ABB 650 series, Technical Manual

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Name Type Values (Range) Unit Description
IDIFF REAL - A Magnitude of fundamental
frequency differential current
ANGLE REAL - deg Direction angle from zero
sequence feature
2NDHARM REAL - - Second harmonic ratio
6.2.7 Operation principle
6.2.7.1 Fundamental principles of the restricted ground fault protection
Restricted fault protection, low impedance function (REFPDIF, 87N) detects ground
faults on grounded power transformer windings, most often an grounded wye winding.
REFPDIF (87N) is a winding protection of the differential type. Since REFPDIF (87N) is
based on the zero sequence current, which theoretically only exists in case of a ground
fault, REFPDIF (87N) can be made very sensitive regardless of normal load currents. It is
the fastest protection a power transformer winding can have. Remember that the high
sensitivity and the high speed tend to make such a protection unstable. Special measures
must be taken to make it insensitive to conditions for which it should not operate, for
example, heavy through faults of phase-to-phase type or heavy external ground faults.
REFPDIF(87N) is of the low impedance type. All three-phase currents, and the neutral
point current, must be fed separately to REFPDIF(87N). The fundamental frequency
components of all currents are extracted from all input currents, while other eventual zero
sequence components, such as the 3
rd
harmonic currents, are fully suppressed. Then the
residual current phasor is calculated from the three line current phasors. This zero
sequence current phasor is then added to the neutral current vectorially, in order to obtain
differential current.
The following facts may be observed from figure 44 and figure 45 , where the three line
CTs are shown as connected together in order to measure the residual 3I
o
current, for the
sake of simplicity.
1MRK 502 048-UUS A
Section 6
Differential protection
121
Technical manual