ABB REG316 4 Series - Page 72

ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-34
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´´= oscIII
21H
for -90° < a < 90°
or is zero
I
H
= 0 for 90° < a < 270°
The angle
()
21
I,I -Ð=a
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
2
1
I
2
I
I
HEST 905 003a C
a
The following vector diagrams then result for a through-fault
2
I
I
I
2
1
I
= 0°
HEST 905 003b
C
a
and an internal fault
2
I
1
I
= 180°
I
2
HEST 905003c C
a
a