ML20086H626
| ML20086H626 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 11/29/1991 |
| From: | Joshua Wilson TENNESSEE VALLEY AUTHORITY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| TAC-M00420B3, TAC-M420B3, NUDOCS 9112090254 | |
| Download: ML20086H626 (4) | |
Text
.
~
i i
d
- u. L, ~
am...
o Jm+ o me n r-19 mt $a tv e f ja e P. t ?
November 29, 1991 U.S. Nuclear Regulatory Commission
_ ATTN:
Document Control Desk Washington, D.C.
20555 Gentlemen:
In the Matter of_
)
Docket Nos. 50-327 Tennessee Valley Authecity
)30-328 SEQUOYAll NUCLEAR PLANT (SQN) - FAULT CURRENT PROBLEM FOR THE 6.9-KILOVOLT -
(kV) UNIT BOARD LOAD BREAKERS (TAC NO A{00420B3)
-Referencew:
1.- TVA letter to NRC dated October 19, 1989, "Sequoyah Nuclear Plant (SQN) - Fault Current Problem For The
-6.9-Kilovolt (kV) Unit Board uoad Breakers" 2.
TVA letter to NRC dated August 10, 1987, "Sequoyah Nuclear Plant (SQN) - Comments on Safety Evaluatloa-l_
Report (SER) -'Short-Circuit Study, Mediu- 'lol tage '
(6.9-kV) System"
-This letter provides the current TVA p an for the resolution of the SQN fault' current problem associated with *,he f.9-kV unit board braakers.
In
!!ef erence __1, TVA cemmitted to install instantaneous overcur r ent relays on-the 6.9-kV unit board incoming breakers to resolva a load breaker fault
-problem. W fault'ptoblem was originally discussed in Reference 2; and althous;h the problem did not constitute a nuc. ear safety concern, TVA commit'.ed to develop a resolution schedule for this condition.
Reference 1_ provided a completion schedule -for -the resolution of the f ault problem by' start-up of Unit 2 f rom the Cycle 5 refueling outage.
A recent-reevaluation of the planned resolution, in. concert with a major electrical supply system modification being implemented for unrelated reasons, determined that the previously planned resolution is not
-required. SQN is presently installing new common station service transformers (CSSTs) with automatic load tap changers. When installation is complete, the CSSTs will be able to provide the units with a better
)( ) 6 I) 91120002",4 9111p
(
PDR A00CK 0:5000327
/'
I P
PDR a
_ _ _. _ _ _ _..... _. _. _ _.. _. ~. -
y
-c-. :
+
7 U.S. Nuclear Regulatory Commission November. 29, 1991 voltage-regulated power supply.
For this reason, SQN will be using the CSSTs-
. inatead of the unit station service transformers (USSTs) as the normal power supply-for the units.
The USSTs will only be used for maintenance purposes af ter this change, which is planned for completion in early _1993 _ To implement this change, operating = procedures will be developed or revised-to accommodate the new breaker alignments for normal CSST power supply and
. operation of_the: automatic load tap changer.
In addition, modifications will be made to the'CSST automatic transfer circuits to provide fast transfer to the spare CSST for transformer faults, which will minimize the impact to the operating units.
The' evaluation of the 6.9-kV unit board fault current has shown that normal alignment to the new CSSTs will reduce the three-phase bolted fault current below the one-time interrupting capability of tbe load breakers. When-connected to the CSSTs the' worst-case-fault current will be 42,/39 ampheres (amps), which translates to 326 mega volt-amps (MVA). This fault current is below the'550-MVA, one-time interrupting capability of the load breakers.
It should.be noted'that without the emergency-diesel generator (EDG) pard leled
-to-;the 6.9-kV shutdown _ board and unit board, this fault current is 40,291 amps and 496 MVA..Tbis_is important because each EDG is normally paralleled to only.one oflthe unit-boards once-a month for approximately one hour at a time.
At'all other. times, the fault current will be.below the 500-MVA design
-rating of the unit ~ board load breaktes.
Taerefore, 6.9-kV~ unit board alignment to the CSSTs will_ reduce 1 fault currents below the one-time
-interrupting' capability of the load breaker at all times and below the breaker
. design rating.for the predominance of operations.
-The significantly reduced potential for this condition, combined with the lack of. nuclear safety-significance, has invalidated the overall merit of the
_previously proposed resolution.
As previously noted, with normal supply from CSSTs, the USSTs would only be used for' occasional maintenance and testing on the 6.9-kV start boards, start buses, or_6.9-kV unit board feeder breakers
-from the CSSTs. Since this use of the'USSTs, along with the unlikely-
-three-phase boltedLfault near the unit board, has little-if any potential for occurring,'the installation of instantaneous ovcrcurrent-devices on unit incoming board breakers is not warranted.
This is based on the high cost of
-specialized overcurrent relays to provide protection for this rarerfault c'ndition on a system alignment that will occur with a minimal frequency.
o
.SQN's. implementation'of 6.9-kV unit board alignment to CSSTs will occur before
~the end of the Unit 1 Cycle 6 refueling outage.
Based on the above. discussions. TVA considers the current plan to install CSSTs with automatic load tap chhngers and to align normal unit board power supply to the CSSTs will result in an appropriate resolution to the outstanding issue on 6.9-kV unit board fault current, i
lt
- m....
-3 d.S. Nuclear Regulatory Commission-November-29.-1991~
A listlof' commitments mede in this letter is included in the enclosure Please direct questions concerning this issue to Keith C. Weller at (615) P43-7527;
-Sincerely, _
{ f){L)\\-
?!v%-
l L= Wilson t\\
' Enclosure.
cc (Enclosure):
-Mr. D. E; LaIsrge, Project Manager
~ U.S.. Nuclear Nogulatory Comnission One. White Flint, North
~
11555'Rockv111e Pike Rockville, Maryland-20852
=
NRC_ Resident Inspector
'Sequoyah. Nuclear Plant 2600;Igou' Ferry Road
- Soddy Daisy, Tennessee 37379 Mr. B. A. Wilson,' Project Chief U.S.' Nuclear Regulatory Commissica Region II 101 Marietta Street._NW, Suite 2900-4
. Atlanta,-Georgia 30323-4
+
ases w
y w
y
+g--evy-.-
ev,---
gm--g--,,gy-yy4--
r-y-
y-w,-y
ENCLOSURE COMMITMENTS 1.
TVA will' implement normal-alignment of the 6.9-kV unit boards to=the cornmon station service-transformers (CSSTs) before the end of the Unit 1 Cycle 6 (U1C6) refueling outage (RO).
2.- TVA will modify the CSST transfer circuits to provide a fast automatic transfer to the spare CSCT for transformer faults before implementing normal alignment'to the 6.9-kV unit boards before the UIC6 RO.
l~
y-r w
a -.
3my
+
W