IR 05000412/1987068

Insp Rept 50-412/87-68 on 871118-21.No Violation Noted. Major Areas Inspected:Root Cause & Safety Significance of 871117 Event & Review of Licensee Corrective Actions, Precursor Events & Potential for Similar Events

ML20234B292
Person / Time
Site:	Beaver Valley
Issue date:	12/16/1987
From:	Anderson C, Durr J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
To:
Shared Package
ML20234B265	List: IR 05000412/1987068 ML20234B261
References
50-412-87-68, NUDOCS 8801050509
Download: ML20234B292 (27)

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U.S. NUCLEAR REGULATORY COMMISSION

REGION I

Report N /87-68 Docket N License N NPF-73 Licensee: Duquesne Light Company P.O. Box 4 Shippingport, Pennsylvania 15077 __

Facility Name: Beaver Valley _ Unit No. 2 Inspection At: Shippingport J Inspection Conducted: o ber 18-21. 1987 Inspector: A & /4G ?

C. J. /@deVson, Chief Plant Systems Section, date .

EB, DRS, RI - Team Leader 1 Other Participants:

P. Kang, Engineer, set'. NRR S. Pullani, Operations Engineer, OB, DRS, RI F. Rosa, Chief, Electrical Systems Branch, SELB NRR 4 F, I. Young, Senior Restdent Inspector, BV-2, PB3, DRP, RI I

Approved by: s

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[J.Durr, Chief,En{' Gin'gering Branch, D(vision

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of Reactor Safetv Inspection Summary: Inspection on Hovember 18-21, 1987 (Inspection R gort No. 50-412/87-68)

Areas Inspectd : Fact fi.1 ding team inspection to followup a loss of effsite power event that occurred at Beaver Vr.lley 2 on November 17, 1987. The it. spec-tion scope included: determination of the root cause and safety significance of the event; review of licensee correttive actions; review of precursor events;' and review of the potential for similar events at Beaver Valley Unit Results; The licensee established the root cause of the even Hardware modifications were inade before startup 13 prevent a recurrence. No violationi were identified. Dr.a unresolved item was identifie I 8801050509 871229 ,

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DETAIM

. Persons Contacted Duquesne Jdght Company D. R. Carothers, Electrical Maintenance Engineer l

, J. D. Crockett, Sr. Manager-Nuclear Operations j

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'K..Grada,fianager of Nuclear Safety

' *W. S. Lacey, Phnt Manager  !

G. Lauct, Electrical Maintenance Engineer l

"J. D.. Sieber, Vice President Nuclear j R. Zupo, Electrical Maintenance Engineer l

-U.S. Nuclear Regulatory Commis,sion

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L *J. Beall, Senior Resident-Inspector  !

l *Denotu those present at the exit raeeting at the plant site on )

! Novender 20, 198 :

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. Description of the_ November 17, 1987 Loss _of Offsite Power Event

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2.1 General Discussion of the Event - j

., On November 17, 1987 at 2:06 p.m., the Peaver VklieyLUn M P tripped from full power:after a technician inadverter,tly tripped a power  ;

switch on the turbine rotor position drawer, causing a turbine /  !

reactor trip due to a spurious?y generate j turbine thrust baring ,;

trip signa Immediately following the trio, a series of brestar  !

automatic operations resulted in loss of offsite power fcr approxi- i p mately seve neen mconds until one of the two offsite power sources

was re-energized automatically. In respoitse to the loss of all AC power, both emergency diesel generators (EDGs) automatically startcd and loaded. Natural circulation was terininated af ter approxitaately five minutes when the reactor coolant pumps were restarted. One EDG  !

L was secured. The second EDG remained runrsing. It provided power for 1- about eight hours following the trip while the licensee perforu d a l- physical ' inspection of the reraining offsit9 transformer. No damage l was identified, and the second offsite power supply wac restored and l the second EDG secured at about 10:00 p.m. Durirg the event, the main generatcr was motorized for a short time by power from the offsite grio through thc offsite and onsite transformer Because of the potential significance of this esent, a team of l:

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Intpectors from Region I and NRR was seat to the site to inde-pendently verify the cause and safety significance of the even The team also reviewed the licensee's short end long term

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corrective actions to ensure that tha root cause of this problem was properly identified and corrected. This inspection report summarizes the team's findings and conclusion Section 2.2 provices a detailed description of the sequence of  ;

events that occurred and is used as a reference throughout the remainder of this report. The remaining sections of this report address the team's findings in the area of event evaluation, licensee corrective actions, and summary of the. t~indings of the tea .2 Detailed Sequence of Events The following is a chronological list of significant events that occurred during the loss of offsite power event. Approximate times are listed for reference onl Exact times may be obtained from the attachments to this repor See Attachment A for the BV-2 One Line Diagra *

Technician inadvertently de-energized a power switch to the turbine votor position drawe *

Technician re-energized the turbine rotor position drawer generating a momentary turbine thrust bearing turbine / generator trip signal, (Time is listed in hour / minutes / seconds / milliseconds: T=14:06:39:302).

Reactor trip , generated due to turbine tri (T=14:06:39:349).

Main generator's excitation field breaker and generator output breakers 352 and 362 opened. (T=39:349 Note: Hour and minute have been dropped and are only shown when necessary for clarity).  !

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4kV bus 2C Breaker 242D opene (T=39:408)

ikV bus 2D Breaker 342D opene kV bus 2A Breaker 42C opene kV bus 2B Breaker 142C opene *

Open indication of reactor trip Breakers note (T=39:505)

• 4kV bus 28 system station service transformer Breaker 142A closed. (T=39:449)

4kV bus 2A system station service transformer Breaker 42A close kV bus 2C system station service transformer Breaker 242B close kV bus 2D system station service transformer Breaker 342B close _ _ _ _ - _ - - _ _ _ -

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4kV bus 2C unit station service transformer (USST) Breaker 2420 reclosed. (T=39:505)

4kV bus 2D Breaker 3420 reclose kV Sus 2A Breaker 42C reclose kV bus 28 Breaker 142C reclose * Transformer 20 tripped on overcurrent by Breakers 42C & 142C opening (l=39:820).

• Transformer 2A tripped on I-30 relay device by Breakers 85, 142A, 42A openin * Transformer 2D tripped on overturrent by Breakers 242D and 342D openin *

Transfo'rmer 28 tripped on overcurrent by Breakers 242B and 342B openin * Emergency diesel generators 2-1 and 2-2 lit off and output Breakers closed in on respective emergency buses (dead bus) and load. (T=14:06:47)

• 138kV Z-30 relay condition cleared. Offsite Breaker 85 reclo; sed due to clearing of Z-30 relay circuitr/. (T=14:06:55)

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4kV bus 2E Breaker 142A close (T=14:06:56, approx. 17 se into the event)

4kV bus 2A Breater 42A close *

2A reactor coolant nump re-energize (T=14: 10 approx. 4 minutes into event).

• 2-1 emergency diesel generator paralleled with the 4kV 2A bus and the loads were transferred from 2-1 diesel back to 2A 4kV bus. (T=15, approx, one hour into ever.t)

• 2-1 diesel generator secure *

Station transformer 2B meggered and resistance checks performe (T=20, approx. six hours into the event).

2B transformer re-energized by closing breaker 94. (T=22, approx. eight hourL itsto the event)

242B and 3428 breakers clcsed re-energizing the C and the D 4kV bu * 2-2 emergency diesel generator paralleled with the 4kV bus and loads shifted back; diesel secure _ _ _ - _ _ _ _ _ _ _ _ __

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5 3.0 Event Evaluation 3.1 . Evaluation of the Event Data The licensee conducted a review of the detailed event data to establish the cause of the loss of off-site power event at Beaver i Valley 2 on November 17, 1987. This data consisted primarily of the strip chart recorder traces of the main generator current and the voltages on the 463 buses 2A, 2B, 2C and 20. (See Attachment A for the breaker, transformer ana bus designat1ons.) The data in the sequence of events log provided additional support to allow a t determination of the events and the underlying design deficiencie However, a greater reliance was placed on the continuous strir chart recorder traces than the sequence of events log, since the evants log contains dicital information collected with a 100 millise:ond sampling interva Thus, the ev(nts log would not providn exact breater position indication for this fast transien The detailed event data including the strip chart recorder traces and data 1' ram the sequence of events log is provfded in Attachment As a result of the licensee's review of this data they arrived at the conclusions discussed below. The inspection team also reviewed this' data and concurred with the licensee's conclusion *

The event was initiated at 14:06:39:302 (i.e., 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br />, 6 minutes, 39 seconds, S32 milliseconds) or November 17, 1987 by an inadvertent momentary turbine thrust bearing signal. The 1 signal was the result of a technician re-energizing the turbine rotor position drawer,

The abcve signal tripped the generator and the reactor. The generator output breakers, the exciter breaker, the Unit Station Service Transformers (USSTs) breakers all opened and the buses fast transferred to the offsite power source through the System Station Service Tra'nsformers (SSSTs). This transfer from the unit generator to offsite power bypasses the nornal 30 second time dela * The .USST breakers immediately reclosed due to an unir,tentional e5ectric breaker contact overlap allowing the tripped generatur to be backfed (motorized) frcm the offsite source through the SSSTs and USSTs. At the time the USST breakers rec 1c sed, the momentary turbine thrust bearing signal was cleare *

The 138kV Breakers 94 and 85 to the grid tripped. Ti e over-current on USST 2D (caused by the motorizing of the generator)

caused all four USST breakers to retrip and lock open. (The 138kV Breaker 94, which had tripped on overcurrent, locked out

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while the 85 breaker, which had tripped on the Midland feeder phase compartson trip (Z-30), reclosed after the trip signal cleared in approximately seventeen seconds into the event),

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Simultaneous with the above, emergency buses 2AE and 20F sensed l' undervoltage and opened their normal supply breakers. The emergency Ciesel generators (EDGs) started and loaded on to the emergency buse *

Buses 2A and 2B were recovered when the 85 Breaker reclosed (as noted previour,1y). Reactor Coolant Pump (RCP) 2A which is on bus 2A was restarted approximately four minutes into the event, thus reestablishing forced circulation through the reactor cor *

After bus 2A was recovered as noted above, emergency bus 2AE was re-energized from bus 2A and the associated EDG was secured in approximately one hour. The second EDG continued to power its emergency bus 2DF for about eight hours until it renergiud from l

the offsite sourc The factors contributing to the event were: The thrust bearing failure trip relay (74 TMAAB) does not seal-i . The protective trip relays (62 TMAABX2, 1) for USSTs, generator output and excitor field breakers do not seal-i . There is an unintentional overlap in the 52S closing coil circuit contacts (see Attachment B and C) on the pallet switches of the USST and SSST breakers. These contacts are of an "early o" design to provide fast transfer from the USST to the SSST and vice versa. The intent of the design is to ensure that one set of breakers cannot reclose if the opposite set is closed and to keep to a minimum the time that the unit and system 4kV Breakers are both open with an unpowered bus. However, the design did not function entirely as intended because of the above contact overlap combined with the fast transfer feature in both direc-tions (i.e. , f ast transfer from the unit generator to the offsite grid or a fast transfer from the offsite grid to the unit generator) and the use of non-seal in type of relays as noted in items 1. and 2. above. When the momentary thrust bearing failure trip signal cleared, the relays rese This allowed the USST breakers to reclose, although the SSST breakers were close _ ~

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3.2 ' Design Deficiencies It was determined that three design deficiencies contributed to the sequence of breaker operations that resulted in the loss of offsite power event. This determination was made by an evaluation of the detailed data collected during the event (see Section 3.1 and Attachment E).

L The first design deficiency 'is in the circuitry which initiates a generator trip and automatic fast transfer of the 4kV buses from the USSTs.to the SSSTs on turbine trip which does not include a j seal-in feature. _ (Refer to Attachment A for breaker, transforme '

' and bus designations). The second deficiency is that feature of the design which provides for automatic fast transfer of the 4kV buses in i.he reverse direction, i .e. , from the SSSTs back to the USST The third design deficiency is the overlap closure of the auxiliary

"early b" contacts of the 4kV breakers. (See Attachment C for a discussion of electrical breaker "early b" contacts.) When the momentary turbine thrust bearing failure signal reset, this reset the trip signal to the USST breakers (42C, 142C, 2420, 3240) which connect the USSTs to the 4kV buses. But the USST breakers had already started to open and by means of their "early b" auxiliary contact had completed the close circuit for the SSST breakers (42A, 142A, 242B, 3428) which co1nnect the SSSTs to the 4kV buse The SSST breakers, therefore, had begun to close but their "early b" auxiliary contact which is closed and is in the close circuit of'the USST side breakers'did not open before the reset of the trip signal to the USST breakcrs. Therefore,.for a momentary interval or over-lap, both sets of 4kV bus breakers simultaneously had a close signa The result was that both sets of breakers (i.e., the USST and SSST breakers) on buses 2A.and 2B were simultaneously closed for approximately 16 cycles and the corresponding breakers on buses 2C and 20 were simultaneously closed for approximately 22 cycles. During these intervals, the main generator with its field deenergized was connected to the 138kV offsite circuits through the intervening USSTs, 4kV buses.and the SSSTs. This resulted in motorizing of the main generator. Approximately 6,000 amps of motorizing current was observec' on the 22kV generator bus. This indicates that over 20,000 amps current was distributed between the 4kV buses and their transformer winding The resulting overload on USSTs 2C and 20 and on SSST 28, and the pha n unbalance condition sensed by the Z-30 relay protection for the 138kV line to Midland, caused tripping of all breakers on the four 4kV buses and the tripping of the 138kV breaker 85. The Z-30 relay sensed the phase difference between the 138kV Midland line an6 the tripped main generator The different time intervals ( M cycles, 22 cycles) cited above are due to the differences in trip characteristics l

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l or the overcurrents seen by the 4kV bus breakers, i.e., the faulted condition was cleared sooner on buses 2A and 2 The licensee determined by tests performed (see Section 3.3) on the 4kV bus breakers that an "early b" contacts closed overlap condition existed in the automatic close circuits of these breaker This condition would permit simultaneous closure of both supply breakers on each 4kV' bus if thE trip signal to the USST side breakers were removed before the SSST side breakers had fully close It is noted that the design deficiencies noted above are in the non safety-related portion of the plant electrical system. Also, the Regulations (10 CFR 50 Appendix A GDC 17) only require immediate access to offsite power (i e., fast transfer to the SSST) in the event of a LOCA. The capability for fast transfer back to the USST is not a design requiremen One other design feature was reviewed to determine whether it contri-buted to the fast transfer malfunction which occurred or had the potential for causing a malfunction. This was the design feature that would prevent a fast transfer if the voltage of the 4kV buses was between 30 and 75 percent of normal voltage, which would be the voltage of the power supply to which the bus is being transferre It was concluded that this feature did not contribute to the malfunc-tion and that it provides equipment protection a3ainst the electrical and nechanical stresses that would be produced ii two such differing voltage sources, with a probable phase difference due to motors on the isolated bus acting as induction motors were conneeted (A bus at less than 30% voltage is considered a dead bus, and greater than 75%

vc hage it is sufficiently close to normal voltage that undue stres-sing would not occur).

3.3 Followup Tests As discussed in Section 3.2 above, a design deficiency involving t:lming of the pallet switches that are a part of the electrical breaker "early b" contacts resulted in simultaneous closure of the 4kV breakers on the unit and the system sides, resulting in motoriz-ing the generator. To confirm this, the licensee performed two io'llowup tests of the pallet switches timing, one on November 19, 1981 for the 2A 4kV bus breakers and another on November 20, 1987 for the 2C 4kV bus breakers. The test results demonstrated that the pallet switches fun':tioned as designed. (i.e., with the design deficiency). The licensee's corrective action for this deficiency was to install knife switches in the closing circuit of the 4kV breakers (see section 4.1). The licensee performed another test on Novemoer 20, 1987 to verify that such corrective action would eliminate the design deficiency. This was done by temporarily opening the closing circuit of one of the 4kV breakers (This is equivalent to an open knife switch). The results of the test

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verified that the licensee's conective action would work, i..e.,

with the set of knife switches on the enit side open and the set on the system side closed, the fast transfer would occur from the unit side to the system sid The unit side breakers would not reclose. This eliminates the possibility of the USST and the SSST '

breakers being closed at the same time and the resulting potential for motorizing the main generato ;

4.0 Corrective Action .1 Description of Modifications Three design deficiencies were identified by the licensen that contributed to the November 17, 1987 loss of offsite power even These deficiencies are discussed in Section These deficiencies included (1) the lack of a seal-in of the s;gnal which initiates generator trip and fast bus transfer to the SSST on turbine trip, (2) the capability for automatic fast bus transfer in the reverse direction, i .e. , f rom the SEST back to the LGST, and , i (3) the overlap operation of the "early b" auxiliary contacts of the i 4kV bus breakers which enabled a simultaneous closure of the USST I side breaker and the SSST side breaker Turbine Trip Seal-In Addition To correct this deficiency, the licensce replaced four Westinghouse MG-6 self reset relays (they are used as a peir in each of two redun, dant turbine trip schemes) with two Electro Switch Type LOR lockout type relays whicn require manual reset. These relays are the final actuation relays for all turbine trip signal inputs and will assure that no turbine trip will self-rese Defeat of the Reverse Automatic Bus Transfer The reverse automatic bus transfer from the SSSTs back to the 'JS$Ts was defeated by inserting a knife switch in the auto closure circuit of each 4kV bus breaker. The switches in the close circuit of the USST side breakers will be kept open, which prevents automatic reclosure of these breakers thus defeating the reverse auto transfer of the buses back to the USSTs. The knife switches in the close circuit of the SSST side breakers will be kept clofed thus retaining the auto transfer from the USSTs to the SSST The knife switches are physically installed in their respective breaker cubicles and will be maintained in the proper pcsition by administrative control Inditator lights in the control room show thac. each of the eight knife switches are in the correct position. The licensee indicated

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i his intant to replace these knife switches with more appropriate components at the earliest opportunity. The proper operation of this modification was verified by tes (See Section 3.3) ,

I It was noted that the knife switches for the SSST side breakers were j not needed to defeat the reverse auto transfer. The licenste l indicated that these switches were installed to prcvide the plant i with flexibility which may oe useful during test or maintenance {

activities during the period prior to replacement of these switches 1 with permanent circuitr ]

Elimination of the "early b" Contact Overlap i

Thc defeat of 'che reverse auto bus transfer by the open knife switches in the close circuit of the USST side breakers effectively 3 eliminated the contact everlap problem. Therefore, no forther I corrective-action was take .2 Tejting and liof f3ation of Electrical Srstem As described in Section 4.1, the licensee performed two 3 modifications to the electrical system for'BV-2. One modified the  !

tripping circuits associated with the transfer scheme of 2A, 2B, 20, and 2D 4kV buses and the other modified the trip initiation

, signals froen the turbine trip system. Team inspector 3 reviewed and f witnessed the post-modification testing for the changes. Visual observations and walkdowns of the newly itutalled lockout relays and knife switches were performed as a part of the inspection. Members of the inspection team also discusted the field modifications with personnel installing the hardware to determine whether the incitvi-duals were knowledgeable of the modifications that were being implemented. The inspectors found that the personnel installing the modifications demonstrated an acceptable knowledge of the change The modification activities were performed in accordance with the applicable preced:tre Af ter the modifications had been functionally tested, the licensee performed an overall system transfer ten to ensure that all modifications worked properly. Members of the inspection team witnessed portions of t!.e test and determined thet the modifications that the licensee instel'ed worked prcperl .3 Safety EvaluLtion and 50.59 Review The licefisee performed the required safety evaluations in accordacce with 10 CFR 50.59 for the two facility modifications discussed in Sections 4.1, i.e., (1) installation of knife switches in the ikV breaker closing circuitry and (2) replacement of the existirg relays

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in the turbine trip circuit with lockout type ralays. The licensee's evaluation concluded that no unreviewed safety questions exist'for the modifications. TFe team reviewed the safety evaluations and verified that suf ficient bases exist for the licensee's conclusion.

l 5.0 Precursor Event _0ctober 24, 1987, Partial Loss of Offsite Power l During the performance of the initial startup test program SNet Load Trip i Test" on October 24, 1987, Beaver Valley 2 experienced a partial loss of of fsite power. This event is discussed in Attachment F and in inspection

reports 50-412/87-63 and 87-64. As noted in Attachment F, the licensee noted that immediately after the off site power fource was connected to the 4kV buses, the unit supply breakers httempted to reclose on the 4kV buses. Sir.ce the November D,1987 event involved a loss of of fsite power and a reclosure of the unit sappiy breakers en the 4kV buses, the inspectors questioned the adequacy of the licensee's initial review of the ;

October 24, 1987 even f Discussions with $1censee management indicated that the October 24, 1987 l es nt was reviewed by engineering to determine the reas'on thess breakers attempted to reclose en an already energized bus.. The ' licensee indicated ;

that using the uformation that was available at the time of the October 24, 1987 evn t, the slow bus transfer seguence appeared to allow the transfer attempt to occur. Because of the test initiai alignment, the plant had been placed in a unique situation which typically does not cccur. From the data, the licensee subsequently concluded that the slow bus transfer sequence was the mechanism that allowed these four additional breakers to attempt to reclose on an energized bus. The review reasonably assured that this was a unique situation and would not occur unless these unique conditians occurred again. Further discussion with the licensee indicated that after reviewing the November 17, 1987 event, engineering went back and reviewed the October 24, 1987 event and determined with the additional data collected from the November 17, 1987 event, that a slow bus transfer did not cause the supply breakers to attempt to reclose on the 4kV buses. The licensee determined that it was in fact the fast transfer scheme that had allowed these breakers to clos i Based on the discuts1ons and an independent review of the data that hed been collected from the October 24, 1987 event, the team concluded that the licensee had reasonably reviewed the data and had come to a reasonable conclusion of the October 24, 1987 occurrence. The team concluded that the licensee's review crocess in this area was adequate to address this type of concern end could adequately resolve technical issues in their review process.

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6.0 Beaver Valley Enft 1: Review l As discussed in Section 4.1, several modifications were made to beaver Valley 2 to ccrrect the design deficiencies that contributed to the November 17, 198/ loss of offsite power event. Discussions were conducted *

with licensee management to determine if similar design deficiencies existed at Beaver Valley 1 and the potential for a loss of offsite power event at Unit I similar to the November 17, 1987 Beaver Valley 2 even As aiscussed in Attachment D, it was determined that a difference exists between the Unit 1 and the Unit 2 thrust ' bearing trip device signal. The 1 thrust bearing trip signal was the initiating event for the November 17, 1987 l event.. In Unit 1, an electronic sigual is generated which doas not losk in. The signal rapf dly decays. However, in Unit I the thrust bearing trip device signal is not electroni Rather, it is generated by a hydraulic i system that does lock in the trip signal. I'nerefore, a similar problem is not an immediate concern for Unit However, some aspects of the design l

'q deficiencies at Unit 2 (see Section 3.2) are potentially applicable to q Unit i i

The licensee committed in their letter of November 30, 1987 from I W. S. Lacey, Duquesne Light to C. J. Anderson, NRC to perform a review of I Unit 1 to establish the need and feasibility of design changes. This review is to be conducted at the upcoming BV-1 refueling outage currently scheduled to start on December 13, 198 The licensee's review of Beaver Valley 1 for potential design deficiencies and the correction of these -

deficiencies is an unresolved item pending licensee completion and NRC

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review of these activities (50-412/86-68-01).

7.0 Summary The team concluded that the licensee identified the primary cause of the short loss of offsite power event that occurred on November 17, 198 Three design deficiencies that contributed to the event included: lack of a seal-in of the signal which initietes a generator trip and fast bus transfer to offsite power on a turbine trip; capability for fast bus transfer in the reverse direction back to the unit; and overlep operation of the auxiliary contacts of the 4kV bus breakers which allowed simul-taneous closure of the USST side and the SSST side breaker These deficiencies were corrected by two modifications. One of these modifi-cations provided a turbine trip seal-in. The other modification consisted of the insertion of knife switches to the auto closure circuit of each 4kV bus breaker to prevent fast bus transfer in the reverse directio !

The insertion of the knife switches eliminated the contact overlap l proble _ _ - __ >

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Prior to startup of Unit E after the event, the licensee committed to make-the above mentioned modifications to correct the specified deficiencie This commitment was made b,v the licensee's management during a telephone call between the liceasce and NRC management on November 20, 1987. This commitnant was documented in a letter frem J. D. Sieber, Duquesne Light to the NRC dated November 25, 198 These modifications wert.made and tested prior to restart of Unit In addition, the license committed to review Beaver Valley Unit 1 for similar deficiencies and make any required modifications, The wnrk for Unit 1 is to be done during the refueling outage scheduled to t.egin on December 11, 190 .0 Un_ resolved Jtems _

Un-esolved items are matters about which more information is required in order to ascertain whether they are acceptable items or violations, An unresolved item identified during this inspection is discussed ir Details, Pa.cagraph .0 Exit Metting The inspectors met with licensee personnel (denoted in Details, Paragraph 1.0) on November 20 198 The in.spectors summari::ed the inspection finding At to time during this inspection was written material given tc the i licensee, i

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• The simplified sketch shown is for the USST breaker 42C and 42A closing coi However, it is typical of all the USST and SSST breakers (42C, 142C, 242D, 3420, 42A, 142A, 24 5 and 3428).

Legend % voltage relay contact (Device 83V1206X): closes when the USST voltage is aoove 9'1% The "early b" contact of each opposite side breaker (e.g., 42A and 42C)

when the breaker begins to open is used to initiate closing of its cppo-site breaker. Thus, when USST breaker 42C starts to open its "early b" contact in the closing circuit of breaker 42A closes initiating closure of 42A before 42C is fully open. Conversely, the "early b" contact of breaker 42A is closed when breaker 42A is open thus providing c close permissive for breaker 42C. This "early b" contact should open when breaker 42A begins to close. The design intent is to provide fast trans-fer from the USST to the SSST and to prevent reclosing of the USST breaker if the SSST breaker is close . 75% voltage relay contact (Device 83V208X2) with time delay of 20 cycles: closes when the USST voltage is greater than 75%. % voltage relay contact (Device 27VA 2200) closes when the USST voltage is less than 30%.

NOTE: Items 3 and 4 above permit the operator to close breaker 42C, provided the USST voltage is within permissible limis (>75% or <3J%) Knife Switch: added in the circuit as a modification to allow fast transfer or.ly in one direction (i.e., from USST to SSST). The knife switches en the USST side breakers will be kept open and those on the SSST side will be closed to permit such a transfe __ _ _ _ _ _ _ _ - _ _ _ _ - _ _ _ _ _ _ _ - _ _ _ . _ _ _ - _ _ _ -

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6ttachment C Electrical Breaker "early b" Contacts Normal contacts associated with the breaker are classified as "a" or "b" contacts, These contacts are mechanically linked to the breaker and open or close with motion of the breaker. When the breaker is closed, the "a" contacts are closed and the "b" contacts are open; when the brnaker is open, the "a" cor, tacts are open and the "b" contacts are closed. When the breaker is moving from the closed to open position, all contacts are open because the breaker is not closed and the "b" contacts are open because the breaker is not fully open.

l Selected breakers at Beaver Valley, Unit 2, are equipped with what is termed as "early b" contacts. The "early b" contacts will close before the normal "b"

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contact in order to shorten the length of time between the indication the first breaker is being opened and an indication that the opposite breaker should clos This arrangement is typically used when a f ast transfer of electrical power is neaded within a few cycles of the first breaker actually tripping ope With this scheme, the first breaker would be tripped and the second breaker could start to go closed before the first breaker has fully opened by use of

",early b" contact. During normal travel under a non-fast transfer, the second yreaker would not be allowed to start to ga closed until the first breaker had truly indicated open by a "b" contact. By use of an "early b" contact, which is typically 22 to 33 degrees ahead of a "b" contact, the second breaker can start to close early on in the actual open cycle of the first breake At Beaver Valley, these "early b" contacts are designed in an arrangement which is known as pallet switche ,

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. Thrust Bearing Trip Description The thrust bearing trip 6avice is designed to measure the axial positioning of the mainfturbirie shaft. The tnrust bearing trip cantrol system detects motion of the thrust bearing in either direction.. The sensing dev1ce for Unit 2 is a

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linear variable transformer-(LVT) type arrangement that generates ar. electrical signal whi;h is directly pret ortional to the thrust bearing motion. This signcl is sent to the plant computer and to the turbine rator position module in the control room. With this circuitry, a turbine trip signal and two control room alarms are generated. The turbine thrust bearing wear alarm, a precursor alarm, is generated when a predetermined setpoint is reached. This alarm is designed to a' low the operator to take immediate action to ' unload the unit prior to reaching a thrust bearing failure. This circuitry also provides a signal tc the Westinghouse emergency cabinat for turbine trip circuitry and

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annunciator window 3F, turbine thrust bearing failure turbine trip. The Westinghouse emergency cabinet circuitry, upon receipt of this signal, is designed to automatically shut down the unit and immediately open generator' i output breakers 352, 362, and the field excitation breake The. Unit 2 thrust bearing trip device does not lock-in the trip signal. There-fore, 'when the electronic signal decays away, the trip signal will clear. The Unit 1 inrust bearirg trip device signal is generated by a hydraulic system that dues lock in the trip signal . Therefore, a similar problem is not an immeciate concern for Unit 1. The above Unit 2 trip device is duplicated, thus incorporating a.one out of two trip coincidence. The' Westinghouse emergency trip cabinet allows for individual circuit testing without causing a turbine tri i

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Attachment F October 24, 1987 Partial Loss of Offsite Power Event The Main Generator output breakers were opened with the unit at full power for the performance of the initial startup test progiam " Net Load Trip Test". A reactor trip signal was received approximately 6 seconds later from a Low-Low Steam Generator level signal followed by a Turbine Trip from Reactor Trip. As per design following a turbine trip, a main generator trip and fast bus transfer to offsite power is initiated 30 seconds after the turbine tri Therefore, the main generator continued to supply the station 4kV buses for 30 seconds after the turbine tri During this time, the main generator speed decreased below 1800 RPM and approximately 11 seconds into the event the Reactor Coolant Pumps tripped on underfrequency (57.5 Hz). Thirty (30)

seconds after the turbine trip, the generator trip signal opened the main generator exciter breaker and a fast bus transfer was initiated. The unit supply breakers to the 4kV buses opened (42C, 142C, 242D, 342D) and the offsite power system supply breakers (42A, 142A, 242B, 3428) closed in on the 4kV buses. About 50 msecs. after the system supply breakers closed in the unit supply breakers for three of the four 4kV buses (42C, 242D, 342D)

attempted to reclose in on the buses and then reopened about 50 msec late After the fast bus transfer to offsite power, the 2A System Station Service Transformer developed an undervoltage condition and tripped the system supply to the 2A and 2B buses. This undervoltage was a result of the transfer of the 4kV buses and the unit to the offsite supply while in a degraded voltage and frequency condition caused by the main generator speed coastdow The offsite supply breaker for the 2A bus reclosed in about 1.5 seconds after opening and the 2B bus supply breaker recloseo about 16.5 seconds after openin The plant was stabilized on natural circulation. The 2A Reactor Coolant Pump was later restarte _ _ _ _ _

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Attachment G List of Documents Reviewed 1. Doc. N Title Revision Elementary Diagrams 12241-E-5A Bus 2A Supply ACB 42C 16 12241-E-5B Bus 2A Supply ACB 42A 19 j 12241-E-5AA Bus 2B Supply ACB 142C 14 '

12241-E-5AB Bus 2B Supply ACB 142A 13 12241-E-5BA Bus 2C Supply ACB 2420 12 12241-E-9A Turbine Controls 14 12241-E-9B Turbine Controls 14 12241-E-5K SSST 2A Undervoltage 9 12201-E-5BL USST 2C Undervoltage 8 i

2. Event Data NA Sequence of Event Log for Tuesday NA 11-17-87, 14.06:09:200 NA sequence of Events 11-17-87 NA (Handwritten summary)

NA Oscillograph Chart of the Event NA 3. Followup Test Results NA Results of Followup Tests on timing NA of pallet switches, performed on November 19, 1987 NA Result of test by temporary opening NA (inserting paper in relay contact)

of the closing circuit of a 4KV breaker, performed on November 20, 1987 4. Miscellaneous bocuments 872005 thru 4 Work Request and Failure Report 0 872012 packages for 8 knife switch installations including their 10 CFR 50.59 safety evaluations 872013 Work Request and Failure Report packages 872014 for replacement of Turbine Thrust Bearing Trip Relay 62-TMAABX1, X2 including their 10 CFR 50.59 safety evaluations

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