ABB REG670 - Page 177

REFPDIF is not sensitive to inrush and overexcitation currents. The only danger is an
eventual current transformer saturation.
REFPDIF has only one operate-bias characteristic, which is described in the table 67
and shown in Figure
63.
Table 67: Data of the operate-bias characteristic of REFPDIF
Default sensitivity Idmin
(zone 1)
Max. base
sensitivity Idmin
(zone 1)
Min. base sensitivity
Idmin (zone 1)
End of zone
1
First slope Second
slope
% IBase % IBase % IBase % IBase % %
30 5 100 125 70 100
The differential protection REFPDIF calculates a differential current and a bias
current. In case of internal earth faults, the differential current is theoretically equal to
the total earth-fault current. The bias current is supposed to give stability to REFPDIF.
The bias current is a measure of how high the currents are and how difficult the
conditions are under which the CTs operate. With a high bias, difficult conditions can
be suspected, and it will be more likely that the calculated differential current has a
component of a false current, primarily due to CT saturation. This “law” is formulated
by the operate-bias characteristic. This characteristic divides the Idiff - Ibias plane in
two areas. The area above the operate-bias characteristic is the operate area, while the
one below is the block area, see Figure
63.
bias current
in per unit
Zero-
sequence
diff. current
in per unit
0
1
2
3
4
5
IdMin
Section 1
Operate
conditionally
I operate
slope = ----------------* 100 %
I restrain
Sec. 2
Section 3
The characteristic
can be moved
up and down
(vertically)
by changing
the setting IdMin
End Section 1
End Section 2
IdMin range: 0.04
–
1.00 IBase
Reset Ratio in all sections:
0.95 (a constant)
Slope 70%
Restrain
Slope 100%
Characteristic if
IdMin = 1.0 pu
IEC98000017-5-en.vsd
D
D
0
1
2
3 4
5
6
IEC98000017-5 V1 EN
Figure 63: Operate - bias characteristic of the Restricted earth-fault protection,
low impedance REFPDIF
1MRK502052-UEN B Section 6
Differential protection
171
Technical manual