Method-B: Relay Co-ordination Considering fault on Transformer LV Side.

To avoid this overlap of Method-A let we have to set HV side relay R3 time as 500 ms for fault on LV side of transformer. With safe time grading margin of 200 ms we adopt relay operation time of 700 ms for R2 and 900 ms for R1. 

Related calculations are tabulated below.

	R3	R2	R1
Relay operating current setting	240	400	600
Fault Current Considered for Calculation of ROT	1377	1377	1377
Remarks for considered fault current	Current reflected on transformer HV side for transformer LV side fault	Current reflected on transformer HV side for transformer LV side fault	Current reflected on transformer HV side for transformer LV side fault
Fault current as multiple of set value	5.74	3.44	2.29
Relay Operational Time with TMS = 1	3.94	5.59	8.36
ROT Required	0.50	0.70	0.90
Remarks for considered ROT	Transformer LV side relay operational time + Time Gradation 0.2 sec	Relay R3 operational time for fault current + Time Gradation 0.2 sec	Relay R2 operational time + Time gradation 0.2 sec
TMS	0.127	0.125	0.108

Validation of adopted settings

After observing relay operation time plotted on common X axis as fault current we may observe that for fault on T/F HV side time gradation margin for relay R1 and R2 is not sufficient. (See Figure)