REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-123
As can be seen from Fig. 3.5.11.2, the relay setting for a zone of
protection of 95 % is 5 % of U
max
. The scheme detects E/F’s on
the generator stator winding, the cables to the step-up trans-
former and the delta-connected windings of the step-up trans-
former.
The capacitances between primary and secondary of the step-up
transformer conduct currents emanating from E/F’s on the HV
side to the LV side and can cause false tripping of the stator E/F
protection. The capacitive coupling of E/F currents on the HV
side takes place regardless of whether the HV star-point is
grounded or not. The capacitance C
12
between HV and LV
windings of the step-up transformer and the capacitance C of the
generator circuit form a potential divider that determines the
potential of the generator star-point (see Fig. 3.5.11.3a). The
value of the capacitance C is usually too low to reliably hold the
star-point below the pick-up setting of the protection. For this
reason, the generator star-point is grounded via a resistor R
E
(see Fig. 3.5.11.4) which ensures that the potential of the star-
point remains below the setting of the protection for an E/F on
the HV power system. Correspondingly, the value of the resistor
R
E
is chosen such that for a given C
12
and an E/F at the HV
terminals of the step-up transformer, the offset of the generator
star-point does not reach the pick-up setting of the 95 % E/F
protection.
HEST 905 030 FL
3 C
12
U
I
E
Star-point
3 C
3
HV
U
U
I
E
R
E
3 C
12
3
HV
U
a) without grounding resistor b) simplified circuit with
grounding resistor
Fig. 3.5.11.3 Generator star-point off-set for an E/F on the HV
side of the step-up transformer
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