Power System Protection and Switchgear
What this domain tests
Questions cover CTs and VTs, relay characteristics, coordination, breaker ratings, arc interruption and protective schemes for generators, transformers, feeders and motors.
Core rules
- Overcurrent relays coordinate by time and current grading. IDMT curves become faster as fault current rises; the time multiplier setting shifts the curve vertically.
- CT ratio defines primary-to-secondary current. Relay pickup in secondary amperes is multiplied by the CT ratio to obtain primary pickup.
- Distance protection uses impedance measured from voltage and current; zones are set to cover most of the line with time delays for selectivity.
- Differential protection compares currents at boundaries and is fast and selective for transformers, generators and busbars.
- Breakers are rated for rated voltage, normal current, breaking capacity and making capacity. Making capacity exceeds breaking capacity because of the asymmetric first peak.
- SF6 and vacuum interrupters extinguish arcs by different mechanisms; SF6 has strong electronegative arc-quenching behaviour.
Worked example
A 400/5 CT feeds an overcurrent relay set to 5 A secondary. Find the primary pickup and the multiple of pickup for a 4000 A fault. Primary pickup = 5 x (400/5) = 400 A. Fault multiple = 4000/400 = 10 times pickup. The relay time is read from the curve at 10 times pickup for the chosen time multiplier setting.
Traps
- Ignoring CT saturation, which distorts secondary current during high faults.
- Forgetting star-delta compensation when CTs are connected differently from the transformer winding.
- Confusing breaking capacity with making capacity.
- Setting distance zones without considering arc resistance and infeed.