1. Instrument Transformer Failures (PT/CT)

• Scenario: RMUs with active distribution network      interfaces typically integrate protection-class PTs (Potential      Transformers) and measurement-class CTs (Current Transformers) for      real-time data acquisition.

• Root      Causes:

• Insulation aging in secondary windings (insulation       resistance <0.5MΩ)

• Magnetic saturation causing measurement errors (CT       ratio error >±1%, phase error >±60')

2. Lightning Protection System Failure

• Failure      Chain: Improper arrester      mounting (e.g., unstable foundation) leads to:

• Cable compartment breakdown (air gap <150mm with       power-frequency withstand voltage <28kV)

• Surface discharge on cable terminations (creepage       distance <20mm/kV)

3. Mechanism Jamming

• Degradation      Mechanism:      Prolonged exposure to >85% RH environments causes:

• Spring stress relaxation (elastic modulus reduction       >15%)

• Abnormal linkage friction (oxidized grease       increases coefficient to μ>0.2)

• Microswitch contact oxidation (contact resistance       >50mΩ)

4. Cable Termination Faults

• Primary      Causes:

• Poor cold-shrink/hot-shrink joint craftsmanship       (interface pressure <0.4MPa)

• Incomplete stress relief in large conductors (e.g.,       300mm² radial shrinkage >5%)

5. Secondary Circuit Abnormalities

• Failure      Modes:

• Loose terminal connections (temperature rise       >65K)

• Multiple grounding of shield layers (circulating       current >10A)

• DC system grounding faults (insulation resistance       <25kΩ)

6. Insulation Degradation

• Key      Manifestations:

• Ceramic insulator tracking (ESDD >0.1mg/cm²)

• Partial discharge in bus supports (PD magnitude       >20pC)

• Surface flashover under condensation (RH >95%       for 4h)

7. SF6 Gas Leakage

• Consequences:

• Arc-quenching capability decline at >0.5%/year       leakage rate (critical pressure <0.35MPa)

• Decomposition byproducts (SO2F2 >5μL/L) from       moisture-induced discharges (>500ppmV)

8. Switching Apparatus Failures

• SFC      Switchgear Risks: In transformer protection applications:

• Transfer current mismatch (e.g., 25kA system with       16kA fuse)

• Mechanical interlock failure (opening speed       <1.2m/s causing restrike)