Low Voltage AC - ESS

Question 1

What is the purpose of teh ESS System?

Answer 1

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

Question 2

What is the normal power flow path for the ESS SYstem?

Answer 2

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

Question 3

What is the alternate power supply to the ESS Static Switch?

Answer 3

Bus 25 (36) via UPS voltage regulator

Question 4

What prevents backfeed from Bus 29(39) to the TB 250vdc MCC 2(3)

Answer 4

set of diodes

Question 5

What is the purpose of a rectifier?

Answer 5

Converts AC to DC

Question 6

What is the purpose of an inverter?

Answer 6

Converts DC to AC

Question 7

What does the ESS UPS Voltage Regulator do?

Answer 7

Maintains the output of Bus 25(36) at an output of 120VAC

Question 8

What provides alternate supply to the ESS system via power seeking ABT?

Answer 8

MCC 28-2 (38-2)

Question 9

What type of auto bus transfer exists in the ESS System?

Answer 9

Power Seeking

Question 10

What is the function of the ESS Static Switch?

Answer 10

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)

Question 11

What kind of transfer does the Static Switch perform?

Answer 11

Bumpless - does not miss a cycle

Question 12

What does the ESS Static Switch monitor for?

Answer 12

Voltage and Synchronization

Question 13

What activates the Power Seeking ABT?

Answer 13

Its own undervoltage device, it will transfer ESS supply to whichever is giving power

Question 14

What type of transfer does the ESS ABT perform?

Answer 14

Break-before make

Question 15

How does the ESS ABT transfer back to the UPS?

Answer 15

Must be reset locally, it is a power seeking and does not care what its normal source is.

Question 16

What happens on a loss of ESS?

Answer 16

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

Question 17

When is the ESS required?

Answer 17

Must be energized in Modes 1, 2, and 3

Question 18

What is required for ESS Bus to be OPERABLE?

Answer 18

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