On 4/21/17, high grid voltage conditions were experienced, resulting in voltages higher than those established for operability of the
offsite circuits. Grid voltages have been observed at approximately 355.8 kV on the nominal
345 kV system. As a result, both qualified circuits between the offsite transmission network and the onsite Class 1E
AC Electrical Power Distribution System were declared inoperable at
0202 hours0.00234 days <br />0.0561 hours <br />3.339947e-4 weeks <br />7.6861e-5 months <br /> [EDT]. Main generator voltage control has been lowered to the minimum possible excitation with the unit operating at 100 percent power.
The inoperability of both offsite circuits results in a loss of safety function in accordance with NRC reporting guidance. The voltage of the onsite Essential busses remains within acceptable values, and both Emergency Diesel Generators are operable. At 0715 [EDT] on 4/21/17, grid voltage has returned to an acceptable value and the equipment was declared operable.
The NRC Resident Inspector has been notified.
- * * RETRACTION FROM ANDREW MILLER TO VINCENT KLCO ON 6/13/17 AT 1128 EDT * * *
Following the reporting of high switchyard voltage on 4/21/17, the licensee-established voltage limits were re-evaluated. The new high voltage limit has been established at 362.94 kV on the nominal 345 kV system, or 105.2 percent of nominal voltage, as compared to the previous maximum grid voltage of 103.3 percent. This new limit is above the 355.8 kV experienced on 4/21/17. Therefore, the equipment remained operable and no loss of safety function existed for the qualified circuits between the offsite transmission network and the onsite Class 1E AC electrical power distribution system, and the notification made per 10 CFR 50.72(b)(3)(v)(A-D) is being retracted.
An evaluation of the past three years of switchyard voltage data was also performed, and it was concluded the AC power system and its connected safety-related equipment remained capable of performing its required safety functions during the three-year evaluation period.
The NRC Resident Inspector has been briefed on the evaluation results and informed of this retraction.
Notified the R3DO (Stone).
