NLS2007006, Response to Request for Additional Information Regarding Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power
| ML070360038 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 01/31/2007 |
| From: | Minahan S Nebraska Public Power District (NPPD) |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NLS2007006 | |
| Download: ML070360038 (7) | |
Text
N Nebraska Public Power District Always there when you need us 50.54 NLS2007006 January 31, 2007 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001
Subject:
Response to Request for Additional Information Regarding Generic Letter 2006-02, "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power."
Cooper Nuclear Station, Docket No. 50-298, License No. DPR-46
Reference:
I. Letter from Catherine Haney, U.S. Nuclear Regulatory Commission, to Holders of Licenses for Operating Power Reactors listed in Enclosure 1 dated December 5, 2006, "Request for Additional Information Regarding Resolution of Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power."
- 2. Letter from Randall K. Edington (Nebraska Public Power District) to U.S.
Nuclear Regulatory Commission dated March 30, 2006, Response to NRC Generic Letter 2006-02, "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power" (NLS2006008)
- 3. NRC Generic Letter 2006-02; "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power," dated February 1, 2006 The purpose of this letter is for Nebraska Public Power District (NPPD) to provide the additional information requested by Nuclear Regulatory Commission (NRC) in Reference 1. Cooper Nuclear Station (CNS) had provided a response in Reference 2 to the questions asked by Generic Letter 2006-02, Reference 3, to enable the NRC to determine compliance with regulatory requirements governing electric power sources and associated training. Subsequently, the NRC requested additional information from licensees in the form of six questions in enclosures to Reference I of which five were asked of CNS. The CNS response to this request for additional information is contained in the attachment to this submittal. NPPD is not making any commitments as a result of this response.
By copy of this letter and its attachments, the appropriate State of Nebraska official is notified.
Copies to the NRC Region IV office and the CNS Resident Inspector are also being provided in accordance with 10 CFR 50.4(b)(1).
COOPER NUCLEAR STATION P.O. Box 98 / Brownville, NE 68321-0098 Telephone: (402) 825-3811 / Fax: (402) 825-5211 www.nppd.com
NLS2007006 Page 2 of 2 Should you have any questions or require additional information, please contact Paul Fleming, Licensing Manager, at (402) 825-2774.
I declare under penalty of perjury that the foregoing is true and correct.
Executed on: _ . O6 (j*.]
- 4 i&J .ygA-.
Date Sincerely, WILMA M.WERNER Sincerely, MY COMMISSION EXPIRES October26, 2010 tewart B. Minahan Vice President-Nuclear and Chief Nuclear Officer
/em Attachment cc: Regional Administrator w/attachment USNRC - Region IV Cooper Project Manager w/attachment USNRC - NRR Project Directorate IV-1 Senior Resident Inspector w/attachment USNRC - CNS Nebraska Health and Human Services w/attachment Department of Regulation and Licensure NPG Distribution w/attachment CNS Records w/attachment
NLS2007006 Attachment Page 1 of 4 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING GENERIC LETTER 2006-02, "GRID RELIABILITY AND THE IMPACT ON PLANT RISK AND THE OPERABILITY OF OFFSITE POWER" The Nuclear Regulatory Commission specific requests for additional information are shown below in italics with the Cooper Nuclear Station (CNS) response immediately following shown in block format.
- 1) SwitchyardMinimum Voltage:
In response to question I (g) you did not identify specific minimum switchyardvoltage limits (k V) that you supplied to the local transmission entity. Pleaseprovide the following.
What is the specific minimum acceptableswitchyardvoltage included in yourprotocol agreement with your grid operator (GO) and what was the basis for this value?
Attachment 2 of the Interface Operating Agreement (IOA) between CNS and the grid operator (GO) identifies conditions of switchyard bus voltage, Real Time Contingency Analysis (RTCA) alarm states, generator output, system frequency, load shedding and system flow which require the GO to notify the CNS Control Room as soon as possible.
Two methods of monitoring the adequacy of offsite power voltage are used. The preferred method (RTCA) uses a minimum acceptable voltage of 3952 volts (v) at the secondary side of the offsite power transformers. The transformers are included in the GO's RTCA and powerflow models and 3952v corresponds to the minimum acceptable secondary side voltage supplying CNS. The alternate method monitors the primary side voltage for the offsite power transfonners and produces alarms to the GO at minimum acceptable switchyard voltages of 167.5 kV for the 161 kV line supplying the Startup Station Service Transformer (SSST) and 70.0 kV for the 69 kV line supplying the Emergency Station Service Transformer (ESST). These alarms are based on annual load studies conducted by the GO. The minimum acceptable primary side voltage for CNS is 167.5 kV and 70.0 kV to SSST and ESST respectively during worst case system conditions, i.e. heaviest line loading, with one line or generating station out of service.
Minimum acceptable post trip voltage for the RTCA has been established based on the upper value of the Second Level Undervoltage Relay (SLUR) Reset Band. The basis for using 3952v on the secondary side is to provide margin above the upper value of the SLUR Reset band which is calculated to be 3915v using a General Electric type of setpoint methodology. The difference between 3952 and 3915 allows for minor discrepancies between actual and calculated post trip voltages and provides for operating margin. The 3952 voltage is a calculated post trip alarm value that GO receives for either source, SSST or ESST. For the purpose of the calculation, CNS provided the worst case calculated station loads to the GO. The GO model assumes a loss of the CNS generator with a concurrent worst case loading to the SSST or the ESST as applicable.
NLS2007006 Attachment Page 2 of 4 The basis for the primary side voltage values of 167.5 kV for the SSST and 70.0 kV for the ESST are derived from an analysis that ensures adequate voltage to supply required loads during the worst case grid loading condition from the previous year. This analysis is reviewed at least annually to ensure the study remains current as required by the bOA.
Minimum acceptable post trip voltage has been established as the upper value of the SLUR Reset Band. This is a conservative voltage since, as confirmed by engineering review of relay drift data, actual drift is much less than the values assumed in the calculation. Using the upper band of the reset setpoint provides additional conservatism since the nominal trip setpoint is typically used for protective relaying.
How is this value related to your technicalspecification degraded voltage relay setpoints?
CNS performs an Off-Site AC Power Alignment surveillance to verify switchyard voltages are greater than minimum acceptable. As noted above, 167.5 kV on the SSST and 70.0 kV on the ESST primary side verifies adequate voltage for required equipment.
The SLUR setpoint is required by Technical Specification (TS) to be greater than or equal to 3828v and less than or equal to 3932 volts, and actual SLUR trip setpoint is calibrated to the midpoint of this range which is 3880 volts. TS are silent with regard to the SLUR reset function. The minimum allowable voltages for the RTCA are selected to provide sufficient margin above the reset function as discussed in the answer to the previous question.
- 2) Loss of Real-Time Contingency Analysis (RTCA) Capabilitv:
Your response to question 2()g did not identify the actions that would be taken if the online analysis tool (softwareprogram) that you rely upon for off ite power operability became unavailable. If the online grid analysis tool that you use to determine i/the offsite power is inoperable becomes unavailable, what actionswould you take to determine ifpost-trip voltage will be acceptable, including any other compensatoly actions, until the post-trip voltage is confirmed to be adequate?
CNS operates using RTCA as the preferred method and uses SSST and ESST primary side voltages as the alternate method. CNS normally uses both methods concurrently. If the GO informs CNS that the RTCA is unavailable for any reason, Emergency Procedure 5.3GRID directs CNS Control Room Operators to verify grid voltage to be above minimums using the SSST and ESST primary side voltages until the RTCA can be restored. No other compensatory measures are required. If the SSST or ESST are supplying any critical 4160v bus whose voltage becomes degraded or is oscillating less than 4050v, then Control Room Operators contact the GO for line status and predicted reliability, and consider reducing loads on the critical busses. If critical bus voltage continues to degrade and cannot be maintained above 3950v (indicated), then the offsite power line and its respective station transformer (161 kV and SSST, or 69 kV and ESST) are declared inoperable and applicable Technical Specification LCO actions are taken.
NLS2007006 Attachment Page 3 of 4
- 3) Verification of RTCA Predicted Post-Trip Voltage:
Your response to question 2(g) indicates that you have not verified by procedure the voltagespredictedby the online grid analysis tool (softwareprogram,) with actual real plant trip voltage values. It is important that the programs usedfor predictingpost-trip voltage be verified to be reasonably accurate and conservative. What is the range of accuracyfor your GO's contingency analysisprogram?
The accuracy of the calculated bus voltages primarily depends on the accuracy of the inputs to RTCA, such as reactive power flow, and will be 0.01% at most buses. The power input mismatch at most buses is much smaller than the tolerance required for the RTCA iterative program to converge. Generally, the post-trip voltage will be quite accurate; 0.01% can be expected at most buses.
Why areyou confident that the post-trip voltage calculated by the GO's contingency analysisprogram (that you are using to determine operabilityof the offsite power system) are reasonably accurateand conservative?
RTCA solutions have correlated well with results from the annual load studies that are the basis for the primary side voltage limits of the SSST and ESST. The annual load studies continue to confinn the validity of the RTCA. When the RTCA is in service and solving acceptably, the primary voltages (alternate method) are still monitored at the same frequency as they were prior to the implementation of the RTCA. Therefore, the combination of continued monitoring, continued evaluation of grid and grid growth by NPPD Transmission Planning, and the favorable comparison between the two methods results in a high degree of confidence with regard to the RTCA.
What is your standard of acceptance?
The acceptance criteria for the offsite power source voltage is the upper band of the SLUR reset function as previously discussed.
- 4) Identification ofApplicable Single Contingencies:
In response to question 3(a) you did not identifj; the loss of other criticaltransmission elements that may cause the offsite power system (OS) to degrade, other than the loss of the nuclear unit. f/it ispossibleforspecific critical transmissionelements (such as other generators,criticaltransmissionline, transJormers,capacitorbanks, voltage regulators, etc.) to degrade the OSP such that inadequatepost-trip voltage could result, have these elements been included in your N-1 contingency analysis?
Yes, the RTCA uses actual grid configuration as a starting point and evaluates the loss of critical transmission elements on the NPPD transmission system, including the worst case element if it is out of service. The alternate methodology evaluated the loss of any transmission line to the CNS 345 kV Switchyard and the corresponding generation
NLS2007006 Attachment Page 4 of 4 stations at the other end of the transmission lines. When more than one generation facility exists, the worst case generation facility is evaluated.
When these elements are inchlded in your GO's contingency analysis model andfailure of one of the transmission elements could result in actuation ofyour degradedvoltage grid relay, is the offsite power declared inoperable?
Yes, the off-site power is declared inoperable in accordance with CNS Emergency Procedure 5.3GRID as noted in the response to Question #2. Exceeding the established voltage limits will result in declaring the appropriate offsite power source inoperable.
If not, what is your basisfor not declaringthe offsite power inoperable?
CNS declares offsite power inoperable based on the appropriate voltage limits for the established contingencies specified in Emergency Procedure 5.3GRID.
- 6) Interface With Transmission System OperatorDuringi Extended PlantMaintenance:
How do you interface with your GO when on-going maintenance at the nuclearpower plant, that has been previously coordinatedwith your GO/for a definite time frame, gets extended past that planned timefirame?
In the event that on-going plant maintenance is extended past the planned time frame and is risk significant to off-site power, CNS contacts the GO and inforns them of the changed work duration. The GO then coordinates the rescheduling of other transmission system work if it is dependent upon the completion of maintenance at CNS. According to the IOA, CNS will coordinate with the GO to jointly plan and schedule upcoming work affecting the CNS Area Grid with CNS outages and on-line station maintenance work.
ATTACHMENT 3 LIST OF REGULATORY COMMITMENTS@
ATTACHMENT 3 LIST OF REGULATORY COMMITMENTS© Correspondence Number: NLS2007006 The following table identifies those actions committed to by Nebraska Public Power District (NPPD) in this document. Any other actions discussed in the submittal represent intended or planned actions by NPPD. They are described for information only and are not regulatory commitments. Please notify the Licensing Manager at Cooper Nuclear Station of any questions regarding this document or any associated regulatory commitments.
COMMITMENT COMMITTED DATE COMMITMENT NUMBER OR OUTAGE None PROCEDURE 0.42 REVISION 20 PAGE 20 OF 27