NG-12-0447, NextEra Energy Duane Arnold, LLC - Response to NRC Bulletin 2012-01 Design Vulnerability in Electric Power System

NextEra Energy Duane Arnold, LLC - Response to NRC Bulletin 2012-01 Design Vulnerability in Electric Power System

ML12307A055
Person / Time
Site:	Duane Arnold
Issue date:	10/25/2012
From:	Richard Anderson NextEra Energy Duane Arnold
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NG-12-0447, BL-12-001
Download: ML12307A055 (14)

Text

NEXTera EN ERG7Y~

ARNOLD October 25, 2012 NG-12-0447 10 CFR 50.54(f)

BL 2012-01 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Duane Arnold Energy Center Docket No. 50-331 Renewed Op. License No. DPR-49 NextEra Energy Duane Arnold, LLC Response to NRC Bulletin 2012-01 Design Vulnerability in Electric Power System

Reference:

(1)

U.S. Nuclear Regulatory Commission, "Design Vulnerability in Electric Power System," NRC Bulletin 2012-01, July 27, 2012 (ML12074A115)

Via Reference (1), the Nuclear Regulatory Commission (NRC) issued Bulletin (BL) 2012-01, "Design Vulnerability in Electric Power System." The BL requested NextEra Energy Duane Arnold, LLC (NextEra) provide information regarding the Duane Arnold Energy Center (DAEC) electric power system design. This request was in response to recent operating experience that involved the loss of one of the three phases of the offsite power circuit at Byron Station, Unit 2. Enclosure 1 contains the NextEra response to BL 2012-01. Please contact Ken Kleinheinz, Engineering Director, at (319) 851-7231, if you require further information.

This letter contains no new Regulatory Commitments and no revisions to existing Regulatory Commitments.

I declare under penalty of perjury that the foregoing is true and correct.

Executed on October 25, 2012.

"*49<e Vice President, Duane Arnold Energy Center NextEra Energy Duane Arnold, LLC Enclosure cc:

Administrator, Region Ill, USNRC Project Manager, DAEC, USNRC Resident Inspector, DAEC, USNRC NextEra Energy Duane Arnold, LLC, 3277 DAEC Road, Palo, IA 52324

ENCLOSURE NEXTERA ENERGY DUANE ARNOLD, LLC RESPONSE TO NRC BULLETIN 2012-01 DESIGN VULNERABILITY IN ELECTRIC POWER SYSTEM SYSTEM DESCRIPTION Items 2., 1.d, 2.a, and 2.c request system information and will be addressed in this section.

Bulletin Item 2.

Briefly describe the operating configuration of the ESF buses (Class 1E for current operating plants or non-Class 1E for passive plants) at power (normal operating condition).

NextEra Response See Attachment 1 for a simplified one-line diagram.

During normal operating conditions the ESF buses (1A3 & 1A4) are powered from the 161kV grid in the switchyard via the three winding 161/4.16 kV Start-Up Transformer (1X003 (also referred to as 1X3 in some plant documents)).

Under various outage, emergency or abnormal operating scenarios the buses can also be powered by the Standby Transformer (Grid powered - 34.5/4.16 kV - 1X004) or the Diesel Generators (1G021 & 1G031).

Bulletin Item l.d.

Describe the offsite power transformer (e.g., start-up, reserve, station auxiliary) winding and grounding configurations.

NextEra Response See Attachment 2, Table 4 for offsite power transformer winding and grounding configurations.

Bulletin Item 2.a.

Are the ESF buses powered by offsite power sources? If so, explain what major loads are connected to the buses including their ratings.

NextEra Response For at power (normal operating condition) configurations, ESF buses are powered by offsite sources. Also note that under normal operating conditions, the ESF buses (1A3/1A4) also provide power to some buses (1 B33, 1 B35, 1 B43, 1 B45) which only supply power to non-safety loads.

See Attachment 2, Tables 1 and 2 for ESF bus power sources. See Attachment 2, Table 3 for ESF bus major loads energized during normal power operations, including their ratings.

Bulletin Item 2.c.

Confirm that the operating configuration of the ESF buses is consistent with the current licensing basis. Describe any changes in offsite power source alignment to the ESF buses from the original plant licensing.

NextEra Response The plant was originally designed to meet the requirements of the Institute of Electrical and Electronic Engineers (IEEE) Criteria for Class 1 E Electrical Systems for Nuclear Power Generating Systems (Standard 308-1971). Each essential bus is capable of receiving power from reliable offsite power sources through either the Startup (normal operating configuration) or Standby Transformers and from one of two diesel generators located in the plant. There have been no significant changes to the offsite power source alignment since original plant licensing.

The following at power (normal operating condition) configurations have been confirmed to be consistent with the current licensing basis:

Both ESF buses (1A3 & 1A4) powered from the offsite grid via the three winding 161/4.16 kV Startup transformer.

There have been no significant changes in the offsite power source alignment to the ESF buses from the original plant licensing.

SYSTEM PROTECTION Items 1., 1.a, 2.b, and 2.d request information regarding electrical system protection and will be addressed in this section:

Bulletin Item 1.

Given the requirements above, describe how the protection scheme for ESF buses (Class 1E for current operating plants or non-Class 1E for passive plants) is designed to detect and automatically respond to a single-phase open circuit condition or high impedance ground fault condition on a credited off-site power circuit or another power sources.

NextEra Response Consistent with the current licensing basis and GDC 17, existing protective circuitry will separate the ESF buses from a connected failed offsite source due to a loss of voltage or a sustained, balanced, degraded grid voltage concurrent with certain design basis accidents. The relay systems were not specifically designed to detect an open single phase of a three phase system. Detection of a single-phase open condition is beyond the approved design and licensing basis of the plant.

An electrical analysis for off-site circuits has not yet been completed with regard to high impedance grounds. Ground protection schemes and a qualitative assessment are covered later in this response.

Bulletin Item 1.a.

The sensitivity of protective devices to detect abnormal operating conditions and the basis for the protective device setpoint(s).

NextEra Response Consistent with the current licensing basis and GDC 17, existing electrical protective devices are sufficiently sensitive to detect design basis conditions like a loss of voltage or a degraded voltage, but were not designed to detect a single phase open circuit condition. See Attachment 2, Table 5 for undervoltage protective devices and the basis for the device setpoint(s).

Existing electrical protective devices are also sufficiently sensitive to detect a ground fault., Table 5 lists ground protection on the ESF buses and the basis for the device setpoint(s).

Bulletin Item 2.b.

If the ESF buses are not powered by offsite power sources, explain how the surveillance tests are performed to verify that a single-phase open circuit condition or high impedance ground fault condition on an off-site power circuit is detected.

NextEra Response Not Applicable - the ESF buses at Duane Arnold Energy Center (DAEC) are powered by offsite power sources. Refer to the responses to items 1.c and 2.e below for additional information concerning detection.

Bulletin Item 2.d.

Do the plant operating procedures, including off-normal operating procedures, specifically call for verification of the voltages on all three phases of the ESF buses?

NextEra Response The current plant operating procedure for the ESF buses (01 304.2), including operating procedures for some off-normal alignments, specifically call for verification of the voltages on all three phases of the ESF buses. The operators can determine if a potential open phase issue exists for the ESF buses by monitoring the phase power lights for the buses in the main control room. If the power available light is out (or abnormal (e.g. dim)) for the same phase on both ESF buses, an open phase condition is highly probable and investigated.

CONSEQUENCES Items 1.b, 1.c, and 2.e request information regarding the electrical consequences of an event and will be addressed in this section:

Bulletin Item 1.b.

The differences (if any) of the consequences of a loaded (i.e., ESF bus normally aligned to offsite power transformer) or unloaded (e.g., ESF buses normally aligned to unit auxiliary transformer) power source.

NextEra Response The installed relays were not designed to detect single phase open circuit conditions. Existing loss of voltage and degraded voltage relays may respond depending on load and possible grounds. An unloaded offsite transformer is not applicable to the DAEC since the ESF buses are not powered from the unit Auxiliary transformer, but are powered from the Startup Transformer during normal operations.

The plant response for a loaded power source cannot be calculated without specifying the amount of loading and the specific loads involved. Further analysis would be required.

The normal power supply for the ESF buses is the Startup Transformer (1X003). If a ground (as measured by high current on the neutral of the Y windings) is sensed on any of the windings (primary or either secondary) the Startup Transformer lockout relay (386/ST) will energize which opens the switchyard supply breaker to the transformer and the ESF loads are transferred to the Standby Transformer (1X004). If the Standby Transformer indicates similar problems, thereby indicating that the problem is in the switchyard or power grid, the ESF buses are transferred to the station's Standby Diesel Generators. Control room alarms also actuate to indicate the condition and the load transfer. The current at which the sensing relay is set is intended to prevent damage to the ESF equipment.

For cases of high impedance grounds, if the impedance is so high that the resulting current is too low to trip the ground sensing overcurrent relay, it is reasonable to assume that the ground current and resulting voltage impacts will be too small to have a noticeably negative impact on the ESF loads. However, this is a complicated power systern analysis problem which needs to be performed to provide further study and analysis.

Bulletin Item 1.c.

If the design does not detect and automatically respond to a single-phase open circuit condition or high impedance ground fault condition on a credited offsite power circuit or another power sources, describe the consequences of such an event and the plant response.

NextEra Response A high impedance ground will have no immediate effect on plant operation. If the ground is sufficiently large to affect plant operation, protective relaying will isolate the ground automatically.

1. The DAEC did not credit in the Current Licensing Basis that the Class 1 E protection scheme (for the emergency safeguard feature (ESF) buses) was designed to detect and automatically respond to a single-phase open circuit condition on the credited off-site power source as described in the UFSAR and Technical Specifications.

The offsite power circuits at the DAEC consist of two independent circuits. The normal circuit supplies power from the 161 kV portion of the transmission grid in the station switchyard, through switchyard breakers 5550 or 5560 to the 161/4.16 kV Startup Transformer (1X003) and on to the ESF buses (1A3 & 1A4). The other supplies a secondary/backup source of power from the 34.5 kV feed off of the T1 transformer in the station switchyard, through switchyard breaker 8490 to the 34.5/4.16 kV Standby Transformer (1X004) and on to the ESF buses (1A3 & 1A4). Reference Tech Spec Bases B3.8.1 and UFSAR Chapters 8.2 and 8.3.

2. Since DAEC did not credit the ESF bus protection scheme as being capable of detecting and automatically responding to a single phase open circuit condition, an open phase fault was not included in the design criteria for either the loss of voltage, the degraded voltage relay scheme or secondary level undervoltage protection system (SLUPS) design criteria. Since open phase detection was not credited in the DAEC design or licensing basis, no design basis calculations or design documents exist that previously considered this condition.

3. Without formalized engineering calculations or engineering evaluations, the electrical consequences of such an open phase event (including plant response), can only be evaluated to the extent of what has already been published by EPRI and Basler; which is a generic overview. The difficulty in applying these documents to the DAEC specific response is that these are generic assessments and cannot be formally credited as a basis for an accurate response. The primary reason is that detailed plant specific models would need to be developed (e.g., transformer magnetic circuit models, electric distribution models, motor models; including positive, negative, and zero sequence impedances (voltage and currents), and the models would need to be compiled and analyzed for the DAEC specific Class 1 E electric distribution system (EDS)).

The loss of a single phase of either the startup or standby transformer would not reliably actuate the loss of power instrumentation degraded voltage relays, to isolate the essential buses from an unbalanced degraded or no voltage condition. Specifically, each bus is monitored for the

Degraded Voltage Function by four relays whose contacts form a coincidence logic matrix such that either of the Al or A2 (A to B phase monitor) contacts and either of the B1 or B2 (B to C phase Monitor) contacts must close to initiate the required actions in the associated division (i.e., one-out-of-two taken twice). Regardless of the loading on the transformer, a loss of the A phase only de-energizes the Al/A2 portion and a loss of the C phase only de-energized the B1/B2 portion of the circuitry. Neither loss would actuate the logic. This instrumentation was installed in accordance with BTP PSB-1 and was not designed to provide isolation due to an open phase connection resulting from a component failure. The loss of voltage and degraded voltage logic is designed to detect a balanced, degraded voltage condition on the essential bus caused by local grid voltage drop due to loss of generation or other three phase transmission issues. DAEC is vulnerable to the event.

DAEC procedures have been revised with interim compensatory actions to:

Require that a designated Operator be utilized in the Main Control Room to trip the Startup Transformer bus feed breaker if the voltmeter indications shows a loss of a single phase on the ESF safety-related buses.

The Operator will monitor the white bus power available indicating lights for each phase of power to the ESF buses on control room panel 1C08. A loss of~a single phase would affect both sets of lights identically. Any abnormal indication of the white lights will be investigated using ESP bus voltmeters.

Further analysis is required to model and analyze the open phase and high impedance ground issues to eliminate or mitigate the impacts of open phase or high impedance grounds in the electrical power supply system on the ESF buses of the DAEC plant.

Bulletin Item 2.e.

If a common or single offsite circuit is used to supply redundant ESF buses, explain why a failure, such as a single-phase open circuit or high impedance ground fault condition, would not adversely affect redundant ESF buses.

NextEra Response Consistent with the Current Licensing Basis and GDC 17, protective circuitry will separate the ESF buses from a failed offsite source due to a loss of voltage or a sustained balanced degraded grid voltage concurrent with certain design basis accidents. The relay systems were not specifically designed to detect an open single phase of a three phase system. Detection of a single-open phase circuit is beyond the approved design and licensing basis of the plant. No calculations for this scenario have been done.

Consistent with the current station design, protective circuitry will protect from a ground fault condition with all three phases intact.

As stated above, the normal power supply for the ESF buses is the Startup Transformer (1 X003). If a ground (as measured by high current on the neutral of the Y windings) is sensed on any of the windings (primary or either secondary) the Startup Transformer lockout relay (386/ST) will energize which opens the switchyard supply breaker to the transformer and the ESF loads are transferred to the Standby Transformer (1X004). Control room alarms also

actuate to indicate the condition and the load transfer. The current at which the sensing relay is set is intended to prevent damage to the ESF equipment.

For cases of high impedance grounds, if the impedance is so high that the resulting current is too low to trip the ground sensing overcurrent relay, it is reasonable to assume that the ground current and resulting voltage impacts will be too small to have and noticeably negative impact on the ESF loads. However, this is a complicated power system analysis problem that would need to be performed to provide further study and analysis.

The applicable DAEC operation procedures have been revised to warn operations personnel of the potential problems associated with an open-phase event and to provide guidance in detecting such an event and isolating the essential electrical power system from it.

ENCLOSURE ATTACHMENT 1 NEXTERA ENERGY DUANE ARNOLD, LLC RESPONSE TO NRC BULLETIN 2012-01 DESIGN VULNERABILITY IN ELECTRIC POWER SYSTEM SIMPLIFIED ONE-LINE DIAGRAM 1 page follows

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(Downto'n or Comntrol House) 1S 1 KV Rk r.s 3110 *'W 6730 "F" 0220 "14 6950 "AR" 2690 "D" 9130 75 10 "C 1/'4 1P2 1kI 345 KV Skr,-i 4730 "R" 2920 "S" D7 10 "T" Breakeis Equipped with Reclosers; 345 KV Bkrs 4730 "R" 28320 S" 0710 "T" 131 KV Rkrts.

5950 "AW" 3110 '"W, 9180 "G" 7510 "C" 8090 "A-Dinconneclt Thanst Can Be Operated By The Control Room:

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To 6~h Sird:eI ~-Gr 5930 "GBs" 0220 "'H' 555D "'J" 5940 "LO" 5330 -GC-4290 "1" 5560 "K" 8490 -M"

ENCLOSURE ATTACHMENT 2 NEXTERA ENERGY DUANE ARNOLD, LLC RESPONSE TO NRC BULLETIN 2012-01 DESIGN VULNERABILITY IN ELECTRIC POWER SYSTEM TABLES 3 pages follow

ENCLOSURE ATTACHMENT 2 - TABLES NEXTERA ENERGY DUANE ARNOLD, LLC REPOSNSE TO NRC BULLTEN 2012-01 DESIGN VULNERABILITY IN ELECTRIC POWER SYSTEM Table 1: ESF Buses Continuously Powered From Offsite Power Source(s)

Description of ESF Bus Power Source ESF Bus Name (normal operating condition).

Original licensing basis configuration (Y/N) 1X003 - 161KV/4160VAC START-UP 1A3 - Division 1 -4160 VAC Essential Switchgear Y

TRANSFORMER 1X003 - 161 KV/4160VAC START-UP 1A4 - Division 2 -4160 VAC Essential Switchgear Y

TRANSFORMER 1A3 1 B3 - Control Building 480VAC Essential Load Center Y

1A3 1 B9 - Intake Structure 480VAC Load Center Y

1 B3 1 B32 - Control Building 480VAC Essential Motor Control Center Y

1 B32 1 B36 - Pump House 480 VAC Motor Control Center Y

1 B3 1 B34 - Reactor Building 786' Level 480VAC Motor Control Center Y

11B34 1 B34A - Reactor Building 786' Level 480VAC Motor Control Center Y

1B34A 1 B37 - Reactor Building 786' Level 480VAC Motor Control Center Y

11B9 1 B91 - Intake Structure 480VAC Load Center Y

1A4 1B4 - Control Building 480VAC Essential Load Center Y

1A4 1B20 - Intake Structure 480VAC Load Center Y

1 B4 1 B42 - Control Building 480VAC Essential Load Center Y

1 B42 1 B46 - Pump House 480VAC Motor Control Center Y

1 B4 1 B44 - Reactor Building 757' Level 480VAC Motor Control Center Y

1 B44 1B44A - Reactor Building 757' Level 480VAC Motor Control Center Y

11B20 1 B21 - Intake Structure 480VAC Load Center Y

Table 2 - ESF Buses Not Continuously Powered From Offsite Power Source(s)

Description of ESF Bus Power Source ESF Bus Name (normal operating condition).

Original licensing basis configuration (Y/N)

N/A - All ESF Buses are normally powered N/A Y

from Offsite

Table 3 - ESF Buses Normally Energized Major Loads ESF Bus Load Voltage Rating Level (HP) 1A3 - Division 1 -

1P089(A/C) General Service Water Pump (1 4160 V 250 HP 4160 VAC Essential of 2 normally energized)

Switchgear 1A4 - Division 2 -

1P089(B/D) General Service Water Pump (1 4160 V 250 HP 4160 VAC Essential of 2 normally energized)

Switchgear 1A3 or 1A4 1P209 (A/B)- CRD Feed Pump (1 of 2 4160 V 250 HP normally energized) 1B3 1V-CH-001A (1 of 2 (See 1B4 Bus) normally 480 V 200 HP energized) 1 B32 1 G051 - Reactor Protection MG Set A 480 V 30 HP 1 B34 1V-EF-18A - Off Gas Stack Exhaust Fan A 480 V 10 HP 1 B34A No major loads normally energized 480 V 1 B36 No major loads normally energized 480 V 1 B37 No major loads normally energized 480 V 1B9 1 P117(A/C) - River Water Supply Pump (1 of 480 V 125 HP 4 (See 1 B20 Bus) normally energized) 1 B91 No major loads normally energized 480 V 1B4 1V-CH-001B (1 of 2 (See 1B3 Bus) normally 480 V 200 HP energized) 1 B42 1 G061 - Reactor Protection MG Set A 480 V 30 HP 1 B44 No major loads normally energized 480 V 1B44A No major loads normally energized 480 V 1 B46 No major loads normally energized 480 V 1 B20 1 P117(BID) - River Water Supply Pump (1 of 480 V 125 HP 4 (See 1 B9 Bus) normally energized) 1 B21 No major loads normally energized 480 V

Table 4 - Offsite Power Transformers Transformer Winding MVA Size Voltage Rating Grounding Configuration (AO/FA/FA)

(Primary/Secondary)

Configuration 1X003 - 161KV/4160VAC Start Wye-Delta Wye 12.5 MVA 161/4.16 & 4.16 kV Neutral Grounded on Up Transformer (OA/FOA/FOA)

(2 Secondary Windings)

Primary (Solid) and Secondary (Resistor) 1X004 - 34.5KV/4160VAC Delta-Wye 7.5 MVA (OA/FA) 34.5/4.16 kV Neutral Grounded on Standby Transformer Secondary(Transformer Resistor)

Table 5 - Protective Devices Protection Protective Device UV Setpoint (Nominal)

Basis for Setpoint Zone Logic Each 4160 Loss of Voltage 2 of 2 2730V = (65% of To actuate upon complete loss of ESF Bus voltage V ESF Bus Relay 4200) condition (1A3/4)

Each 4160 Degraded Grid 1 of 2 3798V = (91.3% of To actuate just above minimum analyzed acceptable V ESF Bus twice 4160) degraded bus voltage value.

(1 A3/4)

Startup Ground Protection 1 of 1 160A (High Side)

High neutral currents used to detect probable ground Transformer 320A (X Winding) faults.

(1X003) 320A (Y Winding)