Protective Relays

Relays that detect abnormal conditions and command breakers to trip — overcurrent, differential and distance protection.

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Modern numerical relays are microprocessor-based, combining many protection functions with communication (IEC 61850). Common functions (with ANSI codes):

50/51 Overcurrent — instantaneous and time-delayed (IDMT curves). Standard inverse IDMT: $t = TMS \times \dfrac{0.14}{(I/I_s)^{0.02} - 1}$ .
87 Differential — compares current entering and leaving a zone (transformers, generators, busbars); very selective.
21 Distance — measures impedance to the fault; zone 1 covers ~80 % of the line instantly, zone 2 and 3 with delays.
27/59 under/over-voltage, 81 frequency, 32 reverse power.

Selectivity is achieved by grading time and current settings so that the breaker nearest the fault trips first.

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