Differential protection operate on difference of primary
and secondary current. Practically load is not balanced and
supply system too and so there will be always current
difference in normal running condition. To avoid
malfunction (during charging also) always differential
protection setting set to protect 70 to 80% zone. 20 to 30%
zone left unprotected with only differential protection.
By using REF, it is possible to protect 20 to 30%
unprotected zone without any maloperation.
Restricted earth fault protection is used for zonal
protection for transformer, generator etc. For earth fault
protection we set 10% or 20% earth fault current hence we
leave 10-20% of winding unprotected. By using REF we provide
protection for that remaining portion of winding.
Differential protection operate on difference of primary
and secondary current. One must note down that construction
of primary and secondary winding keeps them physically
separated. Any earth fault requires to be cleared faster
than line fault/shortcircuit. Difference of current in
primary and secondary is very less in case of internal
So when any fault happens in the transformer say primary
side bushing, due to high set point of differential which
is used for detecting 20% unbalancing, it will trip in more
time or might not trip. To avoid any immediate damage
within the transformer we put REF to detect and trip it
quicker than differential since it works on difference of
current detected by neutral CT and common of all line CT.
Mostly it is used for Dyn11 since star of secondary is
REF IS USED TO PROTECT THE SECONDARY WINDING (WYE
CONNECTED). IT GIVES SENSITIVE PROTECTION AS COMPARED TO
THE DIFF PROTECTION.
TO PREVENT MAL-OPERATION OF DIFF PROTN (DUE TO CT ERRORS /
DIFF IN SATURATION CONDITTIONS ETC) AS WELL AS THE
MIN.SETTINGS POSSIBLE IN THE RELAY, DIFF PROTN. CANNOT
OFFER SENSITIVE PROTN.
whats the basic concept is to provide accurate protection
against fault for the system!
to do so we need to consider the extreme part from top of
the phase to neutral (as potential near neutral and earth is
low the most chances of such fault is to occur near the
neutral). now if the fault occurs near the neutral point,
the covered winding between the fault point and neutral
produce small magnitude of the fault current;
to measure that small signal current we require extreme
sensitive relay system!
if we use such then it ll also detect the unnecessary faults
such as inaccuracy of C.T or saturation of C.T etc. to avoid
malfunction: this is the reason why we protect only 80 to 85
% of winding and left 15% of winding from neutral side
so only the differential protection is not suffice!!
REF: to protect this remaining 15% we need other type of
earth fault protection scheme(neutral connected through
resistor or neutral connected via transformer)! (restricted
earth fault protection is designed to 15% of rated phase
First of all, REF protection is used for the case of
internal faults of the transfomer.Secondly, in REF, usually
when the current flows back form the neutral to the source,
there is much larger current change in the neutral current
as compared to the phase currents.And Differential
protection usually works senses the changes in the phase
currents which is not much larger, that's why differential
protection and REF protection are totally different from
each other.each of them is having different significance
and used for a particular application.
REF Protection- There may be number of feeders down-
stream on the T.F.So if an earth fault occurs in one
of such feeders, that fault should not disturb the
transformer.If the transformer is not protected with REF
what happens is, everytime there is a fault some where
on it's feeders,the T/F will trip and all feeders will
be affected. This will go on happening from feeder to
feeder and consumers get in to trouble unnecessarily. But
with REF ,the transformer will trip only when there is
earthfault within T/F itself, which very very seldom
only occurs.In other words T/F tripping restriced to fault
within cases only.But the case of out-going feeders is
not so safe as to avoid earth fault.They may be in
a jungle where tree branches can touch the conductor
and cause a tripping.So T/F with REF will not trip in
such situation and power interruption will be only in
the feeder affected.
A Restricted Earth Fault (REF) means an earth fault from a
restricted/localized zone of a circuit.
The term "REF protection method " means not to sense any
earth faults outside this restricted zone.
REF is a type of "unit protection" applied to transformers
or generators and is more sensitive than the method known as
An REF relay works by measuring the actual current flowing
to earth from the frame of the unit. If that current exceeds
a certain preset maximum value of milliAmps (mA) then the
relay will trip to cut off the power supply to the unit.
Differential protection can also be used to protect the
windings of a transformer by comparing the current in the
power supply's neutral wire with the current in the phase
wire: if the currents are equal then the differential
protection relay will not operate; if there is a current
imbalance then the differential protection relay operates.
However, REF protection is also applied to transformers in
order to detect ground faults on a given winding much more
sensitively than differential protection can do.
Differntial protection is only use to protect winding
internal faults, It may be used in Generator or
transformer, it protect the paritcular inbetween zone while
we placed CT's. REF protection always used at star winding
side to protect nearby winding of star point which we can
not protect by residual earth fault.
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Can a 50-Hz designed transformer be used at 60-Hz system?
Is the same remains true, when a 60-Hz designed transformer
be used at 50-Hz system? Consider only frequency variation.
The voltage remains as rated value in all cases.
Justify the answer.