IR 05000255/1991019
| ML18059A638 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 01/07/1994 |
| From: | ROGERS D W CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9401240129 | |
| Download: ML18059A638 (13) | |
Text
.. -. -* consumers Power POW ERi Nii /llllCHlliAN"S PROliRESS Palisades Nuclear Plant: 27780 Blue Star Memorial Highway, Covert, Ml 49043 January 7, 1994 Nuclear Regulatory Commission Document Control Desk Washington, DC 20555 David W. Rogers Plant Safety and Licensing Director DOCKET 50-255 -LICENSE DPR-20 -PALISADES PLANT -UPDATE TO ELECTRICAL DISTRIBUTION SYSTEM FUNCTIONAL INSPECTION (EDSFI) FAST BUS TRANSFER SCHEMES -NRC CONCERN No. 8 During the electrical distribution system functional inspection (IR 91019) at Palisades, the NRC expressed a concern (NRC concern No. 8) regarding fast bus transfer schemes. The NRC's concern was resolved by the Palisades staff prior to the NRC exit and was not carried as an NRC open item in the inspection report. A fast transfer event occurred in January 1993. Subsequent evaluation of that event have resulted in new information on the fast bus transfer schemes and is provided below. The NRC's concern on a fast bus transfer from the EDSFI was stated as follows: Operation of the fast automatic transfer on the Class 1£ 2400 V JC and 10 may give rise to transient shaft torques beyond acceptable limits, which in turn may induce early failure of safety related motors due to cumulative effects. The magnitude of the possible transient torques which could be developed has not been determine Should transfer occur during mitigation of a design basis event, the voltage drop through the start-up transformer could cause an unacceptable voltage profile on the safety buses, jeopardizing the safety functio Protective relay misoperation during a fast transfer may jeopardize the safety function under a design basis event. A CMS' ENERGY COMPANY
- Nuclear Regulatory Commission Palisades Plant EDSFI Update to Concern No. 8 Our response to this concern concluded that: 2 The use of Safeguards Transformer 1-1 as the primary power supply fed directly from the 345 kV switchyard has eliminated bus transfers of 2400 volt equipment during turbine generator trips. This has significantly reduced the exposure of safety related motors to cumulative shaft torques or inrush current Bus transfer to the alternate Start-up Transformer 1-2 supply wi77 occur following the Joss of the SGT 1-1 supply. This supply is e7ectrica77y connected to the same 345 kV switchyard minimizing the phase angle difference between the SGT 1-1 and SUT 1-2 supplies prior to bus transfe This minimum offsite phase angle difference coupled with a simultaneous bus transfer scheme minimizes motor shaft torques and inrush currents following transfe No problems are expected on safety related motors in terms of excessive motor shaft torques, inrush currents, degraded voltages, or potential Joss of SUT 1-2 due to overcurrent following bus transfe The overall subject of shaft torques on induction motors is an open industry issue at this time. An EPRI Project (2626-1) is currently underway by General Electric which wi17 provide recommendations for bus transfer criteri Consumers Power Company is following the results of this project for use in bus transfer studies of its fossil and nuclear power plants. BACKGROUND Prior to the installation of Facility Change (FC) 800, "Addition of 2400V Offsite Power," in I988, the normal plant operating configuration for the Palisades station power system was that the Station Power Transformer (SPT) I-2 was connected to the main turbine generator and was supplying power to 2400 volt busses IC, ID, and IE. Transfer of 2400 volt buses IC, ID, and IE to the Start-up Transformer (SUT) I-2 occurred automatically following a turbine trip. Following installation of FC-800, the normal plant operating configuration for the Palisades Plant station power system is that the Safeguards Transformer (SGT) I-I supplies power simultaneously to 2400 volt buses IC, ID, and IE (see Figure I; a single line diagram of the Palisades Plant switchyard and station power system). The SGT I-I supply is connected directly to the Palisades 345 kV switchyar SGT I-I will continue to provide power to 2400 volt busses IC, ID, and IE during and after a turbine trip. The result of the facility change is that bus transfers of 2400 volt loads (when fed by the SPT I-I supply is no longer normally expected to occur since this supply is not used at power. Bus transfers of 2400 volt loads only occur because Qf a loss of the SGT I-I supply, such as a transformer failure or fault within the protective relaying zone. The modification installed via FC-800 has significantly reduced the number of bus transfers and the exposure of safety related motors to cumulative shaft ,torques or inrush current During the development of the design basis document (DBD) for the 2400 volt system in I988, no records were found that documented the actual dead bus transfer time. To resolve this discrepancy in the DBD documentation, a
- Nuclear Regulatory Commission Palisades Plant EDSFI Update to Concern No. 8 3 corrective action document was initiated in October I990 that requested fast bus transfer testing and studies be performed to provide a design basis for the "dead bus" times identified in the design basis document (DBD) and the final safety analysis report (FSAR). In September I992, work was completed on the development of a fast bus transfer computer model of the Palisades station power system and studies were initiated to respond to the corrective action documen Due to the simultaneous fast bus transfer scheme used at Palisades, no initial concerns had been in terms of long "dead bus" times during bus transfers and the possible effects on connected equipmen As a result, analytical resources were directed to higher priority EDSFI issues. On January 28, I993, a loss of the SGT I-I supply occurred during the performance of Technical Specification Surveillance Procedure (TSSP) QO-I, "Safety Injection System," Step 5.2, "Left channel of SIS with Standby Power." The dead bus condition on Buses IC, ID, and IE lasted approximately 7.5 cycles and low voltages resulted in the loss of non-safety related 480 volt loads fed by AC contactors and the starting of both emergency diesel generators
{EDGs). Safety related loads functioned as expected during the event. The printouts from the recorders that were monitoring input voltages to the uninterruptible power supply (UPS) system and transformers for the turbine generator digital electro-hydraulic (DEH) computers indicated voltage levels dropped from IIO VAC to 30 VAC during the event. As a result of this event, another corrective action document was issued to evaluate the adequacy of the fast transfer design. Our fast transfer computer model was used to evaluate the Palisades response to the January 28, I993 event. The results of the computer simulations indicated that, for a simultaneous fast bus transfer scheme, the dead bus time during the fast transfer from SGT I-I to SUT I-2 was abnormally long and had resulted in substantially low station power voltages requiring further investigatio Analysis Figures 2A and 2B are the Logic Diagrams for the safeguards transformer supply. Loss of SGT I-I supply due to the initiation of the 487 safeguards differential relays (I out of 3 logic) requires lockout relays 486P-SG and 486B-P/F to operate before the logic is satisfied to open the 345 kV "F" bus breakers and operate the 486B-X/F relays. Operation of the 486B-X/F relays completes the logic to operate the 383-II and 383-I2 fast transfer relays which simultaneously send trip signals to 2400 volt load bus supply breakers (from SGT I-I) I52-I05, I52-203, and I52-302, and close signals to supply breakers (from SUT I-2) I52-I06, I52-202, and I52-303. Figure 3 is an overall time plot of the event which was used to estimate the "dead bus" time for the Bus IC, ID, and IE transfer scenari As can be seen in Figure 3, the combination of the 345 kV Switchyard logic, opening time of the 345 kV circuit speed of the fast transfer relays, and opening and closing times of the 2400 volt load bus supply breakers result in a relatively long overall bus transfer time of approximately I4 cycles and a "dead bus" time of approximately 7.5 cycles. A "dead bus" time of approximately 7.5 cycles allows the internal voltage and phase angles of Nuclear Regulatory Commission Palisades Plant EDSFI Update to Concern No. 8 4 relatively long overall bus transfer time of approximately I4 cycles*and a "dead bus" time of approximately 7.5 cycles. A "dead bus" time of approximately 7.5 cycles allows the internal voltage and phase angles of connected induction motors to fall significantly out of phase with the incoming SUT I-2 supply prior to re-energizatio Depending on plant operating conditions at the time of transfer, the long "dead bus" time of approximately 7.5 cycles can.result in unacceptable motor shaft torques and unacceptable station power voltages following transfer based a I.33 resultant volts/hertz acceptance criteri Worst case fast bus transfer scenarios were examined and simulated in an engineering analysi The bus transfer scenarios included were: (I) SGT I-1 to SUT I-2 during TSSP QO-I testing (the actual event that occurred on January 28, I993), and (2) SGT I-I to SUT I-2 during LOCA condition * OPERABILITY The results from the engineering analysis indicate the present fast bus transfer scheme from SGT 1-I to SUT does not meet all of the acceptance criteria established for an adequate bus transfer scheme and is considered a design weaknes Fast bus transfer during normal operations and TSSP QO-I testing (Figures 4 and 5) results in a loss of various non-safety related loads fed by AC contactor The manual restart of various non-safety related 480V loads is require The ability of safety related loads to function without operator intervention is not affected during the fast transfer event. This was documented as occurring during the loss of the SGT I-I supply on January 28, I993. Fast bus transfer during LOCA conditions exceeds motor shaft torque criteria and results in low station power voltages and long motor acceleration times (Figures 6 and 7). A review of the simulation results and associated protective relaying indicates that Bus IC will trip on overcurrent and Bus ID will trip because of a sustained undervoltage conditio The second level undervoltage protection schemes will initiate load shed and sequencing of loads onto the associated EDGs. The safety related loads that normally sequence onto each EDG as a result of a safety injection signal (SIS) will automatically sequence onto each EDG without operator intervention, thus, the safety related systems will operate as designed and operability requirements are met. FSAR REQUIREMENTS The FSAR requirements for fast bus transfer from SGT 1-I to SUT I-2 are indirectly stated in section 8.1.I as: The Plant electrical system and the 345 kV switchyard are designed to reliably function and supply power during normal, abnormal and emergency condition This electrical power system is required to meet 10 CFR 50, Appendix A, General Design Criterion 17, for onsite and offsite source requirements
... The electrical system is divided into buses and subsystems to minimize the effects of any electrical fault and maximize the Nuclear Regulatory Commission Palisades Plant EDSFI Update to Concern No. 8 availability of onsite and offsite power sources ... The engineered safeguards electrical system meets the single failure criteria, by which any single failure of a component within the system will not prevent the proper system action when required ... As discussed, the fast bus transfer scheme for SGT 1-1 does not meet all of the acceptance criteria for an adequate fast bus transfer scheme and is considered a design weaknes However, a loss of offsite power due to fast transfer from SGT I-I to SUT I-2 during LOCA conditions " ... will not prevent the proper system action when required ... " The safety related systems will be automatically fed by the EDGs and function as required which meets the above FSAR requirements and, as previously stated, the operability requirements. Palisades electrical distribution system is designed and operated to continuously feed the 2400 VAC Buses IC, ID, and IE from SGT I-I before and after a turbine generator trip. When supplying our 2400 VAC buses from SGT I-I, a fast bus transfer will not occur because of a turbine generator trip. As previously stated, FC 800 has significantly reduced the number of bus transfers and exposure of safety related motors to cumulative shaft torques and inrush current. The fast transfer and resulting actuation of our emergency power system will only occur as a result of failure of SGT I-I or actuation of the SGT I-I protective relaying scheme. Corrective Action* As a result of the fast transfer event on January 28, I993, the following corrective action has been proposed to reduce the dead bus time during a fast transfer:
5 I. A modification is being considered to reduce the dead bus time during a fast transfe The .modification, if implemented, is targeted for completion in the I996 refueling outage. 2. The Operations Department has been notified via letter from NECO Engineering of the expected plant conditions following a fast transfer during normal and emergency operating condition The event was previously discussed with the Operations Grtiup in April/May I993 during their training cycle. The NECO memo was reviewed by the Operations Department and it was determined that department procedures were adequate to address the issues discussed in the NECO mem David W. Rogers Plant Safety and Licensing Director CC Administrator, Region Ill, USNRC NRC Resident Inspector
-Palisades Attachments
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,.,k NO ) 152-116 l 2400V BUS JC t RELAY* (D 487 SG @486P SG @4868 P/F © 4868 X/F tr?:\ 383-11 \;;V 383-12 ESTIMATE OF ACT BUS TRANSFER EVENT 001 TEST INITIATES LOSS OF SGl -1 LOSS 0 SGl-1 SUPPLY JANUARY 28, 1993 DR D-PAL-93-016 383 RELAY SENDS SIGNALS TO SIMULTANEOUSLY OPEN ANO CLOSE 2400V BREAKERS 345 KV "F" BUS CLEARS SGl-1 SUPPLY 2400V BREAKERS FROM SGl-1 SUPPLY OPEN 6.5 10.Cl CYCLES CYCLES 2400V BREAKERS FROM SUT 1.,2 CLOSE BUS TRANSFER COMPLETE 13.Cl CYCLES 3 . 1.0 2.0 3.0 4.0 5.0 I 6.0 7.0 8.0 Cl.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 1 .0 TRIP INITIATE "F" BUS . I I I OPERATING TIME -CYCLES .5 1.0 1.0 I :INITIATE
-4-CYCLE OPEN I I I SG I = 0 -ESTIMATED TRANSIENT OF 3.0 } ASSUMED BASED ON 3.0 7.5 CYCLES & OTHER I . 1 TIME -CYCLES I t I TRIP 5.4-CYCLE CLOSE _, CLOSE I I 152'S I lC,lD,lE -OPEN I I I OPEN CLOSE 152'S 152'S lC,10,lE lC,10,lE -. 7.5-CYCLES " " DEAD BUS TIME *SEE El7 SH.18A RELAY DATA SGICHART.DGN f . 6 0.0 4 .8539 " 6 .6228 2ND LEVEL DROPOUT .9142 2ND LEVEL PICKUP 1. 083 SEC . START EDGS .87.24 (.5 SEC) FIGURE 3 LOSS OF SG 1-1 DURING 001 TESTING 2400V BUS lD VOLTAGE VS TIME .4603 1. START MOTORS -QOl 2. TRIP SG 1-1 VIA 345 KV BREAKERS 3. TRIP 2400V BREAKERS 4. 2400V BUSES ISOLATED 5. CLOSE 2400V BREAKERS TO SUT 1-2 SUPPLY 6. RESULTING TRANSIENT FOLLOWING TRANSFER 7. FINAL STEADY STATE RECOVERY VOLTAGE . 0.60000 1.2000 1. 8000 0.30000 0.90000 1. 5000 2.1000 TI.ME (SECONDS)
2.4000 3.0000 2.7000 0 0 0 :x: z t:"" .. w .. < I to c (/) I I-' 0 QOl F t'Zj H t'-i t'l 'O ....... rt t1 ::i ;.Q 0 I-' p. 0 c rt TUE, JUN 29 1993 LOSS OF SG 0 tU I-' t'-i H ., (/) ., t'l H CJ) z Gl txl .. (/) < t'l (/) 1-( () (/) (/) II t'l I-' t'Zj 0 0 ::0 w l11 txl .. c (/) () 01-'3 ., t'l ::0 H t'l c 00 ()Cf) t'l t'-i ON oo (/) 0 08:16 1-1 < ::s: 0 ., 0 ::0 (/)
2 6 4 5 3 . 464 0.0 AC CONTACTOR PICKUP AT .447 SEC AC CONTACTOR PICKUP .875 AC CONTACTOR DROPOUT .654 FIGURE 4 LOSS OF SG 1-1 DURING 001 TESTING .480 VOLT MCC2 VOLTAGE VS TIME 1. START MOTORS. -QOl 2. TRIP SG 1-1 VIA 345 KV BREAKERS 3. TRIP 2400V BREAKERS 4. 2400V BUSES ISOLATED 5. CLOSE 2400V BREAKERS TO SUT 1-2 SUPPLY 6. RESULTING TRANSIENT FOLLOWING TRANSFER 7. FINAL STEADY STATE RECOVERY VOLTAGE -0.80000 1.6000 2.4000 0.40000 1.2000 2.0000 TIME(SECONDS)
3.2000 2.8000 4.0000 3.6000 0 0 .MCC2 () :i: r:l .. "' w .. :x () () "' F tO 'U 0 )>I ..... H 1-3 en 1-3 tzj Hen z GHtl .. )>I en <: tzj en t-< () en !I:>' hj en H II tzj tzj ..... hj . 0 o::o 'O w Ill <.n OJ I-' .. c rt en 11 () Ill 0 1-3 :::3 ,Q 0 ..... 0. 1-3 tzj 0 ::0 i:: H rt tzj c 00 s;M ()en tzj ON enc <: 3: 0 1-3 0 ::0 en TUE, JUN 29 1993 . 09:41 QOl/LOSE SG 1-1
.7 2ND LEVEL PICKUP .9267 ...... 0 0 0 0.0 +--.8997 .8825 .7806 2ND LEVEL DROPOUT .9142 SUBSEQUENT MOTOR OSCILLATIONS .4842 1.0000 .6899 FIGURE 6 LOCA CONDITIONS
-LOSS OF SG 1-1 AND TRANSFER TO SUT 1-2 SUPPLY 2400V BUS lC VOLTAGE VS TIME 2.0000 3.0000 4.0000 5.0000 TIME(SECONDS)
6.0000 LOADSHED AND SEQUENCE ONTO EDGS AT 6.5 SEC IF VBUS < .9267 8.0000 7.0000 10.000 9.0000 0 0 0 -t"'I t"Cj 0):11 (') t"'4 ):i'H (/) 8E; z [lj 0 (/) H o-3 00 HC OC/l z (/) o-3 t'lj H t"'4 [lj (/) [lj I t'l:J () [lj [lj :r: z :xi z 'O [lj :xi (/) ... Ill : ...... (j) .o-3 (J1 rt HC .. 11 NO Ill [lj H < :;, [lj I ...... 00 (/) tJ:I 0 c c: C/l I (/) 0 I Ill I-' :xi ...... p. () [lj [lj . .. 3: 0 c 0 rt i-< (/) CX> II o I-' < *0 Ot"'I o-3 (') H 3: o-3 :xi N TUE, JUN 29 1993 10:44 .4842 2ND LEVEL PICKUP .9267 ..... 0 0 g 0.0 2ND LEVEL DROPOUT .9142 --TRIP BUS lC FIGURE 7 LOCA CONDITIONS
-TRIP BUS lC AT 3 SEC 2400V BUS lD VOLTAGE VS TIME 2.0000 4.0000 6.0000 1. 0000 3.0000 5.0000 TIME(SECONDS)
LOADSHED AND SEQUENCE ONTO EDGS AT 6.5 SEC IF VBUS < .9267 B.0000 7.0000 10.000 9.0000 0 0 () :i: ... w 'f to c (/) I ..... 0 t'Ej H t1 t<:! 'O Ill ...... rt 11 Ill !j ...... 0 0 Ill rt 11 ...... 'O 0 0 c:: rt () t1 ):ii H (/) z t<:! 0 (/) H t-3 tJj HC QC/) z (/) t-3 ' (/) I t'Ej t<:! t<:! t<:! (j) t-3 HC NO t<:! H t<:! tJj CJ) c Ul I ....... t<:! t<:! ' I< (/) co 11 o I-'<! . 0 Ot-1 t-3 () H 3: t-3 I\) TUE, JUN 29 1993 11:09 TRIP lC AT 3 SECONDS