33kV LINE – RELAYS & ANSI NUMBERS⚡
Main Protection – Core
1. 50 – Instantaneous Phase OC → Trips immediately for high short-circuit current.
2. 51 – Time Delayed Phase OC → Backup with definite/inverse time grading.
3. 50N – Instantaneous Earth Fault → Fast clearing of ground faults.
4. 51N – Time Delayed Earth Fault → Coordinated earth fault protection.
5. 67 – Directional Phase OC → Operates only for forward faults.
6. 67N – Directional Earth Fault → Avoids reverse fault tripping.
7. 79 – Auto Reclose → Restores line after transient faults.
8. 46 – Negative Sequence → Detects unbalance & single phasing.
9. 27 – Under Voltage → Protects during voltage dip.
10. 59 – Over Voltage → Trips on abnormal voltage rise.
System Stability & Grid
11. 81U – Under Frequency → Prevents system collapse.
12. 81O – Over Frequency → Trips on excess generation.
13. 81R – ROCOF → Senses fast frequency change.
14. 78 – Power Swing → Blocks tripping during swings.
15. 25 – Sync Check → Ensures safe breaker closing.
Breaker & Control
16. 50BF – Breaker Failure → Backup if breaker fails.
17. 74TC – Trip Circuit Supervision → Monitors trip coil health.
18. 86 – Lockout → Master trip with manual reset.
19. 52 – Breaker Control → Close/trip logic.
20. 94 – Trip Relay → Trip signal multiplication.
Earth Fault – Special
21. 51G – SEF → Very sensitive earth leakage.
22. 64 – REF → Restricted zone ground protection.
23. 59N – Neutral OV → Detects neutral displacement.
Measurement & Monitoring
24. 55 – Power Factor → PF supervision.
25. 32 – Reverse Power → Important with DG.
26. 60 – Voltage Balance → PT failure check.
27. 49 – Thermal Overload → Conductor heating.
Communication Based
28. 87L – Line Differential → 100% selective protection.
29. 85 – Pilot Communication → Permissive tripping.
30. 21 – Distance → Impedance based reach.
