ABB REG316 4 Series Operating Instructions Manual page 70

ABB Switzerland Ltd
E. Setting instructions
Basic setting
Pick-up ratio
Operating characteristic switching point
Increased basic setting
Pick-up differential current
(uninfluenced by inrush detection)
Pick-up ratio of the inrush detector
Inrush detection time
Amplitude compensation factors
Connection of winding 1
Vector groups of windings 2 and 3
The purpose of the transformer differential protection is to detect
phase faults in the protected zone. It may also detect earth faults
and interturn faults. The protection is sensitive, fast and abso-
lutely discriminative.
Basic setting g
The basic setting "g" defines the pick-up setting of the differential
protection for internal faults.
The lowest possible value should be chosen for "g" (high sensi-
tivity) to enable it to detect transformer earth faults and interturn
faults in addition to phase faults.
The setting of "g" must not be too low, however, to avoid the
danger of false tripping due to:
c.t. errors
·
the maximum off-load transformer current at the maximum
·
short-time system voltage
tap-changer range.
·
The off-load current (magnetising current) of a modern power
transformer is very low, usually between 0.3 and 0.5 % of rated
current at rated voltage. During short-time voltage peaks, e.g.
following load shedding, the off-load current can reach as much
as 10 % or more.
The tap-changer voltage range is usually between ± 5 % and
± 10 %, but ranges of ± 20 % and more occur. Its influence has to
be taken into account regardless of whether the tap-changer is
manually operated or controlled by a voltage regulator.
3-32
REG 316*4 1MRB520049-Uen / Rev. F
g
v
b
g-High
I-Inst
InrushRatio
InrushTime
a1 a2 a3
s1
s2 s3