Information Notice 2017-06, 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) | |
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
ML17228A473 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*
JQuichocho
NAME JDougherty LRamadan KMiller TKoshy
(w/comments)
DATE 9/07/17 9/07/17 9/07/17 9/8/17 9/10/17 NRR/DE/EENB/BC
OFFICE NRO/DCIP/QVIB1/BC* NRR/DE/D* RES/DE/D* NRR/DPR/PGCB/PM
(Acting)*
TMartinez-Navedo JLubinski
NAME TJackson BThomas TMensah
(w/comments) (w/comment)
DATE 9/11/17 9/07/17 9/18/17 9/15/17 9/19/17 NRR/DPR/PGCB/BC
OFFICE NRR/DPR/PGCB/LA* NRO/DCIP/D NRR/DPR/D
(Acting)*
TMcGinty (PKrohn LLund (GBowman
NAME ELee AGarmoe
for) for)
DATE 9/20/17 9/20/17 9/21/17 9/26/17