ML18058B093

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LER 92-038-00:on 920825,reactor Tripped.Caused by Loss of Preferred AC Bus Y-20 Coincident W/Blown Fuse in Second Channel of Reactor Protective Sys.All Fuses in Sys Tested for Proper sizing.W/920924 Ltr
ML18058B093
Person / Time
Site: Palisades Entergy icon.png
Issue date: 09/24/1992
From: HILLMAN C T, SLADE G B
CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-92-038, LER-92-38, NUDOCS 9209300150
Download: ML18058B093 (9)
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  • consumers Power POWERING MICHIGAN'S PROGRESS Palisades Nuclear Plant: 27780 Blue Star Memorial Highway, Covert, Ml 49043
  • September 24, 1992 Nuclear Regulatory Commission Document Control Desk Washington, DC 20555 DOCKET 50-255 -LICENSE DPR-20 PALISADES PLANT GB Slade General Manager LICENSEE EVENT REPORT 92-038 REACTOR TRIP CAUSED BY A LOSS OF THE PREFERRED AC BUS Y-20 WITH A BLOWN FUSE IN A SECOND CHANNEL OF THE REACTOR PROTECTIVE SYSTEM *Licensee Event Report (LER)92-038 is attached.

This event is repqrtable in accordance with 10CFR50.73(a)(2)(iv) as an event that resulted in the automatic actuation of the reactor protective system (RPS).

Gerald B Slade _Genera 1 Manager CC Region III, USNRC NRC Resident Inspector

-Palisades Attachment 9209300150 920924 PDR ADDCK 05000255 S PDR A Q1ltS ENE"RGY COl'v1f'AN'r*

'* NRC Form 366 19*83) U.S. NUCLEAR REGULATORY COMMISSION APPROVED OMB NO. 3160-0104 EXPIRES: 8/31/86 . LICENSEE EVENT REPORT (LERI FACILITY NAME 111 DOCKET NUMBER 121 PAGE 131 Palisades Plant 0 I 5 I 0 I 0 I 0 I 2 I 5 I 5 1 I OF 0 j8 mLEl41 REACTOR TRIP CAUSED BY A LOSS OF THE PREFERRED AC BUS Y-20 COINCIDENT WITH A BLOWN l<'TTC:l<'

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-EVENT DATE 161 LEA NUMBER 161 REPORT DATE 161 OTHER FACILITIES INVOLVEO 181 SEQUENTIAL REVISION FACILITY NAMES MONTH DAY YEAR YEAR NUMBER NUMBER MONTH DAY YEAR N/A o I 6 I .o I o I o I I --ol a 21 s 9 2 912 o I 31 s o lo 019 214 912 N/A 0151010101 I THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR I: ICh<<k on..°'_,,.

of the following/

1111 OPERATING MODE 191 N 20.4021bl 20.4061c1 x 60. 7311112Hivl 73.711bl ......_ -......_ 20.40611111

)(i) 60.361cll1 I 60.731*1121M 73.71 lei: ......_ -......_ POWER LEVEL 110) 20.40611111

)(ii) 60.361cll21

60. 7 311)(2)(viil OTHER !Specify i.n Abltr1c1 -NAME 20.40611111

)(iii) 20.40611111 llivl 20 .406111111 Iv) --......_ -60.731111211il

-60.731*11211ii)

-60. 7 31111211iii)

LICENSEE i;:oNTACT FOR THIS LEA 1121 ......_ 60. 7 31111211viiillAI below and in Text, 60. 731111211viiillBI NRC Form 366AI 60.731*112llxl TELEPHONE NUMBER Cris T.

Staff Licensing Engineer 6 ARIEA,COIDE6 I 11sl41-lalsl1l3 COMPLET.E ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT 1131 MANUFAC* REPORTABLE MANUFAC* REPORTABLE CAUSE SYSTEM COMPONENT TUR ER TO NPRDS CAUSE SYSTEM COMPONENT TURER TO NPRDS I I I I I I I I I I I I I I I I I I I I I I I I I I I I SUPPLEMENTAL REPORT EXPECTED 1141. .MONTH DAY YEAR EXPECTED ... SUBMISSION . YES Vf yu, EXPECTED SUBMISSION DATEI. nNO DATE 1161 1 I 1 2 I a 91 2 ABSTRACT !Limit to 1400 spacu, i.e .. *pproximate/y fihHn sngi.-11><<0 typewritten lino*) 1161 On August 25, 1992 at 0129 hours0.00149 days <br />0.0358 hours <br />2.132936e-4 weeks <br />4.90845e-5 months <br />, with the plant operating at 100% power, and .all systems in a normal full power al igrflient, the reactor tripped. The reactor trip resulted from a loss of the preferred ac bus Y-20,

  • coincident
  • with a blown fuse in another portion of reactor protective system (RPS). The automatic .reactor trip was successfully COllllleted with no major plant equipment .problems identified.

The plant was maintained in hot shutdown for evaluation of the trip.

  • The cause of this event was the loss of preferred ac bus Y-20 that *resulted in the loss of one channel of the reactor pr,otect.ive system (RPS), coincident with a blown fuse in a second chaMel of the RPS, which resulted in the initiation . of an RPS trip signal. Preferred ac bus Y-20 was lost due to the failure of inverter ED-07. It was later determined that improper internal wiring coMecting' the transformers within ED-07 caused accelerated aging of the transformer coils and resulted in the failure of two transformers

.. Corrective*

action for this event included checking all the fuses .in the RPS system for proper sizing, COllllleting a Procurement Engineering Checklist for. the SOLA transformers to identify critical characteristics to be used during the procurement*

process, .. updating plant drawings and vendor manuals of .the inverter's to properly show the use of functionally equivalent SOLA transformers, and reviewing this event* with electrical maintenance department J)e*rsonnel

  • to reinforce the importance of requiring attention to the proper coMect ion of COlll>Onents that have multiple windings.

Additional corrective action regarding the pressurizer level control circuitry is being developed and will be reported in a supplemental licensee event report.

NRC F01m 388A 18-831 FACILITY NAME 111 Palisades Plant EVENT DESCRIPTION LICENSEE EVENT REPORT (LERI TEXT CONTINUATION DOCKET NUMBER 121 LER NUMBER 131 U.S. NUCLEAA REGULATORY COMMISSION APPROVED OMB NO. 3160-0104 EXPIRES: 8/31186 PAGE 141 SEQUENTIAL.

' REVISION YEAA NUMBER NUMBER Q 5 Q 0 0 . 2 5 5 9 2 -

0 3 8 -0 0 0 2 OF Q 8 On August 25, 1992 at 0129 with the plant operating at 100% power and all systems in a normal full power alignment, the reactor tripped. The reactor trip resulted from a loss of the preferred .ac bus Y-20 [ED;PL], coincident with a blown fuse in a second channel of the reactor protective system (RPS). Preferred ac bus provides ac power to the Engineered Safeguards System loads including the reactor protective system (RPS) [JC]. The automatic reactor trip was successfully completed with no major plant equipment problems identified.

The control room immediately initiated emergency operating procedures (EOPs) EOP-1, "Standard Post Trip arid EOP-2, "Reactor Trip Reccivery." The required actions for the EOPs were successfully completed and the* procedures were exited.

  • During the actions taken for the reactor trip, it was noted that the "B" channel of the RPS was de-energized indicating a loss of Y-20 .. Pressurizer level control was operating in the "B" channel which is powered from therefore, the control room immediately swapped pressµrizer level control to the redundant "A" channel, in accordance with the Off-'Normal Procedure (ONP) 24.2, "Loss of Preferred.AC Bus Y-20," and expected additional automatic charging pump starts of the standby charging pumps to be.initiated
  • based bn pressurizer level. After a short period of time with no . additional charging pump starts, the control room operators manually st*rted an additional charging pump in accordance with the EOPs to regain pressurizer level. At the time the control room operators manually started a charging pL1mp, pressurizer level was approximately 32% with the pressurizer level control setpoint at 42%.-The plant was maintained in hot shutdown for evaluation of the trip.*

ac bus Y-20 was re-energized through the bypass regulator at approximately 0330 hours0.00382 days <br />0.0917 hours <br />5.456349e-4 weeks <br />1.25565e-4 months <br /> on August 25, 1992. On August 26, 1992, at 0925 hours0.0107 days <br />0.257 hours <br />0.00153 weeks <br />3.519625e-4 months <br /> preferred ac bus Y-20 was returned to its normal power supply and the limiting condition of operation (LCO) action statement was exited.* This event is to the NRC in accordance with 10CFR50.73(a)(2)(iv) as an event that resulted in the automatic actuation of the reactor protective system (RPS). CAUSE OF THE EVENT The cause of this event was the loss of preferred ac bus Y-20 that resulted in the loss of one channel of the reactor protective (RPS), coiricident with a blown fuse .iri a second channel of the RPSj which resulted in the initiation of an RPS trip signal.

  • Preferred bus Y-20 was lost due to the failure of inverter ED-07. It was later determined that improper internal wiring connecting the transformers within ED-07 caused accelerated aging of the transformer coils and resulted in the failure of two transform*ers.

This involved the failure of equipment important to safety.

NRC FOfm 388A U.S. NUClEAR REGUlATORY COMMISSION APPROVED OMB NO. 3160-0104 EXPIRES: 8131 /86 LICENSEE EVENT REPORT !LERI TEXT CONTINUATION FAClllTY NAME 111 DOCKET NUMBER 12) lER NUMBER 131 PAGE 141 SEQUENTlAl REVISION YEAR NUMBER-NUMBER Palisades Plant Q 5 0 0 Q 2 5 5 9 2 -

Q 3 8 -Q Q Q 3 . OF Q 8 ANALYSIS OF THE EVENT Failed Preferred ac Bus Y-20 Preferred ac bus Y-20 is a 120V ac electrical distribution panel that provides ac power to the Engineered Safeguards electrical loads. Y-20 is normally powered from the station batteries through an inverter.

The iriverters in use at Palisades employ output transformers to filter and*regulate the output of the inverter.*

The inverter uses three, one-third capacity, constant voltage transformers made by SOLA Electric Co. to perform this task. These SOLA transformers are a static magnetic voltage regulator*

that include capacitors as part of a ferro-resonant SOLA supplies their units a complete package; transformer, terminal block, and several capacitors.

History of SOLA The original SOLA transformers were supplied with a single primary winding for input connections.

Current plant drawings and vendor manuals reflect this wiring configuratlon.

  • In 1980, SOLA informed the inverter manufacturer that the original SOLA transformers with a single primary winding were no longer available.

SOLA recommended a substitute with two primary windings.

For the substitute to be used at Palisaed jumpers would be

  • used to prop.erly connect the transformers.

The pl ant drawings or vendor manuals were not changed to of In 1981 another substitution was made by SOLA. The 1981 substitute included multiple primary windings and capacitors.

Again, the use of jumpers to properly connect the transformers was required; however, the plant drawings and vef!dor manuals were not changed. In 1984 transformer T-21 was replaced in ED-07. The replacement SOLA transformer that *was removed from stock was equipped with multiple input connections.

Jumpers were properly attached; however, the source of the information to install the jumpers was not documented in the maintenance order package, and was also not reflected in plant drawings or the vendor manual. In 1985, SOLA suggested that all three transformer/capacitor pairs be replaced as a group. This would ensure that each of the three paralleled SOLAs equally load.

.*. NRC Form 388A .(11-831 FACILITY NAME 111 Palisades Plant LICENSEE EVENT REPORT !LERI TEXT CONTINUATION DOCKET NUMBER 121 LEl'I NUMBEJ'I 131 U.S. i'jUCLEAR REGULATORY.

COMMISSION APPROVED OMB NO. 3160.¢10" EXPIRES: 8/31186 PAGE l"I SEQUENTIAL l'\EVISION . YEAR NUMBER NUMBER Q 5 Q 0 Q 2 5 5 9 2 . -Q 3 8 . -Q Q Q 4 OF Q 8 In July 1986, contrary to the information provided by SOLA, only two transformers (T-20 and T-22), but all sets of capacitors were replaced in ED-07. The work order summary for the replacement did not indicate that transformers T-20 and T-22 had the required jumpers iTistalled.

Current Status Following the *reactor trip on August 25, 1992, troubleshooting revealed that the output voltage from preferred ac bus Y-20 had dropped to 24 volts. Further troubleshooting indicated that inverter ED-07 had failed and that transformers T-20 and T-22 within inverter ED-07 had sh6rted windings and replacement.

Based on equipment history obtained in 1985 concerning replacement of SOLA transformers, all three SOLA transformers T-21.and T-22) their "matching" capacitors would need to be replaced.

Three SOLA transformers were in stock and were used as replacements for the failed transformer components.

Post maintenance testing of the transformers the output voltage of . inverter ED-07 to be unsatisfactorily low at Troubleshooting identified that the replacement*

transformers had four primary windlngs with different connection options. The plant drawings and the controlled vendor had not been updated for correct placement of jumpers to provide the required output voltage, therefore, no jumpers were installed during the replacement of the damaged transformers.

Jumpers were subsequently following engineering direction.and review of the vendor manual that 'was shipped with 'the replacement SOLA transformers

.. The output-voltage was ft.illy acceptable after the installation of the necessary jumpers..

  • Since the SOLA transformers that failed (T-20 and T-22) on August 25, 1992 had been in *service since 1986, a question was raised as to whether or not inverter ED-07 was of providing power putput to meet both normal and emergency loads. An analysis was performed to determine both normal operating and emergency power demands on E0-07. The analysis concluded that the normal continuous load on E0-07 is 4,390 volt-amps while the emergency load on E0-07 adds an additional 1,082 volt-amps for a total of 5,472
  • volt-amps.

During the 1992 refueling outage, technical specifications surveillance procedure (TSSP) RT-80, "Engineered Safeguards System -Right Channel" was performed.

This TSSP tested inverter ED-07 with both normal loads and emergency loads. T_he results of TSSP RT-80 demonstrated that the inverter ED-07 was fully capable of both normal .and emergency loads. Furthermore, preventive maintenance is performed on the .r

  • inverters.using the perioqic and predetermined activity control system (PPACS). PPAC SPS019, which the output voltage of the transformers in inverter ED-07 was last performed on March 17, 1992, reviewed.

This review that the measured output voltage of the transformers in inverter ED-07 were acceptable.

In addition, all three transformers were from inverter ED-07 and were sent to CPCo's Equipment and Services laboratory for testing. Of the three transformers, T-21 had* been properly connected since 1984 therefore, it was available for testing and evaluation.

Transformers and T-22 were examined to determine the cause of their failures.

Preliminary test results fodicate that the transformers operate at a higher*

1 *.** 1 NRC Form 388A 19-831 U.S. NUCLEAR REGULATORY COMMISSION APPROVED OMB NO .. 3160-010' EXPIRES: 8/31186 LICENSEE EVENT REPORT (LERI TEXT CONTINUATION FACILITY NAME 111 DOCKET NUMBER 121 LER NUMBER 131 PAGE l'l SEQUENTIAL REVISION YEAR NUMBER NUMBER Palisades Plant Q 5 Q Q Q 2 5 5 9 2 -

Q 3 8 -Q 0 0 5 OF Q

  • temperature 1 wi th only one primary winding connected which co.ul d cause accel erat.ed aging. Furthermore, the preliminary test results indicate that even with only one primary
  • winding connected, the are able to provide output power of the proper
  • voltage, frequency and wave form. A formal report is being developed.

The. final results of this report will be provided in a supplement to this LER. Incorrectly Sized Fuse in the 11 C 11 Channel RPS Trip Logic* Troubleshooting.performed on the RPS system following the August 25, 1992 revealed a blown fuse in the 11 C 11 channel of the logic circuits for the RPS; The RPS contains six logic ladders representing the 2 ciut of 4 lcigic cbmbinations; AB, AC, AD, BC, BD, & CD. Each logic ladder is powered by two auctioneered 28 volt power supplies.

The power supply and ac feed to the power supply correspond to the matrix ladder; For th_e B-C matrix is powered on one* side by a 11 B 11 channel .28 volt supply which is fed from preferred ac bus X-20. The.other side is powered by a 11 C 11 channel 28 volt power supply from preferred*ac bus Y-30. When the Y-20 bus was lost (due to the failure of ED-07}, three of the six logic ladders that involve the 11 B 11 channel .(AB, BC, & BD} lost one (the B supply} of their auctioneered 28 volt power supplies.

For matrix logic ladders AB and BD, the remaining redundant 28 volt power supplies picked up the load and kept the matrix trip relays For the BC logic ladder, the remaining redundant 28 volt power supply suffered a blown fuse. Although the* power supply was *capable of carrying the load,-the blown fuse prevented the 11 C 11 channel supply from picking up the matrix trip relaYs . for the BC logic ladder, therefore, the BC matrix trip relays de-energized and a full RPS trip was generated.

A review of the vendor supplied wiring diagtams ihdicated that the required fuse size was 1.0 amp; bowever, the blown fuse removed from the circuit was a 0.5 amp fuse. It could not be determined whether the fuse had blown prior to the

  • August 25, 1992.event or blew as a result of the failure of preferred ac bus Y-20 .. A m*ajori ty of the RPS system was rep 1 aced during the 1992 refueling outage. The existence of the 0.5 amp was not determined during post-modification testing. The instrument and control (I&C} technicians assigned to the RPS upgrade project were interviewed and responded ,that they had not replaced any of the 28 volt de power supply fuses. The RPS hardware vendor performed an internal investigation regarding their QA practices during the assembly, inspection and testing of the RPS hardware.

From this the vendor could not conclude whether the incorrect size fuse was or was not installed prior to shipment to CPCo. Failure -0f Charging Pump Auto-start

  • Pressurizer level control wai operating in the channel which is powered from Y-20, the control room operators immediately swapped pressurizer level contfol to the redundant 11 A channel, in accordance with the Off-Normal Procedure (ONP} and expected additional automatic starts initiated by the pressurizer level controller.

After a short period of time no additional charging pump starts,

.( I / ***-' NRC Form 388A 111-831-FACILITY NAME 111 Palisades Plant LICENSEE EVENT REPORT* (LERI TEXT CONTINUATION DOCKET NUMBER 121 . LER NU.MBER 131 U.S. NUCLEAR REGULATORY COMMISSION APPROVED OMB NQ. 3160-0104 EXPIRES: 8/31186 . PAGE 141 SEQUENTIAL REVISION YEAR NUMBER NUMBER 0 5 0 0 0 2 5 5 9 2 -

0 3 8 -0 Q Q 6 OF 0 8 . control rocim operators manually started an additional charging to regain pressuriier level in with the EOPs. At the time the control room operators initiated a manual chargtng pump start level was approximately 32% with the pressurizer level control setpoint at 42%. *

  • System Engineering, Operations and l&C reviewed the electrical prints depicting the pressurizer level control system design. This review concluded that the pumps would not start for plant conditions which existed following the reactor trip on August 25, 1992. The normal level control signal was configured such that no additional charging pumps would start while the back-up level control signal was configured to start additional charging pumps. Testing was performed to verify that the pressurizer level .controllers were operating as designed.

During the event level control was swapped to the "A" channel*

the loss of in accordance with the ONPs. The redundant controller

{in this case the . "A" is normally in the manual mode with a 50% normal level control signal that calls for the additional charging pumps to be tripped. As a result of the trip and the loss of Y-20, with the pressurizer level decreased to the point where the back-up level control signal would have initiated additional charging pump starts. Since the normal pressurizer control signal was still not requiring additional charging pump starts, a standing trip signal was present while the back-up pressurizer level control signal was configured for additional charging pump starts; however, the breakers for the charging pumps equipped with an* anti-pump relay.* With both a start signal and a trip signal fed to the pump breaker, the anti-pump relay activated and w6uld not allow the charging pumps to start. The control room operators . correctly.

identified the failure of the charging pumps to start and placed the charging pumps in manual control, thereby a charging to start. It is important to note that both the manual charging pump start signal and the safety injection system {SIS) charging pump start signal will defeat the anti-pump relay the charging pumps to be started. Further irivestigation revealed that the design discrepancy regarding inability to. start a charging pump with -a standing trip signal energized had been previously identified in D-PAL-90-053 and has not been corrected.

Proposed corrective action for D-PAL-90-053 included modifying the charging pump circuitry so that the.back-up level controls could start the charging pumps without relying on the normal level contrbls to clear the standing trip signal. The modification to the control circuitry has been delayed to the 1994 refueling outage.

NRC Form 388A (9-831 FACILITY NAME 11 I LICENSEE EVENT REPORT (LERI TEXT CONTINUATION DOCKET NUMBER 121 LER NUMBER 131 U.S. NUCLEAR REGULATORY COMMISSION APPROVED OMB 3160-0104 EXPIRES: 8/31 /86 PAGE l'I SEQUENTIAL REVISION YEAR NUMBER *NUMBER Palisades Plant* Q 5 Q Q Q 2 5 5 9 2 -Q 3 8 -Q Q Q 7 OF Q CORRECTIVE ACTION Prior to plant the fuses in the rema1n1ng eleven 28V de power supplies located in RPS Channels A,_B, C, & D were checked and found satisfactory

.. In addition, fuses are in other locations in the RPS circuitry were checked and found satisfactory.

Inverter-ED-07 Further investigation into the reasons why the plant w1r1ng diagrams did not adequately reflect the correct jumper installation for the transformers within inverter ED-07 and why vendbr supplied iriformation was not previously incorporated into plarit design documents was deemed nec*essary.

This information is documented in corrective action document D-PAL-92-227.

The incorrect connection of the two SOLA trarisformers in 1986 was caused by: -* l. Failure to identify during the procurement process that substitute SOLA transformers, although thought to be-functional equivalents, had physical siz_e and electrical connection differences.

2. Fail.ure to perform detailed design engineering during the justification of functionally equivalent replacement parts. 3. Failure to identify on plant drawings and vendor manuals that replacement in stock would require physical and electrical modifications before their use. *
  • 4. Inattention to detail during the installation process. Corrective action for the events pertaining to the failure of inverter ED-07 are documented in D-:-PAL-92-227 and include the following actions.
  • 1. Complete a Procurement Engineering Checklist for the SOLA transf6rmers to identify critical characteristics to be used during the procurement process.'

This will help justify any changes in model and part numbers. 2. Update plant drawings of the inverters to properly show the use of functionally equivalent SOLA transformers.

3. Update plant vendor manual$ for the to properly show.the use of functionally SOLA transformefs.
4. Review this event with electrical maintenance department personnel to reinforce the importance of requiring attention to the proper connection of components that have multiple windings.
  • , "'.------------'------' ,J. t iJ NRC Form 388A lt-83) FACILITY NAME 111 LICENSEE EVENT REPORT (LERI TEXT CONTINUATION DOCKET NUMBER 121 LER NUMBER Ill SEQUENTIAL.

YEAR NUMBER U.S. NUCLEAR REGULATORY .COMMISSION APPROVED OMB NO. 3160-010' EXPIRES:.

8131186 ** REVISION NUMBER PAGE l'I Palisades Plari?t 0500025592-038-0 Q Q 8 OF 0 8 Incorrectly Sized Fuse in the "C" Channel RPS Trip Logic The results of investigation as to the of the 0.5 amp fuse in the RPS circuit as opposed to the required 1.0 amp fuse are documented in corrective action document PAL-92-230 and include the following actions. 1. Check the fuses in the remaining eleven 28V power supplies located in RPS channels A, B, C, & 0. Also, other in the logic were verified to be properly sized. This action was completed prior *to plant re-start and all fuses were found to be correctly

  • 2. Review response from the vendor of the RPS hardware concerning this event and determine if additional corrective action is warranted.

The vendor investigation could not confirm that the incorrectly sized was installed by the vendor. *.Failure of Charging Pump Auto-start The failure of the charging pump*auto-start circuit is being evaluated.

ADDITIONAL INFORMATION None

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