United States Nuclear Regulatory Commission - Protecting People and the Environment

IE Circular No. 79-02, Failure of 120 Volt Vital AC Power Supplies



CR79002 

                             January 11, 1979 

MEMORANDUM FOR:     B. H. Grier, Director, Region I 
                    J. P. O'Reilly, Director, Region II 
                    J. G. Keppler, Director, Region III 
                    K. V. Seyfrit, Director, Region IV 
                    R. H. Engelken, Director, Region V 

FROM:               Norman C. Moseley, Director, POI, IE 

SUBJECT:            IE CIRCULAR 79-02, FAILURE OF 120 VOLT VITAL AC POWER 
                    SUPPLIES 

The subject document is transmitted for issuance on January 16, 1979. The 
Circular should be issued to all holders of Reactor Operating Licensees and 
Construction Permits. 

Also enclosed is a draft copy of the transmittal letter. 


                                        Norman C. Moseley, Director 
                                        Division of Reactor Operations 
                                          Inspection 
                                        Office of Inspection and Enforcement

Enclosures:
1.   IE Circular No. 79-02
2.   Draft Transmittal Letter

CONTACT: V. D. Thomas, IE 
          49-28180 
.

(Transmittal letter for Circular 79-02 to each holder of an NRC Operating 
Licensees.) 

                                                      IE Circular No. 79-02 

Addressee: 

The enclosed Circular 79-02 ia forwarded to you for information. If there 
are any questions related to your understanding of suggested actions, please
contact this office. 


                                        Signature 
                                        (Regional Director) 

Enclosures:
1.   IE Circular No.79-02
2.   List of IE Circulars 
       Issued in the last 
       12 months 
.

                              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 AC POWER 

Description of Circumstances: 

On September 16, 1978, Arkansas Nuclear One - Unit 2, while in hot 
functional testing preceding initial criticality, suffered a degradation of 
both of the independent off-site power sources. the produced an undervoltage 
condition on the Engineering Safety Features (ESF) Busses and caused an 
inadvertent ESF actuation. The license determine that the ESF actuation 
occurred on a loss of at least two of the uninterruptable Immediate 
investigation  revealed that all four of the Solidstate Controls, Inc., 
(SCI) inverter static switches had automatically transferred the alternate 
power supply (the 120 volt 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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IE Circular No. 79-02                                     January 16, 1979 

(c)  The SCI inverter static switch is designed to transfer to an alternate 
     source on inverter output overcurrent or undervoltage. Initially, these
     trips were set at 125 percent overcurrent and 80 percent undervoltage. 
     An explanation for the transfer of two of the inverter static switches 
     is that during one of the transfers of site loads, the instantaneous 
     inductive load caused these trip set points to be exceeded. The 
     icensee has subsequently adjusted the transfer sensing circuitry to 
     higher overcurrent and lower undervoltage trip settings to assure that 
     the inverter will maintain load over the widest possible range of 
     voltage and current. (Note that in this case, one the SCI inverter 
     transfers to the alternated source, it will not automatically transfer 
     back to normal source.) 
     
All holders of operating licenses should be aware of the potential problems 
of the types noted above. It is recommended that the following items be 
considered in you review of this matter: 

1.   Determine whether or not time delay circuitry is used in your inverter 
     units. If so, have they been adjusted to the appropriate set point as 
     required by equipment and the integrated system designs? 

2.   Determine if the AC input voltage and transformer tap settings are 
     optimized to prevent exceeding the inverter component nameplate maximum
     rate DC input voltage in the event of a high AC input voltage transient

3.   If an alternate 120 volt source is used in your design, determine if 
     the protection transfer circuitry of the inverter has been optimized 
     within design limits to ensure maximum possible available  optimized 
     within design limits to ensure maximum possible availability of the 
     inverter system during transient loading conditions An operating 
     history of inexplicable transfer my be indicative of the above. 
     
4.   Determine if the administrative controls employed by your facility 
     ensures operability of safety systems after it subcomponents (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 permits
for their information with regard to preoperational and startup testing. 

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IE Circular No. 79-02                                     January 16, 1979 

No written response to this Circular is required. If you require additional 
information, contact the Director of the appropriate NRC Regional Office. 

Attachment:
Figure 1 

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