ML14118A195

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04/24/2014 Meeting Slide for TVA Watts Bar Nuclear Plant, Unit 2 Presentation Degraded Voltage Protection
ML14118A195
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Site: Watts Bar Tennessee Valley Authority icon.png
Issue date: 04/24/2014
From:
Tennessee Valley Authority
To:
Office of Nuclear Reactor Regulation
Dion J
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ML14132A270 List:
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Download: ML14118A195 (28)


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Watts Bar Nuclear Plant Degraded Grid Voltage ProtectionApril 24, 2014 2AgendaObjectivesHistoryConformance with NRC Guidance PSB

-1Response to Supplemental Safety Evaluation Report 22, Open Item 30Additional Degraded Voltage Protection ScenariosConclusions 3ObjectiveTo describedegraded voltage protection system installed at Watts Bar Nuclear PlantToprovide information on TVA's resolution for Supplemental Safety Evaluation Report 22, Open Item 30 for Watts Bar Unit 2To provide information regarding case studies that addressother degraded voltage scenariosTo obtain alignment on actions needed to resolve Open Item 4History -Industry EventsMillstone 1stEvent -July 5, 1976Adverse effects on Class 1E loads caused by sustained low grid voltageNot detected by LOV relays that were generally 70% (at that time)Millstone 2nd Event

-July 21, 1976Improper voltage protection logic can itself cause adverse effects on Class 1E systems and loadsSpurious load shedding of loads from EDGsSpurious LOOP Due to normal motor starting transientsArkansas Nuclear One Event

-September 16, 1978Degraded voltage conditions can exist on Class 1E buses even with normal grid voltageDeficiencies in design between grid and Class 1E busesStarting transients during LOCA sequencing not considered in circuit design 51977 NRC letterInformation Notice 79

-04 , "Degradation of Engineered Safety Features," on February 14, 1979Generic Letter (GL) 79

-36, "Adequacy Of Station Electric Distribution Systems Voltages on August 8, 1979" Branch Technical Specification PSB

-1, "Adequacy of Station Electric Distribution System Voltages (PSB

-1)," July1981PSB-1 is a compendium and clarification of preceding guidance History -Regulatory Guidance 6TVA installed degraded voltage protection to support the operation of Watts Bar 1 (second level of protection referred to in PSB

-1) Same protective scheme that will support operation of Watts Bar 2Voltage relays (sensors) at Class IE buses (6.9 kV Shutdown Boards)Two out of three trip logicSingular time delayLonger than motor starting transientLess than accident analysis limit for transfer to onsite powerControl room annunciationInitiates transfer to onsite power supplyHistory -Implementation at Watts Bar Nuclear Plant 7 7 8TVA Analysis

-WBN LOV and DVR Relay Settings 80%90%100%95%85%5600V, MV Mot Stall5934V, LOV AL5967.6V, LOV DO6000V, LOV Nom6036V, LOV PU NOM6060V, LOV PU6072V, LOV PU OL6555V, DV AL6570V, DV DO6600V, DV Nom6642V, DV Nom Rst6672V, DV PU6681V, DV PU OL 1 2 3 4 5 6 7 8 10 11 121.14.48.511.5DV TDLOV TDFSAR Limit Note: Loss of Voltage (LOV) and Degraded Voltage Relay (DVR) Settings follow Branch Technical Position PSB

-1 guidanceVoltage(Percent/Volts)Time(Seconds)9 9PSB-1 Technical Position B.1.a:Watts Bar selection of undervoltage and time delay setpoints was determined from analysis of voltage requirements of Class 1E loads at all onsite system levelsPSB-1 Technical Position B.1.b.1 and B.1.b.2Watts Bar degraded voltage protection includes single time delay

-is a more restrictive design than required by guidanceTime delay is

8.5 seconds

(Safety Limit lower value)Time delay provides both alarm and separation functionsTime delay is greater than motor starting transientWatts Bar degraded voltage protection logic does not include SIAS signal input:If grid voltage is degraded, time delay will transfer to onsite powerAny subsequent design basis accident would be powered directly and immediately from onsite power sourceConnected loads demonstrated to not be damaged during time delayWBN Degraded Voltage Protection Conformance with PSB

-1 10PSB-1 Technical Position B.1.cDegraded voltage relays conform to Class 1E and redundant design elements B.1.c.1 -6PSB-1 Technical Position B.1.dTechnical Specification LCO 3.3.5

-Loss of Power (LOP) Diesel Generator (DG) Start Instrumentation Function 2

-6.9 kV Emergency Bus Undervoltage (Degraded Voltage)Trip SetpointAllowable ValueBus UndervoltageTime DelayTechnical Specification surveillance requirementsSR 3.3.5.1; SR 3.3.5.2; and 3.3.5.3WBN Degraded Voltage Protection Conformance with PSB

-1 11PSB-1 Technical Position B.2Watts Bar's degraded protection logic does not cause loads to be shed from the diesel generators PSB-1 Technical Position B.3Station connected to a qualified gridDemonstrates offsite sources are qualified per GDC -17 requirementsVoltage drop of the grid during a DBA is CSST tap changers are functioning properly.All 6.9kV Shutdown Boards are aligned to CSST A (worst

-case alignment).1E buses remain connected to offsite power and 1E components have adequate voltage for starting and running within analyzed time limits.See Next SlidePSB-1 Technical Position B.4Testing was performed during startup of WBN Unit 1WBN Degraded Voltage Protection Conformance with PSB

-1 12Minimum Qualified Grid Case (GDC-17 Offsite Power Analysis)DVR Drop OutDVR ResetLOV Drop Out 13Open Item 30:TVA should confirm that all other safety

-related equipment (in addition to the Class 1E motors) will have adequate starting and running voltage at the most limiting safety related components (such as motor operated valves, contactors, solenoid valves or relays) at the degraded voltage relay setpoint dropout setting. TVA should also confirm that the final Technical Specifications are properly derived from these analytical values for the degraded voltage settings. (Section 8.3.1.2)Essential Elements of Open ItemConfirm adequate performance for running equipmentConfirm adequate starting performanceConfirm Technical specification adequacy Response to Supplemental Safety Evaluation Report, Open Item 30 14Steady-state loading conditions (running load):The DVR dropout setting was previously established using TVA/industry setpoint methodology to confirm that all safety

-related equipment (in addition to the Class 1E motors) have adequate running voltage at the most limiting components (such as MOVs, contactors, solenoid valves and relays) These studies were carried out by maximizing running loads on the Class 1E distribution system (bounding combination of safety system loads), with the voltage at 6.9

-kV Class 1E buses (monitored by the DVRs) at the DVR dropout settingNo credit was taken for any equipment operation (such as Load Tap Changers) upstream of the 6.9

-kV Class 1E buses

.The technical specifications were derived from the analytical values for the DVR dropout and time delay settings

.Response to SSER Open Item 30 15Transient loading conditions (starting load):Because voltage monitoring relays are not suited for protection against inadequate motor starting voltage, TVA performed a sensitivity study to assess the potential impact of considering starting transients on existing DVR setpoints. STUDY-EEB-WBN-12-001 was performed to address adequacy of previously established DVR settings for motor starting voltageThe motor-starting studies were based on the DVR dropout and time delay analytical limitsThe motor-starting studies did not credit any equipment operation (such as LTC transformers) upstream of the 6.9

-kV Class 1E busesResponse to SSER Open Item 30 16Purpose was to study the impact on the DVR setpoint or plant response if the existing TVA DVR setpoint methodology was changed to include motor starting voltage as a basis for establishing DVR dropout setting.The analytical approach used was based on the fact that for any Class 1E motor starting event (single motor, group of motors, or DBA sequence) the voltage at the DVR monitored bus can only respond in one of three ways:

1.The voltage never drops below the DVR dropout setting (i.e. DVR never actuates) 2.The voltage drops below the DVR dropout and never recovers above reset within the DVR time delay (i.e. DVR actuates and times out, transfers loads to the DGs)3.The voltage drops below the DVR dropout but recovers above resetwithin the DVR time delay (i.e. DVR actuates but does not time out, loads remain on offsite power)Analyses were performed for each of these situations. These analyses bound any possible voltage outcome at the Class 1E buses when starting any Class 1E motor(s) required for any scenario.TVA Sensitivity Study for SSER Open Item 30STUDY-EEB-WBN-12-001 17Provided in submittal dated June 7, 2012Starting Voltage Analysis (for Scenario 1)Protective Device Analysis (for Scenario 2)Bounding Voltage Recovery Analysis (for Scenario 3)TVA Sensitivity Study for SSER Open Item 30STUDY-EEB-WBN-12-001 18Starting Voltage Analysis (Scenario 1)The analytical approach chosen demonstrates acceptability of voltages during any motor starting scenario where the DVR monitored bus never drops below the DVR dropout setting. Study also validates that Class 1E components will operate anytime voltage recovers above the DVR dropoutVoltage at the DVR monitored bus set at the DVR dropout analytical limit using a fixed voltageThis simple technique provides conservative bounding voltages for all 1E loads for all times after event initiationNo credit taken for LTCs or any other voltage impact upstreamTwo cases analyzed: DBA load initiation events Individual (manual) motor starting (e.g. required post

-event motor starts)Conclusion:Validates that all Class 1E motors will start when the Class 1E bus voltage is at or above the DVR dropout setpoint.

19Validates that for the period that DVR relay is timing out, Class 1E components:Will not be damaged if they stallWill not be locked out by a protective device (including control power fuses)Validates that the required Class 1E components will successfully start once connected to the DGs.Protective Device Analysis (Scenario 2) 20Bounding Voltage Recovery Analysis (Scenario 3)Station connected to a degraded gridGrid voltage is at 100%.CSST tap changers are not functional.Grid capability is reduced so that at T=0 sec. the voltage is below the LOV drop out but above the Medium Voltage Motor stall voltage (they accelerate).All Shutdown Boards are aligned normally.This case is most representative of a realistic degraded grid condition.Conclusion:During a DBAIf the LOV relay times out, Safety Related busses disconnect from off site power and transfer to the DGsSafety Related components have adequate voltage for starting and running within analyzed time limits.If the LOV relay does not time out, Safety Related busses remain connected to off site powerSR components have adequate voltage for starting and running within analyzed time limits.

21DVR at Dropout Prior to Design Basis Accident Grid Capability at DVR Reset Threshold Additional Case Studies 22DVR at Dropout Prior to DBA (Does Not Reset)Station connected to a degraded gridGrid voltage results in voltage at DVR drop out value (6555V) prior to DBA

.CSST load tap changers are assumed not functioning.Fixed voltage source was modeled at the station switchyardAll Shutdown Boards are assumed aligned normally.Note: Without the load tap changer, this case can not be successfully recovered by offsite power since the post

-event loading is greater than the pre

-event loadingConclusion:Class 1E buses disconnect from off site power and transfer to the DGsBased on previous analysis (protective device analysis), Class 1E components have adequate voltage for starting and running within analyzed time limits 23DVR at Dropout Prior to DBA DVR Does Not Reset

-1E Buses Transfer to DGsDVR Drop OutDVR ResetLOV Drop OutTransfer to DG @ 8.5 -

11.5 SecStays on Off SiteOff Site Volt Not AcceptableTransfers to DGOff Site Volt Acceptable 24Grid Capability at DVR Reset ThresholdStation connected to a degraded gridGrid capability (voltage level and strength) results in initial voltage drop that barely recovers above LOV setpoint prior to 0.5 sec and then recovers to (exactly) the DVR reset value prior to 8.5 sec.CSST load tap changers are not functioningFixed voltage source was modeled at the station switchyardAll Shutdown Boards are aligned normallyNote: This case, while not representative of any actual postulated grid failures, results in the worst conceivable grid that allows Class 1E buses to remain on offsite powerConclusion:This case demonstrates that any conceivable voltage transient caused by motor starting, even at the threshold of LOV timeout AND DVR timeout, will result in recovery of voltage (DVR reset or above) within 4 seconds.Recovery time not significantly different than that shown for offsite power analysis (minimum qualified grid) 25Grid Capability at Degraded Voltage Reset ThresholdWorst Possible Grid that Allows Adequate Recovery of VoltageDVR Drop OutDVR ResetLOV Drop OutDoes Not Transfer to

DGStays on Off SiteOff Site Volt Not AcceptableTransfers to DGOff Site Volt Acceptable 26Overall Conclusion for Additional Case StudiesFor WBN voltage protection setpoints and design, during a DBA:If the LOV relay times outClass 1E buses disconnect from offsite power and transfer to the DGsBased on previous analysis, Class 1E components have adequate voltage for starting and running within analyzed time limitsIf the LOV relay does not time out, then either:The DVR times outClass 1E buses disconnect from off site power and transfer to the DGsBased on previous analyses, Class 1E components have adequate voltage for starting and running within analyzed time limitsORThe DVR does not time outClass 1E buses remain connected to off site powerBased on previous analyses, Class 1E components have adequate voltage for starting and running within analyzed time limits 27Watts Bar followed industry and NRC guidance in the development, design, and settings of the degraded voltage relay scheme.Watts Bars offsite power analysis demonstrates compliance with GDC-17 requirements.The degraded voltage relay scheme ensures protection of safety related motors and componentsThe Watts Bar DVR scheme being reviewed for U2 operation is the same as that licensed for U1 operation.Conclusions and Summary 28Due to the voltage protection system and auxiliary power system design at Watts Bar the most likely outcome of any motor starting event including a DBA sequence is the voltage may decrease as low as the LOV relay setpoint but will still recover above DVR reset well within the DVR timeout.For any cases that do not recover above reset, either the DVR or LOV time delay settings allow successful transfer to the onsite power supply with all required Class 1E loads achieving adequate voltage with required time periods.No revision to degraded voltage relay setpoints requiredConclusions and Summary