IR 05000269/1988014
| ML16125A390 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 06/14/1988 |
| From: | Conlon T, Fillion P NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| To: | |
| Shared Package | |
| ML16125A389 | List: |
| References | |
| 50-269-88-14, 50-270-88-14, 50-287-88-14, NUDOCS 8807050576 | |
| Download: ML16125A390 (5) | |
Text
PV RE(,Z UNITED STATES NUCLEAR REGULATORY COMMISSION
REGION II
101 MARIETTA ST., ATLANTA, GEORGIA 30323 Report Nos.:
50-269/88-14, 50-270/88-14, and 50-287/88-14 Licensee:
Duke Power Company 422 South Church Street Charlotte, NC 28242 Docket Nos.: 50-269, 50-270, License Nos.: DPR-38, DPR-47, and and 50-287 DPR-55 Facility Name: Oconee 1, 2, and 3 Inspection Conducted:
May 16-19, 1988 Inspector:
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4.'.
P. Fillion Date Signed Approved by:
_
_
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T. Conlon, Section hief Date Signed Plant Systems Section
Division of Reactor Safety SUMMARY Scope:
This announced inspection was conducted at the corporate offices in the area of emergency powe Program areas covered in part were engineering, design changes, resolution of design problems, and surveillance testin Results:
No violations or deviations were identified. The licensee's program for resolving design problems was shown to be effectiv,0576 880620 PD ADOCK 05000269
REPORT DETAILS 1. Persons Contacted Licensee Employees
- J. Tannery, Design Engineer, Electrical
- R. Dobson, Design Engineer, Electrical
- J. Stoner, Design Engineer, Electrical
- Attended exit interview 2. Event Report Followup (592700)
The requirements of 10 CFR 50, Appendix A, Criterion 17 are interpreted by the NRC to mean that offsite and onsite power sources considered together must meet the single failure criterion on a system basis without loss of capability to provide power for all safety function However, on April 26, 1988, the licensee reported to the NRC that a condition existed for 16 days whereby the onsite and offsite power supplies would not have been available within a few seconds following a loss-of-coolant accident assuming a single failure (e.g. misoperation of the transformer differ ential relay).
During the Unit 2 refueling outage, which began in February 1988, the 230 KV circuit breakers that control the startup/
standby transformer CT2 were replaced with higher rated breaker The high-voltage windings of transformer CT2 are connected in a wye configu ration with the neutral point solidly grounded. It must be recognized that, for each phase, the circuit breaker contact voltage gradient capacitors are in series with the transformer winding, which is essentially a saturable inductor. With the source bus energized and the source side and load side breakers both open a ferroresonant condition was set up in the capacitive/inductive series circui The ferroresonant condition could produce 4000 volts on the transformer secondary, and cause undervoltage relays to not operate when they would otherwise be expected to operat Therefore, given the pre-existing potential for ferrore sonance to occur, the safety-related systems would have been without any AC power for certain design basis events until operator actions could recover from the failur The inspection was conducted at the corporate offices in Charlotte, NC, and consisted of discussions with the cognizant design engineers as well as review of related document A chronology of events, starting with placement of the purchase order for the new circuit breakers, was established. The theory of ferroresonance and the circuit breaker ratings were reviewed. The listing of documents reviewed and reference documents given in Section 3 is indicative of the depth and scope of the inspectio Possible generic implications of this event were addressed. In order to evaluate the licensee's program in dealing with design engineering problems, the inspection focussed on two questions:
a. Was the engineering analysis at the design phase of the breaker upgrade project adequate? Were the corrective actions taken, after the ferroresonance problem was first identified, adequate and timely?
In order to predict, in the design stages, the problem that occurred, the design engineer would have to have been familiar with this type of problem from previous experience. However, ferroresonance, as manifested in the Oconee event, occurs quite rarel The manufacturer of the circuit breakers (Cogenel-Alsthom)
although aware of the particular application did not mention the possibility of ferroresonance during five contract meeting While it was possible to predict the problem in the design stages, the problem was sufficiently hidden that it would not be fair to conclude that the licensee's design analysis was inadequat On April 1, 1988, the licensee became aware that startup/standby transformer CT2 had been subjected to ferroresonance for several hours on the previous da The ferroresonance condition was confirmed by making oscillograms of the current on the high-voltage side of the transforme The recorded waveshape had periodic impulses separated by periods of essentially zero curren This is the characteristic waveshape of ferroresonance. According to Duke Power Co. general practice, whenever a transformer differential relay operates, the transformer is teste An oil sample analysis, a power factor measurement (Doble test) and ratio test were conducted on April 1, to demonstrate the integrity of the transformer's insulation. The interval for preventive maintenance on the transformer will be reduced in the futur On April 26, the licensee identified the scenario whereby ferroresonance could cause loss of all AC power for certain design basis event A test demonstrated that ferroresonance could cause 4000 volts to appear on the transformer secondary. Since 4000 volts is above the undervoltage relay setpoint, the problem was confirmed to exist. Corrective action taken on April 26, was to re-align the distribution system to preclude ferroresonance from occurring while still maintaining the design basi On May 1, the capacitors on the circuit breakers that control startup/standby transformer CT2 were replaced with capacitors that, when coupled with the inductance, would produce only about 400 volts on the 4160 volt windin This was confirmed by test, and transformer CT2 was returned to servic When one reviews the licensee's actions from April 1 to May 1, 1988 aimed at rectifying a self identified design problem, one may conclude that in the balance their program was effective in resolving the proble Particularly noteworthy was the speed with which the oscillograms were made and interpreted. In hindsight, the problem scenario could have been identified sooner, but then the capacitors were replaced in a timely manner. All appropriate actions were take In NRC Report 88-12, this matter was identified as Licensee Identified Violation 270/88-12-01, Potential Complete Loss of All AC Power Due to 230KV Switchguard Modification The conclusions of this inspection are in agreement with that findin. Documents Reviewed and Reference Documents Qualification Report for Cogenel - Alsthom FX-22 Breakers and IF-242 Current Transformers Rated 230 KV/ 67.5 KA /900KV BIL Approved March 23, 198 Includes:
Qualification statement, Purchase Specification, Seismic Analysis, Short-Circuit Test Data (prototype) and Alsthom QA Progra Drawing, Oconee Nuclear Station Units 1, 2, and 3 One-Line Diagram 6900 V and 4160 V Station Auxiliary System, Dwg No. 0-702-1, Rev.14 Procedure, EPSL Actuation Keowee Emergency Start Test, N PT/2/A/0610/01J, performed on March 28 and 29, 1988 [Note:
Ferroresonance problem affected the completion of this procedure]
Work Request N C, initiated and completed on March 29, 1988, to investigate transformer CT2 lockout which occurred during performance of tes Copies of six oscillograms taken on April 1, 1988, at current transformer 50B/CT Problem Investigation Report Serial No. 4-088-0096 initiated on April 8, 1988, to resolve the "Possibility of resonance build up on startup transformer.......
Project Change Authorization N for Nuclear Station Modification (NSM)
ON-22637, approved on April 4, 1988, to replace the existing capacitors provided with the new PCB's with ones having smaller capacitance ratin Calculation N OSC-3023,
"Investigation of PCB/Startup Transformer Voltages Dependent Upon PCB Contact Capacitance," approved April 29, 198 Data Sheet, Voltage Verification Record, Recorded voltages at transformer secondary after capacitors were replaced, dated May 1, 198 Book, "Magnetic Saturation in Circuits at Rest" Chapter 48, Saturation of Iron in Oscillatory Circuits, Pages 642-65 Paper,
"Analyzing and Understanding Ferroresonance on Distribution Systems" from Transmission and Distribution Magazine, June 1968, by Kilgour, Harlow and Phadk IEEE Paper, "Transient Recovery Voltage and Arc Interruption" by R. Colclaser, Jr., part of IEEE Tutorial Course 75CH0975-3-PW. Licensee Action on Previous Enforcement Matters This subject was not addressed in the inspectio. Unresolved Items Unresolved Items were not identified during this inspectio. Exit Interview The inspection scope and results were summarized on May 19, 1988, with those persons indicated in Paragraph 1. The inspector described the areas inspected and discussed in detail the inspection results listed belo One violation was identified at the exit interview in the area of corrective action or, more specifically, the timeliness for the corrective actions take The licensee stated that the proposed violation was not vali Subsequent to the inspection the matter received further NRC review at the Region II office. An NRC panel concluded that NRC requirements were not violated because the licensee's corrective actions for the matter and time period in question were prudent and reasonabl Therefore, there will not be a citatio The licensee did not identify as proprietary any of the material provided to or reviewed by the inspector during this inspection.