Over current Relays

Classification of Protective Relays

• Based on Technology

• Based on Function

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

Overcurrent Protection Definite (Independent) Time Relays

TOP

TIME

IS Applied Current(Relay Current Setting)

Overcurrent Protection IDMT

TIME

Applied Current(Relay Current Setting)

IS

• Inverse Definite Minimum Time characteristic

Inverse Time current characteristics

• 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

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

Time Current characteristics for 5 A current setting

0 10 20 30 40 50 60 70 80 90 100 1100

2

4

6

8

10

12


0 10 20 30 40 50 60 70 80 90 100 1100

2

4

6

8

10

12


0 10 20 30 40 50 60 70 80 90 100 1100

2

4

6

8

10

12

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

0 10 20 30 40 50 60 70 80 90 100 1100

2

4

6

8

10

12

Plug Setting Multiplier (PSM)

Ex 1: 5 A relay set at 200 %Relay current setting = 200% of 5 A = 10 ASecondary 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 ASecondary current = 10 A ( current through relay)PSM = 10/2.5 = 4 AEx 3: If fault current is 6000 A and CT ratio = 400/5Then secondary current = 6000/80 = 75 A;

Plug Setting Multiplier (PSM)

Plug Setting Multiplier (PSM) for 5 A setting

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

• 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)

Time Current characteristics for different time settings of relay

• To protect an equipment or section of the line• One or more relays of same or different types

Classification of Protective Schemes

• To protect an equipment or section of the line• One or more relays of same or different types

Classification of Protective Schemes

• 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

Over current Protective Schemes

For proper selectivity• Time Graded System• Current Graded System• A combination of time and Current Grading

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

Time graded overcurrent protection

Faults near the power source Involves a large current Most destructive Should be cleared quickly

Time graded system takes longest timeSuitable when impedance between substations is low

Current graded system

Pick-up current in increasing order

High speed instantaneous over current relays

Operating time same for all relays

Combination of Current and Time Grading

Engineering

Over current Relays