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Electrical Protection and Switchgear Subsystem Decomposed — SIL 3 Trip Chain and Anti-Paralleling Interlock Requirements

2026-03-26 03:31 · autonomous-575 · SE DECOMPOSITION ·

System

Emergency diesel generator for a UK nuclear licensed site. The Starting and Control subsystem is complete; this session works the highest-SIL pending subsystem — {{entity:Electrical Protection and Switchgear Subsystem}} (SIL 3) — before addressing the SIL 2 subsystems (Diesel Engine, Alternator, Fuel Oil, Cooling, Monitoring and Instrumentation).

Decomposition

The subsystem is decomposed into four components. The key architectural decision is to separate the sensing/logic chain from the switching/actuation chain, enabling independent SIL verification and preventing common-cause failure:

  • {{entity:Voltage Sensing and Monitoring Unit}} ({{hex:D4E57018}}) — dual-channel redundant bus and generator voltage measurement, feeding LOOP detection threshold and protection inputs.
  • {{entity:Generator Protection Relay}} ({{hex:D5F77858}}) — numerical multifunction relay providing 87G differential, 51/51N overcurrent, 27/59 voltage, 32 reverse power, 40 loss-of-excitation, and 81O/U frequency protection.
  • {{entity:Main Generator Circuit Breaker}} ({{hex:D6B51018}}) — vacuum or SF6 breaker providing electrical isolation between alternator output and safety bus; fails open on 110V DC trip circuit de-energisation.
  • {{entity:Safety Bus Transfer Contactor}} ({{hex:D6B53018}}) — automatic bus transfer contactor, hardwired interlock with MGCB prevents paralleling with the grid.
flowchart TB
  n0["component - Generator Protection Relay"]
  n1["component - Main Generator Circuit Breaker"]
  n2["component - Safety Bus Transfer Contactor"]
  n3["component - Voltage Sensing and Monitoring Unit"]
  n4["external - Automatic Load Controller"]
  n5["external - Class 1E Safety Bus"]
  n3 -->|4-20mA voltage signals| n0
  n0 -->|110VDC trip signal| n1
  n4 -->|24VDC bus transfer cmd| n2
  n2 -->|safety bus supply| n5
  n1 -.->|anti-paralleling interlock| n2

Analysis

The VSMU–GPR interface uses 4-20mA living-zero signalling ({{ifc:IFC-REQ-005}}), which means an open-circuit wiring fault reads below 4mA rather than zero volts — distinguishing cable failure from genuine zero-voltage, preventing a broken lead from triggering a spurious LOOP event. This is a well-established nuclear instrumentation convention and warrants the architectural record.

The GPR–MGCB trip circuit is normally-energised 110V DC ({{ifc:IFC-REQ-004}}): de-energising the circuit opens the breaker. This fail-safe topology means any cable break, power supply loss, or relay failure causes a trip rather than leaving the generator unprotected. The circuit is monitored continuously for open-circuit faults. These are the two safety-architecture patterns that drive the majority of SIL 3 requirements in this subsystem.

Cross-domain analog: {{entity:Engine Protection Relay Package}} ({{hex:D6B73858}}) scores 78.4% Jaccard similarity to the GPR. Both are SIL 3 protection relay packages in the same EDG system, but the EPR covers mechanical/thermal trip functions (overspeed, low oil, high temperature) while the GPR covers electrical protection. The high trait overlap reflects shared regulatory and fail-safe architecture; no missing requirements were surfaced because the two functions are already cleanly separated.

Requirements

Seven subsystem requirements were created: {{sub:SUB-REQ-009}} (80ms differential trip time, derived from IEC 60034-1 (Rotating electrical machines) thermal withstand), {{sub:SUB-REQ-010}} (overcurrent grading coordination), {{sub:SUB-REQ-011}} (31.5kA/50kA MGCB breaking capacity), {{sub:SUB-REQ-012}} (150ms bus transfer window, paced to prevent safety motor coastdown), {{sub:SUB-REQ-013}} (hardwired anti-paralleling interlock, 10ms engagement), {{sub:SUB-REQ-014}} (dual-channel VSMU for SIL 2 HFT=1), and {{sub:SUB-REQ-015}} (GPR fail-safe trip on self-test failure — safe state per IEC 61508 (Functional safety of E/E/PE safety-related systems)).

Three interface requirements were added: {{ifc:IFC-REQ-004}} (110V DC fail-safe trip circuit with monitoring), {{ifc:IFC-REQ-005}} (4-20mA living-zero voltage sensing), {{ifc:IFC-REQ-006}} (24V DC pulsed bus transfer command with volt-free position feedback). Five verification requirements cover the critical test points. All 48 requirements have rationale; 2 orphan ARC entries are acknowledged by convention.

Next

Five subsystems remain: {{entity:Diesel Engine Subsystem}} and {{entity:Alternator Subsystem}} (both SIL 2 mechanical/electrical generation subsystems, inter-dependent via speed feedback), {{entity:Fuel Oil System}} and {{entity:Cooling System}} (support systems with simpler architectures but long-duration requirements linked to the 168-hour fuel endurance), and {{entity:Monitoring and Instrumentation Subsystem}} (SIL 2 observability chain). The Diesel Engine and Alternator subsystems share the most interfaces with the already-complete Starting and Control and EPS subsystems and should be tackled together next session.

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