ML19263B630
| ML19263B630 | |
| Person / Time | |
|---|---|
| Site: | Catawba  |
| Issue date: | 01/16/1979 |
| From: | James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| To: | Dail L DUKE POWER CO. |
| References | |
| NUDOCS 7901220266 | |
| Download: ML19263B630 (1) | |
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ATLANTA, GEORGI A 30303 5.,..v 4p[
o, JAN 161979 In Reply Refer To:
RII:JPO 50-413, 50-414 50-488, 50-489 50-490, 50-491 50-492, 50-493 Duke Power Company Attn:
Mr. L. C. Dail, Vice President resign Engineering P. O. Box 2178 Charlotte, North Carolina 28242 Gentlessen:
The enclosed Circular 79-02 is forwarded to you for information.
If there are any questions related to your undet.,tanding of the suggested actions, please contact this office.
Sincerely, CbY wmes P. O'Reilly D rector
Enclosures:
1.
List of IE Curculars Issued in the last 12 months cc w/ encl:
D. G. Beam, Project Manager Catawba Nuclear Station P. O. Box 223 Clover, South Carolina 27910 J. T. Moore, Project Manager Cherokee Nuclear Station P. O. Box 422 Gaffney, South Carolina 29340 790122 C,26(a
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF 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 thst the ESF actuation occurred on a loss of at least two of the uninter-ruptable 120 volt vital AC power sources.
It:: mediate 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 after 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) On 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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