What is the difference between sensitive earth fault and
restricted earth fault in power transformers?
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Answer Posted / raghavendran
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.
The conductor
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
microprocessor relays.
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