6.4.6
Monitored data
PID-3772-MONITOREDDATA v4
Table 76: REFPDIF (87N) Monitored data
Name
Type
Values (Range)
Unit
Description
IRES
REAL
-
A
Magnitude of fundamental frequency
residual current
IN
REAL
-
A
Magnitude of fundamental frequency
ground current
IBIAS
REAL
-
A
Magnitude of the bias current
IDIFF
REAL
-
A
Magnitude of fundamental frequency
differential current
ANGLE
REAL
-
deg
Direction angle from zero sequence
feature
2NDHARM
REAL
-
-
Second harmonic ratio
6.4.7
Operation principle
IP16290-1 v2
6.4.7.1
Fundamental principles of the restricted ground fault protection
M5447-3 v13
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
unit 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. 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 a differential protection 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 added to the neutral current vectorially, in order to obtain differential current.
The following facts may be observed from Figure
, 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 502 066-UUS B
Section 6
Differential protection
169
Technical manual
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