- 2080 Chaitra · 6 marks
Protection for alternator and step-up transformer
Short answer
Essential alternator protections: differential (87G), stator earth fault 95% and 100% (59N/64G, 27TN/64S), rotor earth fault (64R), loss of excitation (40), negative sequence (46). Essential GSU protections: transformer differential (87T), restricted earth fault (64REF), Buchholz (63), overfluxing (24). Overall unit differential (87GT) and master trip (86) cover both.
List out the most essential protection schemes required for Alternator and Generator Step Up transformer in a mega size power project.
In a mega (large) hydro or thermal project, the alternator and its generator step-up (GSU) transformer are protected by a set of main and backup relays, usually grouped as a unit protection scheme with an overall differential zone.
Generator protections (ANSI number)
| Protection | ANSI | Purpose |
|---|---|---|
| Generator differential | 87G | Stator phase-to-phase faults |
| Stator earth fault 95 % | 59N / 64G | Earth faults on 95 % of winding |
| 100 % stator earth fault | 27TN / 64S | Faults near neutral (3rd harmonic or injection) |
| Rotor earth fault | 64R / 64F | Field winding earth fault |
| Loss of excitation | 40 | Field failure, under-excitation |
| Negative sequence | 46 | Unbalanced load, rotor heating |
| Reverse power | 32 | Motoring of the unit |
| Over/under frequency | 81 | Abnormal speed |
| Over/under voltage | 59 / 27 | Load rejection, AVR failure |
| Overfluxing (V/Hz) | 24 | Core over-excitation |
| Voltage-restrained overcurrent / impedance | 51V / 21 | Backup for external faults |
| Pole slipping | 78 | Loss of synchronism |
| Stator thermal / RTD | 49 | Overload, cooling failure |
| Inadvertent energisation | 50/27 | Breaker closed on a stopped unit |
GSU transformer protections
| Protection | ANSI | Purpose |
|---|---|---|
| Transformer differential | 87T | Internal phase faults |
| Restricted earth fault (HV) | 64REF / 87N | Earth faults near HV neutral |
| Buchholz relay | 63 | Incipient internal faults, gas |
| Oil and winding temperature | 26 / 49 | Overheating |
| Pressure relief device | 63PR | Sudden internal pressure |
| HV overcurrent and earth fault | 50/51, 50N/51N | Backup |
| Overfluxing | 24 | Over-excitation of core |
| Oil level, cooling failure | 71 | Alarm |
Overall scheme
An overall (unit) differential 87GT covers the generator, GSU and unit auxiliary transformer together. Relays are duplicated as Main-1 and Main-2 groups with separate CTs and DC supplies for very large units. Trips go through a master trip relay (86) to the GCB, HV breaker, field breaker and turbine shutdown.
✓ Answer checked by Claude Opus 5.5 (high effort)
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