• Section 1 where 0.0 < I
b
/I
r
<
End section 1
. The differential current required for
tripping is constant. The value of the differential current is the same as the basic
setting (
Low operate value
) selected for the function. The basic setting allows
the appearance of the no-load current of the line, the load current of the tapped
load and minor inaccuracies of the current transformers. It can also be used to
influence the overall level of the operating characteristic.
• Section 2 where
End section 1
< I
b
/I
r
<
End Section 2
. This is called the influence
area of the starting ratio. In this section, the variations in the starting ratio affect
the slope of the characteristic, meaning the required change for tripping in the
differential current in comparison with the change in the load current. CT errors
should be considered in the starting ratio.
• Section 3 where
End section 2
< I
b
/I
r
. By setting the slope in this section,
attention can be paid to prevent unnecessary operation of the protection when
there is an external fault, and the differential current is mainly produced by
saturated current transformers.
The operation of the differential protection is based on the fundamental frequency
components. The operation is accurate and stable and the DC component and the
harmonics of the current do not cause unwanted operations.
Inrush detector
The transformer inrush currents cause high degrees of second harmonic to
the measured phase currents. The inrush detector detects inrush situations in
transformers. The second harmonic based local blocking is selected into use with
the
Restraint mode
parameter. The blocking for the low stage on the local end is
issued when the second harmonic blocking is selected and the inrush is detected.
The inrush detector calculates the ratio of the second harmonic current
I_2H_LOC_A and the fundamental frequency current I_1H_LOC_A. If the line
differential protection is used in normal mode (
Winding selection
is “Not in use”),
the calculated value is compared with the parameter value of the
Start value 2.H
setting. If the calculated value exceeds the set value and the fundamental frequency
current I_1H_LOC_A is more than seven percent of the nominal current, the output
signal BLK2H_A is activated. The inrush detector handles the other phases the same
way.
If the line differential protection is used in the in-zone transformer mode (
Winding
selection
is “Winding 1” or “Winding 2”), the weighted average is calculated for the
2
nd
harmonic ratios in different phases and the weighted ratio is then compared
with the value of the
Start value 2.H
setting. If the calculated weighted ratio value
exceeds the set value and the fundamental frequency current I_1H_LOC_A is more
than seven percent of the nominal current, output signal BLK2H_A is activated.
The locally detected transformer inrush is also transferred to the remote end
as a binary indication signal independently of the local
Restraint mode
setting
parameter value. When the internal blocking of the stabilized low stage is activated,
the RSTD2H_LOC and RSTD2H_REM outputs will also be activated at the same time
depending on whether the inrush has been detected on local or remote end or on
both ends.
1MRS759142 F
Protection functions
REX640
Technical Manual
647
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