ABB RET670 Applications Manual page 153

1MRK504116-UUS C
Application manual
2000
(
= ° + ×
IP 200 0 3 50 60
5
EQUATION1887-ANSI V1 EN
where
200mA is the current drawn by the IED circuit and
50mA is the current drawn by each CT just at pickup
The magnetizing current is taken from the magnetizing curve for the current
transformer cores which should be available. The value at TripPickup is taken.
It can clearly be seen that the sensitivity is not so much influenced by the selected
voltage level so a sufficient margin should be used. The selection of the stabilizing
resistor and the level of the magnetizing current (mostly dependent of the number of
turns) are the most important factors.
Autotransformer differential protection
When Autotransformers are used it is possible to use the high impedance scheme
covering the Autotransformer windings, however not a possible tertiary winding. The
zone and connection of the 1Ph High impedance differential protection HZPDIF (87)
function is shown in figure 41.
)
- ° × - £
3
10 approx .100
Section 3
IED application
A
(Equation 38)
147