Power System Protection and Control
This domain tests how faults are detected, isolated, and coordinated, plus the control loops that keep voltage and frequency stable. Expect CT/VT ratio arithmetic, relay characteristics, and protection zone questions.
Core rules
- Protection zones overlap at circuit breakers so no point is unprotected; the overlap also provides redundancy.
- Overcurrent grading uses current settings and time multipliers; downstream devices must clear first, so each upstream relay is slower by a coordination interval, commonly 0.2–0.4 s.
- Differential protection compares currents entering and leaving a zone; it is fast and selective but must handle CT ratio mismatch and transformer vector group compensation.
- Distance protection measures Z = V/I and is graded in zones, for example 80% of the line in zone 1 and 120% in zone 2 with a time delay.
- CT secondary current = primary current / ratio; relay settings are usually expressed in multiples of the CT secondary rating or the relay nominal current.
Worked example
A feeder CT is 400/5 A. The relay pickup is set at 0.5 × In, where In = 5 A. Secondary pickup = 2.5 A. Primary pickup = 2.5 × (400/5) = 200 A. If the maximum load is 180 A, this setting is acceptable, but the earth-fault element and the downstream relay must still be checked so the settings grade correctly.
Traps
Forgetting the CT ratio when converting a relay setting to primary amperes, or applying star-delta compensation in the wrong direction for a transformer differential scheme, costs marks. Do not assume a relay pickup equals the fault current; it is only a threshold, and actual operating time comes from the curve. Check whether the question uses relay nominal current or CT secondary current.