The usual practice for transformer protection is to set the bias characteristic to a
value of at least twice the value of the expected spill current under through-fault
conditions. These criteria can vary considerably from application to application.
The default settings for the operating characteristic with IdMin = 30% of the
railway power transformer rated current can be recommended in normal
applications. If the conditions are known in more detail, a higher or a lower
sensitivity can be chosen. In such cases, the selection of the suitable characteristic
should be based on the class of the current transformers, the short-circuit power of
the systems, and so on.
The second section of the restrain characteristic has an increased slope in order to
deal with the increased differential current during heavy loading of the transformer
and external fault currents.
The third section of the restrain characteristic further decreases the sensitivity of
the restrained dif
ferential function in order to cope with CT saturation during heavy
through faults.
The unrestrained operation level has a default value of IdUnre = 800% of the
railway power transformer rated current, which is typically acceptable for most of
the standard applications. The overall operating characteristic of the transformer
differential protection is shown in Figure
30:
Section 6 1MRK 506 375-UEN A
Differential protection
80 Railway application RER670 2.2 IEC
Application manual
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