ABB REG670 Applications Manual page 157

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