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WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT
DEIFFERENTIAL PROTECTION SUFFICE???
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Answer Posted / lalit parashar
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
earth fault.
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
physically grounded.
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