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Chapter 7) 7SR23 DAD Applications Guide
©2013 Siemens Protection Devices Limited
Chapter 7 Page 11 of 42
2.1.5
Restricted Earth Fault Protection (87REF)
Restricted earth fault (87REF) protection can be applied to various primary plant connection arrangements, Figure
2-3 shows REF protection applied to both HV and LV windings of a transformer. The 7SR23 provides up to two
high impedance REF elements.
Figure 2-3
AC Connections – Balanced/Restricted Earth Fault Protection
The zone of REF protection is defined by the position of the CTs and the transformer winding. REF protection
provides a low operate current (fault setting) for in zone earth faults and stability during external faults.
To achieve stability of the high impedance REF elements and limit overvoltages a series stabilising resistor and a
non-linear resistor are wired into the scheme.
The operating voltage of the relay/stabilising resistor combination is calculated taking into account the r.m.s. value
of the symmetrical component of the transformer through fault current.
The relay current setting is calculated taking into account: the required operate level for in-zone earth faults (fault
setting).
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