transformers in the feeder circuit (for example, in the transformer bushings). It is often
required to separate the protection zones that the feeder is protected with one scheme
while the T-zone is protected with a separate dif
ferential protection scheme. The 1Ph
high impedance differential HZPDIF (87) function in the IED allows this to be done
efficiently, see Figure
58.
3·87
ANSI05000739-2-en.vsd
ANSI05000739 V2 EN-US
Figure 58: 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 504 163-UUS A
Differential protection
168 Transformer protection RET670 2.2 ANSI
Application manual
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