Low Voltage AC - ESS Flashcards
What is the purpose of teh ESS System?
Provides power for essential low voltage AC load for which even a momentary interruption of power could cause harmful effects. Designed to remain in service with loss of all AC power
What is the normal power flow path for the ESS SYstem?
Bus 29 supplies power to UPS Rectifier to convert 480AC to DC;
Either the Rectifier or 250VDC MCC 2(3) supplies an inverter to convert DC to AC.
Inverter supplies a static switch which sends normal power through a Power Seeking ABT
What is the alternate power supply to the ESS Static Switch?
Bus 25 (36) via UPS voltage regulator
What prevents backfeed from Bus 29(39) to the TB 250vdc MCC 2(3)
set of diodes
What is the purpose of a rectifier?
Converts AC to DC
What is the purpose of an inverter?
Converts DC to AC
What does the ESS UPS Voltage Regulator do?
Maintains the output of Bus 25(36) at an output of 120VAC
What provides alternate supply to the ESS system via power seeking ABT?
MCC 28-2 (38-2)
What type of auto bus transfer exists in the ESS System?
Power Seeking
What is the function of the ESS Static Switch?
It is a normal seeking switch and is normally aligned to the inverter, but will swap to the voltage regulator on inverter failure (Make-Before-Break)
What kind of transfer does the Static Switch perform?
Bumpless - does not miss a cycle
What does the ESS Static Switch monitor for?
Voltage and Synchronization
What activates the Power Seeking ABT?
Its own undervoltage device, it will transfer ESS supply to whichever is giving power
What type of transfer does the ESS ABT perform?
Break-before make
How does the ESS ABT transfer back to the UPS?
Must be reset locally, it is a power seeking and does not care what its normal source is.
What happens on a loss of ESS?
Half group I isolation due to PCIS Channel B isolation trip relay loss
Recirc MG Set scoop tubes both lockout
**SRM & Channel A APRM/IRM/RBM recorders fail as is
**Lose all RPIS indication, but get ROD DRIFT alarm lights up for all CRDs as that comes of Instrument bus
Control Rod Select Matrix INOP
RFP Min FLow valves loses one of power supplies
SJAE suction and CHimney isolation valves fail closed * THIS CAUSES SCRAM* and lose indication
MSL Rad Mon B&D Fail LOW
FWLC lost
RWCU isolation from HELB
SBGT trip or fail to start due to loss of logic (U2 2/3A, U3 is 2/3B SBGT)
Div 2 CCSW Flow control fails downscale
When is the ESS required?
Must be energized in Modes 1, 2, and 3
What is required for ESS Bus to be OPERABLE?
It must be fed from a Div 2 ESS bus (EDG powered) - in other words, if the rectifer fails and MCC 28-2/38-2 or Bus 25/36 is supplying the bus, it is NOT OPERABLE
TS B3.8.7 Background “The 120VAC essential service bus is OPERABLE when aligned to Div 2 Safety Related Power Sources”
Bus 25, 26, and MCCs 28-2/38-2 powering ESS do not meet divisiional separation requirements
