ABB RELION RET670 Applications Manual page 165

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
presence of heavy CT saturation. The principle is based on the CT secondary current
circulating between involved current transformers and not through the IED due to high
impedance in the measuring branch. This stabilizing resistance is in the range of
hundreds of ohms and sometimes above one kilo Ohm. When an internal fault occurs
the current cannot circulate and is forced through the measuring branch causing relay
operation.
It should be remembered that the whole scheme, its built-in components and wiring
must be adequately maintained throughout the lifetime of the equipment in order to be
able to withstand the high voltage peaks (that is, pulses) which may appear during an
internal fault. Otherwise any flash-over in CT secondary circuits or any other part of
the scheme may prevent correct operation of the high impedance differential relay for
an actual internal fault.
87N
ANSI05000164 V1 EN-US
Figure 54: Example for the high impedance restricted earth fault protection
application
For a through fault one current transformer might saturate when the other CTs still will
feed current. For such a case a voltage will be developed across the measuring branch.
Differential protection
en05000164_ansi.vsd
Section 7
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