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.
Id
IEC05000164-2-en.vsd
R
Metrosil
IEC05000164 V3 EN-US
Figure 48: 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. The calculations are made with the worst situations in mind and a
minimum operating voltage U
R
is calculated according to equation 13
1MRK 506 369-UEN B Section 7
Differential protection
Line distance protection REL670 2.2 IEC 131
Application manual
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