ML19291B925

Provides Info on Adequacy of Electric Distribution Sys Voltage.Discusses Nature of Potential Problem

ML19291B925
Person / Time
Site:	Yankee Rowe
Issue date:	11/01/1979
From:	Groce R YANKEE ATOMIC ELECTRIC CO.
To:	Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
TASK-08-01.A, TASK-8-1.A, TASK-RR NUDOCS 7912140370
Download: ML19291B925 (3)

Text

c 6 x 7/ 7 tt f

• Telephone 6l7 366-90ll Twx 710 390-0739 YANKEE ATOMIC ELECTRIC COMPANY g1;1,2, 4L 20 Turnpoke Road Westborough, Massachusetts 01581

,Y A x,ss m.

November 1, 1979 United States Nuclear Regulatory Commission Office of Inspection and Enforcement Region I 631 Park Avenue King of Prussia, Pennsylvania 19401 Attention:

Mr. Boy.e H. Grier, Director

References:

(1) License No. DPR-3 (Docket No. 50-29)

(2) USNRC letter to all power reactors dat-d August 8, 1979 (3) YAEC letter to UP'RC dated October 27, 1979

Subject:

Adequacy of Station Electric Distribution System Voltage As requested in Reference (2), we have been reviewing the electric power systems at the Yankee Nuclear Power Station (Yankee Rowe) to determine if the offsite power system and the onsite power distribution system is of sufficient capacity to automatically start and operate all safety loads, assuming that all onsite power systems are not available.

Although the analysis for Yankee Rowe is not complete, preliminary computer studies have found a potential problem when Yankee Rowe is operating with certain bus tie breakers closed.

Reference (3) reported this potential problem as Licensee Event Report 50-29/79-26.

The following paragraphs provide information on the Yankee Rowe electric system, and also discuss the nature of the potential problem.

Yankee Rowe does not have a conventional auxiliary power system consisting of unit auxiliary transformers, reserve auxiliary transformers and associated transfer facilities between the two c.~'rces.

Attachment No. I shows the main one line diagram at the Yankee Nuclear Power Station.

Electric power from the transmission network to the onsite electric distribution system is supplied by two physically independent circuits. One is the 115 kV Cabot line (Y-177) and the other the 115 kV Harriman line (Z-126).

Both of the lines occupy the same switchyard which is located in close proximity to the turbine generator building.

1502 218 791214 0 3 7D

.q a.

Ur.ited States Nuclear Regulatory Commission November 1, 1979 Attention:

B. H. Grier Page 2 The 115 kV incoming lines (Z-126, Y-177) are tapped on the line sides of the circuit breakers near the switchyard and are connected to the 115/2.4 kV station service transformers No. 2 and 3 respectively, with overhead lines.

The secondaries of the two station service transformers are connected directly to the 2400 volt indoor metal clad switchgear buses No. 2 and 3, and also to the 480 volt switchgear, via two 2400/480 volt station service transformers (No. 5 and No. 6).

The two outer buses (2400 V buses 2 and 3) are thus always connected to offsite power.

The center buses (2400 V bus 1 and 480 V bus 4-1) are connected to the main generator when the main generator is connected to the grid.

When the main generator is not connected to the grid, such as during startup, the center buses are connected to offsite power by closing the normally open bus tie breakers to adjacent buses.

2400 V bus 1 may be tied to 2400 V bus 2 and 480 V bus 4-1 may be tied to 480 V bus 6-3 (or alternately 2400 V bus 1 may be tied to 2400 V bus 3 and 480 V bus 4-1 may be tied to 480 V bus 5-2).

Subsequent to startup, that is, after the generator is synchronized to the grid, the center buses are powered from the main generator and the tie breakers described above are manually opened.

Based on our preliminary computer studies we have found that if the grid voltage is at the minimum expected value (95% of 115 kV), and if the plant is operating with the center buses tied to offsite power as described in the preceeding paragraph, the offsite power system and the onsite distribution system may not be capable of providing the required voltage level at the safety related buses in the event of the receipt of a Safety Injection Actuation Signal.

Our computer studies do not indicate any problem, for all modes of operation when the center buses are fed from the main generator.

We are in the process of confirming these results and arriving at suitable engineering modifications to alleviate the situation.

In the interim we are proposing the following operating procedure for the forthcoming startup which will assure that all safety-related buses are supplied with adequate voltage at all times.

Whenever Yankee Rowe is operating in a mode such that one or more ECCS pump trains are required to be operable, and a center bus is connected to offsite power, a dedicated operator will be stationed in the main control The dedicated operator will be instructed to trip the t'o11owing room.

breakers if a Safety Injection Actuation Signal (SIAS) is received:

(1) bus tie breakers between 2400 V buses 1 and 2 (or between buses 1 and 3)

(2) bus tie breakers between 480 V buses 4-1 and 6-3 (or between buses 4-1 and 5-2)

(3) bus tie breaker BT-2A iSE2 219

United States Nuclear Regulatory Commission November 1, 1979 Attention:

B. H. Grier

• Page 3 The breaker controls are located in the main control room.

These actions will assure that all safety-related buses are supplied with adequate voltage even if the grid voltage is at the minimum expected value.

(This fact has been verified by our computer studies.)

We are presently in the process of completing and then re-running all our computer studies in order to reconfirm the above mentioned problem.

Should this be confirmed we will immediately proceed with the design and engineering of a permament solution.

We wish to point out again that our analysis has revealed no problems whenever the bus tie breakers are opened. With tie breakers opened, the offsite power system and the onsite distribution system is of sufficient capacity to automatically start and operate all required safety loads at the required voltage level.

We trust that you find this information satisfactory; however, should you desire additional information, please contact us.

Very truly yours, YANKEE ATOMIC ELECTRIC COMPANY A 0 YW Robert H. Groce Senior Engineer - Licensing RHG/kaf Attachments

\\cm 223 w

G B

2 1j.

1 i

-e 832 d a J

iii i=.

6 e

es.

3 i.

a g

j s=

s!

1:.

p

!i O

3b

. i s5

~

^

~

p'a f

I y

e

,,<.. o.

}..t;2

!a 5

a g

<o > w>-~=n * *pr h

2 3r a

c i:

W,'.

=

!f a

j j

l

-I 2

=

2 s,,

3 x

s g ; ",

<w o+>

-< M s

+ ' >

E h,

.3 w !;-

'ii' 1

.ns k

p.1 p

=i33 35 e.

a g

--<+-o

.-.mww

)

! OW[

h}

}

12 p,

~:

J s

v n

3 i5j g <+-***--<+*e*/

d, I

f 1

13

"2.

i ali; y

bh 3k is a:

i 1

<o og 53 EI i;'5 d.fi*.

h

1

1 n.4 3

t si i:..

~

sh i

14

~

i m

a=

t 4 +.

>--.e. o>

T 4to 0-+1 2

3g

~.

Q) 3 E h.

^

r I

4 2 ". i s-3

. l.,i eg P.

3f I

e=

3t v

,