The slope of the operating characteristic curve of the differential function varies in
the different sections of the range:
• 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 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
nd
harmonic ratios in different phases and the weighed ratio is then compared with
the value of the Start 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
Section 4 1YHT530004D05 D
Protection functions
348 615 series
Technical Manual
To Next Page
Loading...
