The unrestrained operation level has a default value of IdUnre = 10pu, which is
typically acceptable for most of the standard power transformer applications. In the
following case, this setting need to be changed accordingly:
• When CT from "T-connection" are connected to IED, as in the breaker-and-a-half
or the ring bus scheme, special care shall be taken in order to prevent unwanted
operation of transformer differential IED for through-faults due to different CT
saturation of "T-connected" CTs. Thus if such uneven saturation is a possibility it
is typically required to increase unrestrained operational level to IdUnre = 20-25pu
• For differential applications on HV shunt reactors, due to the fact that there is no
heavy through-fault condition, the unrestrained differential operation level can be
set to IdUnre = 1.75pu
The overall operating characteristic of the transformer differential protection is shown
in figure
25.
1MRK504116-UUS C Section 3
IED application
101
Application manual
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