Safety, Instrument & Vital Power Systems - 125 V DC Systems AND 120 V Instrument AC Systems

Question 1

Discuss the 125VDC function.

Answer 1

Supplies 125 VDC to various control and instrumentation circuits and to DC operated equipment.

Question 2

State the purposes of the Battery.

Answer 2

Provide a backup source of power to the DC bus if the associated battery charger fails
Rating - 2400 amp*hr @ 8 hour discharge rate (4 hour emergency period)

Question 3

What is the Battery/Battery Charger Interlock, (A and B DC buses only)?

Answer 3

Mechanical bar interlock
Prevents the Battery / DC bus breaker and the Load Bank breaker from both being closed

Question 4

What are the DC Bus Interlocks?

Answer 4

UV trip between safety buses (outer tie bkr trips inner tie bkr on UV for AB bus)
2 keys for the 4 safety bus tie bkrs
Admin control of C/AB and D/AB bkr keys

Question 5

Discuss the Battery Chargers.

Answer 5

Supplies trickle (Float) charge to the batteries at 132-135 VDC,
315 amps Max, Warm up time 15 minutes
Equipped with variable timer up to 24 hours for an Equalize charge 137 VDC – No Alarm
156 VDC – Hi Volt Shutdown Relay will trip the Charger Output Breaker. Must be reset inside the cabinet.
136-137.5 VDC – When in Equalizing mode
138-140 VDC - Hi Volt Alarm,
129 VDC - Min TS voltage while in FLOAT,
Low voltage ALARM @ 109 Volts!!! - Tech Spec violation before alarm comes in.

Question 6

Discuss the Battery Charger Ground Test.

Answer 6

3 position switch – POS GND / Normal / NEG GND
The positive or negative light burns dimly when it is selected.
The control room gets a ground alarm when tested.
If a large CHGR ground develops, you get a CR ground alarm & ground light on the charger turns on
With an actual ground on the charger, placing the ground test switch to positive and negative positions will show a difference in brightness. The side with the brightest light has the ground.

Question 7

When performing the Battery Charger Ground Test, what is the indication as to which side has the ground?

Answer 7

The side with the brightest light has the ground.

Question 8

Discuss Placing a Battery Charger in-service.

Answer 8

Close MCC feeder bkr to BC,
Place On/Off switch to on
Close AC input bkr, wait 15 min. – Why wait 15 minutes? Failure to warm up charger prior to being put into service may cause charger to trip
Close DC output bkr
Verify 132-137 VDC,
Close BC bkr to bus
Verify system response

Question 9

Discuss Removing a Battery Charger from Service.

Answer 9

Check system status
Open BC feeder bkr to bus
Verify system response
Place On/Off switch to off
Open AC input bkr
Open DC output bkr
Open MCC feeder bkr to BC

Question 10

Discuss why the 125 VDC Buses Are Ungrounded.

Answer 10

This means that it takes 2 grounds to cause excessive current flow that will damage the Bus.
First Ground can be detected by a Hi Voltage reading on the Bus Ground Detector

Question 11

Discuss the DC Bus Ground Test Device.

Answer 11

“OFF” and “Ground Test” with spring return to “OFF
Bottom line: on bus ground detection, the DIMMEST side has the ground

Question 12

Discuss how to Swap AB DC Bus Power Supplies.

Answer 12

Live bus transfer, the AB charger is placed in-service prior to swapping

AB Battery Charger is normally maintained in Standby Mode – Know what this is

Take AB BC to equalize mode
Close AB BC feeder bkr to bus (live bus transfer)
Verify system response
Open inner tie bkr
Open outer tie bkr
Close other outer tie bkr
Close other inner tie bkr
Take AB BC to Normal mode (float)
Open AB BC feeder bkr to bus

Question 13

Discuss the effects of a Loss of DC bus.

Answer 13

2/4 RPS channels de-energize = Rx trip
2/4 ESFAS channels de-energize = ESFAS actuation (except RAS, CSAS, DSS which are energize to actuate)
1 AFAS channel in trip, 1 AFAS channel in bypass (More detail in AFW / AFAS notes)
Loss of breaker control power
{Loss of AB DC Bus on U1 results in 1C AFW inoperability – 10 min action to swap AB dc bus to energized bus}

Question 14

What are the Unit 1 Battery Charger Power Supplies?

Answer 14

Battery Charger/MCC
A A5
AA A6
B B5
BB B6
AB AB
C A1
D B1

Question 15

What are the Unit 2 Battery Charger Power Supplies?

Answer 15

Battery Charger/MCC
A A5
AA A5 **
B B5
BB B5 **
AB AB
C C
D C

** - Significance of this means that a loss of the A5 (or B5) MCC on Unit 2 results in the loss of both Battery Chargers to the A (or B) DC bus – 2 hour action required by T/S’s

Question 16

What DC bus indications are available in the Control Room?

Answer 16

Unit 1
DC bus voltage

Unit 2
DC bus voltage
DC bus amperage

Question 17

What are the effects of of Abnormal/Emergency conditions on DC busses?

Answer 17

On LOOP – DC buses remained powered from batteries
Once EDG starts & loads – battery chargers regain power, and begin powering the DC buses

Question 18

State the function of 120V Instrument AC.

Answer 18

Supply stable, regulated 120 VAC, 60 Hz power to safety related Instrument buses

Question 19

What are the normal power supplies to the 120V Instrument Inverters?

Answer 19

‘A’ & ‘C’ Inverters – ‘A’ DC Bus
‘B’ & ‘D’ Inverters – ‘B’ DC Bus

Question 20

What are the alternate power supplies to the 120V Instrument Inverters?

Answer 20

Unit 1 –
Powered from A5 & B5 MCC through a Maintenance Bypass Bus (mechanically interlocked to only allow power from one source at a time - either inverter or alternate source)

Unit 2 –
Powered from A5 & B5 MCC through a Manual Transfer Switch (No Maintenance Bus). Manual Transfer Switch is make before break. Will NOT Auto Transfer on Loss of Inverter Power

Question 21

What is the Instrument AC bus response to Loss of a Battery Charger(s)?

Answer 21

Loss of 1 Charger - No impact. 2nd charger on that train picks up the load
Loss of both chargers on a single train – No impact. Battery begins discharging

Question 22

What is the Instrument AC bus response to Loss of off-site power?

Answer 22

Instrument buses unaffected as DC buses are unaffected
If aligned to alternate AC source, power could be lost temporarily as A5/B5 MCC’s lose power

Question 23

What is the Instrument AC bus response to If an Instrument Bus de-energizes?

Answer 23

1/4 RPS channels trip - 4 of 8 Rx trip breakers open. RX does not trip
1/4 ESFAS channels trip (deenergize to actuate only)
1/4 AFAS channel in trip

Question 24

What is the Instrument AC bus response to Inverter Failure?

Answer 24

Associated instrument bus de-energizes (unless aligned to the alternate AC source)

Question 25

What is the power supply for the Qualified Safety Parameter Display System - (QSPDS)?

Answer 25

‘A’ QSPDS: 1MC [2MC-1]
‘B’ QSPDS: 2MD [2MD-1]

Question 26

What is the function of 120V Vital AC?

Answer 26

Provides uninterruptible, stable AC power to vital bus for non-safety related loads:

Turbine Controls
Turbine Supervisory Instrumentation
Feedwater controls.

Question 27

Describe the Typical Static Uninterruptible Power Supply (SUPS).

Answer 27

“RISO”
Rectifier
Inverter
Switches
Output

Question 28

What are the Major SUPS Loads?

Answer 28

SAS SUPS loads – CEDS cabinets for Rod Control, Emergency Response Data Acquisition System (ERDADS)
Fire/security SUPS - Radiation monitoring, Fire detection, Security card readers
Vital SUPS - Turbine Controls, FW controls, FW Heater controls

Question 29

What are the SUPS power supplies?

Answer 29

Vital AC
“ABsD”
Normal AC - ‘AB’ MCC, Alternate AC - ‘AB’ MCC, Backup DC - ‘D’ DC Bus

Fire & Security
“ABCD”
Normal AC - ‘AB’ MCC, Alternate AC - ‘C’ MCC, Backup DC - ‘D’ DC Bus

SAS SUPS
1C / 2A
Normal AC- A1 MCC, Alternate AC - A1 MCC, Backup DC - ‘C’ DC Bus
1D / 2B
Normal AC- B1 MCC, Alternate AC - B1 MCC, Backup DC - ‘D’ DC Bus