IR 05000311/1986029

Insp Rept 50-311/86-29 on 860912-19.No Violation or Deviation Noted.Major Areas Inspected:Followup on Licensee Actions Due to 860911 Failure of Load Ctr & Station Power Transformers Resulting in Reactor & Unit Trips

ML18092B328
Person / Time
Site:	Salem
Issue date:	10/17/1986
From:	Anderson C, Pullani S NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
To:
Shared Package
ML18092B327	List: IR 05000311/1986029 ML18092B326
References
50-311-86-29, NUDOCS 8610270141
Download: ML18092B328 (11)

Text

'I

U.S. NUCLEAR REGULATORY COMMISSION

REGION I

Report N /86-29 Docket N License N DPR-75 Category ~

Licensee:

Public Service Electric and Gas Company P.O. Box 236 Hancocks Bridge, New Jersey 08038 Facility Name:

Salem Nuclear Generating Station, Unit 2 Inspection At:

Hancocks Bridge, New Jersey Inspection Conducted:

12-19, 1986 Inspectors:

Also participating and contributing to the report were:

T. Koshy, Reactor Engineer F. Paulitz, Reactor Engineer C. Woodard, ~c~or Engineer Approved by:

C * UL

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C. nderson, Chief, Plant Systems Section date Inspection Summary:

Inspection on September 12-19, 1986 (Report No.50-311/

86-29)

Areas Inspected:

This special inspection was conducted to followup on licensee 1 s actions following an event due to failure of a load center transformer (LCT 2F) and a station power transformer (SPT 22) resulting in a reactor trip and consequent unit tri Results:

No violations, deviations, or other unacceptable conditions were identifie Three items remained unresolved at the end of the inspection.

(~ 8610270141 861017 II

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• TABLE OF CONTENTS Persons Contacted Summary of the Event Description of the Event 3.1 Plant Electrical Power System 3.2 Plant Conditions Prior to the Event 3.3 Details of the Event Apparent Cause of the Event Failure of LCT 2F 4.2 Failure of SPT 22 Corrective Actions 5.1 Short Term Corrective Actions Long Term Corrective Actions Conclusion Unresolved Items Exit Interview Page No ATTACHMENT ATTACHMENT ATTACHMENT

2

Simplified Electrical One Line Diagram of Salem 1 and 2 Protective Relaying Scheme for SPT 22 and LCT 2F Trip Sequence During September 11, 1986 Event of Protective Relays of SPT 22 and LCT 2F

• ~

DETAILS Persons Contacted Public Service Electric and Gas Company (PSEG)

C. Churchman, Assistant General Manager, E&PB

• L. Corleto, Principal Engineer, Plant Engineering D. Dodson, Engineer, Licensing and Regulation R. Dounges, Engineer, Licensing and Regulation

• L. Hajos, Lead Engineer, Plant Engineering

• L. Miller, Assistant General Manager, Salem Operations P. Mirchandani, Senior Staff Engineer, EP&B A. Nassman, Acting Manager, Plant Engineering

• R. Patwell, Licensing Engineer

• L. Reiter, General Manager, Licensing and Reliability

• J. Zupko Jr~, General Manager, Salem Operations 1.2 Nuclear Regulatory Commission (NRC)

• C. Anderson, Chief, Plant Systems Section

• S. Ebneter, Director, Division of Reactor Safety

• R. Gallo, Chief, Project Branch 2

• K. Gibson, Resident Inspector

• L. Norrholm, Chief, Reactor Projects Section 28

• T. Kenny, Senior Resident Inspector

• Denotes those present at the exit meetin.

Summary of the Event At 1858 hours0.0215 days <br />0.516 hours <br />0.00307 weeks <br />7.06969e-4 months <br /> on September 11, 1986, Salem Unit-2 tripped from 75 percent powe The cause of the reactor trip was loss of reactor coolant pumps 23 and 24 because of de-energization of 4160 Volt group (non-vital) buses 2F and 2 The reason for the de-energization of buses 2F and 2G was trip-ping of all circuit breakers around station power transformer (SPT) 22 including those which feed buses 2F and 2G, by protective relay action to isolate a fault within SPT 2 At the same time, load center transformer (LCT) 2F which is fed from bus 2F also had a fault which was automatically isolated by opening of its own feeder breake The license postulated that an incipient or gradually developing fault existed within SPT 22 prior to the event and the fault within the load center transformer aggravated the SPT 22 fault resulting in relay actuation and tripping of its breaker *

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Vital buses 2A and 2C were on SPT 21 and vital bus 28 was on SPT 22 prior to the even Upon loss of SPT 22, vital bus 28 was automatically trans-ferred over to SPT 21 as expecte All vital equipment operated as expected during the tri The unit was subsequently placed in cold shutdown for replacement of the transformers and other corrective action.

Description of the Event For a better understanding of this event which involved several components in the Plant Electrical Power System, a summary description of this system is presented below before attempting to describe the event itsel.1 Plant Electrical Power System Attachment 1 shows a simplified electrical one line diagram of Salem Units 1 and With respect to Unit 2 side, the normal line up of the system is such that during startup and sh4tdown, 4 KV group buses 2E, 2F, 2G, and 2H and the vital buses 2A, 28, and 2C are supplied from Station Power Transformers (SPTs) 21 and 2 After unit start-up, the group buses are manually transferred to No. 2 Auxiliary Power Transformer (APT), which is the normal power supply for the group buse On a unit trip, the group buses will automatically transfer to the SPTs (offsite source).

When the unit is running, the vital buses continue to be supplied by SPTs 21 and 2 Normally two vital buses will be supplied from one SPT, while the third bus is supplied by the other, with complete transferability between the tw On simultaneous loss of both SPTs, the vital buses are automatically powered by the standby diesel generators to supply engineered safe-guards load The automatic transfer of the group and the vital buses are initiated by various sets of undervoltage relays with appropriate time delays to perform their functions in a coordinated manne Plant Conditions Prior to the Event The plant was operating at 75 percent power, with all 4 reactor cool-ant pumps (RCPs) operatin RCPs 21, 22, 23, and 24 are powered from group buses 2H, 2E, 2F and 2G respectivel Group buses 2H and 2E were being powered from STP 21 and group buses 2F and 2G from SPT 2 With the unit on-line, this is not the normal lineup of group buses which was previously discussed in Section 3.1 of this repor The normal lineup with the unit on-line is such that all 4 group buses will be powered from No. 2 AP The revised lineup was a corrective action to avoid a bus transfer from the APT to the SPTs in the event of a unit trip and consequent degraded voltage conditions on the vital buses and the resulting problems for the diesels to re-energize the vital buses, as was experienced during the event on August 26, 1986 (see Inspection Report 50-311/86-26 for details).

Prior to the event, vital buses 2A and 2C were on SPT 21 and vital bus 28 was on SPT 2 This configuration of the vital buses is their normal lineup with the unit on-line, as explained in Section 3.1 of this repor.3 Details of the Event Attachment 2 shows how the two transformers which faulted and initi-ated the event are connected to their buses and their associated re-laying scheme The two transformers include load center transformer LCT 2F (750 kVA, 4160/230 V, Delta/Wye, ITE make, dry type, self-cooled with provisions for fan cooling, 6.7 percent impedance at rated kVA) and station power transformer SPT 22 (25 MVA, 1.38/4.16 kV, Delta/Wye, Westinghouse make, Class DA/FA/FDA, 5.7 percent im-pedance at 15 MVA) Attachment 3 shows the detailed trip sequence of

• the protective relays which had tripped during the event to clear the fault Summarizing the relay trip sequence, the instantaneous overcurrent relays tripped the feeder breaker 2F5D and cleared the fault on LCT 2 This happened almost instantaneously as was evidenced by not having any flags on the time overcurrent rela~s which are the backup protection for the instantaneous overcurrent relay In normal cir-cumstances, the instantaneous clearing of the fault on LCT 2F should not cause tripping of any upstream breakers as the relay settings are supposed to be well coordinated to clear the faults selectivel However, several upstream breakers around SPT 22 trippe It was concluded by the licensee that this was because of an internal fault within SPT 22 which caused its differential protection relays to trip the breakers around it within a few cycles and was not because of any problem with improper coordination of relay setting After the event, the relay settings and proper functioning of the affected breakers were verified to be correct by the license The tripping of the breakers around SPT 22 de-energized group buses 2F and 2G which power RCPs 23 and 2 Because the reactor was opera-ting at a power level greater than 36 percent, and there was a loss of two RCPs, this resulted in a reactor trip and consequently a unit tri Vital buses 2A and 2C continued to be on SPT 21 which was still energ-ize Upon loss of SPT 22, vital bus 28 was automatically transfer-red over to SPT 21 by the action of the transfer scheme described in Section 3.1 of this.repor All vital equipment operated as expecte The plant stabilized in Mode 3 (hot shutdown).

The plant was later placed in cold shutdown to investigate the cause of the event and take appropriate corrective actions.

5 Apparent Cause of the Event As discussed earlier in this report, the apparent cause of the event is believed to be a fault within LCT 2F which (although LCT 2F was isolated by opening of its feeder breaker) aggravated an already existing or grad-ually developing fault within SPT 2 This in turn tripped the breakers around SPT 22 and isolated its faul The tripping of these breakers caused power failure to RCPs 23 and 2 This in combination with high reactor power caused the reactor trip and consequently a unit tri Thus the faults within LCT 2F and SPT 22 were determined to be the cause of the even These two transformer failures are further discussed in the following section.1 Failure of LCT 2F An examination of the faulted transformer LCT 2F indicates one of the primary phases (delta connected) had experienced a ground faul There were also burn marks on the delta connection The transformer feeder breaker (2F5D) appeared to have opened under heavy short cir-cuit condition The burn marks on the breaker contacts and arc chutes indicated thi The cable connections on the load side of this breaker had burn marks and two of three cables opened up at their crimp connections to their terminal lugs, apparently due to strong magnetic forces induced by the short circuit curren The rear door of the breaker enclosure were blown open and there was hinge damag There were signs that electrical current was induced into the breaker enclosur There were burn marks around the door handl All these observations indicate a heavy short circuit con-ditio However, the root cause of the short circuit will not be known, until the transformer is examined thoroughly in the manufact-urer1 s (ITE) repair sho The licensee is taking prompt action to identify the root cause of failur The licensee is also examining similar previous transformer failures to identify any generic pro-ble This is an unresolved item pending completion of the above licensee actions and its review by NRC (50-311/86-29-01).

Failure of SPT 22 Subsequent to the event, several tests were conducted on SPT 22 to determine the nature and possible cause of its failur The trans-former gas and oil samples were tested, turns ratio checked, and winding power factor and excitation tests were conducte The tests indicated a fault within the transformer, apparently on one phas However, a visual examination of the internals of the transformer could not positively identify the location and nature of the faul The licensee plans to send this transformer to the manufacturer's (Westinghouse) facility for repairs and upgrading to the 1986 design standard The upgrading should eliminate potential problems of a

degradation of winding insulation resistance due to circulating currents within strands, as was experienced within the Unit 1 Auxi-1 iary Power Transformer which is currently being repaire The lic-ensee also expects to identify any similarity between the two trans-former failures, possible root cause of the failures, and any generic implication This is an unresolved item pending completion of the above licensee actions and its review by NRC (50-311/86-29-02). Corrective Actions Based on the apparent cause of the event discussed in Section 4 of this report, the licensee initiated several corrective actions, both short term and long term, which were under various stages of completion at the end of this inspectio The short term actions were generally complete at this tim The licensee plans to develop a schedule for the long term actions depending on the nature of the root cause of the problem (transformer failures) discussed in Sections 4.1 and 4.2 of this repor.1 Short Term Corrective Actions The short term corrective actions fall under two categories: (1)

actions to i..dentify the cause of the problem and (2) actions to correct the problem itsel The actions under the first category included:

0

0 Oil and gas sample test and turns ratio test for SPT 22; Winding power factor and excitation tests for LCT 2F and SPT 22; Hi-Pot tests for 13 kV cables to and from SPT 22; Testing of the relays and breakers associated with LCT 2F and SPT 22 (see Attachment 2 of this report) to verify their func-tion and proper relay coordinatio The actions under the second category included:

0 Replacement of LCT 2F, its feeder breaker 2F5D, the damaged load side cables and associated tests to verify their functio The replacement of LCT 2F was with an identical type and make (ITE).

Replacement of the damaged SPT 22 (Westinghouse) with an elec-trically equivalent but of General Electric make and post ins-tallation testing in accordance with vendor recommended prac-tice The replacement and testing was in progress, but not complete at the end of the inspectio In addition to the above actions, the licensee also performed certain tests (oil and gas sample) on SPT 21 (the sister transformer of SPT 22) which are possible while the transformer is energize This was to verify that a potential problem, similar to that in SPT 22, did not exist in SPT 2.2 Long Term Corrective Actions The licensee's long term actions include:

0 Determination of root cause of the transformer failures (see Sections 4.1 and 4.2 of this report);

Determination of any connections between various transformer failures in the plant and the degraded voltage conditions as a result of bus transfer transients during unit trip This de-termination is expected, once the root cause of the transformer fai*lures is known and the result of the bus transfer transient study for degraded voltage (presently in progress as a correct-ive action for the August 26, 1986 event) are availabl The licensee's corrective actions described in Sections 5.1 and together is an unresolved item pending their satisfactory completion and review by NRC (50-311/86-29-03). Conclusion The inspector had no further questions regarding the licensee's evaluation of the event, the actions to determine root cause of the event, and the corrective action Upon completion of the short term corrective actions the licensee planned to restart the uni.

Unresolved Items Unresolved items are matters about which more information is required to ascertain whether they are acceptable items, violations or deviation Unresolved items identified during this inspection are discussed in Sec-tions 4.1, 4.2 and 5 of this repor.

Exit Interview The inspector met with licensee management representatives (see Section 1.0 for attendees) at the conclusion of the inspection on September 19, 198 The inspector summarized the scope and findings of the inspection at that tim The inspector and the licensee discussed the contents of this inspection report to ascertain that it did not contain any proprietary informatio The licensee agreed that the inspection report may be placed in the Public Document Room without prior licensee review for proprietary information (10 CFR 2.790).

At no time during this inspection was written material provided to the licensee by the tea.....

ATTACHMENT 1 Simplified Electrical One Line Diagram of Salem 1 and 2 SECT s1*************************-.

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ATTACHMENT 3 Trip Sequence During September 11, 1986 Event of Protective Relays on SPT 22 and LCT 2F Fault on a 4160/230V transformer fed from 2F group bus (breaker position 2F5D)

Relay operation determined from the relay targets found by the

~elay Departmen.

Phase A & C overcurrent protection on the high voltage side of the 4160/230V transforme Indicating Instantaneous Trip unit (IIT).

No time-delay unit targets were foun.

Neutral overcurrent protection on the 4160V feeder (IAC Relay-Instantaneous). Phase A & C Instantaneous overcurrent protection (Relay type PJC) on the 4160V feede All these relays trip 2F5D circuit breake The PJC relays operate when the fault current is higher than 5760 amp F group bus has overload protection, set very high both for current and tim However, if the 2F5D breaker should have not operated this protection would have cleared the 4160V group bu The transient data recorder tape indicates that 12 cycles later the bus breakers 22GSD, 2fsD, and 22BSD trippe Targets on phase A & B transformer di~ferential protection were foun B vital bus was transferred from 22SPT to 21SPT (target on the undervoltage transfer-permissive relay 35%).

The transfer was initiated by the 70% undervoltage protection, but no target was foun Apparently, it is a problem with these targets.