BVY 12-074, 90-Day Response to Bulletin 2012-01, Design Vulnerability in Electric Power System

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90-Day Response to Bulletin 2012-01, Design Vulnerability in Electric Power System
ML12306A089
Person / Time
Site: Vermont Yankee Entergy icon.png
Issue date: 10/25/2012
From: Wamser C
Entergy Nuclear Operations
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
BVY 12-074, BL-12-001
Download: ML12306A089 (15)
v • d • e


Text

A Entergy Nuclear Operations, Inc.

Vermont Yankee 320 Governor Hunt Road Entergy Vernon, Vermont 05354 Tel: 802-257-7711 Christopher J. Wamnser Site Vice President BVY 12-074 October 25, 2012 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

SUBJECT:

90-Day Response to Bulletin 2012-01, Design Vulnerability in Electric Power System Vermont Yankee Nuclear Power Station Docket No. 50-271 License No. DPR-28

REFERENCE:

1. NRC letter to Entergy, Bulletin 2012-01 Design Vulnerabilityin Electric Power System, dated July 27, 2012

Dear Sir or Madam:

On July 27, 2012, the NRC issued Bulletin 2012-01 (Reference 1), requesting that each licensee submit a written response in accordance with 10 CFR 50.54(f) within 90 days of the bulletin to provide requested information. This letter provides the Entergy Nuclear Operations, Inc. 90-day response to Reference 1 for Vermont Yankee.

There are no new regulatory commitments contained in this submittal.

Should you have any questions or require additional information concerning this submittal, please contact Mr. Robert J. Wanczyk at (802)451-3166.

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

Executed on October 25, 2012.

Sincerely, CJW/JTM

BVY 12-074 / Page 2 of 2 Attachments: 1. VY 90-Day Response to NRC Bulletin 2012-01

2. Simplified One-Line Diagram
3. Tables cc: Mr. William M. Dean Regional 1 Administrator U. S. Nuclear Regulatory Commission 2100 Renaissance Blvd., Suite 100 King of Prussia, PA 19406-2713 Mr. Richard Guzman, Project Manager U.S. Nuclear Regulatory Commission Mail Stop 08C2A Washington, DC 20555 USNRC Resident Inspector Vermont Yankee 320 Governor Hunt Road Vernon, Vermont 05354 Ms. Elizabeth Miller, Commissioner Vermont Department of Public Service 112 State Street - Drawer 20 Montpelier, Vermont 05620

Docket No. 50-271 BVY 12-074 Attachment 1 VY 90-Day Response to NRC Bulletin 2012-01

BVY 12-074 / Attachment 1 / Page 1 of 7 VY 90-Day Response to NRC Bulletin 2012-01 Overview:

  • System Description - Items 2., 1.d, 2.a, 2.c
  • System Protection - 1., 1.a, 2.b, 2.d
  • Consequences - 1.b, 1.c, 2.e
  • Attachment 2 - Simplified One-Line Diagram
  • Attachment 3 - Tables o Table 1 - ESF Buses Continuously Powered From Offsite Power Source(s) o Table 2 - ESF Buses Not Continuously Powered From Offsite Power Source(s) o Table 3 - ESF Buses Major Loads o Table 4 - Offsite Power Transformers o Table 5 - Protective Devices

System Description

Items 2, 1 .d, 2.a, and 2.c request system information and will be addressed in this section:

2. Briefly describe the operating configurationof the ESF buses (Class 1E for current operatingplants or non-Class 1E for passive plants) at power (normal operatingcondition).

See Attachment 2, for a simplified one-line diagram.

Vermont Yankee has two 4.16 kV Class 1 E buses which are tied to two Non Safety Related (NSR) 4.16 kV buses. The 4.16 kV buses are normally supplied from the main generator through a three winding unit auxiliary transformer (each low voltage winding supplies one NSR and one Class 1 E 4.16 kV bus). Upon generator trip, the 4.16 kV buses are transferred to two startup transformers. Each startup transformer supplies one NSR and one Class 1E 4.16 kV bus. The startup transformers are normally unloaded during station operation except startup transformer T-3B supplies the mechanical draft cooling tower loads during summer months.

The immediate source of offsite power to Vermont Yankee is the Vermont Yankee 115 kV switchyard which supplies power to the two 115 kV/4.16 kV startup transformers. A delayed source of offsite power is established by backfeeding the station 4.16 kV buses from the VY 345 kV transmission system through the generator step up transformer and unit auxiliary transformer. This source is established by opening the generator no load disconnect switch.

1.d. Describethe offsite power transformer(e.g., start-up,reserve, station auxiliary)winding and grounding configurations.

See Attachment 3, Table 4 for offsite power transformer winding and grounding configurations.

BVY 12-074 / Attachment 1 / Page 2 of 7 2.a. Are the ESF buses powered by offsite power sources?ff so, explain what major loads are connected to the buses including their ratings.

For at power (normal operating condition) configurations, ESF buses are not powered by offsite sources. For at power (normal operating condition) configurations, the non-safety buses are not powered by offsite power sources except for cooling tower loads which are normally supplied by offsite power during the summer months.

See Attachment 3, Tables 1 and 2 for ESF bus power sources.

See Attachment 3, Table 3 for ESF bus major loads energized during normal power operations, including their ratings.

2.c. Confirm that the operating configurationof the ESF buses is consistent with the current licensing basis. Describeany changes in offsite power source alignment to the ESF buses from the originalplant licensing The following at power (normal operating condition) configurations have been confirmed to be consistent with the current licensing basis:

ESF buses (4160 V Buses 3 and 4) are normally supplied from the main generator through the unit auxiliary transformer. Upon unit trip the buses are transferred to the two startup transformers (two winding) which are supplied from the 115 kV switchyard (immediate access power source). A delayed access power source is established by backfeed from the 345 kV switchyard through the generator step up transformer by opening the generator no load disconnect switch. A Station Blackout Alternate AC Power Source is available to either 4160 V Bus 3 or4 from the 69kV switchyard from the adjacent Vernon Hydro Electric Station.

The following changes in offsite power source alignment to the ESF buses have been made since the original plant licensing:

1. The original startup transformer (T-3) was a 4 winding transformer (one primary winding at 115 kV and three low voltage windings). One low voltage winding supplied 4160 V NSR Bus 1 and 4160 V ESF Bus 2; the second winding supplied 4160 V NSR Bus 2 and 4160 V ESF Bus 4 and the third low voltage winding supplied 4160 V NSR Bus 5 which supplies the mechanical draft cooling tower loads. This transformer was replaced after it failed with two - two winding transformers. Startup transformer T-3-1A supplies 4160 V NSR Bus 1 and 4160 V ESF Bus 2. Startup transformer T-3-1 B supplies 4160 V NSR Bus 2 4160 V ESF Bus 4 and NSR 4160 V Bus 5.
2. The station was originally licensed with two delayed access offsite power sources. One delayed access source was the backfeed from the 345 kV switchyard to the 4160 V buses through the generator step up transformer and unit auxiliary transformer by removing generator links.

This source could be established within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> as described in the original Technical Specification Bases. The second delayed access source was a connection to the adjacent Vernon Hydroelectric Station. It could be connected to a single ESF bus.

BVY 12-074 / Attachment 1 / Page 3 of 7 The tie to Vernon Hydroelectric Station was credited to meet 10CFR50.63, the Station Blackout Regulation and therefore it is no longer credited as a delayed access source of offsite power. The backfeed through the generator step up transformer and unit auxiliary transformer has been upgraded by the addition of a generator no-load disconnect switch. This backfeed can be manually established within one hour by opening the switch.

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

1. Given the requirementsabove, describe how the protection scheme for ESF buses (Class 1E for current operatingplants or non-Class 1E for passive plants) is designed to detect and automaticallyrespond to a single-phaseopen circuitcondition or high impedance ground fault condition on a credited off-site power circuitor anotherpower sources.

Also, include the following information:

Consistent with the current licensing basis, 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-open phase condition is beyond the approved design and licensing basis of the plant.

The electrical analyses for off-site circuits have been reviewed with regard to high impedance grounds. The effect of a high impedance ground has been analyzed to have no impact on the electrical distribution system if the ground occurs on the secondary side of the offsite power transformers because the 4160 V distribution system is a high impedance grounded power system designed to operate in the presence of a single ground fault.

Preliminary analysis of a single phase high impedance ground occurring on the primary of the offsite power system transformers with all three phases intact indicate that if the fault impedance is as low as 8 ohms, the fault would not be detectable by the existing ground fault relaying associated with the offsite power transformers. In this case, based on unbalanced fault analysis using ETAP, the fault would not impact the ability of the offsite power system to power the Class 1 E buses. [Note: a single ground fault with all three phases intact should be detected by the existing bus differential relay schemes and would result in isolation of the fault.] Analysis of the impact of open phase fault in combination with a high impedance ground fault cannot be completed until the open phase fault analysis is completed.

BVY 12-074 / Attachment 1 / Page 4 of 7 1.a. The sensitivity of protective devices to detect abnormaloperating conditions and the basis for the protective device setpoint(s).

Consistent with the current licensing basis, 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 3, 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 faults which impact the ability of the power system to transfer power to the Class 1 E buses. Ground detection is provided on the non safety related 4160 V buses which normally supply the 4160 V Class 1 E buses. Attachment 3, Table 5 lists ground alarms for the ESF buses and the basis for the device setpoint(s).

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-phaseopen circuit condition or high impedance ground fault condition on an off-site power circuit is detected.

The ESF buses at Vermont Yankee are not normally powered by offsite power sources.

Vermont Yankee has implemented the following as interim measures:

  • Heightened surveillances have been initiated to detect an open phase fault. These include a once per shift visual check of the overhead conductors associated with the offsite power source and enhanced once per shift control room meter readings to check for balanced voltage and currents on lines and buses associated with the offsite power circuits.
  • The heightened surveillances and night orders are being converted to permanent changes to operator rounds and permanent changes to existing station Operating Procedure OPON-3150-01, "Loss of Start Up Transformer(s)"
  • Revision to operator training programs for initial training and requalification training have been initiated to include the Byron event, its consequences and the interim operator response required.
  • Vermont Yankee has reviewed the configuration of the overhead lines associated with the offsite power circuits. Vermont Yankee has concluded, based on the configuration of its lines and insulators (predominantly Lapp suspension insulators) that it is less susceptible to the same failures which occurred at Byron Station.

" Vermont Yankee has initiated an inspection program of the insulators associated with the offsite power circuits as a commitment under license renewal.

" Vermont Yankee shall initiate studies to determine actual currents and voltages which would exist on the primary and secondaries of offsite power transformers under single open circuit conditions at various load levels. Results of these studies are necessary to determine how the open phase fault may be reliably detected.

BVY 12-074 / Attachment 1 / Page 5 of 7 2.d. Do the plant operatingprocedures,including off-normal operating procedures,specifically call for verification of the voltages on all three phases of the ESF buses?

The heightened surveillances implemented as interim compensatory measures described above, specifically call for verification of the voltages on all three phases of the startup transformers which supply the ESF buses. Incorporation of the heightened surveillances into current plant operating procedures (formal operator rounds) is in progress.

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:

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.

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. In general, based on preliminary studies performed by the industry, there will be no plant response for an unloaded (e.g., ESF buses normally aligned to an auxiliary transformer) power source in the event of a single-phase open circuit on a credited off-site power circuit because there is insufficient current to detect a single-phase open circuit for this configuration.

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

As stated above, a single high impedance ground would have no impact on the electrical distribution system if the ground occurs on the secondary side of the offsite power transformers because the 4160 V distribution system is a high impedance grounded power system designed to operate in the presence of a single ground fault. This is true whether the buses or transformers are loaded or unloaded. A high resistance ground occurring on the primary of the unloaded startup transformers would not actuate the ground fault relays associated with transformer protection but may result in transformer or bus differential relay operation. This would not have an immediate impact of ESF buses which are not normally aligned to off-site power.

BVY 12-074 / Attachment 1 / Page 6 of 7 1.c. If the design does not detect and automaticallyrespondto a single-phase open circuit condition or high impedance ground fault condition on a credited offsite power circuit or anotherpower sources, describe the consequences of such an event and the plant response.

A high impedance ground will have no immediate effect on plant operation. A high impedance ground occurring on the secondary of the unit auxiliary or startup transformers will have no impact because the 4.16 kV system is a high impedance grounded system designed for operation with a ground fault. No protective relaying action will occur regardless of ground fault resistance. If the ground fault on the primary of the VY startup transformers is sufficiently large to affect plant operation, protective relaying will isolate the ground automatically.

1. Vermont Yankee did not credit in the Current Licensing Basis (CLB) that the Class 1E 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 Vermont Yankee consist of two circuits. An immediate access offsite power circuit from the transmission system to the Vermont Yankee 115 kV switchyard to the Class 1 E Buses through the startup transformers and a delayed access offsite power source from the transmission system to the Vermont Yankee 345 kV switchyard to the Class 1 E buses by backfeed through the Generator Step Up Transformer and Unit Auxiliary Transformer with the generator no load disconnect switch open.

2. Since Vermont Yankee 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 or the degraded voltage relay (DVR) scheme. Since open phase detection was not credited in the Vermont Yankee 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 Vermont Yankee 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 Vermont Yankee specific Class 1 E electric distribution system).

BVY 12-074 / Attachment 1 / Page 7 of 7

4. As stated previously, Vermont Yankee has implemented interim measures to identify and respond to open phase faults. These measures consist of periodic visual inspection of the overhead lines connecting to the startup transformers, heightened surveillances and revised operating procedures intended to detect and respond to a degraded offsite source due to single open phase condition.

Long term corrective actions consist of completing the plant specific models and completing plant specific open circuit analyses. Results of the analysis can be used to design a protection system to reliably detect and protect the Class 1 E buses from effects of open phase faults.

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 redundantESF buses.

Consistent with the Current Licensing Basis 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.

Docket No. 50-271 BVY 12-074 Attachment 2 VY 90-Day Response to NRC Bulletin 2012-01 Simplified One-Line Diagram

BVY 12-074 / Attachment 2 / Page 1 of 1 SIMPLIFIED ONE LINE DIAGRAM TO 13.2 KV VERNONSTATION SWYD 6

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BUS4 GENERATOR O0-1-lA 3V4 P

Docket No. 50-271 BVY 12-074 Attachment 3 VY 90-day Response to NRC Bulletin 2012-01 Tables

BVY 12-074 / Attachment 3 / Page 1 of 2 Attachment 3 - Tables Table 1 - ESF Buses Continuously Powered From Offsite Power Source(s)

Description of ESF Bus ESF Bus Name (normal Original licensing Power Source operating condition). basis configuration (Y/N)

N/A N/A N/A Table 2 - ESF Buses Not Continuously Powered From Off site Power Source(s)

Description of ESF Bus ESF Bus Name (normal Original licensing Power Source operating condition). basis configuration (Y/N)

UAT T-2-1A X winding 4kv Bus 3 Y UAT T-2-1A Y winding 4kV Bus 4 Y Table 3 - ESF Buses Normally Energized Major Loads ESF Bus Load Voltage Rating (HP)

Level 4kv Bus 3 Service Water Pump P-7-1 B 4000 V 250 HP 4kv Bus 3 Service Water Pump P-7-1 D 4000 V 250 HP 4kv Bus 3 4160/ 480 V Transformer T-8 4160 V 1000 kVA 4kV Bus 4 Service Water Pump P-7-1A 4000 V 250 HP 4kV Bus 4 Service Water Pump P-7-1C 4000 V 250 HP 4kV Bus 4 4160/ 480 V Transformer T-9 4160 V 1000 kVA Table 4 - Offsite Power Transformers Transformer Winding MVA Size Voltage Rating Grounding Configuratio (OA/FA/FOA) (Primary/Secondary) Configuration n

Startup WYE- Delta 15/20/25 115 kV / 4.16 kV Neutral Transformer 28 at 650C Grounded T-3-1A FOA (primay Startup WYE- Delta 15/20/25 115 kV / 4.16 kV Neutral Transformer 28 at 650C Grounded T-3-1 B FOA (primary)

BVY 12-074 / Attachment 3 / Page 2 of 2 Table 5 - Protective Devices Protection Protective Device Logic Setpoint Basis for Setpoint Zone (Nominal) 4 KV ESF Loss of Voltage 2 of 2 1925V To actuate upon complete loss Bus Relay (46% of of ESF Bus voltage condition 4160V) 4 KV ESF Degraded Grid 2 of 2 3700V (89% of To automatically actuate on Bus 4160) degraded voltage at ESF Bus coincident with an accident signal 4 KV ESF Degraded Grid 1 of 2 3700V (89% of To provide degraded grid Bus 4160) voltage alarm for ESF Bus -

not coincident with an accident signal 4 KV ESF Ground Protection 1 of 1 560V across Provide ground fault alarm for Bus Alarm grounding 4kV system indicating resistor (13% presence of single ground on of 4160) high resistance grounded 4 kV system.

115 kV Ground Protection 1 of 1 Ground fault > Detect significant ground fault system 480 Amps on 115 kV system

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