Basler Electric BE1-87T - Manual (Page 134)

A-6 BE1-87T Setting Notes 9171300990 Rev V
Setting Note 6
Inrush vs. Unrestraint Tap
Compare the unrestraint pickup setting defined in NOTE 4 to the transformer in rush current.
The UR tap is set at 70% of IE, the maximum pu through fault current. I
I
T RCH
E
F
=
×




3
The worst case 3-Phase fault occurs when the source impedance is negligible (X
S
=0):
Then
I
X
F
T
= 1
pu at the transformer OA base.
For a X
T
=6%,
I
PU
F
=
=
1
06
16 7
.
.
The unrestraint tap MPU would be set for 0 7 16 7 117 . . . × = pu . (Note that 11.7 pu value is different from
the relay UR tap setting.) The inrush current is generally assumed to be less than 10 times the nominal
transformer current (10 pu on the OA base).
For this worst case example, the maximum inrush current is below the UR threshold. For significant source
impedance values, we assume that the inrush current will decrease in proportion to the decrease in the
fault current and thus maintain security with the recommended settings.
N LOAD
87T
D2751-20
02-03-98
RCL RCH
X
T