Restrained, And Unrestrained Limits Of The Differential Protection - ABB REG650 Technical Manual

Section 5
Differential protection
5.1.3.6
78

Restrained, and unrestrained limits of the differential protection

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 (that is, not
proportional to the bias current). Neither harmonic, nor any other restrain is applied
to this limit, which is therefore allowed to trip power transformer instantaneously.
The restrained (that is, stabilized) part of the differential protection compares the
calculated fundamental differential (that is, operating) currents, and the bias (that
is, 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 IBase in GlobalbaseSelW1)
2. EndSection1 (End of section 1, as multiple of transformer HV side rated
current set under the parameter IBase in GlobalbaseSelW1)
3. EndSection2 (End of section 2, as multiple of transformer HV side rated
current set under the parameter IBase in GlobalbaseSelW1)
4. SlopeSection2 (Slope in section 2, as multiple of transformer HV side rated
current set under the parameter IBase in GlobalbaseSelW1)
5. SlopeSection3 (Slope in section 2, as multiple of transformer HV side rated
current set under the parameter IBase in GlobalbaseSelW1)
The restrained characteristic in figure
1. IdMin
2. EndSection1
3. EndSection2
4. SlopeSection2
5. SlopeSection3
33 is defined by the settings:
1MRK 502 034-UEN -
Technical Manual