ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-32
E. Setting instructions
Basic setting g
Pick-up ratio v
Operating characteristic switching point b
Increased basic setting g-High
Pick-up differential current I-Inst
(uninfluenced by inrush detection)
Pick-up ratio of the inrush detector InrushRatio
Inrush detection time InrushTime
Amplitude compensation factors a1 a2 a3
Connection of winding 1 s1
Vector groups of windings 2 and 3 s2 s3
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.
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