ABB REG316 4 Series Operating Instructions Manual page 80

ABB Switzerland Ltd
A fault on the unit auxiliaries supply system causes a current i
(in p.u.) to flow to the generator star-point.
S
aux.sys .
=
i i
F F 2
S
GN
Example:
It follows from this that "g" must be set higher than 0.3 to avoid
the possibility of a false trip.
Alternative No. 2: C.t's installed in the unit transformer feeder
on the generator side of the unit transformer
These c.t's usually have the same ratio as the generator c.t's
although the rating of the unit transformer is much less. The rea-
son is the high fault level on the generator side of the unit
transformer and the consequentially high thermal and dynamic
stress on the c.t's.
The reference value of the c.t. channel of the protection is de-
termined by the rated current of the generator and not of the unit
transformer.
Alternative No. 3: C.t's installed in the unit transformer feeder
on the auxiliaries side of the unit transformer
this case, the c.t's are specified according to the rated current of
the unit transformer.
Advantages:
clearly defined zone of protection
·
reduced performance required of the c.t's, which are there-
·
fore cheaper.
Disadvantages:
Interposing c.t's may be necessary.
·
reduced sensitivity of the protection for faults in the protected
·
zone, but on the auxiliaries side of the unit transformer.
3-42
REG 316*4 1MRB520049-Uen / Rev. F
S
aux sys . .
=
i 10 ;
F 2
S
GN
= 10 ´ 0.03 = 0.3
i
F
=
0 03 .
(Fig. 3.5.1.4).
(Fig. 3.5.1.4). In
F