Figure. 5.3.9 - 118. Cable end differential when a fault occurs.
If a starting fault occurs in the cable end, the CED mode catches the difference between the ingoing
and the outgoing residual currents. The resulting signal can be used for alarming or tripping purposes
for the feeder with the failing cable end. The user can freely change both the settings and the
sensitivity of the algorithm.
Restricted earth fault protection is usually used in the Y winding of a power transformer. This function is
needed to prevent the main differential protection from being tripped by faults occurring outside the
protection area; in some cases, the function has to be disabled or its sensitivity limited to catch earth
faults inside the protection area. For this purpose, the restricted earth fault function is stable since it
only monitors the side it is wired to, and compares the calculated and measured residual currents.
During an outside earth fault the circulating residual current in the faulty phase winding does not
cause a trip because the comparison of the measured starpoint current and the calculated residual
current differential is close to zero.
A
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Q-T215
-T215
Instruction manual
Version: 2.04
162
