ABB 615 series Technical Manual page 470

Section 4
Protection functions
464
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
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
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 Pickup value 2.H setting. If the calculated value exceeds
the set value and the fundamental frequency current I_1H_LOC_A is more that 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 weighed average is calculated for the 2
harmonic ratios in different phases and the weighed ratio is then compared with the value
of the Pickup value 2.H setting. If the calculated weighed 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.
/I < End Section 2. This is called the influence area
b r
/I . By setting the slope in this section, attention can
b r
1MAC059074-MB A
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615 series ANSI
Technical Manual