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

1MRK 502 048-UUS A
6.1.3.6
Technical manual
Removing the zero sequence current from the differential currents decreases to some
extent the sensitivity of the differential protection for internal ground-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
capable 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. The operate - restrain characteristic is
represented by a double-slope, double-breakpoint diagram, where the operating current is
set against the bias current, as shown in figure
following 5 settings:
1. IdMin (Sensitivity in section 1, multiple of trans. Reference side rated current set
under the parameter IBase in GlobalbaseSelW1)
2. EndSection1 (End of section 1, as multiple of transformer reference side rated current
set under the parameter IBase in GlobalbaseSelW1)
3. EndSection2 (End of section 2, as multiple of transformer reference side rated current
set under the parameter IBase in GlobalbaseSelW1)
4. SlopeSection2 (Slope in section 2, as multiple of transformer reference side rated
current set under the parameter IBase in GlobalbaseSelW1)
5. SlopeSection3 (Slope in section 2, as multiple of transformer reference side rated
current set under the parameter IBase in GlobalbaseSelW1)
Differential protection
32 The characteristic is determined by the
Section 6
99