Information Notice 2017-06, Battery and Battery Charger Short Circuit Current Contributions to a Fault on the DC Distribution System

Battery and Battery Charger Short Circuit Current Contributions to a Fault on the DC Distribution System

ML17228A473
Person / Time
Issue date:	09/26/2017
From:	Louise Lund, Mcginty T Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking
To:
Mensah, T.M., NRR/DPR 415-3610
References
IN-17-006
Download: ML17228A473 (4)

ML17228A473 UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NEW REACTORS

WASHINGTON, DC 20555-0001

September 26, 2017

NRC INFORMATION NOTICE 2017-06: BATTERY AND BATTERY CHARGER

SHORT-CIRCUIT CURRENT CONTRIBUTIONS

TO A FAULT ON THE DIRECT CURRENT

DISTRIBUTION SYSTEM

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 who have permanently ceased operations

and have certified that fuel has been permanently removed from the reactor vessel.

All holders of and applicants for a power reactor early site permit, combined license, standard

design approval, or manufacturing license under 10 CFR Part 52, Licenses, Certifications, and

Approvals for Nuclear Power Plants. All applicants for a standard design certification, including

such applicants after initial issuance of a design certification rule.

PURPOSE

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

addressees of the results of a recent NRC-led battery testing program. The testing program

evaluated the magnitude of direct current (DC) fault current contributions from batteries and

battery chargers to a downstream short-circuit fault on the DC distribution system. The detailed

test results, conclusions, and recommendations are provided in NUREG/CR-7229, Testing to

Evaluate Battery and Battery Charger Short-Circuit Current Contributions to a Fault on the DC

Distribution System (Agencywide Documents Access and Management System (ADAMS)

Accession No. ML17039A869). The NRC expects that recipients of this IN will review the

information for applicability to their facilities and consider actions, as appropriate, for potential

impacts on DC fault studies and other related calculations. The suggestions in this IN are not

NRC requirements; therefore, the NRC requires no specific action or written response.

DESCRIPTION OF CIRCUMSTANCES

On September 6, 2013, the NRC issued IN 2013-17, Significant Plant Transient Induced by

Safety-Related Direct Current Bus Maintenance at Power (ADAMS Accession Number

ML13193A009). It informed addressees of an event involving the loss of one train of the DC

distribution system in a nuclear power plant. Specifically, both the battery and the battery

charger on one DC Class 1E power division tripped on overcurrent when a fault occurred in a

downstream DC panel.

The event demonstrated that the fault impact on the DC distribution system at a nuclear power

plant can have a significant impact, as described in IN 2013-17. While the cause of the battery trip was well understood by the NRC staff, the cause of the battery charger trip was not clear.

The NRC staff assumed that the cause of the battery charger trip could have been because of

the initial higher fault current contribution by the battery charger to the downstream fault, whether connected in parallel with the battery or not. However, this configuration was

inconsistent with the language provided in the Institute of Electrical and Electronics Engineers

(IEEE) Standard (Std) 946-2004, IEEE Recommended Practice for the Design of DC Auxiliary

Power Systems for Generating Stations, Subclause 7.9.2, which states, When the battery

charger is connected in parallel with the battery, the battery capacitance will prevent the battery

charger contribution from rising instantaneously. Therefore, the maximum current that a

charger will deliver on short circuit will not typically exceed 150 [percent] of the charger full load

ampere rating. Instantaneous battery charger current rise should only become a concern during

periods when the battery is disconnected. Therefore, the Office of Nuclear Regulatory

Research (RES) collaborated from 2014 through 2016 in a battery testing program with

Brookhaven National Laboratory (BNL) to validate the assumptions. The purpose of the testing

program was to determine if the battery and battery charger current contributions to the fault on

the DC distribution circuit would be different when connected individually or when connected in

parallel, which could impact the DC system device coordination.

In February 2017, the testing results were documented and published in NUREG/CR-7229.

One of the methods potentially used at nuclear power plants to estimate the short-circuit current

contributions is described in IEEE Std 946-2004. The use of this standard neglects the initial

fault current contribution from the charger. The DC system overcurrent protective device sizing

selection and/or coordination setting could result in a fault not being isolated as intended. This

can lead to undesirable system responses to a fault on the DC distribution system.

BACKGROUND

The DC power system provides power for Class 1E equipment such as breaker control, plant

instrumentation and control, monitoring, lighting (main control room and remote shutdown area),

and other functions. The battery supplies the load without interruption should the battery

charger or associated preferred alternating current source fail.

Criterion 21, Protection System Reliability and Testability, of Appendix A, General Design

Criteria for Nuclear Power Plants, to 10 CFR 50, Domestic Licensing of Production and

Utilization Facilities, states, The protection system shall be designed for high functional

reliability and inservice testability commensurate with the safety functions to be performed.

Proper fault current calculations and protective device setttings on the DC system are important

so that a fault can be isolated as close to the location of the fault as possible, thereby

minimizing the impact on plant operations and safety.

DISCUSSION

Reliability of the Class 1E DC power system is important in a nuclear power plant. The DC

power system is designed so that no single failure of an electrical panel, battery, or battery

charger will result in a condition that will prevent the safe shutdown of the plant.

During the battery testing program, BNL performed various short-circuit tests that simulated fault

conditions on a DC distribution system typical within a nuclear power plant. More specifically, two types of battery chargers were considered, a silicon controlled rectifier (SCR)-type and a

controlled ferroresonant (CF) transformer-type, connected individually and in parallel with three

Class 1E vented lead-acid batteries from different vendors. The three nuclear-qualified batteries from three different vendors are representative of battery models used in more than 75 percent of the nuclear power plants currently in the United States. The SCR and CF battery

chargers represent about 90 percent of the battery charger designs used in nuclear power

plants currently in the United States.

The testing validated that the initial fault current contribution to a downstream fault from a

battery charger (specifically the SCR-type chargers vs. the CF-type) is much higherin the

range of 7 to 10 times the charger full load ampere ratingduring the first 100 milliseconds than

what is currently stated as 150 percent in IEEE Std 946-2004. The test results indicated that

the initial short circuit contribution from the charger is not limited when connected in parallel with

the battery. The SCR-type charger contributed more to the fault current due to the longer

response time of its current limiting circuit than the CF-type. The initial higher short circuit

current contribution from the battery charger could impact the coordination of protective device

settings on the battery charger and downstream devices. If IEEE Std 946-2004 was utilized to

estimate short circuit current contributions in DC distribution systems, licensees should consider

performing a comprehensive review of the entire DC system protection coordination and

assumptions of battery and charger short circuit currents that were used to select their

protection fault interruption ratings and setpoints. Specifically, licensees are encouraged to

review their fault current calculations, make any necessary revision to size, and coordinate the

protective device settings based on the new information documented in NUREG/CR-7229.

Additionally, there are efforts currently underway by the IEEE 946 Working Group to consider

appropriate revisions to the standard. The NRC staff that are involved in IEEE Standard 946 have communicated to the working group the test results, conclusions, and recommendations

provided in NUREG/CR 7229.

CONTACT

S

Please direct any questions about this matter to the technical contact(s) listed below or the

appropriate RES or Office of Nuclear Reactor Regulation (NRR) project manager.

/ra/ (Gregory T. Bowman for)

/ra/ (Paul G. Krohn for)

Louise Lund, Director

Timothy J. McGinty, Director

Division of Policy and Rulemaking

Division of Construction Inspection

Office of Nuclear Reactor Regulation

and Operational Programs

Office of New Reactors

Technical Contact:

Liliana Ramadan, RES/DE

301-415-2463

E-mail: Liliana.Ramadan@nrc.gov

Vijay 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, https://www.nrc.gov, under NRC Library, Document Collections.

ML17228A473; *concurred via email TAC No. MG0062 OFFICE

TECH EDITOR*

RES/DE/ICEEB/TL*

RES/DE/ICEEB/TL*

RES/DE/ICEEB/BC*

NRR/DE/EEOB/BC*

NAME

JDougherty

LRamadan

KMiller

TKoshy

JQuichocho

(w/comments)

DATE

9/07/17

9/07/17

9/07/17

9/8/17

9/10/17 OFFICE

NRR/DE/EENB/BC

(Acting)*

NRO/DCIP/QVIB1/BC*

NRR/DE/D*

RES/DE/D*

NRR/DPR/PGCB/PM

NAME

TMartinez-Navedo

(w/comments)

TJackson

JLubinski

(w/comment)

BThomas

TMensah

DATE

9/11/17

9/07/17

9/18/17

9/15/17

9/19/17 OFFICE

NRR/DPR/PGCB/LA*

NRR/DPR/PGCB/BC

(Acting)*

NRO/DCIP/D

NRR/DPR/D

NAME

ELee

AGarmoe

TMcGinty (PKrohn

for)

LLund (GBowman

for)

DATE

9/20/17

9/20/17

9/21/17

9/26/17