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Chapter 7 Page 28 of 64
2.4
High Impedance Restricted Earth Fault (64H)
Restricted earth fault (REF) protection can be applied to either or both windings of the transformer. The 7SR24
provides a high impedance REF (64H) element for each transformer winding. Low leakage reactance CTs (Class
PX) are required for use with high impedance protection systems.
Figure 2-17: REF Protection Applied to a Delta/Star Transformer
The zone of REF protection is defined by the position of the CTs and/or the transformer winding. REF protection
provides a low operate current (fault setting) for in zone earth faults and stability during external faults.
REF is more sensitive than overall biased differential protection (87BD) to earth faults it can protect against faults
for a greater portion of the transformer windings or where the impedance in the earth fault path is relatively high.
For a solidly earthed star winding, the REF function is roughly twice as sensitive in detecting a winding earth fault,
than biased differential protection.
The stability of high impedance REF schemes depends upon the operate voltage setting being greater than the
voltage which can appear across the element during the maximum assigned through fault conditions. To provide
the required operating voltage an external ‘stabilising’ resistor is wired in series with the 64H current measuring
input. A non-linear resistor is connected in parallel to protect the relay circuit against high over-voltages. REF
connections are shown in Figure 2-18.
Figure 2-18: AC Connections: REF
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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