Document: NUREG-0800
Document ID: 9cfc38bf-7808-446e-a43d-4ca9ff55df2a
Document Type: srp
Title: AC POWER SYSTEMS (ONSITE)
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1007/ML100740289.pdf
Revision Date: 2023-06
Chapter: 8
Section ID: 8.3.1
CFR Part: 
CFR Title: 

Content:
at the plant switchyard. Operating experience has shown that a variety of factors, such as power flow through the transmission grid, reactive power capacity, the plant voltage and frequency protective schemes and setpoints, and weather or temperature conditions in the region, can all affect grid voltage levels and overall stability. BTP 8-6 and References 7 and 13 provide information for the reviewer regarding degraded transmission grid voltage and the effects of grid events on grid voltage at the plant switchyard. Detailed review regarding the analysis of grid operating conditions and stability and their potential interactions with the onsite power system is covered in SRP Section 8.2, "Offsite Power System." In reviewing the mode of operation where both power systems are being operated in parallel (such is the case during full-load testing of standby power supply diesel generator sets), the interlock scheme, including electrical protective relay coordination and settings, is closely examined to verify that the independence of the necessary redundant portions of the onsite power system is established upon a failure in the offsite power system. The event of concern under this mode of operation is an accident concurrent with a LOOP and a single failure preventing the opening of the feeder- isolation breaker through which the paralleling of the power systems was being accomplished. Because the signal to start the diesel generator sets is normally derived from undervoltage relays, and under this situation the voltage is maintained above the trip relay settings by the diesel generator under test, the remaining redundant diesel generators will not be commanded to start running. Consequently, the added capacity resulting from the connection of non-safety-related loads to the diesel generator under test will cause the tripping of this diesel due to overload or underfrequency. The end result could be the total loss of power to the safety buses. However, this power