ABB RELION REX640 Technical Manual page 564

Section 4
Protection functions
558
• Section 1 where 0.0 < I
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
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
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
nd
2 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.
/I < End section 1. The differential current required for
b r
/I < End section 2. This is called the influence
b r
/I . By setting the slope in this section, attention
b r
1MRS759142 C
REX640
Technical Manual