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Catawba Nuclear Station > Main Power Distribution System(EPA) > Flashcards

Main Power Distribution System(EPA) Flashcards

1
Q

What is the purpose of the Main Power Distribution System(EPA)?

A

Supply electric power to each unit’s auxiliaries for normal operation and outages, and to each unit’s protection and engineered safety features systems during abnormal and accident conditions.

ALSO

Supplies power to the Switchyard which is part of Duke’s electrical transmission network for supplying electricity to customers.

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2
Q

What are the major system components of the Onsite Power System?

A
  1. Main Step-Up Transformers(22KV/230KV)
  2. Auxiliary Transformers
  3. Main Generator
  4. Emergency Diesel Generators
  5. Onsite Batteries
  6. DC Power System
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3
Q

Describe the basic layout of the Switchyard?

A
  1. The Switchyard is divided into two electrical buses called “Red” and “Yellow”, located on opposite sides of the Switchyard. The “Yellow” bus is closest to the plant.
  2. The Switchyard contains the Power Circuit Breakers(PCBs) which are normally shut and are aligned in a “breaker-and-a-half” connection scheme. This provides more reliability and flexibility.
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4
Q

Who controls access to the Switchyard?

A

Access into the Switchyard for work related activities or operational activities requires permission from the Transmission Control Center and the Work Control Center SRO.

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5
Q

How many PCBs are in the Switchyard and how are they classified?

A

There are a total of 24 PCBs in the Switchyard classified as either:
1. Switchyard PCBs

Or

  1. Switchyard Unit Tie PCBs
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6
Q

Where can the Switchyard PCBs be operated from?

A
  1. Switchyard Relay House
  2. Remotely from the TCC
  3. Locally at the breaker
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7
Q

Which PCBs are the Switchyard Unit Tie PCBs, and where can they be operated from?

A

Unit 1 1B Transformer - 14 and 15

Unit 1 1A Transformer - 17 and 18

Unit 2 2A Transformer - 20 and 21

Unit 2 2B Transformer - 23 and 24

These PCBs are normally operated from the Control Room, but can be operated from the Switchyard Relay House or locally at the breaker.

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8
Q

How physically are the Switchyard PCBs opened and closed?

A
  1. Opened with spring pressure.

2. Closed with compressed air.

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9
Q

Describe the design of the PCBs?

A
  1. The PCBs are filled with sodium hexafluoride(SF6) gas.
  2. The PCBs have a high pressure side and a low pressure side. The high pressure side provides SF6 for arc blasting/quenching.
  3. During a PCB trip the HP gas moves as its quenching the arc to the LP side through a blast valve.
  4. A portion of the gas on the LP side is pumped back to the HP side following breaker operation.
  5. The LP side provides for breaker insulation.
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10
Q

What is the purpose of the PCB Governor Switch?

A

The Governor is a temperature compensated pressure control switch that maintains constant SF6 gas density.

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11
Q

What is the purpose of the HP Gas System “Low Pressure Cutout” for the PCBs?

A

A temperature compensate pressure switch locks out the PCB from closing, and will trip the PCB if closed, if there is insufficient HP gas for an opening operation(for arc suppression).

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12
Q

What is the purpose of the “Low Pressure Cutout” for the PCBs?

A

The low pressure cutout will prevent the compressor in the HP system from maintaining pressure in case of a leak so that all SF6 isn’t lost.

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13
Q

What is the purpose of the Manually Operated Disconnects?

A

The Manually Operated Disconnects are used to manually isolate the PCBs in the Switchyard.

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14
Q

What is the time sensitive precaution associated with the Manually Operated Disconnects?

A

When the PCBs are opened, the PCB Grading Capacitors are subjected to full line voltage which will degrade the capacitors over time. Therefore when a PCB is opened for more than an hour, the manual disconnects should be opened.

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15
Q

To avoid damaging the Manual Disconnects when should they never be operated?

A

The Manual Disconnects should never be operated when the PCBs are closed. Disconnects are not normally designed to open under load.

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16
Q

What is the purpose of the Bus Line Motor Operated Disconnects?

A

The Bus Line Motor Operated Disconnects are located on the Main Transformers and the Switchyard to allow isolating the unit feeders(supply to the 22KV/6.9KV transformers).

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17
Q

Where can the MODs be operated from?

A

The MODs can be controlled from:

  1. Control Room
  2. Switchyard Relay House
  3. Locally
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18
Q

What are the interlocks associated with operation of the MODs?

A
  1. The MODs are interlocked to prevent operation if either one of the Unit Tie PCBs are closed.
  2. The MODs are interlocked to prevent closing when the Bus Line Ground Disconnects are closed, and vice versa.
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19
Q

What is the purpose of the Bus Line Ground Disconnects?

A

Each unit is provided with a Bus Line Ground Disconnect to ground the bus line when the unit is separated from the grid.

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20
Q

What is the interlock associated with the Bus Line Ground Disconnect?

A

Each Bus Line Ground Disconnect is interlocked with the associated unit MOD, such that both cannot be closed at the same time.

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21
Q

What are the design characteristics of the Main Transformers?

A

Each unit has two 230KV/22KV step up/step down transformers, each 50% capacity. They are each designed for 750MVA each.

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22
Q

How are the Main Transformers cooled?

A

Main Transformer cooling is provided by forced oil and forced air.

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23
Q

What makes up a Main Transformer Cooling Group/Bank, and how many are there?

A

A Main Transformer Cooling Group is comprised of:

  1. One oil pump
  2. Four fans

There are:

  1. 9 Cooling Groups
  2. 2 Cooling Banks
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24
Q

How many isolation valves are there for each Main Transformer Cooling Group?

A

There are two normally open isolation valves

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25
Q

What are the power supplies to the Main Transformer Cooling Banks?

A

The “A” Bank:

  1. Normal power - LXC
  2. Backup power - LXD

The “B” Bank:

  1. Normal power - LXD
  2. Backup power - LXC
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26
Q

How many Main Transformer Cooling Groups are allowed to be removed from service while online?

A

A maximum of three Main Transformer Cooling Groups may be removed from service without the need to reduce Main Generator output.

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27
Q

What is the purpose of the Main Transformer Cooling Group Power Supply Mechanical Interlock?

A

The interlock prevents having both the normal and backup supply breakers closed at the same time.

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28
Q

What type of valve is used for isolating the Main Transformer Cooling Groups from the Main Transformer?

A

A flapper type valve is used, and is held in place using a retaining bolt.

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29
Q

What is the purpose of the Main Transformer Oil Reservoir Surge Tanks, where are they located, and how many are there?

A
  1. The Oil Reservoir Surge Tanks allow for expansion and contraction of the oil in the transformer due to changes in temperature.
  2. There are two Oil Reservoir Surge Tanks located above the Main Transformer Casing.
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30
Q

How do the Main Transformer Oil Reservoir Surge Tanks prevent contact between the oil and air, and why is this important?

A
  1. The Oil Reservoir Surge Tanks have an oil-resistant diaphragm that prevents the contact between the oil and air.
  2. Air entrained in oil can cause corrosion, increased moisture, and lessening of the insulating properties of the oil.
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31
Q

What are the purpose of the Main Transformer Desiccant Dryers, and what are the indications/actions?

A
  1. The topside of the Oil Reservoir Surge Tanks are vented to atmosphere through a desiccant dryer mounted on the side of the Main Transformer casing. The desiccant dryers allow the tank to breathe without allowing moisture to enter the Main Transformer case.
  2. The desiccant color indicates moisture saturation. Healthy desiccant dryers should show a deep blue color in both site glasses. As the desiccant becomes more water laden, the color progresses from blue, to light blue, to pink, and eventually will turn white.
  3. When the desiccant turns pink a Work Request should be generated. When the desiccant turns white service is required.
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32
Q

How do we monitor Main Transformer oil level, and what interlocks are associated with oil level?

A
  1. There are three Oil Level Indicators, each with two level switches(SW1 and SW2). Switch 2 actuates a Liquid Level Relay.
  2. If 2 out of 3 Low Liquid Level switches actuate, that will result in a Transformer Zone Lockout.
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33
Q

Where are local gauge indications for Main Transformer oil level?

A

The local gauge indication for oil level is a LOW and HIGH mark, level should in between.

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34
Q

Where should oil level indication be for the 230KV Bushings?

A

The oil level should be the same between bushings, and between the 1 and 4 O’clock position.

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35
Q

Where should oil level indication be for the Neutral Bushings?

A

The Neutral Bushings have a sight glass, and level should be slightly below the midpoint of the sight glass.

36
Q

What can cause gas accumulation in the Main Transformer?

A

Gas accumulation may be an indication of oil breakdown due to internal faults or gas/air intrusion. Gas accumulation in the top of the transformer may displace oil out of the transformer and into the oil reservoirs.

37
Q

What indications do we have of gas intrusion in the Main Transformer?

A

There are Gas Detector Relays and Indicators located on the top of the Main Transformers. Gauges are scaled 0-450cc, normal level should be less than 100cc. An alarm will actuate at 200cc.

38
Q

What is the purpose of the Fault Pressure Detectors/Relays, and how many are there?

A

A sudden rise in pressure is indicative of an internal fault in the transformer. There are three detectors located at diverse points on the Main Transformer.

39
Q

What is the interlock associated with Main Transformer Fault Pressure?

A

2 out of 3 tripped Fault Pressure relays will result in a transformer lockout.

40
Q

What is the purpose of the Mechanical Pressure Relief Device on the Main Transformer?

A

The Mechanical Pressure Relief Device provides overpressure protection by opening at 10psig as indicated by a yellow flag.

41
Q

How are Main Transformer Winding Temperature and Oil Temperature monitored?

A
  1. There are two gauges, one for Winding Temperature and Oil Temperature. Each gauge has two needles. A light cooler needle indicates the current temperature, and the red needle indicates the highest temperature seen.
42
Q

What are the design characteristics of the Auxiliary Transformers?

A
  1. Each unit has four half-sized unit Auxiliary Transformers that step-down 22KV to 6.9KV to supply the unit’s auxiliary loads.
  2. Transfer capability at the the 6.9KV allows one train of the Unit Main Power System to supply not only its associated auxiliary loads, but also to supply the loads of the other train.
43
Q

What supplies power to the Cooling Tower Fan Transformers and the Auxiliary Electric Boilers?

A

There are separate Auxiliary Transformers which step down 22KV to 13.8KV for the Cooling Tower Fan Transformers and the Auxiliary Electric Boilers. The line for these Auxiliary Transformers comes off of the Main Power System(22KV) on the A Train side.

44
Q

What provides cooling to the Auxiliary Transformers?

A

The transformers are cooled by forced oil and air systems using pumps and fans.

45
Q

What is the power supply configuration for the Auxiliary Transformer Cooling Systems?

A

The power supply scheme is similar to the Main Transformer’s, one normal and one emergency supply.

46
Q

How many Auxiliary Transformers are required to carry all of the auxiliary loads for the unit?

A

Only two are required to carry all of the unit’s auxiliary loads.

47
Q

What types of interlocks do the Auxiliary Transformers have?

A

The Auxiliary Transformers have similar relays to the Main Transformers and when they are actuated they will cause a Zone(A or B) lockout.

48
Q

What is the purpose of the Generator Neutral Ground Disconnects?

A

?????

49
Q

How are the Generator Neutral Ground Disconnects operated?

A
  1. The disconnect can be opened with a hook disconnect operating tool.
  2. The hook is inserted into the Disconnect Eye at the top, pulled until the Disconnect is disconnected, and in the horizontal position.
50
Q

What is required before opening the Generator Neutral Ground Disconnect?

A

The Generator PCBs and the MODs must be open prior to opening the Generator Neutral Ground Disconnect.

51
Q

What are the design characteristics of the Generator Power Circuit Breakers?

A
  1. The Generator PCBs are similar in construction to the Switchyard PCBs. Each GPCB is rated for approximately 50% load.
  2. The two major differences between the Switchyard PCBs and the GPCBs are:
    A. The air compressors on the GPCBs can be cross-connected unlike the Switchyard PCBs.
    B. The GPCBs are air closed and air opened unlike the Switchyard PCBs which are air closed and spring opened.
52
Q

Where can the GPCBs be operated from?

A
  1. Control Room

2. Locally

53
Q

What are the advantages provided by the GPCBs?

A
  1. The need for a Startup Transformer is eliminated.
  2. Auxiliary power is not transferred between sources during startup, shutdown, or unit trip.
  3. The breakers provide a means of isolating the two trains such that we have two independent and redundant power circuits.
  4. Faulted equipment on one train can be isolated without a generator and/or reactor trip, thus improving reliability, availability, and safety.
54
Q

What is the purpose of the Generator Motor Operated Disconnects?

A

The generator MODs provide a method of disconnecting the Main Generator and GPCBs from the grid for isolation and maintenance.

55
Q

What type of disconnect are the Generator MODs, and where are they operated from?

A
  1. The Generator MODs are motor operated, sliding disconnects located on both sides of the GPCBs. There is one disconnect per phase on each side.
  2. The Generator MODs are normally controlled from the Control Room, but they can be controlled from a local control cabinet near the GPCBs.
56
Q

What is the purpose of the Isolated Phase Bus?

A

The phase busses that connect the Main Generator to the Auxiliary Transformers, the Auxiliary Transformers to the 6.9KV busses, and the Main Transformers to the Auxiliary Transformers are all isolated busses. Each phase bus is encased in a metal housing. A cooling system circulates air through the metal housings to remove the heat from current flow in the 22KV portions of the Isolated Phase Bus.

57
Q

What makes up the cooling portion of the Isolated Phase Bus System?

A

The cooling portion of the IPB system is made up of:

  1. 4 fans
  2. Heat Exchangers cooled by the Recirculated Cooling water system(KR)
58
Q

What is the flow path of the IPB Cooling System?

A
  1. 2 fans provide forced air to the X phase
  2. 2 fans provide forced air to the Z phase
  3. Air is returned to the fans through the Y phase.
59
Q

How is a Switchyard PCB removed from service?

A
  1. Open the PCB, and verify it’s open by observing the local mechanical flag indicator.
  2. Place the local control switch in “TRIP” and “PULL TO LOCK”.
  3. Open the two local manually operated disconnects and lock them open.
60
Q

How is a Switchyard PCB returned to service?

A
  1. Unlock and close the manually operated disconnects around the PCB.
  2. Removed the local control switch from the “PULL TO LOCK” position.
  3. Close the PCB
61
Q

What is the purpose of placing the PCB local control switch in the “PULL TO LOCK” position?

A

Placing the local control switch in the “PULL TO LOCK” will prevent the PCB from operating.

62
Q

Describe the operation of a Manually Operated Disconnect?

A
  1. Manually Operated Disconnects are cycled prior to locking them in the open position to identify any problems with the disconnects.
  2. The ends(3) of the disconnect are called “Beavertails” because of their physical appearance.
  3. The rotation of the beavertail in the stationary contacts will cause the outer portion to the beavertail to wipe the stationary contacts on both sides clean of any debris or corrosion.
63
Q

What is the precaution associated with operating the Manually Operated Disconnects?

A

When using the hand-crank the operator must be careful to avoid stressing the mechanical stop.

64
Q

Describe the operation of the Generator Power Circuit Breakers(GPCB)?

A
  1. Operation is from the Control Room or locally.
  2. The CLOSE pushbutton is operable only when GPCB control is in the LOCAL position and the MOD lever is in the SAFE position.
65
Q

Describe how the GPCBs can be tripped locally?

A
  1. There are two local pushbuttons.
  2. The local trip pushbutton that is located behind the red placard is always active.
  3. The second local trip pushbutton is only operable when GPCB control is in the LOCAL position.
66
Q

What is the effect of placing the GPCB control in LOCAL?

A

When the GPCB control is in LOCAL all protective relaying is bypassed except the breaker failure relay.

67
Q

Where can you find local position indication for the GPCBs?

A
  1. Locally by each pole contact
  2. The GPCB Cabinet
  3. The Generator MOD Cabinet
68
Q

What is the interlock associated with closing the GPCBs?

A

To close the Generator Power Circuit Breakers:

  1. The Generator MODs must be closed.
  2. The Exciter Field Breaker must be closed.
  3. The Main Generator speed must be greater than 99%.
69
Q

What happens when the GPCBs trip?

A

The Generator MODs will open also.

70
Q

What are the positions of the Generator Motor Operated Clutch Lever, and where is it located?

A
  1. The Generator Motor Operated Disconnect Clutch Lever is located in the Generator MOD local control panel.
  2. The positions of the Generator MOD Clutch Lever are:
    A. Auto
    B. Safe
    C. Man
71
Q

What is the effect of placing the Generator MOD Clutch Lever in AUTO?

A
  1. Allows the Generator MODs to be controlled from the Control Room or locally by OPEN-REMOTE-CLOSE handle.
  2. The Generator MODs will open anytime the Generator PCBs trip.
  3. All interlocks are active in AUTO.
  4. The Disconnect Control switch(Pistol Grip) OPEN-REMOTE-CLOSE positions are operable in the AUTO position.
72
Q

What is the effect of placing the Generator MOD Clutch Lever in SAFE?

A
  1. Blocks all Generator MOD operations.
  2. If a Generator MOD is closed when the lever is taken to SAFE it will open.
  3. The SAFE position send a momentary trip signal to the GPCBs and the Generator MODs to open.
  4. The GPCB cannot be operated from the Control Room with the lever in SAFE.
  5. If the generator is online when the lever is moved from AUTO to SAFE the GPCB will trip open.
  6. The SAFE position allows the GPCB to be closed for testing when the GPCB control is in LOCAL.
73
Q

What is the effect of placing the Generator MOD Clutch Lever in MAN?

A
  1. The MAN position allows the Generator MODs to be racked open/closed locally using a hand crank.
  2. The Generator MODs cannot operate electrically in the REMOTE or LOCAL position.
74
Q

What is the purpose of the Generator MOD Emergency Stop pushbutton, and where is it located?

A
  1. The Generator MOD Emergency Stop Pushbutton is located in the Generator MOD local panel.
  2. The Emergency Stop Pushbutton will stop MOD travel when pushed.
75
Q

Describe the normal operation of the Isolated Phase Bus cooling?

A
  1. During normal operation the IPB has one fan running per phase on the X and Z phases.
  2. IPB operation is controlled from a local panel.
  3. Due to condensation in the buses when the unit is shutdown, the IPB system should be left in operation if the unit is to be shutdown for greater than 24 hours.
76
Q

What will cause an IPB fan to automatically trip?

A

IPB fans will trip on:

  1. Any Zone G lockout actuation signal
  2. Zone A or Zone B Ground Fault
  3. Differential Relay actuation
77
Q

What is the normal IPB operating temperature, and how long will it take to reach limits without cooling?

A
  1. Normal temperature is 60-65C
  2. Maximum bus temperature of 105C will be reached in less than or equal to 10 minutes at full load without forced cooling.
  3. Design maximum temperature of 125C will be reached in less than or equal to 20 minutes at full load without forced cooling.
78
Q

How will a Loss of Main Transformer Cooler Power affect plant operation?

A
  1. For a loss of either feeder power supply to the Main Transformer Auxiliaries a mechanical interlock must be operated inside the Main Transformer Auxiliaries Control Cabinet to align the alternate source.
  2. If this is done within 12 minutes a turbine runback on a loss of one of the feeders(LXC and LXD) should be prevented.
79
Q

What GPCB trip function does the Loss of Cooler Power Relaying perform?

A

Trips the associated GPCB if:
1. Both cooler power supplies for the associated Main Transformer are lost for 3 minutes if the GPCB is closed
OR
2. One cooler power supply for the associated Main Transformer is lost for 15 minutes and that transformer is tied to the grid.

80
Q

What Runback function does the Loss of Cooler Power Relaying perform?

A

Causes a Runback of the Main Turbine if:
1. Both cooler power supplies for the associated Main Transformer are lost with both GPCBs closed
OR
2. One cooler power supply for the associated Main Transformer is lost for 12 minutes when both GPCBs are closed.

81
Q

What Zone function does the Loss of Cooler Power Relaying perform?

A

If both cooler power supplies are lost for the associate Main Transformer for 15 minutes and that transformer is tied to the grid, then a Zone A or Zone B Lockout will be generated.

82
Q

How will the Isolated Phase Bus Cooling Fans react to protective zone faults and why?

A
  1. The IPB fans are automatically stopped by a lockout relay following a protected zone fault trip covering the generator and transformer zones.
  2. The IPB fans will trip on a Zone G, and on a Zone A or B Lockout caused by a Zone Differential or Bus Ground Relay.
83
Q

What is the purpose of tripping the Isolated Phase Bus Cooling Fans during a fire?

A

Tripping the fans during a fire condition will prevent the spread of contaminates leading to a loss of both of the Unit’s Main Transformers.

84
Q

What is required on a loss of Isolated Phase Bus cooling?

A

Additional monitoring is required to prevent exceeding and maximum or design maximum limits.

85
Q

What is the most critical section of bus work in the Isolated Phase Bus system?

A

The most critical section of the bus to be monitored is the generator bus segment prior to the split to the A and B Main Step-Up Transformers.

86
Q

What are the maximum temperatures associated with the Isolated Phase Bus System?

A
  1. Maximum bus temperature of 105C will be reached in less than or equal to 10 minutes at full load without forced cooling.
  2. Design maximum temperature of 125C will be reached in less than or equal to 20 minutes at full load without forced cooling.

Catawba Nuclear Station (72 decks)

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