Restricted Earth Fault Protection - Toshiba GRT100 Series Instruction Manual

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2.4 Restricted Earth Fault Protection

Restricted earth fault protection (REF) is a zero-phase current differential scheme and applied for
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 residual current derived from the three-phase line currents and the neutral current in
the neutral conductor as shown in Figure 2.4.1
REF
Figure 2.4.1 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 models to provide 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 2.4.2 shows the scheme logic of 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.
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