B. Protective relaying
Differential (87) + distance (21) + zone protection for transformers, motors, generators, lines, buses.
Lessons
Differential (87) and distance (21) protection
KCL-based unit protection plus impedance-plane mho zones for transmission lines — the two fastest, most selective protections on the grid.
Motor protection — ANSI 49 / 50 / 51 / 46 / 27
Layered relay protection for industrial motors: thermal overload (49), instantaneous + time overcurrent (50/51), ground (50G/51G), negative-sequence (46), undervoltage (27), phase reversal (47), start-counter (66), motor differential (87M).
Generator protection — 30+ ANSI functions
Zoned protection for large synchronous generators: stator (87G, 64G, 27TN, 50/51V), rotor / field (40, 64R), system (32, 81, 24, 78), mechanical (38, 49, 46, 87T). All in one microprocessor relay (SEL-300G / GE G60 / ABB REG670).
Transformer protection — 87T / 87REF / 63
ANSI 87T differential primary, 87REF restricted earth fault, percentage-restraint bias + 2nd/5th harmonic blocking, sudden-pressure (63), Buchholz, thermal (26/49). One modern numerical relay (SEL-787 / GE T60 / RET670 / 7UT85).
Line protection — 87L + 21 + 67 + pilot schemes
Transmission-line protection: 87L line differential (100% coverage via comms), 21 distance multi-zone (Z1≤80% / Z2 120% / Z3 220%), 67 directional OC, pilot schemes POTT / DCB / DUTT / DTT. SEL-411L / 421, GE D60, REL670, 7SA8.
Bus protection — ANSI 87B differential
Substation bus differential applying KCL across all feeder CTs. High-impedance vs low-impedance numerical 87B (SEL-487B / REB670 / B30 / 7SS8), zone-interlocked OC for distribution, multi-zone topology, 87B + 50BF coordination.