A substation steps voltage up or down and provides switching, protection, and metering for the connected network. Railway traction sub-stations (TSS) additionally convert grid power to the traction supply — see the TSS/Feeding Posts chapter for that specific application.
Key Substation Equipment
- Power transformers — step voltage up/down; equipped with tap changers (on-load or off-load) to regulate secondary voltage.
- Circuit breakers (CB) — interrupt fault current; common types include air-blast, SF6, and vacuum circuit breakers.
- Isolators — provide visible physical disconnection for maintenance safety; not rated to break load current.
- Current transformers (CT) and potential transformers (PT) — step down current/voltage to safe levels for metering and protection relays.
- Lightning/surge arresters — divert lightning and switching surges to earth, protecting connected equipment.
Isolator vs. Circuit Breaker — Common Confusion
- A circuit breaker can safely interrupt load or fault current; an isolator cannot — it is only operated after the breaker has already opened.
- Correct switching sequence: open breaker first, then open isolator; reverse order when closing.
Exam Focus
Expect scenario-based questions on correct switching sequence (isolator vs breaker) and on which protective device responds to which fault type.