while the T-zone is protected with a separate differential protection scheme. The 1Ph high
impedance differential HZPDIF (87) function in the IED allows this to be done efficiently,
see Figure 53.
ANSI05000165 V1 EN
Figure 53: 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-UUS -
Differential protection
146 Line distance protection REL670 2.2 ANSI
Application manual
To Next Page
Loading...
