05000263/FIN-2012007-06
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Finding | |
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| Title | Failure to Analyze Effect of System and Transient Harmonics on Proper Operation of Degraded Voltage Relays |
| Description | The Monticello degraded voltage protection scheme features three ITE Type 27N relays for each 4.16kV safety bus, arranged in a two out of three tripping scheme. BBC Instruction Bulletin 7.4.1.7-7 states, the relay employs a peak voltage detector, and harmonic distortion on the AC waveform can have a noticeable effect on the relay operating point and the measuring instruments used to calibrate the relay. The bulletin also notes that the relay is available with an internal harmonic filter for applications where waveform distortion is a factor. The inspectors noted Calculation 92-220, Instrument Set-Point Calculation 4.16KV Degraded Voltage, identified the relays as a model not equipped with harmonic filters, but did not address the basis for excluding harmonic distortion as a factor affecting relay accuracy. The inspectors were concerned that persistent harmonics on the 4.16kV system could cause the relays to fail to actuate at the set point specified in Technical Specifications, and that transient harmonics, caused by switching operations, could cause the relays to spuriously reset during the time delay that occurs during an actual degraded voltage condition concurrent with a design basis accident. Persistent harmonics can be produced by factors external to the nuclear site or by internal phenomena. A typical internal source of harmonics at nuclear power plants is defects in rotating equipment. Persistent harmonics could cause dropout set point shift, and mask an actual degraded voltage condition. Transient harmonics could cause the relays to spuriously reset during an actual degraded voltage event, thereby delaying the protective function beyond the nominal 10 seconds (9.2 seconds Allowable Value) stipulated in Technical Specifications Table 3.3.8.1-1 and the 15 seconds assumed in the accident analysis. The relay is susceptible to this type of mal-operation because it features an instantaneous voltage sensor that could reset in less than two cycles in the presence of harmonics, thereby reinitiating the relays internal timer. Operating Experience available to the inspectors from another nuclear station indicated the transient voltages that occur during the operation of medium voltage circuit breakers could cause the relays to spuriously reset during an actual degraded voltage condition. The inspectors were specifically concerned that if a degraded voltage condition occurred concurrently with an accident, the automatic switching operations that occur on the 4.16kV electrical system following the onset of an accident could cause the relay to spuriously reset, thereby delaying the transfer to a reliable source of power beyond the required time. In response to the inspectors concerns, the licensee provided information regarding condition monitoring of large motors consisting of periodic measurement and analysis of motor bearing vibration from which various defects that may produce harmonics could be identified. However, the inspectors noted, there was no guidance in design documents that linked the presence of harmonics produced by motors during testing to mal-operation of the degraded voltage scheme. Although the motor vibration data taken by the licensee may be used to predict the presence of harmonics to some extent, the reason or the intent of the tests was not to evaluate or monitor harmonics. Also, the test data was not interpreted to evaluate and document the presence of harmonics. The inspectors further noted during normal bus voltage conditions, i.e., when voltage is above the degraded voltage relay reset set point, harmonics would shift system peak voltage away from the degraded voltage relay operating set point rather than closer to it, and so the presence of harmful harmonics would not self-reveal by spurious actuations. Also, there was no instrumentation or testing that demonstrated transient harmonic would not be presented during switching operations. The licensee has entered this item into their corrective action program as AR 01331618. This issue is unresolved pending consultation with NRC Headquarters for clarification of equipment qualification requirements of degraded voltage relays to withstand the effects of harmonics. |
| Site: | Monticello  |
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| Report | IR 05000263/2012007 Section 1R21 |
| Date counted | Jun 30, 2012 (2012Q2) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | C Zoia B Jose G Skinner M Jones J Gilliam A Stone N Feliz-Adornob Josed Krause G Hausman K Riemer P Zurawski S Bell |
| INPO aspect | |
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Finding - Monticello - IR 05000263/2012007 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Monticello) @ 2012Q2
Self-Identified List (Monticello)
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