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Electrical Distribution (Class 1E AC + Vital DC)

The Class-1E electrical system supplies power to safety-related loads during accidents. Two redundant trains (A and B), each fed from offsite power, the unit's main turbine, or an emergency diesel generator. Loss of all AC power coincident with offsite-power loss is station blackout (SBO); response in ECA-0.0. Vital DC batteries supply control and instrumentation power for the duration of the SBO coping window.

Function

Energizes Class-1E pumps, valves, and instrumentation under all design- basis events. Loss of any single AC train must not preclude mitigation; single-failure assumption is the design basis (Vogtle UFSAR §8.3.1).

Components

flowchart LR
    OFFSITE["Offsite Grid<br/>(230/500 kV)"]
    MAIN["Main Generator<br/>(via UAT)"]
    SAT["Startup Auxiliary<br/>Transformer"]
    UAT["Unit Auxiliary<br/>Transformer"]
    DG_A(("«DG-A»<br/>EDG"))
    DG_B(("«DG-B»<br/>EDG"))
    BUS_A["4kV Emerg Bus A<br/>«BUS-A-EMERG»"]
    BUS_B["4kV Emerg Bus B<br/>«BUS-B-EMERG»"]
    LOADS_A["Train A loads<br/>(HHSI-A, CHG-A,<br/>RHR-A, MD-AFW-A, ...)"]
    LOADS_B["Train B loads<br/>(HHSI-B, CHG-B,<br/>RHR-B, MD-AFW-B, ...)"]
    DC_A["Vital DC Bus A<br/>«DC-BUS-LVL»"]
    DC_B["Vital DC Bus B"]
    BAT_A[("Battery A<br/>(~125 V)")]
    BAT_B[("Battery B")]
    INV["Vital AC Inverters<br/>(instrumentation)"]

    OFFSITE --> SAT
    MAIN --> UAT
    SAT --> BUS_A & BUS_B
    UAT --> BUS_A & BUS_B
    DG_A --> BUS_A
    DG_B --> BUS_B
    BUS_A --> LOADS_A & DC_A
    BUS_B --> LOADS_B & DC_B
    BAT_A --> DC_A
    BAT_B --> DC_B
    DC_A & DC_B --> INV

    classDef src stroke:#856404,stroke-width:2px
    classDef bus stroke:#1f618d,stroke-width:2px
    classDef gen stroke:#943126,stroke-width:2px
    class OFFSITE,MAIN,SAT,UAT src
    class BUS_A,BUS_B,DC_A,DC_B bus
    class DG_A,DG_B gen
  • Two emergency diesel generators — «DG-A», «DG-B»; each rated ~6500 kW (Vogtle); start automatically on undervoltage or SI; sequenced loading of safety loads over ~60 seconds.
  • Two 4kV emergency buses — «BUS-A-EMERG», «BUS-B-EMERG»; one per train; supplied by either offsite source or own DG.
  • Two vital 125 V DC batteries — sized for 4-hour SBO coping per 10 CFR 50.63 (Vogtle), extendable via load-shed and FLEX (NEI 12-06).
  • Vital AC inverters — DC-powered AC for instrumentation channels; surviving under SBO.
  • Offsite power sources — typically two independent 230/500 kV lines from different switchyard buses.

Instrumentation

  • Emergency 4kV bus statuses: «BUS-A-EMERG», «BUS-B-EMERG»
  • DG statuses: «DG-A», «DG-B»
  • Vital DC bus voltage: «DC-BUS-LVL»

Setpoints

Parameter Value Source
DG start (undervoltage) bus voltage < 75% nominal Vogtle Tech Spec 3.3.5
DG start (SI signal) automatic with SI Vogtle UFSAR §8.3.1.1
DG load-sequence stagger ~5-15 s per safety load Vogtle UFSAR §8.3.1.1
Battery LCO minimum 70% capacity per 6-hour discharge Vogtle Tech Spec 3.8.4
SBO coping duration (Tech Spec) 4 hours 10 CFR 50.63
FLEX extension target 72 hours NEI 12-06 / Order EA-12-049

Normal alignment

  • Both 4kV emergency buses energized from offsite via SAT
  • DGs standby (in auto-start mode)
  • Vital DC buses energized from battery chargers (AC-supplied); batteries floating
  • Inverters supplying vital AC channels

Failure modes

  • Loss of one bus / one train — single-failure tolerated; other train carries safety loads. Cross-tie under SS authorization.
  • Loss of all AC (SBO) — both DGs failed AND offsite power lost. TDAFW + DC instrumentation only. Response ECA-0.0.
  • DG failure to start on demand — common cause: fuel system, control logic. Single-failure assumption protects against one DG failure.
  • Battery depletion during prolonged SBO — DC load-shed extends battery life; FLEX portable diesel restores AC.
  • RCP seal injection lost on SBO — no charging without AC; seal cooling time ~5-10 min. Seal leakage becomes a small-break LOCA source under prolonged SBO.

References

  • Vogtle UFSAR §8.3 (Onsite power systems)
  • Vogtle UFSAR §8.3.1.1 (Emergency diesel generators)
  • Vogtle UFSAR §8.3.2 (Vital DC systems)
  • Vogtle Tech Spec 3.3.5 (LOP/Undervoltage instrumentation), 3.8 (Electrical Power)
  • 10 CFR 50.63 (Station Blackout Rule)
  • NEI 12-06 (FLEX coping strategies)
  • NRC Order EA-12-049 (post-Fukushima beyond-design-basis mitigating strategies)