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Exam 1 > 13.8 KV > Flashcards

13.8 KV Flashcards

1
Q

Describe how the 13.8KV distribution system is protected from damage resulting from full phase to ground faults

A

a. A grounding resistor rated for 600A for 10 seconds has been installed to allow the ground fault relays to function before the ground damages the transformer

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2
Q

State the function of the UAT pressure-vacuum bleeder valve and describe operation

A

a. Designed to accommodate season and operational changes in temperature/gas space pressure, opens to relieve pressure

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3
Q

Draw and label 13.8KV one-line

A

see drawing PDF

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4
Q

State the function of the UAT/RAT tap chargers

A

a. Allows no-load charging of the transformer primary to secondary voltage ratio without having to break the transformer seal.

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5
Q

Describe how the UATs and RATs are cooled, including flow-paths and auto-control features

A

a. UATs: 8 oil-to-air heat exchangers with isolation valves and 4 fans, auto on at 70C and off at 65C, powered from MCC 131/261
b. RATS: 12 fans with 4 HXs auto on at 70C of <70C, powered from 131/261

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6
Q

Describe the three general types of fault detection (including sensing devices and parameters sensed) common to both the UATs and the RATs

A

a. UAT Primary Protection (86UP)
i. Differential Current only (87UP) – senses current flow imbalance from UAT supply to isophase bus ducts to swgr sides of the 13.8/4.16kv busses.
b. UAT Backup Protection (86UB)
i. Timed OverCurrent (51)
ii. Ground Fault (51N) – 600 amps for 10 seconds. Uses single current transformer on grounding resistor of transformers 13.8kv secondary winding. If current flow to grounding resistor exceeds setpoint for GFR, the relay is actuated.
iii. Sudden Pressure Relay (63FPX) – 2 of 3 logic
1. Used to detect excessive rate of oil pressure increase within the transformer, as a result of internal arcing.
2. An internal transformer fault that causes arcing vaporizes the insulating oil, resulting in a rapid pressure increase.
3. Gradual temperature changes will not result in actuation.
c. Overcurrent (51) – specific to UAT only. Uses current transformers of each phase of 25kv-segregated bus to monitor current flow. If current exceeds setpoint, relay is actuated.

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7
Q

Describe how and electrical fault within the UAT and RAT will affect the station electrical distribution system

A

a. UAT: Trips Turbine, Opens MGB, Opens Exciter Field Breaker, Isolates Buses 1-6 UAT supply breakers, Opens SY PCB 11 and 12, Isolates UAT cooling Auxiliaries
b. RAT: Trips open Buses 1-6 RAT supply breakers, Opens 345 KV PCBs 52 and 695, isolates RAT cooling fans

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8
Q

Describe how the gas mixture in the UAT gas space is maintained inert

A

The air space above the oil is charged with nitrogen

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9
Q

State the function of the UAT/RAT mechanical pressure relief device and describe its operation

A

a. UAT: Relieves excess tank pressure, opens at 11.5#, seats are 8#
b. RAT: Relieves excess tank pressure, opens at 10#, seats at 5#

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10
Q

Describe the UAT/RAT fire protection Schemes

A

AT 190F foam insulation is melted and two wires come into contract activating the detection circuit which opens the associated deluge valve

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11
Q

Describe the operation the RAT oil preservation system

A

0.5# overpressure is maintained on oil

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12
Q

Describe the routing of the 13.8KV bus ducts from the UAT/RATs to the 13.8KV buses

A

From UATS/RATS in non-segregated bus ducts through the TB to SWGR

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13
Q

Describe the motive power for operation of the 13.8KV incoming supply breakers and load feeder breakers

A

Closing springs charged via motor powered from 125VDC (control power)

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14
Q

Describe all the uses for 125VDC power within a 13.8KV breaker enclosure, including the impact of losing DC control power

A

a. Closing springs charging motor
b. Undervoltage protective relaying
c. Breaker control power

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15
Q

State the function of the 13.8KV breaker closing spring charging motor

A

Compresses the closing springs until fully charged, the springs discharge and close the breaker

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16
Q

Describe the interlock which prevents racking out a 13.8KV breaker which is closed

A

If the breakers is closed the breaker is mechanically interlocked to prevent it from moving

17
Q

Describe the actions that must be taken to operate a 13.8KV breaker locally at the breaker enclosure

A

a. Local/remote switch allows opening and closing of the breaker in TEST position only.
b. If 125VD is lost (no Control power) trip pushbutton still works as does the closing lever and manual charging of the springs

18
Q

Explain the meaning of each of the indicating lights associated with the MCB and local control switches for the 13.8KV UAT and RAT incoming supply breakers

A

a. Red: breaker closed
b. Green: breaker open
c. Amber: Switch in NAC with breaker open
d. White light (local only): 86 device reset with power

19
Q

State the function of the closing relay (52X) and anti-pump relay (52Y) relay located in the 13.8KV incoming supply breaker closing circuit

A

a. Anti-pump relay: prevents inadvertent cycling of the breaker
b. Closing relay: when energized the closing springs discharge and closes the breaker

20
Q

Describe the 5 general fault conditions which will cause the 13.8KV UAT incoming supply breakers to Auto trip open

A

a. Bus Duct fault (86UP or 86UB)
b. GSU 25KV isophase fault (86SP or 86 SP)
c. 346KV bus 6 (or GSU connector fault)
d. Switchgear enclosure fault
e. Breaker failures (345KV 11/12 or MGB)

21
Q

Describe the 2 sets of conditions that will normally cause the 13.6KV bus 1 or 2 RAT incoming supply breaker to automatically shut

A

a. If the UAT incoming supply breaker trips open (assuming no bus fault) then RAT breaker will auto close
b. RAT breaker will not close if UAT incoming breaker is still closed, operator action is required at this point.

22
Q

Describe the 3 general fault conditions which will cause the 13.8 KV RAT incoming supply breakers to automatically open

A

a. RAT/bus duct faults
b. 345KV bus 5 faults
c. SWGR enclosure Fault

23
Q

Describe the automatic actions that occur as from an extended under-voltage condition on 13.8KV bus 1 or 2

A

a. If bus voltage drops below 70% for 1/3 sec bus stripping occurs, RCPS go first at 20 cycles (1/3 of second)
b. After 1.5 sec CW pumps trip

24
Q

Describe the actions that have to be taken to manually transfer the 13.8KV bus 1 power supply from the UAT to the RAT and back to the UAT

A

a. Close in the other breaker onto the bus, as the breaker closed the other breaker auto trips open. Synch switch must be on and breaker selected

Exam 1 (10 decks)

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