SBK-L-12220, Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System
| ML12299A467 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 10/22/2012 |
| From: | Walsh K NextEra Energy Seabrook |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| SBK-L-12220, BL-12-001 | |
| Download: ML12299A467 (25) | |
Text
NEXTera" ENERGY 10 CFR 50.54(f)
Docket No. 50-443 SBK-L- 12220 October 22, 2012 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555 - 0001 Seabrook Station Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System Pursuant to Section 50.54(f) of Title 10 of the Code of Federal Regulations, NextEra Energy Seabrook, LLC (NextEra) is submitting its response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System.
The enclosure to this letter provides NextEra's response to Bulletin 2012-01.
Should you have any questions regarding this letter, please contact Mr. Michael O'Keefe, Licensing Manager, at (603) 773-7745 Sincerely, NextEra Energy Seabrook, LLC Kevin T. Walsh Site Vice President cc: NRC Region I Administrator NRC Project Manager NRC Senior Resident Inspector i
NextEra Energy Seabrook, LLC, P.O. Box 300, Lafayette Road, Seabrook, NH 03874
T....
SEABROOK AFFIDAVIT U
SEAMOOK STATION UNIT 1 * ,
Xac ItI' Operat Ing License NP1 o DIocket No. 50-443 4 NRC Betin22 Design Vunerabiity inEecric er System I, Kevin T. Walsh, Site Vice President of NextEra Energy Seabrook, LLC hereby affirm that the information and statements contained within this response to NRC Bulletin 2012-01 are based on facts and circumstances which are true and accurate to the best of my knowledge and belief.
Sworn and Subscribed before me this J)- day of _ .____
1,2012 Kevin T. Walsh Ntr ublic Site Vice President
Enclosure to SBK-L-12220 Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System Bulletin Response Simplified One-Line Diagram Byron vs. Seabrook Station Switchyard Comparison Tables
- Table 1 ESF Buses Continuously Powered From Offsite Power Source(s)
- Table 2 ESF Buses Not Continuously Powered From Offsite Power Source(s)
" Table 3 ESF Buses Normally Energized Major Loads
" Table 4 Offsite Power Transformers
" Table 5 Protective Devices Enclosure to SBK-L-12220 Attachment I Bulletin Response
System Description
Bulletin items 2., l.d, 2.a and 2.c are addressed in this Section.
- 2. Briefly describethe operatingconfigurationof the ESF buses (Class 1Efor current operatingplants or non-Class 1Efor passiveplants) atpower (normaloperating condition).
Seabrook Station Response:
The following description provides a brief overview of Seabrook Station's onsite and offsite electrical power systems and normal operating configuration of Seabrook Station's emergency buses.
The transmission grid connections that provide offsite power to Seabrook Station consist of three 345kV transmission lines, as shown in Attachment 2, Simplified One-Line Diagram. These lines are designed and built to provide the electrical and structural independence necessary to ensure continuity of offsite electrical power to the station. At Seabrook Station, the three lines terminate at separate terminating structures. From the terminating structures each circuit is routed in metal-enclosed, Sulphur Hexafluoride (SF6) gas-insulated bus to a common switching station as shown in Attachment 2. Two separate and independent safety-related 4.16kV emergency buses and associated diesel generators are the source of power for the plant auxiliaries, protection system and Engineered Safety Features (ESF) loads during normal, abnormal and accident conditions. The normal supply to Train A (Bus E5) and Train B (Bus E6) 4.16kV emergency buses is supplied from separate Unit Auxiliary Transformers (UATs). When the unit is operating, the turbine generator output is supplied through the generator circuit breaker and the UATs to the emergency buses. When the unit is shutdown, the generator breaker is opened and the current flow is reversed through the Generator Step-Up Transformer (GSU) and supplied to the UATs. The Reserve Auxiliary Transformers (RATs) provide an alternate source of power to the emergency buses. On loss of normal supply (UAT) to a bus, the alternate supply (RAT) is automatically connected. In addition to a normal and alternate supply, each emergency bus has an emergency power supply in the form of an emergency diesel generator.
The offsite electric power system complies with General Design Criterion 17 (ref. UFSAR Section 8.2.1.5). General Design Criterion 17 requires two physically independent circuits to supply electric power from the transmission network to the onsite electric distribution system, designed and located so as to minimize to the extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions.
One of the required, independent offsite circuits is connected to the onsite distribution system, including all of the emergency buses, through the unit auxiliary transformers. The supply to the unit auxiliary transformers can be traced from the transformers through the generator isolated 2
Enclosure to SBK-L-12220 Attachment 1 Bulletin Response phase bus, the generator step-up transformers, the gas-insulated isolated phase bus of the 345kV switching station, and then to an offsite transmission line. The second required, independent offsite circuit is connected to the onsite distribution system, including all of the emergency buses, through the reserve auxiliary transformers. This circuit can be traced from the reserve auxiliary transformers through a different portion of the gas-insulated isolated phase bus of the 345kV switching station, and then to another offsite transmission line.
The connections from the transformers to the onsite distribution system of these two circuits are made with separate nonsegregated phase bus ducts, which provide the necessary separation to minimize the likelihood of simultaneous failure of these circuits to the extent practical.
Both of these circuits are designed for immediate access to the onsite distribution system, thus meeting the preferred design of Regulatory Guide 1.32.
In addition to two 4.16kV ESF buses, the onsite AC power systems include two 4.16kV non-ESF and two 13.8kV non-ESF buses. In addition to one ESF bus, each UAT provides normal supply to one 4.16kV non-ESF bus and one 13.8kV Non-ESF bus. Each RAT is the reserve source for one 4.16kV ESF, one 4.16kV non-ESF and one 13.8kV non-ESF bus.
1.d. Describe the offsite power transformer (e.g., start-up,reserve, station auxiliary) winding and groundingconfigurations.
Seabrook Station Response:
See Attachment 4, Table 4, Offsite Power Transformers, for offsite power transformer winding and grounding configurations.
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.
Seabrook Station Response:
For at power (normal operating condition) configurations, ESF buses are not powered by offsite sources. For ESF buses major loads energized during normal power operation, see Attachment 4, Table 3, ESF Buses Normally Energized Major Loads.
2.c. Confirm that the operatingof the ESF buses is consistent with the currentlicensing basis.
Describe any changes in offsite power source alignment to the ESF busesfrom the original plant licensing.
Seabrook Station Response:
The operating configurations of the Seabrook Station ESF buses have been confirmed to be consistent with the current licensing basis. The normal supply to Train A (Bus E5) and Train B (Bus E6) 4.16kV emergency buses is supplied from separate Unit Auxiliary Transformers 3
Enclosure to SBK-L-12220 Attachment 1 Bulletin Response (UATs). When the unit is operating, the turbine generator output is supplied through the generator circuit breaker and the UATs to the emergency buses. When the unit is shutdown, the generator breaker is opened and the current flow is reversed through the Generator Step-Up Transformer (GSU) and supplied to the UATs. The Reserve Auxiliary Transformers (RATs) provide an alternate source of power to the emergency buses. On loss of normal supply (UAT) to a bus, the alternate supply (RAT) is automatically connected.
The Seabrook Substation Reliability Upgrade Project was undertaken in 2009 with the main goal of improving the reliability of the five key transmission elements interconnected at Seabrook Substation (main generator/GSU bus, RAT bus, and three transmission lines). The first phase of the switchyard reliability upgrade project was implemented in 2009, which involved installing five new Gas Insulated Substation (GIS) breakers, three new breaker positions and two replacements. This project improved the reliability of the Seabrook Station 345kV substation by changing the switchyard topology and by replacing aging equipment with new equipment of a more reliable design. The second phase of the switchyard reliability upgrade project was implemented in 2011, which involved relocating the main generator/GSU bus connection from the existing 345kV Bus-3 breaker bay to 345kV Bus-6 located in a new breaker bay. Overall, the switchyard reliability upgrade project implemented under 10 CFR 50.59 to improve the GIS reliability and improve the availability of the main generator and offsite power supply for the Seabrook station does not make significant change to the original licensing in regard to the offsite power source alignment to the ESF buses.
System Protection Bulletin items 1, 1.a, 2.b and 2.d are addressed in this Section.
- 1. Given the requirements above, describe how the protection scheme for ESF buses (Class JE for Current Operatingplants or non-Class 1Eforpassiveplants) is designed to detect and automaticallyrespondto a single-phase open circuitcondition or high impedance ground fault condition on a credited off-site power circuit or anotherpower sources.
Seabrook Station 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 schemes 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 Seabrook Station protection scheme does not have sufficient sensitivity to detect and automatically respond to all single-phase open circuit or high impedance ground fault conditions on off-site power supplies to ESF buses. The Seabrook Station safety-related 4.16 kV Bus Loss-4
Enclosure to SBK-L-12220 Attachment 1 Bulletin Response of-Voltage and Degraded-Voltage relay undervoltage protection schemes are similar to Byron in that they require a 2-out of -2 logic to initiate undervoltage protection. However, contrary to Byron Unit 2, the Seabrook Station design and configuration of 345kV switchyard and station buses are unique. Seabrook Station utilizes the Unit Auxiliary Transformers (UATs) to run electrical auxiliaries, with the Reserve Auxiliary Transformers (RATs) remaining unloaded.
Based on Seabrook Station's unique design and configuration of 345kV switchyard and station buses, the loss of single phase on a supply circuit to or from any of the auxiliary transformers (RATs or UATs) is not a credible event. See Attachment 3, Byron vs. Seabrook Station Switchyard Comparison, for a discussion on why an initiating event similar to the Byron event is not credible at Seabrook Station due to Seabrook Station's unique design.
In addition, the following provides a discussion on why a high impedance ground fault condition will not result in unbalanced voltages on the ESF buses.
Under normal operation, the main generator supplies electrical power to the 345kV grid through the generator step-up transformer and to the plant through the unit auxiliary transformers.
During startup and shutdown, auxiliary power may be taken from the 345kV system in one of two ways:
- Back-fed through the generator step-up transformer and unit auxiliary transformers when the generator circuit breaker is open
" From the reserve auxiliary transformers 345kV System:
Seabrook Station's 345kV system is a solidly grounded system and the neutral connections of the RATs and the GSU are solidly grounded. The Seabrook Station GSUs and RATs are connected to the 345 kV switchyard via SF6 gas-insulated isolated phase bus. Based on Seabrook Station's 345kV switchyard design, a high resistance ground fault on the gas-insulated isolated phase buses is not credible. A ground fault on the 345kV gas-insulated isolated phase buses from the air termination yard to the gas-insulated switchyard will be detected by the 345kV bus differential protective devices, whose function is to isolate the faulted portion of the bus by tripping the appropriate breakers. A ground fault between the switchyard power circuit breakers (PCB) and the GSU will be detected by the 345kV Bus 6 differential protective devices, whose function is to isolate the GSU by opening the switchyard PCBs and the feeder breakers to the station's 13.8kV and 4.16kV buses.
The GSU neutral ground fault relay (Device 5 IN) monitors for ground fault current from any portion of the GSU transformer's high voltage windings or its connected bus returns to the GSU transformer through the neutral ground tap. This relay activates the GSU transformer backup protection lockout relay (86SB), whose function is to actuate various tripping (and block- close) relays including the following pertinent functions:
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Enclosure to SBK-L-12220 Attachment 1 Bulletin Response
" trip the connecting 345kV switchyard breaker and initiate the associated 345kV breaker failure circuit
" trip the generator circuit breaker and initiate the associated breaker failure circuit
" trip the all 13.8kV buses incoming breakers (Buses 1 and 2)
- trip the 4.16kV buses incoming breakers (Buses 3, 4, E5 and E6)
Each RAT has two ground fault relays. One of the ground fault relays (Device 51NH) is placed on the grounded neutrals of the primary, and the other (Device 51NX) on the neutral grounding resistor off the transformer's 13.8 kV secondary winding. If the current flow to the ground exceeds the setpoint for the ground fault relays, the relays operate. Relays 5 1NH or 5 1NX will activate the RAT transformer primary protection lockout relay (86RP), whose function is to actuate various tripping (and block- close) relays including the following pertinent functions:
" trip the connecting 345kV switchyard breaker and initiate the associated 345kV breaker failure circuit
" trip the all 13.8kV buses incoming breakers (Buses 1 and 2)
" trip the 4.16kV buses incoming breakers (Buses 3, 4, E5 and E6)
Other local and remote relaying on the 345kV transmission lines protects the lines from ground faults. If the ground on the 345kV system is sufficiently large, it would create momentary voltage dips and voltage unbalance on the ESF buses; however, a fault of this magnitude will be isolated quickly by the activation of appropriate protective relays and there will be no adverse affect on the energized equipment. A single high impedance ground fault which does not generate sufficient current to actuate the differential and ground fault relays cannot result in a voltage imbalance due to this section of the system being directly connected to the 345kV grid; the entire grid voltage would have to be distorted (imbalanced) as a result of the high impedance fault, which is not credible.
25kV System:
The turbine generator armature winding is connected to ground through a high impedance circuit made up of a neutral grounding transformer, neutral resistor and neutral reactor. When the generator breaker is closed, the neutral grounding equipment detects a ground anywhere on the 25kV section of the generator distribution system (Devices GP1/TG1 & 64 TG1). The components of this circuit are sized such that a full phase-to- ground fault will result in only 5 Amp of current flow to ground. If the ground is sufficiently large to affect plant operation, protective relaying will isolate the generator. Since the system is connected to ground through a high impedance circuit, a single high impedance ground fault with all three phases intact will not result in unbalanced phase to phase voltages on the ESF buses and have no affect on the energized equipment.
During times when station buses are back-fed through the GSU and UATs with the generator circuit breaker open, the ground fault voltage relay (Device 64B) associated with the isolated 6
Enclosure to SBK-L-12220 Attachment I Bulletin Response phase bus monitors the portion of the bus extending from the generator breaker to the GSU transformer and UATs. This relay activates the Generator/Transformer Overall Protection Lockout Relays (86GT), whose function is to actuate various tripping (and block- close) relays including the following pertinent functions:
" trip the connecting 345kV switchyard breaker and initiate the associated 345kV breaker failure circuit
- trip the generator circuit breaker
- trip the UAT supply breakers to all 13.8kV buses (Buses 1 and 2)
" trip the UAT supply breakers to all 4.16kV buses (Buses 3, 4, E5 and E6 While in the back-feed mode, a single high impedance ground fault in this portion of the 25kV distribution system (high impedance grounded) will have no affect on the phase to phase voltages on the 4.16kV ESF buses or the energized equipment.
13.8kV System:
The 13.8kV System is a low-resistance grounded system. The ground fault relays (Device 51NX) on the neutral grounding resistor off the UATs and RATs 13.8 kV secondary winding protect the transformers and connecting buses from ground faults. Ground fault relays have also been provided on each motor feeder and load center feeder and each incoming line. The feeder ground fault relays are set low enough (1 OAmp at 0.1 second) to isolate the faulted portion of the system by tripping appropriate circuit breakers. A single high impedance ground fault on the 13.8kV system will not result in unbalanced phase to phase voltages on the 4.16kV ESF buses and will have no affect on the energized equipment on the ESF buses.
4.16kV System:
The 4.16kV system is a high resistance grounded system. In order to detect grounds, ground sensors have been provided on each motor circuit and load center feeder and each incoming line, including the diesel generator. Inputs from ground sensors and ground detection circuits are furnished to the station computer. The grounding scheme used allows single ground faults to be alarmed only and the equipment to continue operation. Therefore, a single ground fault with all three phases intact on the 4.16kV system will not result in unbalanced phase to phase voltages and will have no affect on the energized equipment.
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Enclosure to SBK-L-12220 Attachment I Bulletin Response l.a. The sensitivity ofprotective devices to detect abnormaloperatingconditions and the basis for the protective device setpoint(s).
Seabrook Station 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 or a high impedance ground fault condition. See Attachment 4, Table 5, Protective Devices, for undervoltage and ground fault protective devices and the basis for the device setpoint(s).
2.b. If the ESF buses are not powered by offsite power sources, explain how thie surveillance tests areperformed to verify that a single-phase open circuit condition or high impedance groundfault condition on an off-site power circuit is detected.
Seabrook Station Response:
As discussed in the response to question 2, Seabrook Station's ESF buses are not powered by offsite sources during normal operation and the loss of single phase on a supply circuit to or from any of the auxiliary transformers (RATs or UATs) is not a credible event at Seabrook Station due to the unique design of the Seabrook Station switchyard. Although, the loss of single phase is not a credible event, the following actions were performed in response to the Byron single open phase event:
In response to NRC NRR Event Notice EN47636, on 2/7/2012, Design Engineering conducted a review of the ability of the undervoltage protection relay schemes at Seabrook Station to detect a similar loss-of-phase event. Upon completion of this review, Engineering provided technical guidance to the site Operations Department in order to aid plant operators in the detection and response to such an event. Operations issued this guidance as a briefing document to alert operators to the event. Operations Department also initiated ARI 732579 to review and revise the Loss-of-Power (voltage)/Low (Degraded)-Voltage alarm response procedures identified in the technical guidance from Engineering. The alarm response procedures for emergency Buses E5 and E6 Loss of Power (F73 00, "Bus E5 Loss of Power", F73 10 "Bus E6 Loss of Power") and Bus E5 and E6 Low Voltage (F7301, "Bus E5 Voltage Low", F7311 "Bus E6 Voltage Low")
were revised to aid plant operators in detecting and responding to such an event. INPO IER-L2-12-14, Automatic Scram Resulting from a Design Vulnerability in the 4.16-kV Undervoltage Protection Scheme, and changes to the Loss-of-Voltage/Degraded-Voltage alarm response procedures were covered in Lesson Plan SBK LOP L5076C123, LORT Phase 12-03 Self-Study/Reading Package, during License Operator Requalification Training Phase 12-03. This event was also covered in Lesson Plan SBK LOP L3554C122, Transformer Fires, during License Operator Requalification Training Phase 12-02.
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Enclosure to SBK-L-12220 Attachment 1 Bulletin Response The following procedures have been revised to help operators diagnose and respond to an open phase condition.
" Procedure OX1446.01, "AC Power Source Weekly Surveillance", was revised to verify currents on each phase of the offsite transmission lines. No current on any phase could be indicative of either a line out of service or loss of a phase and help operators diagnose open phase condition on the offsite transmission line.
" Procedure OX1446.03, "Electric Bus Weekly Operability (Operating)", was revised to verify voltages on each phase of the ESF buses.
" Procedure OS 1246.02, "Degraded Vital AC Power (Plant Operating)", was revised to help operators diagnose an open phase condition and respond to this condition.
The following Video Alarm Response Operating Procedures (VPROs) have been revised to help operators diagnose open phase condition and respond to this condition.
m F7300, Bus E5 Loss of Power
- F7301, Bus E5 Voltage Low
- F73 10, Bus E6 Loss of Power mF73 11, Bus E6 Voltage Low The following Main Control Board Hardwire Annunciator Alarm Response Procedures have been revised to help operators diagnose an open phase condition and respond to this condition.
" MM-UA-54 E-4, 480V Bus 51/52/53 Volts Low
" MM-UA-55 E-4, 480V Bus 61/62/63 Volts Low Based on Seabrook Station's unique design and configuration of 345kV switchyard and station buses, the high impedance ground fault condition is not expected to result in unbalanced voltages on the 4.16kV ESF buses. Since high impedance ground faults above the setpoints of conventional protection systems such as ground fault, overcurrent, differential, etc., usually cannot readily be detected; at present there are no surveillance tests being performed to verify the high impedance ground fault condition. However, a weekly PM activity performs detailed inspections of the Seabrook Station 345kV SF6 gas-insulated switchyard and air termination yard. This PM activity looks for abnormal conditions on any equipment. In-house switchyard qualified resources are utilized to perform these maintenance activities. This weekly surveillance activity most likely will detect equipment issues in the air termination yard, and also, based on gas insulated bus design, a high impedance ground fault is not a credible event at Seabrook Station.
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Enclosure to SBK-L-12220 Attachment 1 Bulletin Response 2.d. Do the plant operatingprocedures,including off-normal operatingprocedures, specifically callfor verificationof the voltages on all three phase of the ESF buses?
Seabrook Station Response:
In response to the Byron single open phase event, the following procedures have been revised to specifically include verification of the voltages/currents on all three phases of the ESF buses/
offsite transmission lines to help operators diagnose an open phase condition and respond to this condition.
" Procedure OX1446.01, "AC Power Source Weekly Surveillance," was revised to verify currents on each phase of the offsite transmission lines. No current on any phase or current unbalance could be indicative of either a line out of service, loss of a phase or high impedance ground fault and help operators diagnose these conditions on the offsite transmission line.
" Procedure OX1446.03, "Electric Bus Weekly Operability (Operating)," was revised to verify voltages on each phase of the ESF buses.
The following Video Alarm Response Operating Procedures (VPROs) have been revised to verify voltages on each phase of the ESF buses.
" F7300, Bus E5 Loss of Power
" F7301, Bus E5 Voltage Low
" F73 10, Bus E6 Loss of Power
- F73 11, Bus E6 Voltage Low The following Main Control Board Hardwire Annunciator Alarm Response Procedures have been revised to verify voltages on each phase of the ESF buses.
" MM-UA-54 E-4, 480V Bus 51/52/53 Volts Low
" MM-UA-55 E-4, 480V Bus 61/62/63 Volts Low 10
Enclosure to SBK-L-12220 Attachment I Bulletin Response Consequences Bulletin items 1.b, 1 .c, and 2.e are addressed in this section.
1.b. The differences (if any) of the consequences of a loaded (Le. ESF bus normally aligned to offsite power transformer)or unloaded (e.g., ESF buses normally aligned to unit auxiliary transformer)power source Seabrook Station Response:
Installed relays were not designed to detect single phase open circuit or high impedance ground fault conditions. Existing loss of voltage and degraded voltage relays may respond depending on load and possible grounds. In general, there will be no plant response for an unloaded power source (e.g., ESF buses normally aligned to UATs) in the event of a single phase open circuit or high impedance ground fault condition on a credited off-site power circuit from the Reserve Auxiliary Transformers (RATs) because there is insufficient current to detect a single-phase open circuit or high impedance ground for this configuration. The plant response for a loaded power source cannot be calculated without the amount of loading and the specific loads involved. However, based on Seabrook Station's unique design and configuration of 345kV switchyard and station buses, the loss of a single phase on a supply circuit to or from any of the auxiliary transformers (RATs or UATs) is not a credible event and the high impedance ground fault condition is not expected to result in unbalanced voltages on the 4.16kV ESF buses.
1.c. If the design does not detect and automatically respondto a single -phase open circuit condition or high impedance groundfault condition on a credited offsite power circuit or anotherpower sources, describe the consequences of such an event and the plant response.
Seabrook Station Response:
Seabrook Station 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 or high impedance ground fault condition on the credited off-site power source as described in the UFSAR and Technical Specifications.
The offsite electric power system complies with General Design Criterion 17 (ref. UFSAR Section 8.2.1.5). General Design Criterion 17 requires two physically independent circuits to supply electric power from the transmission network to the onsite electric distribution system, designed and located so as to minimize to the extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions.
II
Enclosure to SBK-L-12220 Attachment 1 Bulletin Response One of the required, independent, offsite circuits is connected to the onsite distribution system, including all of the emergency buses, through the unit auxiliary transformers. The supply to the unit auxiliary transformers can be traced from the transformers through the generator isolated phase bus, the generator step-up transformers, the gas-insulated isolated phase bus of the 345kV switching station, and then to an offsite transmission line. The second required, independent, offsite circuit is connected to the onsite distribution system, including all of the emergency buses, through the reserve auxiliary transformers.
Since Seabrook Station did not credit the ESF bus protection scheme as being capable of detecting and automatically responding to a single phase open circuit or high impedance ground condition, an open phase fault or high impedance ground was not included in the design criteria for either the loss of voltage, the degraded voltage relay (DVR) scheme or secondary level undervoltage protection system (SLUPS) design criteria. Since open phase or high impedance ground detection was not credited in the Seabrook Station design or licensing basis, no design basis calculations or design documents exist that previously considered this condition.
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 Seabrook Station 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 Seabrook Station specific Class lE electric distribution system (EDS)). However, based on Seabrook Station's unique design and the configuration of the 345kV switchyard and station buses, the loss of single phase on a supply circuit to or from any of the auxiliary transformers (RATs or UATs) is not a credible event and a high impedance ground fault on a supply circuit to or from any of the Seabrook Station auxiliary transformers is not expected to result in unbalanced voltages on the 4.16kV ESF buses. No interim or long term actions are being pursued to provide automatic protection from single-phase open circuit or high impedance ground fault conditions for off-site power sources supplying Class lE vital buses.
2.e. If a common or single offsite circuitis used to supply redundantESF buses, explain why afailure, such as a single-phase open circuit or high impedance groundfault condition, would not adversely affect redundantESF buses.
Seabrook Station 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 12
Enclosure to SBK-L-12220 Attachment 1 Bulletin Response single-open phase condition is beyond the approved design and licensing basis of the plant. No calculations for this scenario have been done. Also, consistent with the current station design, protective circuitry will protect from a ground fault condition with all three phases intact.
Seabrook Station uses a common or single offsite circuit to supply redundant ESF buses.
However, based on Seabrook Station's unique design and configuration of the 345kV switchyard and station buses, the loss of single phase on a supply circuit to or from any of the auxiliary transformers (RATs or UATs) is not a credible event and a high impedance ground fault on a supply circuit to or from any of the Seabrook Station auxiliary transformers is not expected to result in unbalanced voltages on the 4.16kV ESF buses.
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Enclosure to SBK-L-12220 Attachment 2 Simplified One-line Diagram SIMPLIFIED ONE-LINE DIAGRAM WARDHILL / WEST AMESBURY SCOREPOND NEWINGTON/ TIMBERSWAMP LINE 394 LINE 363 LINE 3S9 t t t ILINE TERMINATION YARD (AIR) r- -- " - - - -
1
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Enclosure to SBK-L-12220 Attachment 3 Byron vs. Seabrook Station Switchyard Comparison Byron Station recently had a switchyard event where failure of an insulator stack in their air-insulated switchyard resulted in a loss of one phase in the circuit supplying their System Auxiliary Transformer. This resulted in a voltage imbalance for the station auxiliary buses. This voltage imbalance was not detected due to the design of the emergency bus undervoltage relaying scheme.
An initiating event similar to the Byron event is not likely at Seabrook Station because of the design differences associated with the switchyard configuration and the bus runs to and from the auxiliary transformers.
The Seabrook Station 345kV switchyard uses an SF6 gas-insulated isolated phase bus design. The switchyard ring bus is connected to the grid by three overhead air-insulated transmission lines. Each transmission line, associated 345kV buses and circuit breakers have the capacity to supply the power requirements of station auxiliaries under all pant conditions. Failure of one phase on one of the air-insulated transmission lines would not result in an imbalance at the station auxiliary buses because balanced phase voltages would be maintained by the remaining two transmission lines. A simultaneous open circuit failure on the same phase of all three transmission lines in not a credible event.
The Seabrook Station Generator Step-Up Transformers (GSUs) and the Unit Auxiliary Transformers (UATs) are connected to the ring bus through two circuit breakers (11 & 12). The Seabrook Station Reserve Auxiliary Transformers (RATs) are also connected to the ring bus through two circuit breakers (52
& 695). As such, it is not credible that a single failure in one breaker would result in a loss of one phase in the switchyard supply circuit to either the GSU/UAT circuit or the RAT circuit.
The Seabrook Station GSUs and RATs are connected to the 345 kV switchyard via SF6 gas-insulated isolated phase bus. Each single-phase bus section consists of two concentric tubes of aluminum. The inner tube is the conductor and the outer tube is the solidly grounded enclosure for each isolated phase. Two types of insulators, gas-barrier and non-barrier, are used to support the center conductor within the enclosure. The gas-barrier type is a full circumference type insulator and the non-barrier type is of a tri-post design. These type insulators keep the conductor centered and fully supported within the enclosure.
The conductors in adjacent bus sections are interconnected using a plug and socket design that is not susceptible to open circuit failure. The conductor connection at the GSU and RAT bushings is a manually operated sliding link (plug & socket) design that is closed during normal operation. The bus enclosure sections are bolted together. This design is not susceptible to the same failure mode as the Byron event where an air-insulated line insulator failed and dropped the conductor. Based on this design, it is not credible that a single insulator failure will result in an open circuit. The conductor bus will remain in position due to the balance of remaining insulators and the rigidity of the plug and socket connections.
With the SF6 bus design, should an internal insulator failure occur, it is postulated to initiate a phase-to-ground fault between the conductor and the enclosure and result in isolation of all phases in the affected circuit.
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Enclosure to SBK-L-12220 Attachment 3 Byron vs. Seabrook Station Switchyard Comparison Motor-operated and manually-operated disconnect switches in the 345kV SF6 GSU and RAT circuits incorporate similar design features as the SF6 bus. These switches are maintained in the closed position during normal operation.
The Seabrook Station Unit Auxiliary Transformers (UATs) are connected to the 25 kV system via air-insulated isolated-phase bus. Each single-phase bus section consists of two concentric tubes of aluminum.
The inner tube is the conductor and the outer tube is the solidly grounded enclosure for each isolated phase.
Insulators (bus support assemblies) are used to support the center conductor within the enclosure. This type insulator keeps the conductor centered and fully supported within the enclosure. Conductorjoints are welded except for connections to equipment which utilize multiple flexible links with bolted connections.
Based on this design, it is not credible that a single insulator failure will result in an open circuit. The conductor bus will remain in position due to the balance of remaining insulators and the rigidity of the bolted connections. This design is not susceptible to the same failure mode as the Byron event where an air-insulated line insulator failed and dropped the conductor. With the air-insulated isolated-phase bus design, should an internal insulator failure occur, it is postulated to initiate a phase-to-ground fault between the conductor and the enclosure and result in isolation of all phases in the affected circuit.
The connections from the RATs and UATs to the station's 13.8 kV and 4.16 kV buses are via three-phase non-segregated phase bus duct. This design consists of all three phase conductors being supported within a common metal enclosure. The bus bars are covered with insulation sufficient to withstand the full line voltage rating. The bus bars are supported internally within the enclosure by either polyester glass insulating channels (5 kV bus) or porcelain insulators (15 kV bus). To connect one section of bus to another, the bus bar conductors are mechanically bolted together. This provides a rigid connection where failure by open circuit is not credible. Multiple flexible links with bolted connections are used to connect the bus conductors to other equipment such as transformer bushing terminals. Again, with this construction, an open circuit failure is not credible. With this design, should an internal insulator failure occur, it is postulated to initiate a phase-to-ground or phase-to-phase fault. For the low resistance grounded 13.8 kV buses, either a phase-to-ground or phase-to-phase fault would result in isolation of the affected circuit. For the high resistance grounded 4.16 kV buses, a phase-to-phase fault would result in isolation of the affected circuit. A phase-to-ground fault would initiate an alarm only.
In addition, extensive Preventive Maintenance (PM) activities have been developed for the Seabrook Station 345kV SF6 gas-insulated switchyard and air switchyard. PMs are included and scheduled in accordance with the Seabrook Station PM program. In-house switchyard qualified resources are utilized to perform these maintenance activities. Included in the scope of the Electrical Maintenance PM activities is an activity for detailed switchyard rounds inspections. This weekly PM activity performs detailed inspections of the entire switchyard looking for abnormal conditions on any equipment. Any physical degradation or failure of SF6 gas-insulated bus work system or structures would be observed and reported during these inspections. The results of weekly switchyard rounds are reviewed by Electrical Maintenance Supervision prior to closeout. Action Requests (AR) are initiated for any unanticipated conditions.
Based on the above, due to Seabrook Station's unique design, configuration of 345kV switchyard and station buses, and proper operating and maintenance practices, the initiating event similar to the Byron event is not credible at Seabrook Station.
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Enclosure to SBK-L-12220 Attachment 3 Byron vs. Seabrook Station Switchyard Comparison Single Line Drawing for Seabrook Station 394 363 369 Tewksbury / Scobie Newington /
Ward Hill Pond Timber Swamp t t NLine t Termination Yard (Air)
GIS Bus New Equipment Installed in 10/09, operable in 04/11 New Equipment Installed in 12/09, operable in 04/11 (Note: all dates shown are approximate) UAT 2A UAT 2B UNIT #1 17
Enclosure to SBK-L-12220 Attachment 4 Tables TABLE 1 - ESF BUSES CONTINUOUSLY POWERED FROM OFFSITE POWER SOURCE(s)
Description of ESF Bus ESF Bus Name (Normal operating Original licensing-Power Source condition). basis configuration-(Y/N)
N/A N/A N/A Note: Seabrook Station's ESF or non-ESF buses are not continuously powered from offsite power sources, therefore, Table 1 is not applicable to Seabrook Station.
TABLE 2 - ESF BUSES NOT CONTINUOUSLY POWERED FROM OFFSITE POWER SOURCE(s)
Description of ESF Bus Power ESF Bus Name (Normal operating Original licensing Source' condition). basis configuration (Y/N)
UAT 2A 4.16kV bus E5 Y UAT 2B 4.16kV bus E6 Y 18
Enclosure to SBK-L-12220 Attachment 4 Tables TABLE 3 - ESF BUSES NORMALLY ENERGIZED MAJOR LOADS ESF Bus Load Voltage Rating Level E5/E6 CHARGING PUMPS CS-P-2A or 2B 4kV 600 HP E5 PCCW PUMPS CC-P- I1A or 1IC 4kV 700 HP E6 PCCW PUMPS CC-P- 11 B or 1ID 4kV 700 HP E5 SERVICE WATER PUMPS SW-P-41A or 4kV 600 HP 41C E6 SERVICE WATER PUMPS SW-P-41B or 4kV 600 HP 41D E5 480V UNITSUBSTATION AND 480V VARIOUS ASSOCIATED MOTOR CONTROL CENTER SUPPLIED LOADS E6 480V UNITSUBSTATION AND 480V VARIOUS ASSOCIATED MOTOR CONTROL CENTER SUPPLIED LOADS Note: Seabrook Station's ESF buses are not powered by offsite sources during normal operation (at power).
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