6.9 kV to 480V Distribution

Question 1

Describe the design criteria of the 6.9 kV to 480V 1E distribution system

Answer 1

1. Two independent offsite sources or the train-related diesel generator.
2. Single-failure criteria - Redundant safety loads are divided between trains such that the loss of one does not inhibit safe shutdown.
3. Train Separation - No automatic or manual connections between trains
4. Only electrical equipment inside switchgear rooms (minimize equipment damage due to fire and water).
5. Electric motors are able to accelerate to loaded speed within 5 seconds with as little as 80% rated voltage
6. Trip relays implemented such that
   - Breakers will not trip due to low voltage caused by starting large motors
   - Built in time delays;
   - Allow enough time for breakers to trip and isolate a fault

Question 2

State the loads and their associated bus in relation to the non-safeguards 6.9 kV distribution system.

Answer 2

1A1 - CW Pump; RCP 1-01; HDP 1-01
1A2 - CW Pump; RCP 1-02; HDP 1-02
1A3 - CW Pump; RCP 1-03; TPCW Pump 1-01; Condensate Pump 1-01
1A4 - CW Pump; RCP 1-04; TPCW Pump 1-02; Condensate Pump 1-02

Question 3

What kind of grounding is utilized by 6.9 kV buses?

Answer 3

Low-impedance grounding.

Question 4

What kind of grounding is utilized by 480V buses

Answer 4

High-impedance grounding.

Question 5

What kind of grounding is utilized by the Diesel Generator

Answer 5

High-impedance grounding.

Question 6

Describe the ‘synchronization switch’ interlock associated with closing a supply supply breaker (e.g: 1EA1-1, 1EG1, 1EA1-2, etc.)

Answer 6

To prevent paralleling two power sources out of phase, the synchronization switch must be energized (switch taken to ON) to close the breaker. If the switch is left OFF, the breaker will NOT close.

Question 7

Describe the ‘anti-parallel’ interlock associated with closing a supply supply breaker (e.g: 1EA1-1, 1EG1, 1EA1-2, etc.)

Answer 7

As soon as the oncoming feeder breaker is closed, the previously closed feeder breaker will automatically open, preventing the two power sources from being paralleled for greater than 1 second.

This interlock does NOT apply to the EDG breaker.

Question 8

Briefly describe the required conditions and the sequence of events following an undervoltage condition on a 1E 6.9 kV bus.

Answer 8

Required Conditions:
1. STP control switch in CR
2. Alternate power breaker HS in AUTO
3. Voltage available to Alternate power

Sequence of Events
1. Once a UV condition exists, two TDPUs start; a 0.5 permissive to close the alternate breaker and a 1.0 permissive to start the diesel
2. After 0.5 seconds, loads are shed from the bus and if the alternate breaker automatically closes within 0.5 to 1.0 second, the diesel will not start and the BO sequencer will sequence loads back onto the bus.
3. If the alternate power breaker does not close within one second, the diesel will start and the BO sequencer will begin to sequence loads when the diesel is up to rated speed and voltage.

Question 9

Briefly describe the function and sequence of events following a secondary undervoltage condition.

Answer 9

Secondary undervoltage relays protect class 1E equipment from sustained low voltage / high current conditions.

1. If a degraded voltage condition (<88%) exists for greater than 46 seconds, the primary feeder breaker will open and the alternate will close.
2. If the alternate breaker does not close within 1.9 seconds following the secondary undervoltage relay actuation, the alternate breaker will be blocked from closing and the diesel will start.