Rockwell Automation 865 Differential Protection Relay - Manual (Page 143)

Chapter
8
865-UM001A-EN-P – July 2009
Applications
Restricted earth fault (REF) protection is a sensitive way to protect
a zone between two measuring points against earth faults. See
Figure 8.1.
Note: The CT secondaries are wired to
cancel each other's currents during through
faults and to drive all to the relay when the
fault is inside the protected zone. (Saturation
of the CTs makes the situation a little more
complicated than that.) The stabilizing
resistor R
S guarantees that the relay will not
trip during a through fault. The VDR is used
to protect the CTs and the wiring by limiting
the voltage V
S during heavy inside faults.
Figure 8.1 – Principle of restricted earth fault protection
When there is a fault outside the protected zone the CT secondaries
will cancel each other's currents. This is partly true even if both or
only one of the CTs saturates, because the impedance of a saturated
CT secondary will collapse to near zero. The non-zero wiring
impedance and CT impedance will however cause a voltage V
S
, but
the resistor R
S
will prevent the relay from tripping. RS is called the
stabilizing resistor.
During an inside fault the secondary currents of the two CTs have no
other way to go than the relay. The relay will trip when the current I
= V
S
/R
S
exceeds the setting I
S
of the relay. The voltage dependent
resistor (VDR, varistor, METROSIL) is used to protect the CTs and
wiring by limiting the voltage V
S
during heavy inside faults.
The resistance of the secondary loop connecting the CTs together
should be as low as possible.
Restricted Earth Fault
Protection