ABB REG670 Technical Manual page 129

1MRK502052-UEN B
Technical manual
quantities, but zero sequence currents can flow in the earthed star- connected winding.
In such cases, an external earth-fault on the star-side causes zero sequence current to
flow on the star-side of the power transformer, but not on the other side. This results
in false differential currents - consisting exclusively of the zero sequence currents. If
high enough, these false differential currents can cause an unwanted disconnection of
the healthy power transformer. They must therefore be subtracted from the
fundamental frequency differential currents if an unwanted trip is to be avoided.
For delta windings this feature shall be enabled only if an earthing transformer exists
within the differential zone on the delta side of the protected power transformer.
Removing the zero sequence current from the differential currents decreases to some
extent the sensitivity of the differential protection for internal earth -faults. In order to
counteract this effect to some degree, the zero sequence current is subtracted not only
from the three fundamental frequency differential currents, but from the bias current
as well.
Restrained and unrestrained limits of the differential protection
The power transformer differential protection function uses two limits, to which
actual magnitudes of the three fundamental frequency differential currents are
compared at each execution of the function.
The unrestrained (that is, non-stabilized, "instantaneous") part of the differential
protection is used for very high differential currents, where it should be beyond any
doubt, that the fault is internal. This settable limit is constant and not proportional to
the bias current. Neither harmonic, nor any other restrain is applied to this limit, which
is therefore allowed to trip the power transformer instantaneously.
The restrained (stabilized) part of the differential protection compares the calculated
fundamental differential (operating) currents and the bias (restrain) current, by
applying them to the operate - restrain characteristic. Practically, the magnitudes of
the individual fundamental frequency differential currents are compared with an
adaptive limit. This limit is adaptive because it is dependent on the bias (that is,
restrain) current magnitude. This limit is called the operate - restrain characteristic. It
is represented by a double-slope, double-breakpoint characteristic, as shown in
figure 33. The restrained characteristic is determined by the following 5 settings:
1. IdMin (Sensitivity in section 1, multiple of trans. HV side rated current set under
the parameter RatedCurrentW1)
2. EndSection1 (End of section 1, as multiple of transformer HV side rated current
set under the parameter RatedCurrentW1)
3. EndSection2 (End of section 2, as multiple of transformer HV side rated current
set under the parameter RatedCurrentW1)
4. SlopeSection2 (Slope in section 2, as multiple of transformer HV side rated
current set under the parameter RatedCurrentW1)
5. SlopeSection3 (Slope in section 2, as multiple of transformer HV side rated
current set under the parameter RatedCurrentW1)
The restrained characteristic in figure
Differential protection
33 is defined by the settings:
Section 6
123