ML20059H340
| ML20059H340 | |
| Person / Time | |
|---|---|
| Issue date: | 01/11/1994 |
| From: | Alexander S, Cwalina G Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20059H333 | List: |
| References | |
| REF-QA-99900786 NUDOCS 9401270106 | |
| Download: ML20059H340 (7) | |
Text
p U.S. NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION DIVISION OF REACTOR INSPECTION AND LICENSEE PERFORMANCE ORGANIZATION:
GE Electrical Distribution and Control (GE-ED&C)
Plainville, Connecticut REPORT NO.:
9990786/93-01 CORRESPONDENCE Electrical Distribution and Control ADDRESS:
41 Woodford Avenue Plainville, Connecticut 06062 NUCLEAR INDUSTRY Manufacturer and Supplier of Commercial Grade, ACTIVITY:
Low-Voltage Distribution Equipment INSPECTION CONDUCTED:
August 26-27, 1993 at GE-ED&C August 30-31, 1993 at Maine Yankee ASSIGNED
~
(
INSPECTOR:
Stephen D. Alexander, Date Reactive Inspection Section-2 (RIS-2)
Vendor Inspection Branch (VIB)
OTHER INSPECTORS:
Kamalakar R. Naidu, RIS-2, VIB APPROVED BY:
L Gregory C. Cwalina, Section Chief Date-RIS-2, VIB INSPECTION BASES 10 CFR Part 21, 10 CFR Part 50, Appendix B INSPECTION SCOPE:
Obtain information on RMS-9 overcurrent trip devices,_
THMK, THJK, and TJJ molded-case circuit breakers, CR124 overload relays, and certain.other issues PLANT SITE Maine Yankee, Browns Ferry,'Oconee,' Point Beach, and APPLICABILITY:
all other plants utilizing RMS-9 trip devices, GE' molded-case circuit breakers, CR124s, etc.
9401270106 940121 PDR GA999 EMVGENE 99900786 PDR
=-
1 INSPECTION
SUMMARY
1.1 Violations No violations were identified during this inspection.
i 1.2 Nonconformances No nonconformances were identified during this inspection.
1.3 Unresolved Items No unresolved items were identified during this inspection.
1.4 Inspector Followup Itemt 1.4.1 (93-01-01)
RMS-9 Tripping (see Section 2.1) 1.4.2 (93-01-02) RMS-9 Flux Shifter (see Section 2.2) i 1.4.3 (93-01-03) THKM Molded-Case Circuit Breakers (see Section 2.3) 1.4.4 (93-01-04)
CR124 Overload Relays (see Section 2.4) 1.4.5 (93-01-05) THJK and TJJ Molded-Case Circuit Breakers (see Section 2.5).
2 INSPECTION FINDINGS AND OTHER COMMENTS The principal purpose of this inspection was to obtain information regarding-I the incidents of unwanted tripping of GE AK-type circuit breakers equipped with RMS-9 digital trip units at the Browns Ferry Nuclear Plant (Browns Ferry) and the Maine Yankee Atomic Power Station (Maine Yankee).
Also discussed during this inspection were problems with resetting of RMS-9 flux shif ter trip -
mechanisms, out-of-tolerance tripping of THMK-type molded case circuit-breakers at Oconee, CR-124 relay problems, and THJK-type breaker problems at Point Beach.
In addition, the inspectors learned of planned testing on a l
nonsafety-related 480-volt bus at Maine. Yankee, with the assistance of GE Nuclear Energy (GE NE), intended to attempt to reproduce and analyze the transients'that appear to have caused the RMS-9 trips.
Immediately following the visit to GE-ED&C, the inspectors went to Maine Yankee to review the test plan, examine the test setup and observe the testing.
2.1 Undesirable Trips With RMS-9 Units 2.1.1 Backaround Licensees of several nuclear. power plants have installed i
RMS-9 overcurrent trip units manufactured and distributed by GE Electrical i
D;stribution and Control (ED&C).
InLmany cases, the trip units were part of conversion kits to replace the electro-mechanical EC-type series overload trip units that were previously used in GE AK-type, low-voltage circuit breakers.
Problems with unwanted tripping of these units include the following:
4 2.1.1.1 On August 4, 1993, the NRC was informed that Maine Yankee had experienced a condition that the licensee described as sympathetic tripping of two safety-related, RMS-9-equipped breakers on July 30,.1993, one-of which was.
a load breaker (with longtime and instantaneous trip functions) and the other a motor control center (MCC) feeder breaker (with longtime-shorttime trips).
I 2
i m
- h...
l Both of the affected breakers were fed from the plant's delta-connected, 480-volt, engineered safeguards buses which are of an ungrounded design like those at Browns Ferry.
The trips may have been initiated by current spikes caused by an intermittent ground fault on the boric acid makeup tank startup i
heaters, a nonsafety-related load on that bus; although the heater breaker did i
not trip. Also, the trips occurred during the process of ground isolation which may have created or contributed to transients on the buses. However, the transients were not sufficient to trip other RMS-9-equipped breakers, some i
of which, acting as Class IE isolation devices, are meant to protect the safety-related buses from faults and overloads on nonsafety-related circuits.
Because of the concern with common-mode fault initiators, the fact that many of these circuits are also not environmentally qualified is significant.
i 2.1.1.2 In October 1992, the Tennessee Valley Authority (TVA) reported to the NRC pursuant to Part 21 of Title 10 of the Code of Federal Reaulations (10 CFR Part 21) (Log. No.93-258) that the ungrounded, delta-connected, 480-volt distribution system at Browns Ferry had experienced short-duration, high-amplitude current transients (possibly high-frequency electrical noise spikes) that caused unwanted tripping of some GE AK-type circuit breakers fitted with GE RMS-9 solid-state digital trip units.
These units have a low-pass filter (described by some as a "holdoff circuit") that is supposed to attenuate (and effectively exclude) most transients of this sort. However, TVA reported that testing of the trip units revealed that the instantaneous trip function of the trip unit would respond to current transients as short as 100 microseconds and trip the breaker when the peak amplitude of the current pulse or spike was sufficiently above the instantaneous trip setpoint of the RMS-9 unit. When a ground f ault occurs intermittently, e.g., by means of insulation breakdown or-flash over, in an ungrounded system, it can cause spikes of a type to which the RMS-9 can respond.
TVA has also postulated that this disturbance may have caused the Browns Ferry breakers to trip.
TVA also reported that it has been investigating with GE NE development of RMS-9 trip units that are less sensitive to such transients, but may be backfitting some AK breakers with EC-type series overload trip units in the interim.
The NRC is concerned that a common-mode initiator of ground faults such as a loss-of-coolant accident or high-energy line break could conceivably cause propagation of short-duration, high-current transients to multiple portions of an electrical distribution system which could result in spurious tripping of RMS-9-equipped breakers and the attendant loss of vital loads, possibly in more than one train.
2.1.2 Results of the inspection During the inspection, cognizant personnel representing GE ED&C and GE NE Power Delivery Services of King of Prussia, Pennsylvania, shared with the NRC inspectors the information that they had been able to obtain thus far from Maine Yankee and Browns Ferry. However, the i
cause of the unwanted tripping was not yet fully understood. GE NE personnel had agreed to assist Maine Yankee in performing tests on a nonsafety-related, 480-vac' bus that was ungrounded and delta-connected, similar to the one on
~3 which the trips had occurred, to try to reproduce them and capture the characteristics of the transients to which some of the RMS-9 trip units had responded. On August 30-31, 1993, VIB inspectors observed Maine Yankee personnel perform electrical switching operations on a nonsafety-related 480-3 l
volt bus after introducing a ground. These tests were intended to reproduce electrical transients that may have caused unwanted tripping of GE AK-type circuit breakers equipped with RMS-9 type digital overcurrent trip units.
Unfertunately, the transients to which the RMS-9s had responded were not able to be reproduced, so the results were-inconclusive.
GE NE, ED&C, and TVA are considering conducting similar tests at Browns-Ferry in the near future.
The tripping events at Maine Yankee also had some other unexplained aspects that suggest that the transients may not merely be interpreted by the RMS-9s as so-called "trippable events," but may also somehow affect the operation of the RMS-9s.
Some of.the units that tripped at Maine Yankee did not exhibit the popped-out " targets" or trip mode indicators that would have been expected under the circumstances.
Certain proprietary features of the circuit design would be consistent with an instantaneous trip without its target popping out.
However, ED&C was not able to explain why this occurred in a unit without an instantaneous trip function that tripped instead on its short-time function.
This issue is designated Inspector Followup Item 93-01-01.
2.2 Problems RMS-9 flux Shifters On January 29, 1993, GE NE informed the NRC pursuant to the requirements of 10 CFR Part 21 (LOG No.93-058) that the flux shifters (trip actuators) triggered by RMS-9 devices mounted on electrically-operated AK-25:and AKR-305 breakers were not resetting after tripping the breakers so that the breakers can be reclosed.
During the refurbishing of two electrically-operated AK-25 type circuit breakers at GE's switchgear service shop in Hammond, Indiana, the technician observed that following a trip, the flux shif ter did not reset.
However, with a heavier spring, the flux shifter did reset following a trip.
GE procured several heavier springs and distributed them to tb3ir service facilities but had not used them so far.
A GE NE representative associated with the GE switchgear service shop in Philadelphia stated that in his experience, all the instances of flux shifters not resetting could be attributed to worn, dirty, and/or misaligned parts.
ED&C also stated that this problem could occur (although it had not yet to their knowledge) on manually operated breakers of this type as well.
However, they had not tested the heavier flux shifter springs on manually operated breakers.
Breaker factory personnel in Bloomington, Illinois, were concerned that the heavier spring may detract excessively from the opening force-margin. This is less than that of-the electrically operated breakers because the manual breakers have slightly weaker operating springs.
GE NE and ED&C advised that licensees who either have experienced problems with flux shifter resetting (or who are concerned that they might) should contact their GE NE field-service representative to help address the problem.
This issue is designated Inspector followup Item 93-01-02, 2.3 problems With Trio Units on THKM type Molded Case Circuit Breakers On June 1, 1993, Duke Power Company (Duke) personnel informed the NRC that they had observed GE THKM-type molded-case circuit' breakers at their Oconee Nuclear Station (0conee) tripping below the manufacturer's published values.
ED&C manufactures and distributes THKM type breakers and their accessories such as shunt or undervoltage trip units. According to Duke's information, 4
i Duke, Farwell and Hendricks (an organization in Cincinnati, Ohio, that performs dedication and qualification testing services), and the GE NE test facility in San Jose, California, tested the breakers and obtained results which were outside the tolerances published in GE's literature.
Some of the breakers even tripped outside the field tolerances given in National Electrical Manufacturers Association (NEMA) Publication AB 4-1991.
The following breakers were tested:
Specimen 1:
THKM 826 F000 equipped with M6-magnetic-only instantaneous trip adjustable from 2400 to 8000 amperes (A)..GE publication GET-27798, " Application and Selection Molded-Cue Circuit Breakers for Industrial-and Commercial Requirements - 1969," indicates that these types'of. breakers should trip within 10% of the set point.
Specimen 2:
THKM 826 F000 with a TKMA836T800 thermal magnetic trip unit.
The time-current curves for these types of breakers, GE diagram GES-61110, indicate that the breaker should trip between 5760A (90%) and 7200A (112.5%) when set on HI (6400A).
Specimen 3:
THKM 1200 magnetic-only with a THKMA3TM612 trip unit.
GET-2779B indicates that the trip unit is adjustable between 2400-8000A with 10% tolerance.
SPECIMEN 1 SPECIMEN 2 SPECIMEN 3 (set at 8000A)
(set at 6400A)
(set at 8000A)
Duke 62-85%
49%-69%
Farwell & Hendricks 68.6-78.6%
GE NE 75-92%
52-81%
The above data indicate that none of the breakers tested tripped within the minus-10% manufacturer's trip current tolerance.
However, ED&C pointed out-that the 10-percent tolerance is a design value meant to be applicable _ for installed breakers with all three phases loaded under plant service conditions. Nevertheless, some of the trips occurred outside the NEMA AB-4
- recommended field test tolerances as well.
Duke uses these breakers in
' applications where time versus current coordination.is important, and any reduction in the instantaneous current trip point below the published _ values could result in an unwanted trip of these breakers which would be a safety concern if a Class lE load was lost.
The issue was discussed during the inspection in a conference call with Duke and was resolved to the point'that Duke.was to send to ED&C several of the MCCB; that either failed tests or were of the same lot, but had not yet been tested or installed so-that._they.could' be inspected and tested at ED&C.
This issue is designated Inspector Followup Item 93-01-03.
5
t i
't b
2.4 CR124 Overload Relavs-Temperature Compensation Problems On April 2, 1991, GE NE issued a " Germane to Safety" (GTS) letter concerning GE CR124 Overload Relays (Tracked under Part 21 Log No.91-033).
The manufacturer, ED&C-Bloomington, had discovered that on relay models CR124K028, K128, LO28, and L128, manufactured before October 1990, many of the ambient temperature compensating bimetal elements had been installed upside down due to a problem with marking of the elements.
These overload relays are typically used in conjunction with starters or motor controllers, many of which may serve safety-related loads.
The ambient temperature compensating bimetal element or spring is intended to adjust the trip forces inside the relay so that the trip time as a function of overload current is consistent with the design characteristic curves over a wide range of ambient temperatures at the relay. According to GE, the improperly installed ambient compensating bimetals will permit the overload relays to work correctly within a temperature range of 15-20*C (59-68*F). However, at low or high ambient temperatures such as 0-15"C (32-59*F) or 20-40*C (68-104*F), the ambient compensation would cause trip times outside of published specifications.
GE NE stated that any of the affected models made before October 1990 should be considered suspect and should be replaced. To identify when the relays were built, they are marked with date codes consisting of two letters.
The first letter, "N" through "Z" (skipping "Q"), indicates the month of manufacture. The second letter indicates the year with "E" meaning 1990, "F" meaning 1991, etc.
Hence relays built in September 1990 and earlier (date codes "WE" and earlier) are affected.
ED&C also recommended testing installed and in-storage relays at least every 5 years at room temperature (e.g.
25*C/77*F) and also after thermal soaking at 40*C/104*F and verifying that the trip times at some overload level (e.g. 300%) are within 10% of each other.
1 If not, ED&C recommended replacement because these relays cannot be repaired.
ED&C reportedly corrected the problem on these models in September 1990 so that date codes for October 1990 and later, e.g."XE" (October 1990), "YE" (November 1990), "ZE" (December 1990), and "NF" (January 1991), should not be affected. During this inspection, ED&C could provide no further information on this issue except to confirm that none of its Service Advice Letters (SALs) had been issued. Also, the GE NE representative stated that none of its Service Information Letters (SIls) had been issued in addition to the GTS to BWRs and the NRC. This issue is designated Inspector followup Item 93-01-04.
2.5 Replacement GE TJK426400 and TJJ426400 MCCBs for Point Beach In a December 13, 1991, letter to the NRC reporting the status of MCCB replacements pursuant to NRC Bulletin 88-10 at Point Beach Nuclear Plant (Point Beach), Wisconsin Electric Power Company stated that GE had encountered problems with the internally-mounted auxiliary switches ordered by Wisconsin _
Electric and with performance testing of the MCCBs. During this inspection, l
the inspectors inquired of ED&C and GE NE as to the status of the testing, nature of the problems, etc.
ED&C agreed to research the matter and provide an update to the NRC as soon as possible.
Subsequent to the inspection, the inspectors received information from ED&C that GE NE had shipped the MCCBs to Point Beach in December 1991 and June and July 1992. No information was 6
E s.
provided regarding the reported problems. This issue is designated Inspector
,i Followup Item 93-01-05.
r 3 PERSONNEL CONTACTED GE-ED&C Dougherthy, J.J., Manager. Systems
- Mallon, J., Application Engineer Reiler, S.M., Development Engineer Sailer, H.P., Development Engineer Saunders, R.E., Product Manager, Industrial Breakers Sheen, Ray, Manager, Distribution Components Smith, J.I., Development Engineer i
St. John, S., Quality Control Engineer GE-NE Sanders, G., Lead Engineer, Power Delivery Services i
}
i e
1 1
7
~
J i
.