ABB REC670 Applications Manual page 132

Section 7
Differential protection
126
R
Id Metrosil
IEC05000164 V3 EN-US
Figure 48: 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 measuring
branch. The calculations are made with the worst situations in mind and a
minimum operating voltage U
(
> × +
UR IF max Rct Rl
EQUATION1531 V1 EN-US
where:
IF max
is the maximum through fault current at the secondary side of the CT
Rct
is the current transformer secondary winding resistance and
Rl
is the maximum loop resistance of the circuit at any CT.
is calculated according to equation
R
)
1MRK 511 401-UEN A
IEC05000164-2-en.vsd
13
(Equation 13)
Bay control REC670 2.2 IEC
Application manual