How it works
Overcurrent relays (OCR) trip when current exceeds a set value; the IDMT (Inverse Definite Minimum Time) characteristic gives operating time t = TMS × K/(I/Is − 1)^n, where TMS is the time multiplier setting, Is the pick-up current, and for standard inverse K = 0.14, n = 0.02. A relay at a 33 kV feeder might have Is = 200 A (twice full-load), TMS = 0.1, giving a very fast trip at 10× fault current. Distance (impedance) relays measure Z = V/I at the relay point; Zone 1 covers 80% of protected line with instantaneous trip, Zone 2 covers 120% with 0.4 s delay, Zone 3 covers 220% with 1.2 s delay.
Key points to remember
Differential relay compares currents entering and leaving the protected zone — operates when |Idiff| > Is (typically 10–20% of full-load current). Biased differential relay handles CT mismatch and through-fault current by requiring Idiff > k × Ibias. Current transformer (CT) ratio must be chosen so that secondary current does not exceed 5 A at maximum fault current; a 400/5 A CT feeds relay with 5 A secondary. Coordination time interval (CTI) between primary and backup relay is typically 0.3–0.5 s to allow primary relay to clear before backup operates. Buchholz relay in transformers detects internal faults through gas accumulation.
Exam tip
Every Anna University protection paper asks you to calculate the operating time of an IDMT relay given TMS, plug setting, and fault current — substitute directly into the standard inverse formula and show the units clearly.