ML18078B293
| ML18078B293 | |
| Person / Time | |
|---|---|
| Site: | 05000000 |
| Issue date: | 01/16/1979 |
| From: | NRC OFFICE OF INSPECTION & ENFORCEMENT (IE) |
| To: | |
| References | |
| IEC-79-2, NUDOCS 7901290277 | |
| Download: ML18078B293 (4) | |
Text
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Circular r10. 7~-r:2
,'>-'>,.;/Date" *January 11=
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OF 120. VOLT VlT_AL AC POt~ER SUPPLI.ES
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on of Circumstances:
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Septemberl6,-1978i. Arkansas* Nuclear One -
U~it-2,. while ir. hct anal testing preceding ini_tial criticality, suffered a deg!"a~a:ian*
both.* of the. independent off-s.i te power* sources. This produc_ed an
.undervoltage conditiorr on the Engineering Safety Features (ESF) Buses
- and* caused* an inadvertent ESF actuation. The licensee determined tha:.
the* ESF actuation occurred on a loss of at least two of the unintel""-
. -*~ ruptable 120 volt vital AC power sources.
Ir.mediate investiga:ion
'\\' revealed that all four of the Solidstate Controls, Inc. 1 (SCI) inverter
- ' s.tati c switches had automatically transferred to the alternate power
.. *supply (the 120 volt* AC being supplied by transformers directly off t~e.
. c:.
- '.:*,. ESFbuses. rather than from the inverter output). Figure l (atta-chec~
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... shows. the. functional interconnection of the 120 volt AC power supply
-~system.
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A~ingle conclusive cause.of the undesired SCI inverter static switch
- transfer* could not be identified. However, the followin~ probier.s were
..,_discovered during subsequent investigation.
~:
(a.)* The SCI inverters have circuitry to monitor the incor.iing 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 settings of these time delay relay~ were not verified either preoperational testing or subsequent maintenance.
time delay 1s necessary fn order to accorrrnodate transien~
. conditions which may be encountered.
during The loading (b)
On one SCI inverter, a DC fuse within the inverter component was found blown.
The vendor indicated that thf s 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 maxilTlJm rating fn the event of a high AC input voltage transient.
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.sci.* inverter static sw1ich.. is designed to transfer to' an*
terna te* source on.inverter output overcurrent or undervo 1 tag*~. * *
- tially, these trips were set at 125 percent ~vercurrent and
- 80. percent undervoltage.
An ~xplanation for the transfer of t-.'~
'the inverter static switches is that during one of the trans~~rs
'*/:/of site loads,. the instantaneous inductive load caused these.t!"i:;
- .set points to be.exceeded.
The licensee has subsequently ad~:;:;:~~:
the transfer sensing c.ircuitry to higher overcur.rent and lowe!"'
undervol tage trip settings to assure that the inverter wi 11 mai.ntain load over the widest possible range of voltage and current~
(Note. that: in this~ case, once the SCI inverter transfers to the.
.*: ~OaJternate source,. it will not automatically transfer back tc nor=:-a.l
- * *. sour*ce.)
.._.
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-.All holders of operating licenses should be aware of the potential problems of the types* noted above. It is reconr..ended that the follcwing
- .. *.* i:tems be considered in. you.r review of this matter:
l..... Detennine whethe.r* or not time delay circuitry is. used in your
~ *fnverter unit~. If so. have they been adjusted to the aporocriate.
set point as required by equipment and the integrated syste!':'l
. 2.
designs*?
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Detennine if the AC input voltage and transfome!" tap settir.~s
_are optimized to prevent ex~eeding the inverter component name-
. pl ate maximum rated DC input voltage in the event of a high AC input voltage transient.
- 3. * *If an alternate 120 volt source is used in your desi;n, detemine
- .if the protection transfer* circuitry of the inverter has bee!'l optimized* within design limits to ensure rr.aximu~ possible avail-ability of the inverter system during transient loading conditions.
An operating history of inexplicable transfers may be indicative of the above.
- 4.
Oetennine if the administrative controls emoloyed by your facility ensures operability of safety systems after its subcomoonen.ts (e.g., time delay relays. switches, etc.) have been subjected to maintenance or testing.
This Circular is also being forwarded to all holders of construction pennits *for their information with regard to preoperational and startup testing. *
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,,~g,:'l~;~tT IE Circular tlo.
Date:
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No,w~-itten response this Circular is required~ If you. requi,.i:
additional information regarding this matter, contact the Directoro7
<the appropriate NRC Regional Office.
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Attachment:
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