ML19289C734
| ML19289C734 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 01/16/1979 |
| From: | James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| To: | Mary Johnson SOUTH CAROLINA ELECTRIC & GAS CO. |
| References | |
| NUDOCS 7901220192 | |
| Download: ML19289C734 (1) | |
Text
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pa micti UNITED STATES y,.
jo, NUCLEAR REGULATORY COMMISSION
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101 M ARIETT A STR E ET, N.W.
A4 ATLANT A, GEORGIA 30303 o
'%.*....l JAN 161979 In Reply Refer To:
RII:JPO 50-395 South Carolina Electric and Gas Company Attn:
Mr. M. C. Johnson, Vice President and Group Executive - Special Services and Purchasing P. O. Box 764 Columbia, South Carolina 29218 Gentlemen:
The enclosed Circular 79-02 is forwarded to you for information.
If there are any questions related to your understanding of the suggested actions, please contact this office.
Sincerely,
\\
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w,ad C_,N h.L mes P. O'Reilly D ector M
Enclosures:
1.
.)
2.
List of IE Curculars Issued in the last 12 :nonths cc w/ encl:
H. I. Donnelly, Jr.
Quality Assurance P. O. Box 8 Jenkinsville, South Carolina 29065 T. B. Conners, Jr.
Conners, Moore and Corber 1747 Pennsylvania Avenue, N. W.
Washington, D. C.
20006
,901220I Q
UNITED STATES NUCi2AR REGULATORY COMMISSION OFFIC'i 0F INSPECTION AND ENFORCEMENT WASHINGTON, D.C.
20555 January 16, 1979 IE Circular No. 79-02 FAILURE OF 120 VOLT VITAL AC POWER SUPPLIES Description of Circumstances:
On September 16, 1978, Arkansas Nuclear One - Unit 2, while in hot functional testing preceding initial criticality, suffered a degrada-tion of both of the independent off-site power sources. This produced an undervoltage condition on the Engineering Safety Features (ESF) Buses and caused an inadvertent ESF actuation.
The licensee determined that the ESF actuation occurred on a loss of at least two of the uninter-ruptable 120 volt vital AC power sources.
Immediate investigation revealed that all four of the Solidstate Controls, Inc., (SCI) inverter static switches had automatically transferred to the alternate power supply (the 120 volt AC being supplied by transformers directly off the ESF buses rather than from the inverter output.)
Figure 1 (attached) shows the functional interconnection of the 120 volt AC power supply system.
A single conclusive cause of the undesired SCI inverter static switch transfer could not be identified. However, the following problems were discovered during subsequent investigation.
(a)
The SCI inverters have circuitry to monitor the incoming DC voltage level which, on sensing a low voltage (nominally 104 DC volts for these inverters) will trip both input breakers to the inverter component af ter an adjustable time delay.
The setting of these time delay relays were not verified during either preoperational testing or subsequent maintenance. The time delay is necessary in order to accommodate transient loading conditions which may be encountered.
(b) fa one SCI inverter, a DC fuse within the inverter component was found blown.
The vendor indicated that this fuse will blow due to an excessive DC voltage to the inverter component caused by a transient on the 480 AC input.
The licensee has subsequently reset the taps on the 480V to 120V three-phase AC input transformer to limit the DC voltage to the inverter to less than the nameplate maximum rating in the event of a high AC input voltage transient.
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