ML20004A816
| ML20004A816 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom, Arkansas Nuclear  |
| Issue date: | 04/17/1981 |
| From: | Rosa F Office of Nuclear Reactor Regulation |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20004A817 | List: |
| References | |
| NUDOCS 8105260247 | |
| Download: ML20004A816 (7) | |
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e April 17,1981 NOTE TO:
PSB Members FROM:
F. Rosa Acting Chief, PSB SUB.1ECT:
BRANCH TECHNICAL POSITION PSB 1 " ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTAGES"
REFERENCE:
Memorandum to PSB Members from R. Fitzpatiick, dated March 11,1981 Enclosed is the finalized branch technical position (BTP) on the e!'.svo subject.
Thank you for your 'aput in response to the above reference in enabling us to finalite tha rc:,ition in a timely fashion.
This BTP is effective imediately.
It is a preferred method over the old PSB Review Remincer 13 but it in no way detracts from the acceptability of designs in accordance with Review Reminder #3.
.'he guidance of part 4 concerning an acceptable verification testing program should be applied across-the-board to all plants [0Rs OLs & cps] that have not already either provided or proposed on accepted verification testing program.
You should bring the revised criteria of part 1 to the attention of those lict,ees/ applicants that have designs now under review simply for their infornation and inform them that they may opt for the revised design criteria if they so choose. However, this should not become a mechanism to substantially delay implementation of a final design.
This BTP will be part of our Chapter 8 submittal of revised SRPs scheduled for compietior sy May 1, 1981.
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Faust Rosa, Acting Chief
<f,j Power Systems Branch, DSI
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i BRANCH TECHNICAL POSITION PSB 1 ADEQUACY OF STATION ELECTRIC DI5TRIBUTION SYSTEM VOLTAGES e
A.
BACKGROUND i
Events at the Millstone station have shown that adverse effects on the l
Class 1E loads can be caused by sustained low grid voltage cond:tions l
i when the Class 1E buses are connected to offsite power.
These low voltage i
conditions will not be detected by the.?ss of voltag relays (loss of off-site power) whose low voltage pickup setting is generally in the range of
.7 per unit voltage or less.
The above everits also demonstrated that improper voltage protection logic l
can itself cause adverse effects on the Class 1E systems and equipment such as spurious load shedding of Class 1E loads from the standhy diesel generators and spurious separation of Class 1E systems from offsite power due to normal motor startino transients.
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i A more recent event at Arkansas Nucle'ar One (ANO) st. tion and the subsequent t
analysis performed disclosed the possibility of degraded voltage conditions existing on the Cla'ss lE buses even with normal grid voltages, due to deficiencies in equipment between the grid and the Class 1E buses or by the starting transients experienced during certain accident events not originally considered in the sizing of these circuits.
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B.
BRANCH 1ECHNICAL POSITION 1.
In addition to the undervoltage scheme provided to detect loss of offsite t
power at the Class 1E buses, a second level of undervoltage protection with time' delay should also be~provided to protect the Class 1E equipment; this
- second level of undervoltage protection shall satisfy the following criterf a:
a) The selection of undervoltage and time delay setpoints shall be determined from an analysis M the voltage requirements of the Csass 1E loads at all onsite system distribution levels; b) Two. separate time delays shall be selected for the second level of undervoltage protection based on the following conditions:
- 1) The first time delay should be of a duration that establishes the existance of a sustained degraded voltage condition (i.e.,
something longer than a motor : tarting transient).
Following this delay, an alarm in the control room should alert the operator to the degraded condition. The subsequent occurrence uf a safety injection actuation signal (SIAS) should immediately separate the Class 1E distribution system from the offsite power system.
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- 2) The second time delay should be of a limited duration such that the permanently connect J Class IE loads will not be damaged.
Followin,j this delay, is the operator has failed to restore adequate voltages, the Class 1E distribution system should be j
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.. i automatically separated from the offsite power system.
Bases and justification must be provided in support of the actual j
delay chosen.
c) The voltage sensors shall be designed to satisfy the following j
i applicable requirements dirived from IEEE Std. 279-1 971,
" Criteria for Protection Systems for Nuclear Power Generating
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Stations":
- 1) Class 1E equipment shall be utilized and shall be physically
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located at and electrically connected to the Class 1E switchgear.
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- 2) An independent scheme shall be provided for each division of j
the Class 1E power system.
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- 3) The undervoltage protection shall include coincidence logic on a h
per bus basis to preclude spurious trips of the offsite power i
f source; l
- 4) The voltage sensors shall automatically initiate the disconnection l
of offsite power sources whenever the voltace set point and time delay limits, (cited in item 1.b.2 above) have been exceeded;
- 5) Capability for test and calibration during power operation shall be t
i provided.
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- 6) Annunciation must be provided in the control room for any bypasses incorporated in the design.
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c d) The Technical Specifications shall include limiting conditions for operations, surveillance requirements, trip setpoints with minimum and l
maximum limits, and allowable valuet for the second-level voltage protection sensors and essociated time delay devices.
2.
The Class 1E but load shedding scheme should automatically prevent shedding during sequencing of the emergency loads to the bus.
The load shedding feature should, however, be reinstated upon completion of the load sequencing action. The technical specifications must include i
a test requirement to demonstrate the operability of the automatic l
bypass and reinstatement features at least once per 18 months during shutdown.
In the event an adequate basis can be provided for retaining the load I
shed feature during the above transient conditions, the setpoint value l
in the Technical Specifir,ations for the first level of undervoltage I
protection (loss of offsite power) must specify a value having maximum and minimum limits.
The basis for the setpoints and limits selected must be documented.
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3.
The voltage levels at the safety-related buse.s should be optimized for j
the maximum and minimum load conditions that are expected throughout e
i the anticipated range of voltage variations of the offsite power sources l
by appropriate adjustment of the voltage tap settings of the intervening trans formers.
The tap settings selected should be based on an analysis
.. of the voltage at the term'nals of the Class 1E loads.
The analyses l
l perforned to determine minimum operating voltages should typically consider maximum unit steady state and transient loads for events such j
as a un't trip, loss of cooler.t accident, startup or shutdown; with the offsite power supply (grid) at minimum anticipated voltage and only the offsite source being considered available. Maximum voltages should be i
analyzed with the offsite power supply (grid) at maximum expected voltage j
l concurrent with minimum unit loads (e.g. cold shutdcwn, refueling). A
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separate set of the above analyses should be performed for each available
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connection to the offsite power supply.
I 4.
The analytical techniques and assumptions used in the voltage analyses
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cited in item 3 above must be verified by actual measurement. The j
verification and test should be performed prior to ir.itial fill power reactor operation on all sources of offsite power by:
j a) loading the station distribution buses, including all Class 1E
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buses down to the 120/208 v level, to at least 30%;
b) recording the existing grid and Class 1E bus voltages and bus loading down to the 120/208 volt level at steady state conditions and during the starting of both a large Class 1E and non-Class 1E
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motor (not concurrently);
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Note: To minimize the number of instrumented locations, l
1 (recorders) during the motor starting transient tests, the bus voltages and loading need only be
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recorded on that string of buses which previously i
showed the lowest analyzed voltages from item 3 above.
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... c) using the analytical techniques and assumptions of the previous l
1 voltage analyses cited in item 3 above, and the measured existing grid voltage and bus loading conditions recorded during conduct of the test, calculate a new set of voltages for all the Class 1E buses down f
to the 120/.'")8 volt level g d) compare the analytically derived voltage values against the test results.
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With good correlation bet.
9 the analytical results and the test results, j
the test verification requirement will be met. That is, the validity of j
the mathematical model used in performance of the analyses of item 3 will t
have been established; therefore, the validity of the results of the j
t analyses is also established.
In general the test results should hat be l
i more than 3% lower than the analytical results; however, the difference between the two when subtracted from the voltage levels determined in the original analyses should never be less than the Class lE equipment f
rated voltages.
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C.
REFERENCES l
1.
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IEEE Std. 279, " Criteria for Protection Systems for Nuclear Power I
Stations" I
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3.
Millstone Unit No. 2. Safety Evaluation Supporting Amendment No.16 to
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license No. DPR-65 l
A.
NRC Summary of Meeting for Arkansas Nuclear One Incident of September 16,
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1978, dated February 9,1979.
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