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
36. 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 36 is defined by the settings:
1.
IdMin
2.
EndSection1
3.
EndSection2
4.
SlopeSection2
5.
SlopeSection3
1MRK 502 066-UUS B Section 6
Differential protection
127
Technical manual
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