ML19344D894
| ML19344D894 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 08/20/1980 |
| From: | Counsil W, Fee W NORTHEAST NUCLEAR ENERGY CO. |
| To: | Crutchfield D Office of Nuclear Reactor Regulation |
| References | |
| A01115, A1115, NUDOCS 8008260276 | |
| Download: ML19344D894 (7) | |
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August 20, 1980 Docket No. 50-245 A0lll5 Director of Nuclear Reactor Regulation Attn: Mr. Dennis M. Crutchfield, Chief Operating Reactors Branch #5 U.S. Nuclear Regulatory Commission Washington, D.C.
20555
Reference:
(1) D. M. Crutchfield letter to W. G. Counsil, dated June 27, 1980.
(2) W. G. Counsil letter to D. L. Ziemann, dated November 15, 1979.
Gentlemen:
Millstone Nuclear Power Station, Unit No. 1 Station Electric Distribution System Voltages Reference (1) requested additional information to that previously submitted by Northeast Nuclear Energy Company on November 15, 1979, regarding station electric distribution system voltages. Attached are responses to items 2, 3 and 4 of the Reference (1) letter.
Items 1 and 5 will require additional time and we expect to submit them by November 14, 1980.
A sketch of the Millstone Point Unit No. 1 auxiliary bus system was included in the Reference (2) submittal.
The nomenclature for the 480-volt bus shown in that sketch is incorrect. A corrected copy is attached.
Should you have any questions please contact us.
Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY W. G. GTounsil Senior Vice President By:
W. F. Fee Executive Vice President Attachment 8 008260 9 76 e
I Attachment i
Station Electric Distribution System Voltages
References:
1.
NRC letter (D. M. Crutchfield) to Northeast Utilities (W. G. Counsil) dated June 27, 1980.
- 2. of Northeast Utilities letter (W. G.
Counsil) to NRC (D. L. Ziemann), dated November 15, 1979.
3.
NRC letter (W. Gammill) to all Power Reactor Licensees, dated August 8, 1979.
Question 2.
Ref. 2, Pages 3 and 4 identifies four separate conditions when the +10% overvoltage capability of the motors on the 480-volt buses is exceeded.
Installation of overvoltage monitors is planned.to initiate operator corrective action.
Credit will be given for this corrective action only if the overvoltage monitors and alarms are Class lE, and in the interim period of correction the overvoltage condition does not shorten equipment life or affect the Class lE equipment's ability to perform the required function.
Provide documentation which demonstrates the equipment can meet these overvoltagc conditions.
Response 2.
We have reviewed these conditions and find that one case can be eliminated since it doesn't exceed the limiting voltage. This case is identified below with the data on
-the other three overvoltage conditions.
Case a.
The unit is shut down and the Normal Station Service Transformer (NSST-1), supplied from the main generator step-up, is carrying minimum station auxiliary load (3.33 MVA).
Minimum Load on Plant Auxiliary System is:
4.16 KV Buses A & B - 0 MVA 4.16 KV Buses C & D - 3.33 MVA @.83 pf 480 V Buses
- 4 Buses x 0.33 MVA @.85 pf Bus Voltage Limits (0vervoltage) 4.16 KV Buses - 4420 V 480 V Buses
- 494 V Voltage Reached for System Maximum (362 KV) 4.16 KV Buses A & B - 4256 V 4.16 KV Buses C & D - 4206 V 480 V Buses
- 494 V The 480 V bus voltages reach but do not exceed their limit. The 4160 V buses remain well below their limit.
. Case b.
The unit is shut down and the Reserve Station Service Transformer (RSST-1) is carrying minimum station auxiliary load.
The bus loading and voltage limits in this case are the same as those in Case a.
Voltage Reached for System Maximum (362 KV) 4.16 KV Buses A & B - 4353 V 4.16 KV Buses C & D - 4314 V 480 V Buses
- 507 V None of the 4.16 KV buses experience a voltage in excess of their upper limit. The voltage value on the 480-volt buses is 2.9% above bus maximum.
Case c.
The unit is fully loaded and the NSST-1 is carrying normal station auxiliary load (34.7 MVA).
In this case, the iaaximum voltage is determined by the main generator maximum rather than the system maximum. The voltage limits in this case are the same as those in Case a.
Normal Load on Plant Auxiliary System is:
4.16 KV Buses A & B - 19.44 MVA @.85 pf 4.16 KV Buses C & D - 15.25 MVA 0.83 pf 480 V Bus 12C
.72 MVA 0.79 pf 480 V Bus 120
.71 MVA 0.79 pf 480 V Bus 12E
.39 MVA 0.77 pf 480 V Bus 12F
.79 MVA 0.85 pf Voltage Reached for Generator Maximum (25.2 KV) 4.16 KV Buses A & B - 4287 V 4.16 KV Buses C & D - 4238 V 480 V Bus 12C
- 491 V 480 V Bus 120
- 492 V l
480 V Bus 12E
- 496 V l
480 V Bus 12F
- 491 V The 4.16 KV bus maximum is not exceeded.
The 480-volt bus maximum is exceeded on bus 12E by 0.5%.
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. Case d.
The emergency diesel loads are being carried by the alternate offsite supply (SDT-1). The same voltage limits apply in thit case as in Case a.
Emergency Bus Loading 4.16 KV Bus C or D - 2.85 MVA 0.85 pf 480 V Bus 12E or 12 F
.025 MVA 0.85 pf Voltage Reached for System Maximum (24.15 KV) 4.16 KV Buses C & D - 4249 V 480 V Bus 12E
- 500 V 480 V Bus 12F
- 500 V The 4.16 KV bus maximum is not exceeded in this case. The maximum for the 480-volt bus voltages is 1.4% above the rated maximum.
However, since the plant is at the end of this feeder, it is very unlikely that this maximum would ever be reached.
Also, it is unlikely that this source would ever be used since it is used to supply emergency loads only after the RSST-1 and both emergency onsite power sources have become unavailable.
Case b requires the coincidence of minimum plant auxiliary load with maximum switchyard voltage to reach the level identified. The extent of the overvoltage lessens as the switchyard voltage decreases and the station auxiliary load increases.
The target (normal) voltage for the Millstone switchyard is 357 KV.
Since the station is in a minimum load condition for short periods and the switchyard voltage is normally held at about 357 KV, the motors experience these overvoltages during a small percentage of their life.
Since the overvoltages are of relatively short duration and the magnitude of overvoltage is small, we conclude that the effect of these small overvoltages on the life of the motors is negligible.
The overvoltage monitors to be installed will be Class lE equipment which will alarm through non-Class IE annunciators.
Inasmuch as the overvoltages are marginal and the operators are generally aware of the auxiliary bus voltage levels, Class lE annunciation is not necessary.
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. Question 3.
Ref. 2, Page 5, Paragraph 1 states that the setpoint of the second level of undervoltage protection will be reset to the new value of 336 KV (switchyard voltage). The design of the second-level of undervoltage protection must meet the requirements of IEEE 279-1971.
Provide the undervoltage and time setpoints in terms of Class 1E nominal bus voltage and compare these setpoints as required in Guidelines 10 an,' 12 (Ref. 3).
Response 3.
The level-two undervoltage relays are connected to monitor voltage on the 345 KV side of the RSST-1.
The relays are set at a value of 112 volts secondary, 336 KV primary, based on the level of switchyard voltage required to maintain at least 3620 volts on the 4160-volt buses and 416 volts on the 480-volt buses for all loading conditions.
The 336 KV level cannot be translated into a fixed voltage on the Class lE buses unless a particular loading condition is specified.
If we assume the worst case loading (normal plant load plus running of all LOCA loads) the level-two undervoltage setpoint corresponds to the following voltage levels on the Class 1E buses:
345 KV System Voltage
- 336 KV 4.16 KV Buses 14C & 14D (lE) - 3646 V 480-volt Bus 12E (1E)
- 420 V 480-volt Bus 12F (lE)
- 417 V The time delay associated with the level-two undervoltage shceme is 1.0 second. This time delay is required to prevent false operation of the level-two scheme during voltage trans-ients on the auxiliary bus sytem and transmission system fault clearing. More time delay is not needed to override the starting of large motors because the 345 KV voltage is not affected by these relatively small loads.
We should restate at this time that the loss-of-offsite power and degraded voltage schemes (level-one and level-two under-voltage schemes) are being redesigned to provide monitoring directly from the 4.16 KV Class lE buses.
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, Question 4.
Ref. 2, Page 5, Paragraoh 2 refers to the addition of a second-level of undervoltage protection when the trans-former RSST-1 is carrying Millstone #2 shutdown loads and Millstone #1 Normal and LOCA loads. The design of the second-level of undervoltage (NRC Staff Position 1, June 2,1977 letter) is to protect all Class 1E equipment from grid voltage degradation under all_ modes of operation.
Explain in detail why this second second-level protection scheme is necessary.
The'present logic for the Unit 2 level-two undervoltage Response 4.
scheme will trip the preferred offsite supply for Unit 2 if a degraded voltage is detected on the associated Class lE buses.
If these Class lE buses experience a degraded voltage while being supplied from RSST-1 (the alternate offsite supply) the Unit 2 level-two scheme will still call for a trip of ti.e preferred offsite supply (this supply is already open).
No automatic action would take place to disconnect from the alternate offsite supply.
Recognizing this, we then proposed a second level-two degraded vcitage scheme for Unit 2 which would perform the proper switching to disconnect the alternate offsite supply if it was the source of the degraded voltage.
Upon further investigation, we have found that the level-two sensing relays may be comon for either the preferred or alternate offsite supplies since the voltage minimum on the Unit 2 Class lE buses will be the same for either connection.
However, the logic associated with the level-two scheme must be redesigned to recognize which offsite supply is feeding the Class lE buses and perform the appropriate automatic switching to disconnect the offsite supply which is actually being used. This redesign of the logic asso-ciated with the Unit 2 level-two scheme in no way effects the Unit 1 level-two scheme.
The only relationship between the two schemes is that the Unit 1 preferred offsite source (RSST-1) is the Unit 2 alternate offsite source.
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