From memory, explain the purpose of the Electrical Distribution system in accordance with FSAR Chapter 8 and DBD Sections 3.00 and 6.00.
Designed to supply and distribute electrical power needed to preserve the three fission product barriers: Fuel Cladding, Primary Coolant System, Containment Building. Under ALL plant conditions, including emergencies.
From memory, draw and label a one line diagram of the Electrical Distribution system showing system interfaces and the following major components:
- 345kv busses (062 K4.08)
- 24kv busses (062 K4.08)
- 4160v busses (062 K4.07)
- 2400v busses (062 K4.07)
- 480v busses supplied from safety related busses (062 K4.07)
- Supply breakers (062 K4.07)
- Crosstie breakers (062 K4.07, 062 K4.08)
- Transformers (062 K4.07, 062 K4.08)
in accordance with P&ID E1 and WD1421.
What is the operational design of the switchyard?
Two buses (Front and Rear), 6 transmission lines connect SWYD to grid with 2 circuits on each of 3 towers.
Breaker‑and‑one‑half scheme, any breaker may be removed from service without affecting switchyard operation.
What are switchyard connections to plant?
SWYD connections to Plant:
- (Front Bus) Underground line to Safeguards Bus via MOD24F1
- Overhead line to Main Generator (via MOD26H5)
- (Rear Bus) Overhead line to Startup Power via MOD24R2.
What supplies switchyard AC power?
Bus 1C and 1E through two 2400‑240 volt transformers, EX-50 and EX-51. Each transformer supplies half the power requirements for the switchyard
What supplies switchyard DC power?
60‑cell battery (ED-51) located in relay house. Eight hour supply without recharging.
What is the operational design of the 4160 VAC system?
A Bus: 350 MVA, SP 1-1 at power, SU 1-1 off line: P-50A/C, P-2A, EX-17
B Bus: 350 MVA, SP 1-1 at power, SU 1-3 off line: P-50B/D, P-2B,
F Bus: 350 MVA, SP 1-3 at power, SU 1-1 off line: P-39A, LCC-71, 73 & 75
G Bus: 350 MVA, SP 1-3 at power, SU 1-3 off line: P-39B, LCC-72, 74 & 76
What is the operational design of the 2400 VAC system?
- Safeguards Xfmr 1‑1
- 4 ungrounded 2400V buses (1C, 1D, 1E and Safeguards). Ungrounded advantage is that an inadvertent ground on one phase will not disable any bus.
- Station Power Xfmr 1‑2
- Start‑Up Xfmr 1‑2
How do tap settings impact system operation?
- Variations in voltage due to load changes are corrected by an automatic load tap changer on Safeguards Xfmr low side.
- Operation is normally automatic. (Can be manual but only at local cabinet.). Each step on the LTC will change 2400V bus voltage by ~15 volts
What is the operational design of the safeguards bus?
•1st immediate access circuit: Safeguards Xfmr 1‑1 (Front bus via MOD)
•2nd immediate access circuit: A 345 kV transmission line between Switchyard and plant Start Up Xfmr 1-2 (Rear Bus via MOD)
•One onsite power source (Station Power Xfmr 1-2 via Main Generator)
•One delayed access circuit Main Xfmr can be used to backfeed 2400V power through Station Power Xfmr 1-2. (must open MOD 389)
Enclosed by its own switchgear building. Separated from the second independent offsite power source (startup transformers) by a fire barrier wall
Panel C04 is the center for 2400V System control. Electrical parameters such as bus voltage, supply voltage, incoming amperes, megawatts, kilowatt hours, and ground fault are indicated. Synchronizing and breaker operation and lockout switches, meters and status lights.
What is the operational design of C bus?
150 MVA, Class 1E (seperaton point is incoming breaker SG Bus)
- P-52 A & C
- P-54 B & C
- LCC 11 & 19
- Switchyard Xfmr No 2
- LCC 77 & 13
What is the operational design of D bus?
150 MVA, Class 1E (seperaton point is incoming breaker SG Bus)
P-7A & C
PZR Heaters Xfmr No 16 (480V Bus No 16)
Xfmr No 12 (480V Bus No 12)
Xfmr No 20 (480V Bus No 20)
What is the operational design of E Bus?
Non-Class 1E, 150 MVA
P-10 A & B
Xfmr No 200 (480V AC Bus No 200)
Xfmr No 14 (480V AC Bus No 14)
Switchyard Xfmr No 1 (AC Power)
Xfmr No 78 (480V AC Bus No 78)
Xfmr No 90 (480V AC Bus No 90)
Xfmr No 91 (480V AC Bus No 91)
Xfmr No 15 (480V AC Bus No 15)
Service Building Expansion Xfmr No 160
What are the Control Room alarms for the switchyard?
EK-0328, GEN ACB “CLOSE” CONT AT SWYD. Initiated when 25F7, 25H9 control
selector switch at Relay House in LOCAL position.
EK-0329, PILOT WIRE TROUBLE, Loss or Failure of DC Monitoring
EK-0330, SWYD PWR PLT INTERPOSING CONTROL CKT UNDERVOLTAGE, Loss of DC Voltage, Alarm indicates loss of primary transfer trip and loss of manual control of Generator ACBs 25F7 & 25H9. Backup transfer trip from switchyard will still function.
EK-0333, SWITCHYARD 125 VDC AND 240 VAC TROUBLE,
EK-0334, SWITCHYARD CRITICAL TROUBLE
EK-0335, SWITCHYARD NONCRITICAL TROUBLE
EK-0336, SWITCHYARD ACB TRIP
What are the Control Room controls and indications for the switchyard?
The only breakers in the Switchyard that can be operated from the control room are the Main Generator Breakers 25F7 and 25H9.
MOD26H5: Can be electrically opened from Control Room or Relay House or manually (handcrank) at the MOD
Switchyard Breaker Status Lights (green/red)
Describe the design features and interlocks that provide the following Electrical Distribution system functions- Paralleling of AC sources
Main Generator breakers:
- Sync scope is only required when closing a breaker.
- At Panel C‑01, ENERGIZE Sync scope for breaker to be operated. (This is performed by inserting sync switch handle into receptacle on Panel C-01.)
- WHEN correct synchronization is indicated, THEN OPERATE breaker control switch to desired position.
- DE‑ENERGIZE Sync scope by turning to OFF and removing handle.
Consequences of operating under various conditions Grounds
Four ungrounded 2400V buses (1C, 1D, 1E and Safeguards). Ungrounded advantage is that an inadvertent ground on one phase will not disable any bus.
However if a ground exist prompt action must be taken to isolate the ground since the reliability of the system is reduced. If a second ground were to develop a possible outcome could be a phase to phase short resulting in loss of the bus.
What are the consequences of Bus Control Power not available.
1) No control power to bus
2) Cannot operate breakers from Control Room
3) No fast transfer available
4) Prevent D/G from automatically loading the bus
5) Stored energy breakers can still be operated locally.
What breakers on "C" Bus have RLTS?
152 103 Service Water Pump P-7B
152‑106 Startup Transformer 1-2 Incoming Breaker
152‑107 D/G 1-1 Incoming Breaker
152-108 Station Power Transformer No 13
152‑110 Station Power Transformers 51 and 77
What are the consequences of NOT transferring between S/U and Station Power?
During startup operations, electrical power is typically transferred at 20% power to Station Power Transformers, this makes Coastdown available.
What Load Control Centers (LCCs) can be supplied from two different sources?
- LCC 11 and 12
- LCC 13 and 14
- LCC 77 and 78
- LCC-90 and 91
- 240 VAC SWYD buses No. 1 and No.2
What concerns are there with cross connecting load centers?
Concerns with cross connecting Load Centers
- Cross-connecting two safeguards busses (eliminating safeguards train redundancy).
- If LCC11 and 12 are crosstied LCOs apply.
- Transformer and cable Loading concerns
If LCC 11 & 12 are crosstied, what happens if C or D bus experience and undervoltage condition?
C-Bus undervoltage will open 52-1118. LCC-12 remains powered.
D-Bus undervoltage will open 52-1217. LCC-11 remains powered.
What prevents S/G 1-1 or S/U 1-2 from being overloaded during SIAS?
- E-Bus Load Sheds
- C-Bus load sheds the following:
What happens if Rear Bus receives a fault?
- Primary (487B-P/R) - R Bus Differential isolates R bus (trips R bus breakers) and initiates a transfer trip to trip all S/U xfmr low side breakers
- Zone of Protection - From each R bus breaker to high side of each S/U xfmr
- Operates aux relay (486S-X) which initiates a trip to trip all S/U xfmr low side breakers).
- Backup (487B-B/R) - same function as primary.
What happens if a Fault on Front Bus, S/G 1-1, or cabling to 152-401 occurs?
- Primary scheme (486B-P/F) - initiated by bus differential voltage and S/G transformer differential current and ground relays
- Backup scheme (486B-B/F) - initiated S/G transformer overcurrent and sudden pressure relays, Load Tap Changer Pressure relief Device and LTC hand crank removal.
- Relays initiate:
- a) Trip of F bus breakers
- b) Trip of Safeguards Bus Feeder breaker 152‑401
- c) Fast transfer of 2400V busses to S/U 1-2
Fault on Safeguards and Line between Safeguards Bus to 2400V Buses
1) Differential relays (187-1)
2) Trips 152‑401 or 152‑402 breaker
3) Initiates fast transfer of 2400V Busses when 152‑401 is closed.
4) If 152-402 ( from SP 1-2) is closed and fails to trip open when this differential relay operates, a breaker failure relay on this breaker will detect this and will initiate a unit trip.
Fault on Main Xfmr 1-1
Differential protection (487U) (C04 panel)
- Zone of Protection - Main Xfmr, Main Gen., and high voltage of S/P 1-1 and 1-2
- Operates the 386B lockout relay which trips Main Gen.
Sudden Pressure (463M)
- Detects a sudden rise in gas pressure caused by a fault internal to the xfmrs.
- Aux relay 463MX actuates 386P (plant trip)
- Aux relay 463XX causes an alarm and actuates deluge system.
- Actuates 386B (trips the Main Gen)
Fault on Line between Main Xfmr and SWYD
- Primary (487LU1, 450PU1) (Switchyard to Plant) actuates 386P and B (Main Gen trip)
- Secondary Protection (450BU1&421U1, 467U1) (Switchyard to Plant). Actuates 386P & B (Plant trip)
Ground Protection (451MN)
- Actuates 386B (trips the Main Gen
Fault on Station Power Transformers
- S/P Xfmr 1-1 - (287)
- S/P Xfmr 1-2 - (187)
- S/P Xfmr 1-3 - (487-SP3)
- Zone of Protection - From the high side of the transformer to the bus side of station power breakers.
- Actuates 386B (plant trip)
2) Sudden Pressure
- S/P 1-2 (163), S/P 1-1 (263), S/P 1-3 (463) will operate a 163X, 263X, 463FPX aux. relay, which initiates:
- Deluge system
- Actuates the 386P relay.
3) S/P Xfmr 1-2
With the 152‑402 closed a fault will trip the plant. 2400 V buses will transfer to S/U power.
Differential Fault on S/U Transformers
- Zone of Protection - from transformer to low side of S/U breakers
- Primary Protection - differential relays (487-S1, 487-S2 and 487-S3); S/U 1-1, S/U 1-2, S/U 1-3, respectively
- Opens all R bus breakers, and S/U low side breakers (isolates R bus)
- Backup Protection (Phase Protection 450-451/S1): Actuates the same respective lockout relay (486-11, 486-12 or 486-13) as the (487-S1, 487-S2, 487-S3) primary differential relay
Sudden Pressure Fault on S/U Transformers
(SP463-S1, SP463-S2, SP463‑13)
1) Trips R Bus, trips all S/U Xfmr low side breakers, and actuate deluge valve.
2) Is actually a backup to other protective relaying. Sudden pressure condition would occur only as a result of some huge fault within the transformer that would cause oil vaporization (winding to ground, or winding to winding if the distance were far enough to draw a large arc) - but on a fault of this nature, other relaying should isolate the fault before sudden pressure actuates.
Ground Fault on S/U Transformers
1) Each xfmr is equipped with one overcurrent relay connected to the 345 KV side of each xfmr (451‑SN1, 451-SN2, 451-SN3).
2) Trips R Bus and trips all S/U Xfmr low side breakers.
Important Point: Any fault on any S/U Xfmr will actuate protective relaying which de-energizes ALL three S/U Xfmrs
Overcurrent Fault on 4160V Bus
- Each 4160V incoming breaker has phase time overcurrent relays and one ground time overcurrent relay to protect against switchgear bus faults and to provide backup protection for feeder circuit faults.
- When supplied from S/U transformer Buses 1A, 1B, 1F, & 1G overcurrent relay trip their associated incoming breaker.
- When supplied from S/P transformer Buses 1A, 1B, 1F & 1G overcurrent relay trip their associated incoming breaker, block fast transfer and block manual closure.
Undervoltage Fault on 4160V Bus
Each bus has inverse time undervoltage relay that trips PCPs, Cooling Tower Pumps and condensate pumps to prevent damage to the motors.
Relays on breakers 252-101 and 252-201, 252-301, 252-401 actuate a lockout relay to trip and lockout the incoming breaker, and prevent a fast transfer to the start-up transformer. The purpose for this is to prevent connecting a source of supply to a faulted bus.
Overcurrent on 2400V bus?
- Phase overcurrent relays protect against switchgear bus faults and to provide backup protection for feeder circuit faults.
- In the event of a 2400V bus fault, relays on Breakers 152‑105, 152-203 and 152-302 (incoming breakers from the Safeguard Bus) actuate a lockout relay to trip and lockout the incoming breaker, D/G output breaker will not close. The basis for this is to prevent connecting a source of supply to a faulted bus.
- 2400V AC breakers supplying motors have overcurrent relays (protect against overload, locked rotor and faults). Actuation of the relay isolates the faulted motor from the 2400V bus.
Discuss ground detection on 2400V bus?
Ground relays are not used for system protection in the 2400V AC distribution system since this is an ungrounded system. However, ground relays are used to alarm and indicate the existence of a ground fault on the 2400V AC system.
Ground Detection Lights are mounted adjacent to the relay.
- Normally dimly lit.
- In the case of a ground on the bus, the intensity of the lights would vary.
- Alarms only
If busses are fed from the same source the ground could be on any of the connected busses. Relays on all connected busses would likely target. Isolate to identify.
Discuss undervoltage protection on 2400V Busses C & D.
First Level (127-1, 2)
- Set at 77% of rated voltage with an inverse time relay. The greater the voltage drop, the quicker the relay response. Protects against a sudden loss of voltage as sensed on the bus.
- Set at 93% of rated voltage. Protects against sustained undervoltage conditions on the bus.
- When under voltage condition exists for 0.65 second, they initiate a Diesel Generator start. If under voltage continues for an additional six seconds a load shed occurs and the Diesel breaker closes. The NSD or DBA sequencer then sequences on loads.
- Trip respective incoming bus breakers
- Initiates load shedding (D/G breaker must be open)
- Starts respective diesel