The team identified an unresolved item (
URI) regarding the degraded voltage relays. Specifically, the effect of system and
transient harmonics on proper operation of degraded voltage relays was not analyzed. The Robinson degraded voltage protection design features three ITE Type 27N relays for each 480V emergency bus E1 and E2, 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; however, harmonic filters are not installed on the degraded grid voltage relays based upon their model number and specification package. The inspectors questioned if persistent harmonics on the 480V system could cause the relays to fail to actuate at the set point specified in
Technical Specifications 3.3.5, and if
transient harmonics 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 value stipulated in
Technical Specifications 3.3.5. 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. The licensee has entered this item into their corrective action program as
NCR 601203. This issue is unresolved pending inspector consultation with NRC headquarters technical staff for clarification of license basis design requirements of degraded voltage relays to withstand the effects of harmonics. This issue is identified as
URI 05000261/2013007-07, Questions Regarding License Basis Design Requirements for Degraded Voltage Relays.
