Table 666: Different sections of the range and their operation
Sections Operation
Section 1 In section 1, where 0 percent Ir < Ib < End section 1, with
End section 1 being fixed to 50 percent of Ir, the differential
current required for tripping is constant. The value of the
differential current is the same as the
Low operate value
se-
lected for the function. The
Low operate value
setting allows
for the no-load current of the power transformer and small
inaccuracies of the current transformers, but it can also be
used to influence the overall level of the operation character-
istic. At the rated current, the no-load losses of the power
transformer are about 0.2 percent. If the supply voltage of
the power transformer suddenly increases due to operation-
al disturbances, the magnetizing current of the transformer
increases as well. In general, the magnetic flux density of the
transformer is rather high at the rated voltage and the rise in
voltage by a few percent causes the magnetizing current to
increase by tens percent. This should be considered in
Low
operate value
.
Section 2 Section 2, where End section 1 < Ib <
End section 2
, is called
the influence area of
Slope section 2
. In this section, the
variations in the starting ratio affect the slope of the charac-
teristic, that is, how big a change in the differential current
is required for tripping compared to the change in the load
current. The starting ratio should consider CT errors and
variations in the transformer tap changer position (if not
compensated). Too high a starting ratio should be avoided,
because the sensitivity of the protection for detecting inter-
turn faults depends on the starting ratio.
Section 3 In section 3, where Ib >
End section 2
, the slope of the char-
acteristic is constant. The slope is 100 percent, which means
that the increase in the differential current is equal to the
corresponding increase in the biasing current.
1MRS759142 F Protection functions
REX640
Technical Manual
727
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