Time
Site:	Callaway
Issue date:	11/07/2019
From:	Bianco F Ameren Missouri, Union Electric Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
ULNRC-06538 LER 2019-001-01
Download: ML19312B669 (8)

LER-2019-001, Unplanned Loss of Switchyard Bus B Results in System Actuation

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(5) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(iv)(B), System Actuation
4832019001R01 - NRC Website

text

Ameren MISSOURI November 7, 2019 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-000 1 Ladies and Gentlemen:

ULNRC-06538 10 CFR 50.73(a)(2)(iv)

Callaway Plant DOCKET NUMBER 50-483 CALLAWAY PLANT UNIT 1 UNION ELECTRIC CO.

RENEWED FACILITY OPERATING LICENSE NPF-30 LICENSEE EVENT REPORT 2019-001-01 EMERGENCY AC ELECTRICAL POWER ACTUATION On June 6, 2019, Callaway Plant submitted Licensee Event Report (LER) 2019-001-00 in accordance with 10 CFR 50.73(a)(2)(iv)(A) to report the automatic actuation of emergency ac electrical power.

The enclosed LER supplement, LER 2019-001-01, is submitted to update the causes and corrective actions for the same condition.

This letter does not contain new commitments.

Enclosure Sincerely, sc Fred Bianco Senior Director, Nuclear Operations 8315 County Road 459 Steedman, MO 65077 AmerenMissouricom

ULNRC-06538 November 7, 2019 Page 2 of 3 cc:

Mr. Scott A. Morris Regional Administrator U. S. Nuclear Regulatory Commission Region IV 1600 East Lamar Boulevard Arlington, TX 76011-4511 Senior Resident Inspector Callaway Resident Office U.S. Nuclear Regulatory Commission

$201 NRC Road Steedman, MO 65077 Mr. L. John Klos Project Manager, Callaway Plant Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Mail Stop 09E3 Washington, DC 20555-0001

ULNRC-06538 November 7, 2019 Page 3 of 3 Index and send hardcopy to QA file A160.0761 Hardcopy:

Certrec Corporation 6100 Western Place, Suite 1050 Fort Worth, TX 76107 (Certrec receives ALL attachments as long as they are non-safeguards and may be publicly disclosed.)

Electronic distribution for the following can be made via LER ULNRC Distribution:

F. M. Diya B. L. Cox F. J. Bianco T. E. Herrmann S. P. Banker R. C. Wink T. B. Elwood Corporate Oversight NSRB Secretary Performance Improvement Coordinator Resident Inspectors (NRC)

STARS Regulatory Affairs Mr. Jay Silberg (Pillsbury Winthrop Shaw Pittman LIP)

Missouri Public Service Commission

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 03/37/2020 (04-2018)

, the NRC may not conduct or sponsor, and a person is not equired to respond to, the information collection.

1. Facility Name

. Docket Number

3. Page CallawayPlantUniti 05000483 i 0F5

4. Title Unplanned Loss of Swchyard Bus B Results in System Actuation

5. Event Date

6. LER Number 7.Report Date

8. Other Facilities Involved Month Day Year YEAR Squeitial Rev Month Day Year Facility Name Docicet Number 4

1 7 201 9

- 001

- 01 11 7

201 9 Facility Name Docket Number 9.OpeiathigMode

11. Thi.e Report is Submitted Pursuanttothe Requirementsof JO CFR §: (Check all that appfr)

C 20.2201 (b)

D 20.2203(a)(3)(i)

Q 50.73(a)(2)(ii)(A)

Q 50.73(a)(2)(viii)(A)

N 20.2201(d)

D 20.2203(a)(3)(ii)

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

Q 50.73(a)(2)(viii)(5)

D 20.2203(a)(1)

Q 20.2203(a)(4)

Q 5073(a)(2)(iii)

Q 50.73(a)(2)(ix)(A) i: 20.2203(a)(2)(i)

D 50.36(c)(1 )(i)(A) 0 50.73(a)(2)(iv)(A)

Q 5073(a)(2)(x)

10. PowerLevel Q 20.2203(a)(2)(ii)

Q 50.36(c)(1)(ii)(A)

Q 50.73(a)(2)(v)(A)

Q 73.71(a)(4)

D 202203(a)(2)(iii)

D 50.36(c)(2)

Q 50.73(a)(2)(v)(B)

Q 73.71(a)(5)

D 20.2203(a)(2)(iv)

C 50.46(a)(3)(ii)

Q 50.73(a)(2)(v)(C)

Q 73.77(a)(1) 000 C 20.2203(a)(2)(v)

D 50.73(a)(2)(i)(A)

Q 50.73(a)(2)(v)(D)

Q 73.77(a)(2)(i)

D 20.2203(a)(2)(vi)

Q 50.73(a)(2)(i)(B)

Q 50.73(a)(2)(vii)

Q 73.77(a)(2)(ii)

I: 50.73(a)(2)(i)(C)

Q OTHER Specify in Abstract below or in NRC Form 366A

12. Licensee Contact for this LER Licensee Contact frelephone Number (Include Area Code)

TB. Elwood, SupeMsing Engineer, Regulatory Affairs and Licensing 314-225-1 905 BSTRACT (Limitto 1400 spaces, i.e., approamately 74 single-spaced typewritten lines)

Dn April 1 7, 201 9 at approximately 01 37 CDI, a Valid actuation of the A emergency diesel generator (EDG) at the Callaway plant occurred due to inadvertent tripping of the B switchyard bus. The plant was in No Mode (defueled) when he switchyard 345-kV main generator output breaker (MDV53) was closed to backfeed the unit auxiliary transformer.

MDV53 immediately reopened due to a ground located on a set of current transformers that set up an unwanted path for urrent to flow between the protection circuits for switchyard breakers MDV53 and MDV55. De-energization of the B switchyard bus resulted in de-energization of two transformers in the switchyard circuit connection to the 4.16-kV NBOY safety bus. The loss of voltage on the NBO1 bus triggered the A EDG actuation.

the following root causes were identified:

1 Ineffective management by the Transmission Substations department of a design scope change introduced a design error, i.e., an extraneous jumper that enabled the unwanted current path to the breaker protection circuits.

2.

Engineering supervision assumed the Transmissions Substations design process included an independent review, and thus, an independent review was not requested.

Corrective actions to prevent recurrence include:

1 Revising the Engineering Design Quality Review Process procedure to require an independent review of Callaway specific schematic and wiring diagrams.

2.

Establishing procedural requirements for reviewing the design review documentation from Transmission Substations.

the automatic actuation of the emergency diesel generator was a valid actuation and is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A). There was no impact to the health and safety of the public as a result of this event. Systems responded as expected (i.e., per design) in response to de-energization of the B switchyard bus.

i: Yes ( yes, comete 1 5. EXPECTED SUBMISSON DATE)

No

15. Expected Submission Date NRC FORM 366 (04-2018)

1.

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

Switchyard System Description:

I As depicted in the figure provided on page 5 of this LER, the 345-kV Callaway switchyard [EIIS System: FK] consists of circuit breakers, disconnect switches, buses, transformers, and associated equipment. The offsite circuit connections to the onsite switchyard for Callaway are arranged such that there are four transmission lines incoming and connecting to the switchyard via two physically separated rights of way (i.e., each with two 345-kV circuits). Versatility for connecting these offsite circuits to the onsite plant power system is provided via a dual bus arrangement within the switchyard (i.e.,

345-kV Bus A and 345-kV Bus B [EIIS Component: BUJ) for which a breaker-and-a-half arrangement is utilized. A 345/1 3.8-kV safeguard transformer [EIIS Component: XFMR] is connected 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) a 13.8/4.1 6-kV engineered safety feature (ESF) transformer at the plant. The safeguard transformers are sized so that either transformer A or B has the capacity to handle the design shutdown or the design basis loss of coolant accident load.

For the two offsite circuit connections required to satisfy the AC source requirements of Technical Specification (TS) 3.8.1, AC Sources

- Operating, during plant operation (i.e., Modes 1, 2, 3, and 4), two electrically and physically separated circuits provide AC power from the switchyard to the two 4.1 6-kV Class 1 E safety buses (NBO1 and NBO2) via the ESF transformers, XNBO1 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 XNBO1, which in turn powers the NBOY ESF bus through its normal feeder breaker. The other required offsite circuit consists of the startup transformer which is normally fed from the switchyard through breaker PAO2OY and feeds power (from one of the startup transformers two secondary windings) to ESF transformer XNBO2, which in turn powers the NBO2 ESF bus through its normal feeder breaker.

For shutdown conditions (i.e., Modes 5 and 6 and during movement of irradiated fuel assemblies), either one of the above-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 during plant operation and shutdown conditions. During refueling outages, however, a backfeed alignment is employed tc provide power to the non-safety 13.8-kV PAO1/PAO2 bus(es) via the unit auxiliary transformer. The unit auxiliary transformer is backfed from the switchyard via the main power step-up transformers (used normally to couple the main generator 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 irradiated fuel in the Fuel Building. Both trains of ESW and CCW were in service and the A spent fuel pool cooling pump was running. The B spent fuel pool cooling pump was available but not running. The protected train was the A train, and the B emergency diesel generator was out of service. Both NBOY 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 (off the B switchyard bus) via ESF transformer XNBO1, and the NBO2 bus was being supplied from the startup transformer via ESF transformer XNBO2.Page 2 of 5U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 3/31/2020 (04-2018) *

, the NRC may not conduct or httpi/www.nrc.gov/reading-rm/doc/collections/nuregs/staff/srl 022/r3/)

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

1. FACILITY NAME

2. DOCKETNUMBER

3. LERNLLIBER YEAR SEQUENTIAL REV Callaway Plant Unit 1 05000-483 NUMBER NO.

2019

- 001

- 01 3.

EVENT DESCRIPTION

on April 1 7, 201 9, Operations personnel were attempting to energize the unit auxiliary transformer from the switchyard via a backfeed through the main power transformers, as a planned refueling outage activity, utilizing Callaway procedure OTS-MA-00001, Main Step-Up Transformer Backfeed

- Infrequently Performed Test or Evolution. At 01 37 CDT, step I

5.2.9 of the procedure was performed to close 345-kV main generator output breaker MDV53 [EIIS Component: BKR] in order to energize the unit auxiliary transformer. By local observation, and upon closure of the MDV53 breaker, the main and unit auxiliary transformers started to hum and the breaker immediately reopened. The B switchyard bus cleared, causing a loss of power to the B safeguard transformer and thus to ESF transformer XNBO1. The A train Shutdown I

Sequencer actuated, and the A emergency diesel generator (NEO1) [EIIS Component: DGJ started and auto-connected to the NBO1 bus via the 4.16-kV bus switchgear. Equipment on the NBOY bus was shed as expected, and the A essential service water (ESW) pump, the A component cooling water (CCW) pump, and the A control room air conditioning unit all automatically restarted.

Off-normal procedure OTO-NB-00001, Loss Of Power To NBO1, was entered to respond to the actuation. The A spent fuel pool cooling pump was restarted per procedure at 01 49. During this period, spent fuel pool temperature rose from 1 02 degrees Fahrenheit to 1 03 degrees Fahrenheit. XNBO1 was re-energized from switchyard Bus A, and power to the NBOY bus was transferred back from the A emergency diesel generator to ESF transformer XNBOY.

It should be noted that there are two breakers that feed the main generator step-up transformers in a backfeed lineup, i.e.

MDV53 and MDV55. A set of current transformers (CTs) on MDV53 are wired to a set of current transformers on MDV5I so that current to the main generator step-up transformers can be determined. These main CTs also feed the breaker failure and breaker flashover protection circuitry. For the backfeed that was attempted on April 1 7, 201 9, MDV53 was closed to energize the main generator step-up transformers while MDV55 was still not closed. Due to a ground jumper associated with the MDV55 breaker (explained further in section 6 of this LER), the current in the MDV53 CTs was fed over to a set of CTs on the MDV55 breaker which was still open. This current flow in the MDV55 breaker CTs looked like a breaker failure and flashover since the open breaker was not supposed to be allowing current to flow through the open set of main contacts (breaker failure) or from the main contacts to the breaker outside tank (breaker failure). This protection then cleared another set of 345-kV breakers to isolate MDV55.

4.

ASSESSMENT OF SAFETY CONSEQUENCES

Safety systems responded as required and the plant was in No Mode during this event.

5.

REPORTING REQUIREMENTS

This LER is submitted pursuantto 10 CFR 50.73(a)(2)(iv)(A) to report an automatic actuation of the emergency diesel generator. Specifically, 10 CFR 50.73(a)(2)(iv) states in part that the licensee shall report:

(A) Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph 10 CFR 50.73(a)(2)(iv)(B) of this section (B)The systems to which the requirements of paragraph 10 CFR 50.73(a)(2)(iv)(A)of this section apply are:

(8)

Emergency ac electrical power systems, including: emergency diesel generators (EDG5)...

For the eventthat occurred on April 17, 2019, loss of power to the NBO1 bus resulted in an undervoltage condition and subsequent safety system actuation of the A EDG, including a sequencer actuation.Page 3 of 5U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 3/31/2020 (04-2018)

, the NRC may not conduct or httx//www.nrc.gov/reading-rm/doc/collections/nuregs/staff/sr1022/r3/)

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

1. FACILITY NAME Z DOCKErNUMBER

3. LER NUMBER YEAR SEQUENTIAL REV Callaway Plant Unit 1 05000-483 NUMBER NO.

2019

- 001

- 01 6.

CAUSE OF THE EVENT

A ground wire was inadvertently added on circuit breaker MDV55 as part of a design change under modification package 1 6-0021 that was implemented in the fall of 201 7 (during Refueling Outage 22) in which several switchyard breakers were replaced with a new and different type manufactured by Mitsubishi. When MDV53 was closed for the backleed on April 1 7, 201 9, this additional ground created a path for current to flow through the protective relays associated with MDV55, and this caused the protection circuitry for MDV55 to detect an unexpected current surge due to the main transformer windings drawing current in order to charge the transformers field windings. The protective relay actuated on this inrush and initiated a B Bus fault clearing of the other breakers. Specifically, the activation triggered circuit breakers MDV45, MDVZ5, 52-3, and 52-4 to open and clear the bus of load. The opening of 52-3 isolated the B safeguard transformer from the B switchyard bus, which in turn de-energized ESF transformer XNBOY which was powering the NBO1 bus and caused an actuation of Engineered Safety Features equipment.

The following root causes were identified that led to the automatic actuation of the A emergency diesel generator:

1 Ineffective management, by the Transmission Substations department, of a design scope change introduced a design error, i.e., an extraneous ground jumper, in connection with the breaker replacement modification. This extraneous ground jumper created a parallel path for current to flow through protective relays associated with MDV55, which detected an unexpected current surge when closing MDV53.

2.

Engineering Supervision assumed the Transmission Substations design process included an independent review, and thus, an independent review was not requested.

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 switchyard breakers. That review confirmed no similar situations exist that would lead to a similar plant event.

Corrective actions to prevent recurrence are as follows:

1 The Transmission Substations department head will add another layer of defense in their design package development process by revising the Engineering Design Quality Review Process procedure.

It will require an independent System Protection Engineer to review of all of the Callaway specific schematic and wiring diagrams.

In addition, the design review documentation will be reviewed by Callaway.

2.

The Major Modifications department will revise the Electrical Design Criteria/Inputs document to establish requirements for reviewing the design review documentation from the Transmission Substations department.

3.

The Major Modifications department will revise the Electrical Design Criteria/Inputs document to include a verification by a Design Supervisor of the independent System Protection Engineer review.

In addition, the Design Supervisor will be required to determine if an independent third-level review is needed.

8.

PREVIOUS SIMILAR EVENTS

A review of Callaway LERs for the past five years did not identify any other examples of system actuations caused by the same or similar reasons for the event.

9.

OTHER INFORMATION:

A figure ofthe Callaway switchyard is on the next page.Page 4 of 5U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 3/31/2020 (04-2018) *,

, the NRC may not conduct or http//www.nrc.gov/reading-rm/doc/collections/nureqs/staff/srl 022/r3/)

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

1. FACILITY NAME

2. DOCKETNUMBER a LERNLRIBER YEAR SEQUENTIAL REV Callaway Plant Unit 1 05000-483 NUMBER NO.

2019

- 001

- 01 REV. 15 10107 UNION ELECTRIC COMPANY CALLAWAY PLANT FINAL SAFETY ANALYSIS REPORT FIGURE 8.2-5 345KV ONE LINE AND GENERAL ARRANGEMENT CALLAWAY PLANT I

U.G.

CABLES 345KV SWITCHYARD 1.----------.-.-*J SITE FEEDER SEE FIGURE 8.3-I GEN-I I 13.8 KV NRC FORM 3665 (04-20171 Page 5 of 5