ABB REG316 4 Series - Page 79

REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-41
Factors a1, a2 and a3:
a
I
I
TN
B
1
131
131
1
1
1
== =
a
I
I
TN
B
2
577
722
0 799
2
2
== =.
a
I
I
TN
B
3
458
2291
0 200
3
3
== =.
C.t’s in the unit transformer feeder
When designing the overall differential protection for a genera-
tor/transformer unit, there are the following alternative methods
of taking account of the c.t’s or lack of c.t’s in the unit trans-
former feeder (Fig. 3.5.1.4).
Alternative No.1: No c.t’s in the unit transformer feeder.
This alternative is mainly needed for hydroelectric power plants,
which have a relatively low auxiliaries requirement. The disad-
vantage is that the zone of protection is unlimited in the direction
of the unit transformer feeder with the possible hazard of false
tripping for a fault on the unit auxiliaries supply system. This
hazard can be avoided by correspondingly setting “g”.
HEST 905 009 C
Unit
transformer
i
F2
S
aux. s
y
s.
i
F
GS
GN
i
F