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.
ANSI05000164 V1 EN
Figure 49: 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
V
R
is calculated according to equation
16
1MRK 506 369-UUS - Section 7
Differential protection
Line distance protection REL670 2.2 ANSI 137
Application manual
To Next Page
Loading...
