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Question
What is the difference between sensitive earth fault and 
restricted earth fault in power transformers?
 Question Submitted By :: Y_dhruv
I also faced this Question!!     Rank Answer Posted By  
 
  Re: What is the difference between sensitive earth fault and restricted earth fault in power transformers?
Answer
# 1
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.
 
Is This Answer Correct ?    14 Yes 1 No
Raghavendran
 
  Re: What is the difference between sensitive earth fault and restricted earth fault in power transformers?
Answer
# 2
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.
 
Is This Answer Correct ?    10 Yes 2 No
Raghavendran
 
 
 
  Re: What is the difference between sensitive earth fault and restricted earth fault in power transformers?
Answer
# 3
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
transformer
 
Is This Answer Correct ?    2 Yes 1 No
Surendar
 

 
 
 
Other Electrical Engineering Interview Questions
 
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name of good resistance material   1
what are the main parts of UPS what is the function of that? HCL 20
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Just a silly question but just to make my curious mind bac to normal - Why was not 67 and 67N relays built into a single relay as i have noticed that the R,Y,B CT wiring end up into the 67 relay while the common of the R,Y,B CT wiring end up into the 67N relay. Then it was better if a single relay was manufactured just like the distance relay (21). Another question i would like to ask is - how do we know where the 67 and 67 N relay has to be placed.why is it not placed at the transformer bay? In the substaion that i am working -i have noticed that it is placed between a breaker and a common double bus bar and this line bay is connected to another substation at a remote place which is furthur connected to a load. B U S______CT for 67 & 67N ______Breaker _____ remot s/s. B A R   1
what should be the minimum rating for using HT Motors? BSL 1
How we connect capacitors in Fixed Series compensation of AC transmission lines. Whether in parallel or in series?   3
I am getting differential earth fault in my transformer realy , i have checked all its CT & control wiring but do not get fault, Please help   6
How do you connect the capacitors on polyphase circuits?   4
i have to use generator in my project,which gen. i can use?   1
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