ABB REG670 - Operation Principle; Monitored Data

6.4.6 Monitored data
Table 66: REFPDIF Monitored data
Name
Type Values (Range) Unit Description
IRES REAL - A Magnitude of fund. freq.
residual current
IN REAL - A Magnitude of fund. freq.
neutral current
IBIAS REAL - A Magnitude of the bias
current
IDIFF REAL - A Magnitude of fund. freq.
differential current
ANGLE REAL - deg Direction angle from
zerosequence feature
I2RATIO REAL - - Second harmonic ratio
6.4.7 Operation principle
6.4.7.1 Fundamental principles of the restricted earth-fault protection
Restricted earth-fault protection, low impedance function (REFPDIF) detects earth
faults on earthed power transformer windings, most often an earthed star winding.
REFPDIF is a unit protection of the differential type. Since REFPDIF is based on the
zero sequence current, which theoretically only exists in case of an earth fault,
REFPDIF can be made very sensitive regardless of normal load currents. It is the
fastest protection a power transformer winding can have. 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 earth faults.
REFPDIF is a differential protection of the low impedance type. All three-phase
currents, and the neutral point current, must be fed separately to REFPDIF. 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 added to the neutral
current vectorially, in order to obtain differential current.
The following facts may be observed from Figure
61 and Figure 62, where the three
line CTs are shown as connected together in order to measure the residual 3I
o
current,
for the sake of simplicity.
Section 6 1MRK502052-UEN B
Differential protection
168
Technical manual