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Protection of a medium size generator
Short answer
Core electrical protections: differential (87G), stator earth fault (64G), rotor earth fault (64R), loss of excitation (40), negative sequence (46), reverse power (32), voltage-restrained overcurrent (51V) or impedance (21), thermal overload (49), over/under voltage/frequency, overfluxing (24), out-of-step (78). Mechanical: overspeed, bearing/vibration.
List out the necessary protection schemes to be employed for a medium size generator in a power plant.
A medium size generator (roughly 5–100 MVA) is protected against internal faults, abnormal operating conditions and system disturbances. Typical schemes, with ANSI device numbers:
| ANSI | Protection | Purpose |
|---|---|---|
| 87G | Generator differential | Fast clearing of stator phase-to-phase faults |
| 87GT / 87T | Overall gen-transformer / transformer differential | Faults in GSU and connections |
| 64G (59N + 27TN) | Stator earth fault (95% neutral overvoltage + 100% third harmonic) | Earth faults in stator winding (high-resistance grounded via NGT) |
| 64R | Rotor (field) earth fault | First earth fault in field winding |
| 40 | Loss of excitation (field failure) | Prevents running as induction generator and drawing VArs |
| 46 | Negative phase sequence | Unbalanced loads/open phase; rotor heating |
| 32 | Reverse power | Motoring when turbine power is lost |
| 51V / 21 | Voltage-restrained overcurrent / impedance | Backup for external faults |
| 49 | Stator thermal overload (RTDs) | Overheating |
| 59 / 27 | Over/under voltage | Load rejection, AVR failure |
| 81O / 81U | Over/under frequency | Speed and grid frequency deviations |
| 24 | Overfluxing (V/Hz) | Protects generator and GSU core |
| 78 | Pole slipping (out of step) | Loss of synchronism |
| 50/27 | Inadvertent energisation | Breaker closed at standstill |
| 50BF | Breaker failure | Backup tripping of adjacent breakers |
Mechanical/hydraulic protections
Overspeed, bearing and oil temperature, vibration, cooling water failure, low oil pressure, shear pin failure.
Tripping logic
Faults like 87G, 64G, 64R (second stage) trip the GCB, field breaker and turbine (emergency shutdown); abnormal conditions like 46, 49 or 81 first alarm, then trip after a time delay.
✓ Answer checked by Claude Opus 5.5 (high effort)
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