ABB REG316 4 Series Operating Instructions Manual page 75

REG 316*4 1MRB520049-Uen / Rev. F
Situations occur during normal system operation which cause a
higher differential current, e.g.
increased magnetising current as a consequence of a higher
·
system voltage (switching operations, following load shed-
ding, generator regulator faults etc.)
large variation of current ratio (tap-changer at the end of its
·
range)
Providing special conditions of this kind are detected by a volt-
age relay or a saturation relay, the corresponding signal can be
used to switch the differential function from "g" to "g-High". The
recommended setting is g-High = 0.75 I
The reset ratio following a trip remains unchanged at 0.8 g.
Differential current I-Inst
The differential current setting I-Inst. facilitates fast tripping of
high internal fault currents (inhibits the detection of an inrush
current).
The setting must be higher than any normal inrush current to be
expected.
A typical value for power transformers of low to medium power is
I-Inst. = 12 I .
N
Pick-up ratio for detecting inrush
The setting of this ratio determines the sensitivity of the function
for detecting inrush.
Generally the ratio of 2nd. harmonic to fundamental is greater
than 15 %. Allowing a margin to ensure that an inrush condition
is detected, a setting of 10 % is recommended.
Duration of active inrush detection
The setting for how long the inrush detection function should be
active depends on how long the danger of false tripping due to
an inrush current, which only flows through one winding, exists.
A typical setting is 5 s.
ABB Switzerland Ltd
.
N
3-37