Method-C: O/C Relay Co-ordination by Proper Use of HS Element

If we wish to avoid overlap as observed by Method-A and Method-B we have to set HV side relay R3 time as 500 ms for fault on LV side of transformer. 

Hence to arrive at practical solution we may adopt HS instantaneous setting of relay R3 as 1377*1.5 = 2065 or say 2000 Amp. And will adjust operational time setting of relay R2 and R1 accordingly to have proper time gradation for transformer LV side as well as HV side faults. Such worked out relay current setting and TMS are as tabulated below and corresponding relay operational time graph is also shown in  Figure.

Related Calculations are tabulated below.

	R3	R2	R1
Relay operating current setting	240	400	600
Fault Current Considered for Calculation of ROT	1377	2000	3385
Remarks for considered fault current	Current reflected on transformer HV side for transformer LV side fault	HS Setting for relay R3	Fault current for fault at Bus-C
Fault current as multiple of set value	5.74	5.00	5.67
Relay Operational Time with TMS = 1	3.94	4.28	3.97
ROT Required	0.50	0.63	0.67
Remarks for considered ROT	Transformer LV side relay operational time + Time Gradation 0.2 sec	Relay R3 operational time for fault current 2000 A + Time Gradation 0.2 sec	Relay R2 operational time for fault current 3385 A + Time gradation 0.2 sec
TMS	0.127	0.147	0.169

Validation of adopted settings

                Relay operating characteristics for the relay R1, R2 and R3 on common fault current on X axis is plotted for Case-C as shown below. After observing the graph we may see that for T/F LV side fault relay R3 operates with time delay of 500 ms which is sufficiently more than transformer LV side operational time. Also for T/F HV side fault relay R1, R2 and R3 operate with sufficient time gradation margin. (See Figure-7)

Comments

Thus with Inverse characteristics of the relay we may obtain required time grading and system safety by properly selecting relay current setting and time multiplier setting correctly. Also we may adopt instantaneous or definite time delay in conjunction with inverse characteristics depending on our need.