ABB RELION REX640 Technical Manual page 646

Section 4
Protection functions
640
Table 592: Different sections of the range and their operation
Sections Operation
Section 1 In section 1, where 0 percent Ir < Ib < End section 1, with End section 1 being fixed
to 50 percent of Ir, the differential current required for tripping is constant. The value
of the differential current is the same as the
function. The
transformer and small inaccuracies of the current transformers, but it can also be
used to influence the overall level of the operation characteristic. At the rated
current, the no-load losses of the power transformer are about 0.2 percent. If the
supply voltage of the power transformer suddenly increases due to operational
disturbances, the magnetizing current of the transformer increases as well. In
general, the magnetic flux density of the transformer is rather high at the rated
voltage and the rise in voltage by a few percent causes the magnetizing current to
increase by tens percent. This should be considered in
Section 2 Section 2, where End section 1 < Ib <
Slope section 2 . In this section, the variations in the starting ratio affect the slope of
the characteristic, that is, how big a change in the differential current is required for
tripping compared to the change in the load current. The starting ratio should
consider CT errors and variations in the transformer tap changer position (if not
compensated). Too high a starting ratio should be avoided, because the sensitivity
of the protection for detecting inter-turn faults depends on the starting ratio.
Section 3 In section 3, where Ib >
slope is 100 percent, which means that the increase in the differential current is
equal to the corresponding increase in the biasing current.
Low
operate
Curve
value
I [%I ]
d r
1
50 %Ir
2 20 %Ir
300
3 5 %Ir
200
100
100
GUID-A8116F5B-DFA0-43C9-927F-E7315B0D33F8 V1 EN
Figure 353: Setting range for biased low stage
If the biasing current is small compared to the differential current or if the phase angle
between the currents of two windings with the highest phase current is close to zero
(normally, the phase difference is 180 degrees) or the phase angle between the
compared phase currents (in case of two-winding transformer) is close to zero, a fault
Low operate value setting allows for the no-load current of the power
End section 2 , is called the influence area of
End section 2 , the slope of the characteristic is constant. The
Slope End
section section
2
2
50 % 100 %Ir
30 % 150 %Ir
10 % 500 %Ir
Curve 1
Curve 2 ( Default settings )
200 300
1MRS759142 C
Low operate value selected for the
Low operate value .
Curve 3
400 500
Technical Manual
I [%I ]
d r
REX640