The previous example assume 1:1 transformation ratio of power transformer, Y/Y connection , and equal current transformation ratio. Practically, all these may be changed. Assume a power transformer as shown in figure (39) with the following data:-
25 MVA 66/11 KV , Delta-Star11
C.T.R. (66 KV Side) = 400/5
C.T.R. (11 KV Side) = 1500/5
For the Delta-Star transformer, There is a phase angle difference between primary and secondary equal to -30°. So, an aux current transformer (matching) is installed in the secondary circuit of 11 kV current transformer side to compensate the magnitude and phase.
For equilibrium of differential relay:-
Current of 11 kV of differential relay must be equal to current of 66kv side of differential relay = 2.73
But, Input current of matching = I11S = 4.37
Out put current of matching (input relay to diff relay) must be equal to I66S = 2.73
Current of 11 kV of differential relay must be equal to current of 66kv side of differential relay = 2.73
But, Input current of matching = I11S = 4.37
Out put current of matching (input relay to diff relay) must be equal to I66S = 2.73
Note:
Matching connection must be Star/Delta to compensate the original angle of power transformer
( IS_Matching = ILine / sqrt( 3 ))
