Protection and Switchgear: Relays, CT/VT, Coordination and Breaker Ratings
This domain tests protective relaying philosophy, current and voltage transformers, coordination, and switchgear rating and arc interruption - the core of PGCIL substation engineering work.
Relaying fundamentals
- A CT must be matched to relay burden and accuracy limit factor; saturation distorts secondary current and can cause maloperation.
- IDMT relays use standard curves - for the IEC normal-inverse characteristic, t = TMS × 0.14/(PSM^0.02 − 1), where PSM = secondary fault current / plug setting.
- Differential protection compares currents at zone boundaries; the bias (stabilizing) characteristic restrains tripping for through faults and CT mismatch.
- Distance relays: Zone 1 covers about 80% of the line with no intentional delay, Zone 2 about 120–150% with a time step, Zone 3 provides backup. Mho elements are directional.
Switchgear
Breaking capacity is the RMS symmetrical current a breaker can interrupt at rated voltage; making capacity is about 2.55 times the symmetrical peak at 50 Hz to cover the DC offset. Restriking voltage and RRRV govern arc behaviour; SF6 and vacuum designs dominate at EHV and distribution levels respectively.
Worked example
A 400/1 A CT feeds a relay with 50% plug setting and TMS 0.1. For a 4000 A primary fault, secondary current = 4000/400 = 10 A, so PSM = 10/0.5 = 20. Then t = 0.14 × 0.1/(20^0.02 − 1) = 0.014/0.0617 ≈ 0.23 s.
Traps
Forgetting the CT ratio, confusing making with breaking capacity, and setting Zone 1 beyond the protected line (it must not overreach).