ABB REG316 4 Series Operating Instructions Manual page 72

ABB Switzerland Ltd
Operating characteristic
The restraint current in the case of a three-winding power
transformer is derived from the currents of two windings and not
three. In the interest of the best possible through-fault stability,
the two largest currents of the three windings are used for this
purpose.
The restrain current is either defined by the equation
= ´ ´ a
I I I c os
H 1 2
or is zero
I = 0
H
The angle
( )
a = Ð , I -
I
1 2
The following vector diagram of the current on primary and sec-
ondary sides and of the differential current measured for a
transformer on load was assumed.
I
I 2
1
a
2 I
I
HEST 905 003a C
The following vector diagrams then result for a through-fault
I 2
I
2 I
and an internal fault
a
I 2
3-34
REG 316*4 1MRB520049-Uen / Rev. F
for -90° < a < 90°
for 90° < a < 270°
1 I
a
= 0°
HEST 905 003b C
1 I 2 I
a
= 180°
HEST 905003c C