If in the cable end should occur any starting faults the cable end differential catches the difference in
between of the ingoing and outgoing residual currents and the resulting signal can be used for
alarming or tripping purpose for the feeder with failing cable end. The sensitivity of the algorithm and
settings are freely user settable.
Restricted earth fault protection is normally used in Y winding of a power transformer. This function is
needed to prevent the main differential protection from tripping in outside protection area faults in some
cases it has disabled or limited sensitivity to catch inside area earth faults. For this purpose, restricted
earth fault function is stabile since it monitors only the side it is wired to and compares the calculated
and measured residual currents. In case of outside earth fault the circulating residual current in the
faulty phase winding is not causing tripping because the comparison of measured starpoint current
and calculated residual current differential is close to zero.
Figure. 5.4.8. - 44. Restricted earth fault on outside of Y winding transformer.
If the fault is located inside of the transformer and thus inside of the protection area the REF function
catches the fault with high sensitivity since the measured residual current directions are now opposite
for the outside fault situation the measured differential current is high.
AQ-T257
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
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