ML18085A844
| ML18085A844 | |
| Person / Time | |
|---|---|
| Site: | Salem  |
| Issue date: | 01/19/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML18085A843 | List: |
| References | |
| NUDOCS 8102200212 | |
| Download: ML18085A844 (9) | |
Text
1.0 INTRODUCTION
SAFETY'EVALUATION REPORT SALEM GENERATING STATION UNIT NOS. 1 ANO 2 DOCKET NOS. 50-272 AND 50-311 DEGRADED GRID VOLTAGE PROTECTION FOR THE SAFETY RELATED A-C POWER SYSTEM Enclosure Operating experience has shown that adverse effects on the safety related power system and safety-related equipment and-loads can be caused by susta foed 1 ow or high grid voltage cond_ition.
We therefore requested the Public Service Electric and Gas (PSE&G) to assess the susceptibility of the safety-related electric system at the Salem Generating Station to sustained voltage degradation of the offsite sources and to address three Staff Positions.
After comparing the current design to the Staff Positions, PSE&G was required
- to. either propose modifications to satisfy the positions and criteria or furnish an ana.lysis to substantiate-that~the existing-facility-design has equivalent capabilities.
By letters dated August 1, 1977 and March 3, 1980, ~SE&G proposed certain design modifications to satisfy Staff Positions and criteria. These modifications con-sist of the install at ion of a second-level undervol tage* protection system for the safety-related equipment.
We have* also required that the set'points, sur-veillance requirements, test requirements and allowable limtts be included by PSE&G* in the Salem Unit l Technical Specifications (position number 3) *
. Proposed plant Technical Specifications that address these requirements and
_ refle~t the, proposed design modifications have not been submitted by the licensee for our approval*.
- 2.0 DESIGN BASIS CRITERIA The design.basis criteria that were applied in determining the acceptability of the system modifications to protect the safety-related equipment from a sustained degradation of the offsite grid are: *
(l) General Design Crherion 17 (GDC 17),. "Electrical Power Systems,"
of Appendix A, "General Design Criteria for 'Nuclear Power Plants, 11 of 10 CFR 50.
(2)
IEEE Standard 279-1971, "Protection Systems for Nuclear Power Generating Stations.
11 (3) Staff Positions as detailed in a letter sent to the licensee, dated June 2~ 1977.
(4)
IEEE Standard 308-1974, "Class 1E Power Systems for Nuclear Power Generating Stations.~
(5)
ANSI Standard C84. l-l 977, "Voltage Ratings for Electrical Power Systems and Equipment (60Hz)."
3.0 DISCUSSION.
This section provides a. brief descri.ption of the. onsite distribution system, the existing undervoltage protection available to the Salem Generating. Station and a description of the 1icensee'spr.oposed modifications for the second-level undervoltage protection.
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3.1 OnSite Distribution System The onsite distribution is arranged so that two 4.16 kv vital buses are connected to one Station Power Transformer and the third is connected to the other Station Power Transformer.
The in-feed breakers for each vital bus from the two station power transformers are electrically interlocked to pre-vent paralleling both*~ources through a vital bus.
The breakers also provide the means for transferring between sources in the event of an interruption of power from one source (power transformer).
3.2 Existing* Undervoltage Protection (First Level:. Loss of Power)
The present. design uses undervoltage relays to sense the loss of offsite power.
These relays monitor the 4160 V vital buses.
When the voltage on these buses drops below 70% of its rated~ voltage, the undervoltage relays drop out.
The drop-out action of the relays isolates the buses from the offsite sources, starts.the emergency diesel generator, initiates load shedding, and permits closure of the diesel generator breakers.
. 3. 3
- Modification We required the licensee to comply with the three positions in our letter dated June 2, 1977, by doc_umenting their modified. design and committing to install a second* level of undervoltage protection on the emergency* buses *.
By letters dated August 1, 1977 and.March 3, 1980, the ~icensee proposed.the fostal 1 at ion of* a second level undervol tage protection scheme in response to.Staff Position l. This scheme is. in addition to the first level under-voltage protection scheme described in section 3~2 above.
The second level' undervoltage* protection.scheme will be comprised of two sets of relays: a set for und~rvoltage 11 tr~nsfer 11 and a s_et for generating "blackout" signals.
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! The second level undervoltage protection relays will react instantaneously when the voltage drops below the setpoint of 91% of rated voltage. There will also be an external timer with adjustable time delay.
The time delay of the second level undervoltage "transfer" relay will be 10.5 seconds when the output of the station power transformers is below 91% of the rated voltage.
The second level undervoltage "blackout~ relays will consist of three under-voltage relays per unit, one relay for each of the three 4160 V vital buses and the time delay of the 11blackout 11 relays will be 13 seconds when the voltage on the affected bus (or buses) is below 91% of the rated voltage.
The output from the time in each bus will energize three auxiliary relays.
One auxiliary relay output from.each bus will be combined in a two-out-of-three matrix with
-its redundant--co1.mte~pu.t--from the other two buses.
One of the three two-out-of-three matrices thus formed is assigned to each emergency bus.
The output from the two-out-of-three matrices s_i gnifi es that an undervol tage condition has occurred on at least two buses.
This intelligence is input to each of the three independent safeguards equipment controllers which will act to disconnect the offsite power source from the emergency buses.
When* an undervoltage condition at a 4160 V vital bus (or buses) persists below 91% of the rated voltage for at least 10.5 seconds~ the affected bus_ (or buses) wi_ll be automatically transferred to the alternate source by the action of the vital bus "transfer" relay.
The vital bus "transfer" relays allow the affected.
bus (or buses) to be transferred to the remaining station power transformer be-fore the bus "blackout" relays are actuated. If the supply voltage to the vital buses falls below 91% of the* rated voltage and a transfer is not accomplished,
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I l the second level 11blackout 11 relays will provide a signal to start the diesel generators.
4.0 EVALUATION 4.1 Position 1 - Second Level of Undervoltage Protection The first position of the June 2s 1977 letter required that a second level of undervoltage protection for the onsite power system be provided.
The letter also went on to identify certain criteria that the undervoltage protectfon must meet.
Each criterion has been evaluated against the licensee's proposal and is addressed below.
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{l) The licensee's proposed setpoint of 91% of 4160 V rated system voltage is 94.5% of 4000V the motor rated voltage. This setpoint reflected down to the 480V buses will be greater than 90% of the 1 I.
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i I motor rated voltage of 460 V.
As the motors are the most limit-ing equipment in the system, this setpoint is acceptable.
{2)
The proposed time delay will not be beyond the thennal capability of the safety-related equipment.
The setpoint is within voltage ranges recoTTJTiended by ANSI C84.l-l973 for sustained operation.
(3) The licensee's proposed time delay is long enough to override any short inconsequential grid disturbances. Further, we have reviewed the licensee's analysis and agree with the licensee's finding that any voltage dips caused from the starting of large motors will not trip the offsite source.
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The proposed modification incorporates a two out of three coincidence logic scheme to preclude spurious trips of the offsite sources and thereby, satisfies the criterion.
(5)
The licensee has stated in his proposals that the modifications are designed to meet the necessary requirements of IEEE Standard 279-1971.
The licensee has proposed that the second level undervoltage protection relaying scheme be administratively bypassed during the start of any reactor coolant pump (RCP).
The inrush current associated with the start of a 6000 horsepower RCP motor would *cause the 4 kv vital bus voltage to drop below the 91% setpoint for about 25 seconds, which would needlessly actuate the protection scheme.
Ihe manual bypass of the undervoltage protection scheme on the vital buses will be reset by the control room operator upon completion of any RCP start and an alarm in the control room will be provided for the bypassed status.
In addition to manual reinstatement by the control room operator, a timer of two minutes will be provided to automatically reset the andervoltage protection system. This bypass scheme is in accordance with our requirements and is acceptableo 4.2 Position 2 Interaction of Load Shed Feature The design complies with our position that requires automatic prevention of load shedding of the eme_rgency buses once the onsite sources are supplying.
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- power to all sequenced loads on the emergency buses.
However, the design does not include the capability of having the load shedding feature to be automatically reinstated if the diesel generator incoming breakers are tripped. The Safeguard Equipment Control System that governs the load shedding and sequencing operation provides indication in the control room of whether the 1 oad shedding feature has been reinstated: This feature in each diesel generator is manually reinstated from the ~ontrol room.
The provisions of the design to have reinstatement indication and control of the load shedding feature available to the operator in-the control room will minimize administrative procedural errors from interfering with the avail-ability of emergency power.
Therefore, we conclude that the manual reinstate-ment of the load shedding feature is acceptable.
5.0 CONCLUSION
Based on the information provided by PSE&G, it has been determined that the proposed modifications comply with positions l and 2, and have met all of the staff's requirements and design basis criteria. The modifications, when implemented, will protect the safety.related equipment from a sustained degraded volt.age condition of the offsite power source. Therefore, we conclude that the licensee's proposed design modifications are acceptable.
The licensee* indicated that the Techni.cal Specifications.will 'be modifi~d.
upon our approval of the above'des.ign modifications.
We will issue a separate safety evaluation of the proposed Technical Specifications upon receipt and eva 1 ua ti on.
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6.0 REFERENCES
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Lear, letter dated April 25, 1977 Lear, letter dated August 1, 1977 Gammill, letter dated October l 0, 1979 Schwencer, letter dated March 3, 1980
. JAN 2 9 1961