Over current Relays

IT IS A HELPFUL TOPIC ON RELAYS

1. Classification of Protective Relays Based on Technology Based on Function

2. Over current Relay Definite time over current relay Instantaneous over current relay Inverse time over current relay Inverse definite minimum time (IDMT) over current relay Very inverse –time over current relay Extremely inverse time over current relay

3. Overcurrent Protection Definite (Independent) Time Relays T OP TIME I S Applied Current (Relay Current Setting)

4. Overcurrent Protection IDMT TIME Applied Current (Relay Current Setting) I S Inverse Definite Minimum Time characteristic

5. Inverse Time current characteristics

6. By suitable design of magnetic circuit Saturation above a certain value of actuating current Below saturation inverse characteristic Above saturation straight line characteristic Saturation at pick up value – Definite time characteristic Saturation at later stage – IDMT characteristic Saturation at still later – very inverse Saturation at a very late stage – extremely inverse Realization of Time current characteristics using EM relays

7. Relay Current Setting Over current relay – Provided with tappings Phase to phase fault protection – 50%-200% -steps of 25% If rating of the relay is 5 A then 2.5A, 3.75A, 5A, 6.25A,…………, 10A Earth fault protection - 20%-80% - steps of 10% Normal current rating of earth fault relay 1 A Time-current characteristic -current in ampere on X-axis Different curves for the same relay for different settings

8. Time Current characteristics for 5 A current setting

9. Time Current characteristics for 5 A current setting

10. Time Current characteristics for 10 A current setting

11. Time Current characteristics for two different settings of relay (5 A & 10 A)

12. Plug Setting Multiplier (PSM)

13. Ex 1: 5 A relay set at 200 % Relay current setting = 200% of 5 A = 10 A Secondary current = 10 A ( current through relay) PSM = 10/10 = 1 Ex 2: If relay set at 50 % ; Relay current setting = 50% of 5 A = 2.5 A Secondary current = 10 A ( current through relay) PSM = 10/2.5 = 4 A Ex 3: If fault current is 6000 A and CT ratio = 400/5 Then secondary current = 6000/80 = 75 A ; Plug Setting Multiplier (PSM)

14. Plug Setting Multiplier (PSM) for 5 A setting

15. Plug Setting Multiplier (PSM) for 5 A & 10 A

16. Different operating times possible for the same relay By adjusting the angular distance by which the moving part of the relay travels for closing the relay contacts Can be set up to 10 steps from 0.1,0.2,…..,0.9,1 Ex 4: For, TMS =1, if operating time = 4 s then with TMS = 0.5 it will be 4x0.5= 2 s with TMS = 0.2 it will be 4x0.2= 0.8 s Note :All these op. times are for the same value of current Time Multiplier Setting(TMS)

17. Time Current characteristics for different time settings of relay

18. To protect an equipment or section of the line One or more relays of same or different types Classification of Protective Schemes

19. To protect an equipment or section of the line One or more relays of same or different types Classification of Protective Schemes

20. Widely used for protection of distribution lines Radial feeder sectionalized – 2 or 3 relays used Relays must be selective with each other Over Current Protective Schemes

21. Over current P rotective Schemes For proper selectivity Time Graded System Current Graded System A combination of time and Current Grading

22. Time Graded system Definite time overcurrent relays Preset time –independent of fault current Definite time relays are cheaper than IDMT Relays Starts a timing unit to trip the circuit breaker Operating time is in increasing order Difference in time settings of adjacent relays To cover operating time of circuit Breaker To cover errors in relay usually 0.5 sec More accurate relays and fast CB can reduce the difference in the time

23. Time graded overcurrent protection Faults near the power source Involves a large current Most destructive Should be cleared quickly Time graded system takes longest time Suitable when impedance between substations is low

24. Current graded system Pick-up current in increasing order High speed instantaneous over current relays Operating time same for all relays

25. Combination of Current and Time Grading