IEC05000165 V2 EN-US
Figure 52: The protection scheme utilizing the high impedance function for the
T-feeder
Normally this scheme is set to achieve a sensitivity of around 20 percent of the
used CT primary rating so that a low ohmic value can be used for the series
resistor.
It is strongly recommended to use the highest tap of the CT
whenever high impedance protection is used. This helps in utilizing
maximum CT capability, minimize the secondary fault current,
thereby reducing the stability voltage limit. Another factor is that
during internal faults, the voltage developed across the selected tap
is limited by the non-linear resistor but in the unused taps, owing to
auto-transformer action, voltages induced may be much higher than
design limits.
Section 7 1MRK 506 369-UEN B
Differential protection
140 Line distance protection REL670 2.2 IEC
Application manual
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