ABB REG670 - Page 157

Id
IEC05000164-2-en.vsd
R
Metrosil
IEC05000164 V3 EN
Figure 59: 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
R
is calculated according to equation 21
( )
max
UR IF Rct Rl> × +
EQUATION1531 V1 EN
(Equation 21)
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.
1MRK 502 071-UEN - Section 7
Differential protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 151
Application manual