Method-A: O/C relay settings co-ordination considering fault at transformer HV side.

               As shown here under we will set the relay R1, R2 and R3 for operation time of 600 ms, 400 ms and 200 ms respectively considering fault at T/F HV Side.

For R1, R2 and R3 relay setting calculations carried out considering fault at T/F HV side. Detail steps for deciding relay current and time multiplier settings are as below

1)    Deciding Relay Current Setting

a.    Relay R3: 240 A which is 110% of transformer full load current

b.    Relay R2: 400 A which is sufficient to allow transformer load plus other loads at Bus-C

c.    Relay R1: 600 A which is sufficient to allow load of Bus-B and Bus-C

2)    Deciding relay multiple of fault current for fault at transformer HV side: Here same fault current (3385 A) will measured by each relay but as each relay has different current setting; multiple of fault current sensed by relay will be different and as calculated below

a.    Relay R3: 3385/240 = 14.1

b.    Relay R2: 3385/400 = 8.46

c.    Relay R1: 3385/600 = 5.64

3)    Deciding relay operational time for TMS = 1: Using standard inverse curve we can decide relay operation time with TMS = 1 (Y axis value corresponding to X axis value as above)

a.    Relay R3: Time of operation with TMS = 1 is 2.8 sec

b.    Relay R2: Time of operation with TMS = 1 is 3.2 sec

c.    Relay R1: Time of operation with TMS = 1 is 4.0 sec

4)    Deciding Time Multiplier Setting for the relays: As per required time gradation of 200 ms TMS settings will be

a.    Relay R3: TMS = 0.2/2.8 = 0.075

b.    Relay R2: TMS = 0.4/3.2 = 0.125

c.    Relay R1: TMS = 0.6/4.0 = 0.150

Validation of adopted settings

It is always necessary to check correctness of the settings adopted for different situation so as to avoid overlapping of relay operating characteristics and there by chances of un-desired relay operation. To do this let us plot the relay operating characteristics for the relay R1, R2 and R3 on common fault current on X axis. Such type of graph for above case is plotted and shown in the figure. After observing the graph we may see that for T/F LV side fault relay R3 will operate during 300 ms. this time of operation will overlap transformer LV side relay operational time.