Fault clearing process with trip circuit

Fault clearing is the sequence of events from the instant the fault occurs to the final interruption of fault current by the circuit breaker. It uses the trip circuit, which links the relay to the breaker.

Trip circuit

  Line ═══[ CB main contacts ]═══════
        │                       ▲
       CT                       │ opens
        │                       │
  ┌─────▼─────┐      ┌──────────┴──┐
  │ Relay coil│      │  Trip coil  │
  └─────┬─────┘      └──┬──────────┘
        │ closes        │
  (+)───┤ Relay contact ├──[52a]──(−)
       DC battery         aux. switch

• CT/VT: reduce line current and voltage to relay level.
• Relay operating coil: energised by the CT secondary.
• Relay contact: in the DC trip circuit.
• Trip coil: solenoid that releases the breaker mechanism.
• Auxiliary switch (52a): closed when breaker is closed; opens with the breaker and breaks trip-coil current.
• Battery (station DC supply): independent of the AC system, which may collapse during the fault.

Fault clearing process

1. Fault occurs: current in the line rises sharply.
2. Sensing: the CT secondary current rises proportionally and flows through the relay coil.
3. Relay operation: when current exceeds the pick-up setting, the relay (after its set time) closes its contacts. This time is the relay time.
4. Trip coil energised: DC current flows from the battery through relay contacts, trip coil and the auxiliary switch.
5. Breaker opens: the trip coil plunger releases the latch; stored energy (spring/pneumatic) separates the main contacts. This is the opening time.
6. Arc extinction: an arc forms between the contacts and is extinguished at a current zero by the arc-quenching medium (SF₆, vacuum, oil, air). This is the arcing time.
7. Trip circuit reset: the auxiliary switch opens, de-energising the trip coil to prevent it burning.

Total fault clearing time = relay time + breaker opening time + arcing time, e.g. 1 cycle + 1.5 cycles + 0.5–1 cycle ≈ 3–4 cycles (60–80 ms at 50 Hz) for a modern EHV breaker.