RELAY
1. REF RELAY :
• OPERATION WITH CT CURRENT FLOW DIRECTION.
• WHY REF RELAY WHEN DIFFERENTIAL RELAY IS THERE.
• REF RELAY STABILITY CHECK BY PRIMARY INJECTION –
WHICH POINT FOR CUR RENT INJECTION.
• WHY REF RELAY NEED STABILISING RESISTOR. IF
RESISTOR SHORTED OR BYPASSED, WHAT WILL HAPPEN.
• WHICH TYPE OF RELAY FOR REF, HIGH IMPEDANCE OR LOW
IMPEDANCE.
• IF NEUTRAL CT IS NOT CONNECTED IN THE CIRCUIT, WHAT
WILL HAPPEN.
• REF PROTECTION ACTS ON WHICH OF FOLLOWING:
SINGLE PH. TO E/F; PH-PH TO E/F; THREE PH. TO E/F.

2. DISTANCE RELAY :
• BASIC PRINCIPLE AND TYPES
• SWITCHED MODE PROTECTION – WHY IS IT CALLED SO.
• SCHEMES USED
• WHY ZONE-1 SET AT 80%
• SETTING RANGE FOR ZONE-2 AND ZONE-3
• HOW SINGLE PHASE TO GROUND FAULT MEASUREMENT IS
DONE.
•
3. WHAT IS POLARITY. CT LOCATION IF INTERCHANGED WHAT
WILL HAPPEN.

4. SOLKOR RELAY:
1. WHY SOLKOR IS USED.
2. CIRCUIT DIAGRAM OF SOLKOR RELAY
3. DIFFERENT PARTS OF RELAY
4. HOW IT WORKS.
5. EXPLAIN BY VECTOR DIGRAM.
6. WHAT ADVANTAGE FOR USING SUMMATION TR.
7. DIFFERENCE BETWEEN R & RF WITH CIRCUIT.
8. ADJUSTMENT OF PADDING RESISTANCE.
9. WHAT IS LEAKAGE CURRENT AND HOW TO MEASURE.
10. WHAT HAPPEN IF LINK IS LEFT OPEN.
11. 5KV/15KV INSULATION TR. ARE USED IN WHICH SYSTEM.
12. WHAT PRECAUTION TAKEN DURING TESTING.

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13. PROCEDURE BEFORE TESTING AND AFTER NORMALISATION.
14. HOW RELAY IS TESTED IN LIVE CONDITION.
15. RANGE OF SPILL CURRENT ANS SETTING OF RELAY
16. IF MEASURED CURRENT IS 5MILLI AMPS, CAN RELAY
OPERATE.
17. EACH FAULT APPROXIMATE VALUE OF PICK UP.
18. WHAT IS THE REASON SOLKOR-R MODE PILOT SHORTED
CONDITION RELAY CURRENT PRESENT.
19. AFTER SHUTDOWN, WHY NEED TO OPEN SOLKOR PILOT, IF
NOT WHAT WILL HAPPEN.
20. AFTER NORMALISING WHAT CURRENT TO BE MEASURED, WHAT
IS ACCEPTABLE LIMIT.
21. WHAT HAPPENS IF PILOT OPENS FROM RELAY, WHEN FEEDER
IS LOADED.
22. CAN WE USE THIS RELAY AS O/C OR E/F RELAY.
23. ANY RELAY OTHER THAN PILOT CABLE TYPE.
24. IF FEEDER TRIPS ON SOLKOR, WHAT TO DO.
25. WHAT HAPPENS IF PILOT POLARITY REVERSE.
26. WHAT HAPPENS WHEN CT SHORTED AT ONE END.
27. WHAT HAPPENS IF PILOT SHORTED – STATUS OF RELAY
28. WHAT HAPPENS TO RELAY ,IF POWER CABLE GETS OPEN.
29. WHAT HAPPEN IF PADDING RESISTOR IS SHORTED AT ONE
END.


5. TR. DIFFERENTIAL:
1. CIRCUIT DIAGRAM
2. DIRECTION OF FAULT CURRENT.
3. WHEN THE FAULT IS IN ZONE AND OUT OF ZONE.
4. CT CONNECTION IN PY. AND SY. SIDE.
5. BIAS SETTING AND BIAS CURRENT.
6. WHICH BIAS SETTING USED IN TRS. FOR HIGHER TAPS.
7. PURPOSE OF RESTRAINING BIAS COIL.
8. WHY BIASING REQUIRED.
9. EXPLAIN IPCT, FOR EXAMPLE, DYN10 VECTOR GROUP. WHY
MATCHING TR. USED.
10. WHY NEED 2ND HARMONIC BLOCKING DURING INRUSH.
11. WHICH HARMONIC IS MORE 2ND OR 3RD.
12. HOW TO AVOID FALSE TRIPPING IN DIFF. PROTECTION WHEN
INRUSH CURRENT FLOW DURING TR. IS ENERGISED.


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6. DIRECTIONAL RELAY:
1. CIRCUIT FOR USE OF DIRECTIONAL PROTECTION SYSTEM.
2. WORKING ZONE / NON-WORKING ZONE.
3. HOW THE CORRECT DIRECTION OF FLOW OF CURRENT IS
DETERMINED.
4. CT & PT CONNECTION DIAGRAM.
5. SETTING OF PHASE & EARTH FAULT ANGLE AND
OPERATING REGION.
6. WHICH DIRECTION IT IS PROTECTING.

7. WHAT PROTECTION AVAILABLE FOR POWER TR.

8. WHY CT CONNECTION IS STAR, IF TR. WINDING IS DELTA.

9. DIFFERENCE BETWEEN REF AND DIFF. RELAY

10. WHERE IS O/C & E/F RELAY CONNECTED EITHER HV SIDE
OR LV SIDE AND HOW E/F RELAY WORKS IF TR. WINDING IS DELTA
CONNECTED.

11. IN DELTA WINDING OF TR., IF ONE WINDING IS CUT AND
NOT EARTHED, WHICH PROTECTION WILL ACT.

12. IF DIFF. & REF RELAY ARE NOT WORKING, WHAT IS BACK
UP PROTECTION IN TR.

13. WHAT IS PURPOSE OF NGR AND HOW STANDBY E/F RELAY
WORKING, TESTING PRECAUTIONS IF COMMON NGR IS USED.

14. WHAT IS PROTECTIVE ZONE OF STANDBY E/F RELAY.

15. WHAT ARE PROTECTION AVAILABLE FOR FEEDER.

16. AFTER TR. IS NORMALISED, WHAT CURRENT TO BE
MEASURED.

17. BUSBAR PROTECTION:
a. BASIC PRINCIPLE AND DIAGRAM WITH CURRENT DIRECTION.
b. WHY OVER LAPPING CT’S REQUIRED, IF NOT WHAT WILL BE
THE EFFECT.
c. WHY CT SUPERVISION REQUIRED, HOW TRIPPING IS
AVOIDED, WHAT IS CT SUPERVISION SETTING VALUE.
d. HIGH IMPEDANCE & LOW IMPEDANCE TYPE
e. TEST PROCEDURE(STEP BY STEP)
f. AFTER NORMALISING, SPILL CURRENT VALUE AND LIMIT.

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18. CIRCUIT BREAKER FAILURE – FUNCTION, WHY NEEDED, HOW
TRIPPING ARRANGEMENT.

19. WHY INSTANTANEOUS ELEMENT IS NOT USED IN THE
NETWORK.


20. WHEN FEEDER IS LIVE AND BY MISTAKE LINE EARTH
SWITCH IS CLOSED, WHICH RELAY OPERATES – O/C OR E/F.


21. DURING TR. SWITCHING ON, IF DIFF. RELAY IS BLOCKED
BY 2ND HARMONICS, WHAT ARE THE OTHER PROTECTION IN SERVICE.


22. WHICH OF FOLLOWING PROTECTION OPERATES FAST, WHEN
11KV CABLE COMPARTMENT GET FLASH OVER: CABLE DIFFERENTIAL;
OVERCURRENT & EARTH FAULT; ARC PROTECTION.


23. WHICH PROTECTION OPERATES WHEN ANY ONE OF THE OHL
PHASE IS CUT AND HANGING, NOT TOUCHED WITH EARTH.


24. WHAT ARE DIFFERENT TTB ALARMS – HOW YOU KNOW THERE
IS ANY ALARM.


25. WHAT IS CIRCULATING CURRENT AND HOW IT WILL COME.


26. IF ONE TR. TRIPPED ON DIFF. RELAY WHAT YOU WILL DO.


27. IF SUB-STATION TRIP ON BUSBAR PROTECTION, HOW YOU
WILL RESTORE.


28. WHAT IS CT SATURATION. HOW IT IS CARRIED OUT.


29. D.C EARTH FAULT – HOW YOU WILL FIND OUT.

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30. PILOT WIRE SUPERVISION – HOW IT WORKS

31. IN 11KV SWITCHGEAR DBB SCHEME, ONE E/F RELAY IS
FAULTY IN ONE FEEDER AND THERE IS NO SPARE FEEDER – HOW YOU
WILL NORMALISE THE FEEDER.

32. WHICH PROTECTION IS USED FOR LONG CABLE FEEDER

33. DIFFERENCE BETWEEN TRAFO. DIFF. AND CABLE DIFF.
PROTECTION.

34. WHAT TYPE CHARACTERISTIC AVAILABLE FOR O/C AND E/F
RELAY.

35. WHAT IS THE DIFFERENCE BETWEEN STATIC AND ELECTRO-
MECHANICAL O/C RELAYS.

36. WHAT IS DEAD ZONE

37. HOW TRIPPING OF OTHER ZONE AVOIDED WHEN BUS
SECTION CB IN OPEN CONDITION FOR DEAD ZONE FAULT.

38. IF OUT OF STEP ALARM COMES WHILE ALL TR. IN SAME
TAP POSITION, WHAT YOU WILL DO.

39. WHAT YOU WILL DO IF AN 11KV FEEDER FAILED TO TRIP
ON: O/C or E/F or SOLKOR.
HOW YOU RESTORE THE SUPPLY.














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