Monitored Data; Operation Principle; Fundamental Principles Of The Restricted Ground Fault Protection - ABB RELION 670 Series Technical Manual

1MRK 502 066-UUS B
6.4.6

Monitored data

Table 76:
Name
IRES
IN
IBIAS
IDIFF
ANGLE
2NDHARM
6.4.7

Operation principle

6.4.7.1

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
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
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
shown as connected together in order to measure the residual 3I
simplicity.
Technical manual
REFPDIF (87N) Monitored data
Type Values (Range)
REAL
REAL
REAL
REAL
REAL
REAL
Unit
- A
- A
- A
- A
- deg
- -
rd
harmonic currents, are fully suppressed. Then the residual
64 and Figure 65, where the three line CTs are
Differential protection
Description
Magnitude of fundamental frequency
residual current
Magnitude of fundamental frequency
ground current
Magnitude of the bias current
Magnitude of fundamental frequency
differential current
Direction angle from zero sequence
feature
Second harmonic ratio
current, for the sake of
o
Section 6
PID-3772-MONITOREDDATA v4
IP16290-1 v2
M5447-3 v13
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