480v

Question 1

STATE the two (2) major functions of the 480 VAC Distribution System

Answer 1

• Supply and distribute 480 VAC power to the vital and non-vital 480 VAC loads.
• Provide 480 VAC that is transformed to 120/240 VAC for lighting, heating, receptacles, and other low voltage uses

Question 2

Given a 480 volt unit substation or MCC bus identification number

Answer 2

TRACE its normal power supply back to the proper 4.16 kV or 13.8 kV bus.
• Example US 51, 52 or US 21, 22
• First number - 13.8 or 4.16 KV supply
• Second number - unique identifier

Question 3

STATE the actions required to reclose a 13.8 kV unit substation supply breaker that has tripped by means of an 86 relay actuation

Answer 3

• Determine and correct cause of trip (instantaneous, timed overcurrent, or ground fault-13.8 kV only)
• Reset “targets” per ODI-17 “Tracking Protective Relay Actuation’s”
• DO NOT RESET 86 device without control room permission.
• Normal/Reset switch
• 86 relay trips breaker and prevents reclosure
• Red/Orange flag/white light out-relay tripped
• Black flag/white light on-relay reset
• picked up on 50 & 51 relays

Question 4

STATE the only 480 VAC Load Center load powered from the Unit One Electrical Distribution System

Answer 4

• LC-3 powered from 4.16 kV bus 4

Question 5

DESCRIBE the operation of the switchyard 480 VAC automatic transfer switch, and be able to LIST two loads that are powered from the switchyard 480 VAC Distribution System

Answer 5

• Switchyard 480 VAC Automatic Bus Transfer Switches (ABTs)
• when normal power is lost, solenoid switching assembly automatically opens normal supply breaker and closes emergency supply breaker.
• when normal supply is restored, solenoid switching assembly transfers back to normal
• Battery Chargers (System 1 & 2)
• 345KV Breaker Auxiliaries
• 120/240 V Distribution Panels (1SD1 & 1SD2)

Question 6

STATE the two basic categories that the 480 volt unit substation breakers fall into

Answer 6

• electrically operated (remote)

* manually operated

Question 7

DESCRIBE how to crosstie the 480 volt unit substation buses by station procedure.

Answer 7

Several buses can be cross connected
–allows power to be maintained in the event of supply failure
or maintenance.
–consideration must be given to loading.
–non-vital substations can be supplied from another bus:
•11 and 23, 12 and 25, 13 and 26
•14 and 21, 14 and 16, 15 and 24
Limited buses can be cross connected:
–allows power to be maintained in the event of supply failure or maintenance.
–vital substations can only be supplied from same bus