ABB RELION 670 Series Applications Manual page 124

Section 6
Differential protection
Id
IEC05000164 V3 EN-US
Figure 39: 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
>
UR IF
EQUATION1531 V1 EN-US
where:
IF max
Rct
Rl
The minimum operating voltage has to be calculated (all loops) and the IED function is set
higher than the highest achieved value (setting
the connection point from each CT, it is advisable to do all the CT core summations in the
switchgear to have shortest possible loops. This will give lower setting values and also a better
balanced scheme. The connection in to the control room can then be from the most central
bay.
118
R
Metrosil
( )
× +
max Rct Rl
is the maximum through fault current at the secondary side of the CT
is the current transformer secondary winding resistance and
is the maximum loop resistance of the circuit at any CT.
IEC05000164-2-en.vsd
19
U>Trip ). As the loop resistance is the value to
1MRK 504 152-UEN B
R
(Equation 19)
Application manual