ABB RET670 Applications Manual page 142

Section 3
IED application
136
87N
ANSI05000164 V1 EN
Figure 36: 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 stabilising resistor.
The calculations are made with the worst situations in mind and a minimum operating
voltage V is calculated according to equation
R
(
> × +
VR IF max Rct
EQUATION1531-ANSI V1 EN
where:
IF max
is the maximum through fault current at the secondary side of the CT
Rct is the current transformer secondary resistance and
Rl
is the maximum loop resistance of the circuit at any CT.
34
)
Rl
1MRK504116-UUS C
en05000164_ansi.vsd
Application manual
(Equation 34)