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Systems > 4KV

4KV

⚠️ DRAFT

4KV

Function

The 4160V AC distribution system is the backbone of plant AC power distribution. It supplies both non-safeguards Group Buses and safety-related Vital Buses, ensuring power to safeguards equipment during accident response. The Class 1E distribution system meets the intent of IEEE 308-1971. (UFSAR 8.3.1.2)

Group Buses (Non-Safety)

  • 4 sections
  • Feed plant auxiliaries (non-safeguards loads)
  • Powered from station power transformers (normal) or auxiliary power transformer (during operation)
  • On unit trip, 4160V group buses automatically transfer from auxiliary transformer to station power transformer (UFSAR 8.3.1.2)
  • 3 sections per unit (A, B, C)
  • Feed safeguards equipment (ECCS pumps, containment spray, etc.)
  • Each vital bus fed from station power transformers (T13/T14 for Unit 1, T23/T24 for Unit 2)
  • Two station power transformers per unit — each can supply vital buses
  • In-feed breakers electrically interlocked to prevent paralleling through a vital bus
  • Each vital bus backed by a dedicated diesel generator (UFSAR 8.3.1.2)

Safeguards Sequence

On SI signal with loss of offsite power:

  1. Diesel generators auto-start
  2. Vital bus loads shed
  3. Diesel generators connect to vital buses
  4. Safeguards loads sequenced on in prescribed order and time delays
  5. Ensures diesel generators not overloaded during accident response (UFSAR 7.3, 8.3)
Exam — 2023 Q93
CW bus loss scenario: when the 4KV infeed breaker to the 24 CW bus fails open, the bus auto-transfers to the 23 CW bus. However, all 'B' circulator breakers trip open on undervoltage during the transfer and must be manually restarted. With one CW pump already C/T, this results in 4 or more circulators out of service. Per AB.CW-0001 Attachment 1, with 4+ circulators OOS at P-10 or above: trip the reactor and enter EOP-TRIP-1.
Exam — 2022 Q47
EDG paralleled to grid during surveillance: when LOOP occurs, SEC actuates in Mode 2 (Blackout). EDG output breaker opens, diesel remains running, SEC opens both infeed breakers, strips all loads from the 4KV vital bus, then automatically recloses the output breaker and re-starts loads using the SEC load sequencer. The output breaker does NOT remain closed during a LOOP even though the EDG was already paralleled. The SEC strips and re-sequences the bus regardless of EDG test status.

SPT Transfer Relay Behavior

Exam — 2019 Q21
When 23ASD (23 SPT Supply Breaker to 2A 4KV Vital Bus) spuriously opens while 23 SPT is at normal voltage, the 23SPT transfer relay remains energized. This prevents an automatic transfer of the 2A 4KV Vital Bus to the 24 SPT. The 2A 4KV Bus de-energizes and the 2A EDG starts and powers the bus via the 2A SEC operating in Mode II*. Trap: auto transfer to the alternate SPT only occurs if the SPT transfer relay de-energizes (indicating SPT voltage loss). With the SPT still energized but the breaker open, the relay stays energized and blocks the transfer.

Voltage Protection

Exam — 2020 Q47
Degraded voltage relays actuate at < 95% of nominal 4160V (sustained for > 13 seconds). Fast bus transfer relays actuate at < 70% of nominal. When degraded voltage is detected on a single bus (e.g., 3600V from failing SPT), and SI and SECs have been reset, the SEC actuates in Mode II* (Single Bus Degraded UV) — strips the bus from offsite power and loads it onto its EDG. Mode III and Mode IV require an active SI signal — after SI/SEC reset, SEC can only actuate in Mode I (Blackout) or Mode II* (Single Bus Degraded UV). 3600V is above the 70% fast transfer threshold (2912V) so no fast transfer occurs.

1A 4KV Bus Loads

Exam — 2020 Q53
Unit 1 — 1A 4KV Vital Bus powers 15 and 16 SW Pumps (associated with #3 SW Bay). A 1A Bus Differential with #3 SW Bay already C/Ted has no additional impact on SW operation because 15 and 16 SW Pumps are already isolated. 12 SW Pump (C Bus) auto-starts on low SW header pressure when 13 SW Pump trips, maintaining SW flow. Correct procedure entry: AB.4KV-0001, Loss of 1A 4KV Vital Bus. Unit difference: on Unit 2, the 2A bus powers 21 and 22 SW pumps (not associated with a single bay) — a 2A bus loss impacts operating SW pumps directly.

2B 4KV Bus Loads

Exam — 2018 Q22
Confirms 2B bus powers 22 RHR Pump and 2C bus powers 22 SI Pump. With EDG 2B unable to start following RT/SI + LOOP: 2A and 2C buses powered from their EDGs, 2B bus de-energized. Result at 15 minutes: 22 RHR Pump stopped (2B bus), 22 SI Pump running (2C bus).
Exam — 2019 Q12
Key loads on the 2B 4KV Vital Bus include: 22 RHR Pump. During a Reactor Trip + SI coincident with LOOP with EDG 2C unavailable, 2A and 2B buses are powered from EDG 2A and EDG 2B. 22 RHR Pump (2B bus) is running; 22 SI Pump (2C bus) is stopped because EDG 2C cannot be started. Common confusion: 22 RHR is on the B bus (not C), and 22 SI is on the C bus (not B). Pump number does NOT always match bus letter.

2C 4KV Bus Loads

Exam — 2020 Q29
Key loads on the 2C 4KV Vital Bus include: 22 Charging Pump, 22 SI Pump, 22 Containment Spray Pump. On loss of 2C bus (bus differential), these loads are lost. Charging pump availability: 21 CV Pump (2B bus) and 23 CV Pump (2A 460V bus) remain available. 23 CV Pump is powered from the 2A 460V bus, NOT from a 4KV bus — it remains available on loss of any 4KV bus.

Tech Spec LCOs

JPM — 2022 Sim-g
Loss of 2A 4KV Vital Bus response per AB.4KV-0001: A EDG auto-starts. 23 Charging Pump was providing seal injection/charging to Unit 2 (lost on bus loss). Close 2CV55, then start 21 Charging Pump. Alternate path: 21 Charging Pump trips on start → start 22 Charging Pump to restore RCP seal injection and charging flow. Note: 2CV55 has ~2 minute stroke time.
Exam — 2020 Q9
MSLB + LOOP with 2C 4KV Vital Bus lost (OHA J-3, 2C 4KV VTL BUS DIFF PROT): confirms 2C bus loads lost include 22 CS Pump, 23 CFCU, and 25 CFCU. Only 21 CS Pump (A bus) and 22 & 24 CFCUs (B bus) remain for containment cooling — design bases NOT met (requires 3 CFCUs + 1 CS pump).
JPM — 2018 Sim-g
4KV Group Bus transfer from SPT to APT (S2.OP-SO.4KV-0008, R13): at 18% power / 160 MWe, Rod Control in Manual (D-147), Steam Dumps in MS Pressure Mode-Auto at 970 psig. P&L 3.3: OHA J-39 (GROUP BUS XFER FAIL) illuminates then clears on normal transfer. Alternate path: 2G Group Bus fails to transfer, de-energizing bus and tripping 24 RCP. OHAs: J-8 (2G 4KV GRP BUS DIFF/OVRLD), J-38 (4KV GRP BUS UNDRVOLT), J-39, D-31 (24 RCP BKR OPEN/FLO LO). ARP J-39 → J-38 Step 3.5 or D-31: IF ANY RCP trips → TRIP Reactor → GO TO EOP-TRIP-1.
JPM — 2020 Sim-h
4KV Group Bus transfer from SPT to APT (S2.OP-SO.4KV-0008): verify APT voltage 4.22 - 4.36KV. P&L 3.3: OHA J-39 (GROUP BUS XFER FAIL) illuminates then clears during normal transfer. Alternate path: 2G Group Bus fails to transfer, de-energizing bus and tripping 24 RCP. OHA alarms: J-8 (2G 4KV GRP BUS DIFF/OVRLD), J-38 (4KV GRP BUS UNDRVOLT), J-39, D-31 (24 RCP BKR OPEN/FLO LO). ARP for J-38 or D-31: IF ANY RCP trips → TRIP Reactor → GO TO EOP-TRIP-1.
Exam — 2018 Q19
SPT failure to 0 VAC transfer behavior: when 23 SPT fails to 0 VAC, the SPT transfer relay de-energizes, enabling automatic fast bus transfer. The 23 SPT Supply Breaker (23ASD) opens at < 70% of normal voltage (fast bus transfer relay setpoint). After 23ASD opens, the 35% permissive allows 24ASD to close, transferring the 2A 4KV Vital Bus to 24 SPT. Trap: EDG 2A does NOT power the bus in this case — the alternate SPT (24 SPT) is available and the SPT transfer relay de-energized (enabling auto transfer). EDG start only occurs when BOTH SPT infeed breakers are open. 35% is the permissive setpoint for the alternate SPT breaker to close, NOT the trip setpoint for the normal SPT breaker.

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