The slope of the differential function's operating characteristic curve varies in the
different sections of the range.
• In section 1, where 0 percent Ir < Ib < End section 1, End section 1 being fixed
to 50 percent Ir, the differential current required for tripping is constant. The
value of the differential current is the same as the
Low operate value selected for
the function.
Low operate value basically allows the no-load current of the power
transformer and small inaccuracies of the current transformers, but it can also
be used to influence the overall level of the operating 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 rated
voltage and a rise in voltage by a few percent causes the magnetizing current to
increase by tens of percent. This should be considered in
Low operate value
• In section 2, where End section 1 < Ib/In <
End section 2, is called the influence
area of
Slope section 2. In this section, 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 in comparison with 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 basically on the starting ratio.
• In section 3, where Ib/In >
End section 2, the slope of the characteristic can be
set by
Slope section 3 that defines the increase in the differential current to the
corresponding increase in the biasing current.
Figure 273: Setting range for biased low stage
If the biasing current is small compared to the differential current of the phase
angle between the winding 1 and winding 2 phase currents is close to zero (in
a normal situation, the phase difference is 180 degrees), a fault has most likely
Protection functions
1MRS757644 H
530
620 series
Technical Manual
