Answer / ramji

The differential protection is used for protecting the
transformer from internal winding fault,internal winding
shorts etc.,.It operates through three sets of CT's
connected in and out of transformer.It may or may not
operate during minor earth faults.REF is used to detect the
earth fault at restricted zone where its protection CT is
connected.It will surely operate during earth fault.But not
in windings short.

Answer / gunasekaran

both are using for internal fault but differential
protection only sensitive up to 92% winding of the
transformer if any fault occur in balance 8% winding diff is
not operating so we r using REF protection

Answer / nikhil thakkar

REF and Differential both pritection are for internal zone and SEF for external zone. Actual all protections are voltage/current/power based and they are back to each other. DIFFERENTIAL AND REF both protection is for internal fault i.e trafo winding . differential sense unbalance between both end of trafo. /Gen. . REF sense unbalance caused between NCT and phase CTdue to earth fault abd actual backup for REF is SEF.

Answer / ashish

as per my knowledge ref protections hav to be assigned
separately on both sides of traffo it protects 90% of
winding.
it is unit protection and its very sensitive.

while talking about diff protection two cts in each phase
on both sides of trafo is there any diff in incoming and
outgoing current will leads to trip so both are equally
required.

Answer / raveendra nath

Differential protection provides excellent protection for
phase- to-phase and most phase - to - earth faults, this
element is less sensitive for single- phase-to-earth faults
close to the earth point in solidly earthed transformers.

For these faults (i.e fault at transformer winding turns
nearer to neutral point ), phase current changes very
little, but large current flows in the neutral.

Answer / gunasekaran

at internal earth fault condition diffrentional sensitive
only 85% only remaining 15% covered by REF protection

Answer / munish

REF 64T is used for protect the 95% winding of T/F.it sens
only the internal fault of T/F and 51NT circulating earth
fault relay used for detect the other area.

87T diff. is act only when the difference B/W both side
current is more than setting value

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