Sensitive Earth fault protection: Normally used on systems
where the earth fault current is limited to a very low
value. Scheme consists out of a ring CT (Core Balance CT)
around the three phase conductors, and the relay connected
to the secondary side of the CT. A very low setting is
possible, even sometimes lower then 1% FLA. Need to
coordinate with similar relays downstream. This method is
based on primary current vector addition or flux summation.
cables pass through the center hole of the core balance CT.
Secondary current is the system zero-sequence current
3I_0.This scheme provides increased sensitivity with no
false residual currents. However,
care must be taken to prevent false tripping from unbalanced
inrush currents that may
saturate the current transformer core.
residual Earth Fault Relay:Restricted Earth Fault Protection
Is a type of zone of protection, or a differential scheme.
You get two types, low impedance and high impedance. Low
impedance is normally current operated and high impedance
voltage operated. High impedance REF is the most commonly
used. Relay connected in parallel with three phase CTs and
one neutral CT. It will look just for faults inside the
transformer, and need not to coordinate with relays
downstream (can operate very fast). With this kind of
protection you can protect over 90% transformer windings.
Need to use special class CTs, previously called class X
CTs.low impedance REF, are incorporated into some the new
A Restricted Earth Fault (R.E.F.) has nothing to do with the
magnitude of the current. It is in fact a normal earth fault
that is restricted to a 'zone'. That is, the R.E.F.
protection is only looking for earth faults occuring in a
given zone. Any earth fault outside the zone (restricted
area) is ignored by the R.E.F. protection. It is typically
used on power transformers, with the protected zone
typically being between the star-point current transformer
and the phase transformers of the star-wound winding.
Restricted earth fault function only protect the specified
zone .But sensitive earth fault function can protect all
earth faults(only applicaple for after start of ct point).
both protection analog inputs taking from neutral ct of
power transformer.but in ref taking ct reference for all
phases of ct for making protection of specified area .
setting of ref lesser then sef.sef also back up for ref.
both protection only applicable for star side for