ML20195G571
| ML20195G571 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 10/14/1987 |
| From: | Trevors G NEBRASKA PUBLIC POWER DISTRICT |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| CNSS876107, NUDOCS 8710200502 | |
| Download: ML20195G571 (6) | |
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Nebraska Public Power District CNSS876107 October 14, 1987 g
a o C C SG U. S. Nuclear Regulatory Commission M 55 Washington, DC 20555
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> C" Attention:
Document Control Desk p
Subject:
Unplanned Automatic Actuations of Diesel Generators y
Reference:
Letter from W. G. Long to G. A. Trevors, dated August 26, 1987; Same Subject
Dear Mr. Long:
In accordance with the request for further information regarding unplanned automatic actuations of the Diesel Generators (DGs) contained within the referenced letter, the following response is provided.
For clarity in presentation, the specific information requests are noted and are followed by our responses.
Requested Information 1.
Provide a description of your reporting criteria and changes thereof relating to reportability of Diesel Generator starts.
Response
Prior to January 1,1984, the CNS Technical Specifications did not require reporting of unplanned automatic actuations of an Engineered Safety Feature (ESF).
Hence, no Licensee Event Reports (LERs) specifically addressing unplanned automatic DG actuations were submitted.
However, with the issuance of 10CFR50.73 and its subsequent incorporation into the CNS Technical Specifications, a significant change in reporting criteria was initiated.
Our current reporting criteria fully complies with the requirements soecifieJ in 10CFR50.73.
In an effort to further investigate the noted absence of I.ERs associated with unplanned automatic DG actuations subsequent to Januarr 1, 1984, but prior to the event reported in LER 86-012, a detailed review of the Control Room logs was conducted.
From this review, seven (7) instances were found of which five (5) had not been reported.
This reporting failure on our part has since been corrected, and attached to this letter is a summary description of each of these five (5) unplanned automatic DG actuations.
8710200502 871014 00 PDR ADOCK 05000298 0
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LER 85-013, dated November 21, 1985, reported an unplanned automatic DG actuation due to inadvertent de-energizing of a 125V DC Distribution Panel while shut down on October 22, 1985. This event was documented in the Control Room log and was the last of the previously noted seven (7) instances found during our review.
Documented as part of the corrective action on the nonco'iformance report (NCR) submitted for this event ns the notation that any unplanned actuation of an ESF was to be reported as an LER.
Prior to this time frame, actuation of the Diesel Generator as an ESF included not only starting the units, our present understanding, but also tying the units to their respective 4160V critical switchgears.
This sequence takes about ten (10) seconds and will only occur if the respective critical switchgear is de-energized for a like amount of time. We are confident that, subsequent to the event on October 22, 1985, all unplanned automatic DG actuations have been documented as NCRs and appropriately dispositioned.
Our review of Control Room logs after the October 22, 1985 event, through June 10, 1986, when an unplanned automatic DG actuation next occurred and was reported (LER 86-012) confirms Cooper Nuclear Station staff awareness and compliance to existing 10CFR50.73 reporting criteria.
This "awareness" on the part of appropriate station staff members is a result of a better understanding of the NRC's reporting requirements as provided by NUREG 1022.
Requested Informatig 2.
Provide a description of the instrumentation which results in automatic DG startup without loading when disturbances occur in the offsite power sources.
Response
Disturbances which occur in the off-site power sources are sensed by undernitage relays designed to monitor voltages on transformer secondary windings and 4160V AC buses, including the IF and 1G critical buses.
In the event of a disturbance on the 69KV Transmission System, which normally is not supplying power to the IF or 1G 4160V critical buses, undervoltage relays 27/ET-1 and 27/ET-2 may trip.
These relays are listed in Table 3.2.B of the CNS Technical Specifications with setpoint limits specified to be 2900 15% V, five (5) il second delay.
Th? time delay feature of these relays is applicable for the voltage setpoint noted; i.e. if voltage should decrease to 2900 5% V, relay activation will be delayed for five (5) 1 seconds.
l However, if voltage is lost or if line voltage is substantially less than 2900 15% V, relay actuation is for all practical purposes, instantaneous.
Tripping these relays result in starting of the Diesel Generators.
Similar undervoltage protection and relay logic is provided for the Startup Transformer which is powered from the 161KV Transmission System.
Upon loss of voltage, the Diesel Generators are automatically started.
Although the Emergency Diesel Generators receive an automatic start signal from the above noted relay logic, the generators are not connected to the IF and 1G critical buses unless both the normal and emergency power supplies to the IF and 1G 4160V critical buses are lost.
When off-site disturbances l
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October 14 1987
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-which only affect the 69KV Transmission System occur, the normal power supply breakers to the IF and 1G 4160V critical buses are not affected and therefore, remain closed. As a result, the conditions required to cause the Diesel Generators to function as the power source for the critical buses do not exist.
Requested Information 3.
Provide a description of any measures being taken to reduce the nunber of such events in the future.
Responsa Since January 1,1984, to date, a total of 19 instances of unplanned automatic DG actuations have been recorded. Nine (5) of these have been due to offsite power disturbances of the 69KV transmission line, one each during 1984 and 1985, three in 1986, and four, thus far, in 1987.
To date, disturbances of the 161XV Transmission System due to lightning have not been experienced. As evidence of our concern with the increased frequency of lightning induced voltage fluctuations / outages of the 69KV Transmission System, an evaluation of lightning protection afforded the 69KV transmission line was noted in LER 87-017 (July 31, 1987) to be in progress. The evaluation has revealed the following:
a)
The lines are shielded and appear to be in good condition, b)
The right-of-way is well maintained and free of interference.
In summary, the outages experienced do not appear to have been caused by inadequate lightning protection or substandard design and construction.
Subsequent to completion of this evaluation, an engineering effort was initiated to evaluate the undervoltage relay scheme, setpoints, and logic for the 69KV transmission line. This effort is ongoing, at present.
It should also be noted that no design changes have been made at CNS that would have led to the increased event frequency experienced in 1986 and 1987.
To further investigate the appannt increased frequency of offsite power disturbances due to lightning, an effort was made to determine the incidence of thunderstorms in the southeastern Nebraska area during 1986 and 1987 as contrasted with 1984 and 1985.
Information was obtained from the National Climatic Data Center and National Weather Service Forecast Offices for the closest reporting stations to this area; namely, Omaha, 70 miles to the north; Lincoln, 70 miles to the northwest; and Kansas City, 120 miles to the south.
l The numerical data obtained is in the form of thunderstorm days. To explain, I
one thunderstorm may occur on one day; three on another.
In each case, one thunderstora day is tabulated. The information obtained included the Mean Values for the 30 year period extending from 1951 through 1980 and data for the years 1985, 1986, and 1987. Data from 1984 was not immediately availabic.
i Tabulated below is the data provided which reflects cumulative thunderstorm days for the months of May through August at each reporting station for the 1951-1980 time frame and years 1985, 1986, and 1987.
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October 14 1987 Page 4 Thunderstorm Days (May-August)
Time Period Lincoln Omaha Kansas City 1951-1980 (Mean Values) 30.9 32.8 33.4 1985 30 28 33 1986 37 40 40 1987 31 37 37 The data indicates an increased frequency (in the order of 20%) in 1986 over the 30 year Mean Value for all three reporting locations.
For 1987, a frequency increase (in the order of 12%) was documented in two of the three reporting locations, Lincoln being the exception.
The relevance of this data to the area surrounding CNS (and the route taken by the 69KV transmission line) is somewhat suspect since thunderstorm activity is considered to be a localized meteorological event. Nevertheless, it does reflect conditions experienced in areas which surround the site on three of four sides. Substantive changes in weather conditions at those stations, as is reflected in the data, may well be typical of weather pattern changes in this localized area.
Hence, the apparent increased frequency of lightning induced voltage fluctuations on the 69KV line experienced these past two years may well reflect an increased incidence of thunderstorms in this area.
As noted previously, an engineering effort, with the overall objective of reducing the number of challenges to the Diesel Generators due to offsite power disturbances, is in progress. We will advise you of the outcome of this effort upon its completion.
In the meantime, snould you have any further questions regarding the information provided, please contact Mr. G. R. Horn at Cooper Nuclear Station.
Sincerely, h
G. A. Trevors Division Manager Nuclear Support GAT:DLR:tlb(COMM16)
Attachment cc:
U.S. Nuclear Regulatory Commission Regional Office - Region IV Arlington, Texas NRC Resident Inspector Cooper Nuclear Station t
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g-UNPLANNED. AUTOMATIC-DIESEL GENERATOR ACTUATIONS.
r IN 1984 AND 1985 WHICH WERE NOT PREVIOUSLY REPORTED
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June 24, 1984:.
12:03 p.m. - Both #1 and #2 Diesel Generators automatically s
started due to a momentary power interruption on the. 69 kV emergency power transmission line..
While not-noted in the log, due to the fact that a.69 kV power supply breaker operation was recorded and noted as being momentary, lightning is suspected to have been the cause.
At the time, the plant was in operation at approximately 80 percent power. Operation was not affected by this event.
May 3, 1985:
11:52 a.m.
- Upon re-energizing the 125V DC B Distribution Panel which had inadvertently -been de-energized prior to transferring its loads, both Diesel Generators started.
Due to design of 4160V AC relaying, a loes and subsequent restoration of 125V DC control power will result in 4160V Breaker actuation.
At the time of this event, DC control power for both 4150V Breakers 1FA and 1GB, the Normal Power Supply Breakers for 4160V 1F and 1G critical switchgear, respectively, were being supplied from the B Distribution Panel.
Upon its restoration, after being inadvertently de-energized, both breakers tripped and both Diesel Generators started.
Also, upon the trip of Breakers 1FA and
- 1GB, 4160V Breakers IFS and 1GS atitomatically closed, re-energizing the 4160V 1F and 1G critical buses from the Emergency Transformer.
When this event occurred, the plant was in cold shutdown, with the refueling / pipe replacement outage in progress.
Design change activities associated with Equipment Qualification concerns were in progress.
Their implementation required de-energizing of the 125V DC Battery Buses, one at a time.
May 24, 1985:
9:38 a.m.
- Upon re-energizing the 125V DC B Distribution Panel subsequent to automatic tripping of its supply breaker, 4160V Breaker 1GB tripped, causing #2 Diesel Generator to start.
The trip of the 1GB Breaker occurred as a result of the 4160V AC relaying logic design discussed previously.
The cause of the problem with the 125V DC B Distribution Panel was due to a ground which was determined to have been caused by incorrect terminations of wiring for the Turbine Control System.
At the time, the plant was in cold shutdown, with the refueling / pipe replacement outage in progress.
A Turbine Control System computer changeout project was in progress.
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r-July 30,.1985:
8:09 a.m. - Both #1 and #2 Diesel Getierators automatically started due to an apparent voltage fluctuation on the 69 ' kV emergency power transmission line.
As noted-in the log, lightning was believed to have been the cause.
There was no 69 ' kV Breaker operation recorded.
At the
- time, the plant was in cold
- shutdown, with the refueling / pipe replacement outage.in progress.
August 15, 1985:
11:33 a.m.
- 2 Diesel Generator automatically started while performing design change acceptance-testing.
The design change was associated with re-routing of control power cables for relay contacts which cause 4160V Breaker 1GS to trip and 1GE to close when an undervoltage condition on the Emergency Station Transformer is sensed.
This design change was required as part of the Appendix R compliance program.
At the time, startup preparations were in progress to return the station to service upon completion of the refueling / pipe replacement' outage.
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