Name Type Values (Range) Unit DescriptionIDIFF REAL - A Magnitude of fundamentalfrequency differential currentANGLE REAL - deg Direction angle from zerosequence feature2NDHARM REAL - - Second harmonic ratio6.2.7 Operation principle6.2.7.1 Fundamental principles of the restricted ground fault protectionRestricted fault protection, low impedance function (REFPDIF, 87N) detects groundfaults on grounded power transformer windings, most often an grounded wye winding.REFPDIF (87N) is a winding protection of the differential type. Since REFPDIF (87N) isbased on the zero sequence current, which theoretically only exists in case of a groundfault, REFPDIF (87N) can be made very sensitive regardless of normal load currents. It isthe fastest protection a power transformer winding can have. Remember that the highsensitivity and the high speed tend to make such a protection unstable. Special measuresmust be taken to make it insensitive to conditions for which it should not operate, forexample, 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 neutralpoint current, must be fed separately to REFPDIF(87N). The fundamental frequencycomponents of all currents are extracted from all input currents, while other eventual zerosequence components, such as the 3rd harmonic currents, are fully suppressed. Then theresidual current phasor is calculated from the three line current phasors. This zerosequence current phasor is then added to the neutral current vectorially, in order to obtaindifferential current.The following facts may be observed from figure 44 and figure 45, where the three lineCTs are shown as connected together in order to measure the residual 3Io current, for thesake of simplicity.1MRK 502 048-UUS A Section 6Differential protection121Technical manual
