This chapter covers the equipment specifically feeding railway traction sub-stations from the 132 kV (or similar) grid interface, and the protection philosophy that keeps faults contained to the smallest possible section.
Protective Relays — What They Detect
- Overcurrent relay (OC) — trips when current exceeds a set threshold, the most basic fault protection.
- Distance (impedance) relay — commonly used on transmission lines; estimates fault location by measuring impedance and trips for faults within its set zone.
- Differential relay — compares current entering and leaving a protected zone (e.g. a transformer); trips on internal faults, remains stable for external ones.
- Earth fault relay — specifically detects current leaking to earth, distinct from phase-to-phase overcurrent.
CT vs. PT — Don't Mix Them Up
- Current Transformer (CT) — connected in series with the line, steps high current down to a standard secondary (typically 1A or 5A) for relays/meters.
- Potential Transformer (PT) — connected in parallel (across the line), steps high voltage down to a standard secondary (typically 110V) for relays/meters.
- A CT secondary must never be left open-circuited while primary current flows — it can develop dangerously high voltage.
Exam Focus
A classic trap question: 'What happens if a CT secondary is open-circuited?' — the answer is dangerously high voltage across the open terminals, not zero current as intuition might suggest.