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2.3 Restricted Earth Fault Protection
Restricted earth fault protection (REF) is a zero-phase current differential scheme applied to a
star-connected winding whose neutral is earthed directly or through a low impedance. It gives
highly sensitive protection for internal earth faults.
REF employs a low impedance current differential scheme which detects the differential current
between the zero-sequence current I
0
derived from the three-phase line currents and the neutral
current I
N
in the neutral conductor as shown in Figure 2.3.1.
REF
I
N
Ia+Ib+Ic
Figure 2.3.1 Restricted Earth Fault Protection
REF and the overall differential protection DIFT use the three-phase line currents in common.
GRT100 has two or three REF elements depending on the model, providing separate protection
for all star-connected and neutral-earthed windings.
The elements have the same percentage restraining characteristics and are stable for all faults
outside the protected zone.
Figure 2.3.2 shows the block diagram of the REF element which is composed of REF_DIF and
REF_DEF. The REF_DIF has a percentage restraining characteristic while the REF_DEF
provides a directional check feature to discriminate between internal and external faults. When the
REF_DEF is “ON”, the REF_DEF element is used. The REF_DEF element provides additional
security against incorrect operation of the REF element in the event of saturation of the neutral
CT. The REF_DEF is blocked when the maximum phase current exceeds 2 × kct × (Rated current
of neutral CT), since the REF element is used for earth fault protection of transformer winding.
For details, see Section 2.10.3. In case of terminal current larger than that, the DIFT element
provides tripping. The REF_DEF can be disabled by setting the scheme switch [REF_DEF] to
“OFF”.
internal fault detection
REF_DIF
Ires≦2.0×Max_kct
REF
&
REF_DEF
&
[REF_DEF]
OFF
ON
≥1
+
Figure 2.3.2 Block Diagram of REF
Figure 2.3.3 shows the scheme logic of the restricted earth fault protection when three REF
elements are applied. Each REF element can perform instantaneous or time-delayed tripping of up
to five breakers. Any of the five breaker tripping signals 1REF-1 to 3REF-5 are enabled or
disabled by the scheme switch [1REF1] to [3REF5] settings.
Note: Models 203 and 204 are not provided with 1REF-4, 1REF5, 2REF-4, 2REF-5, 3REF-4 and