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withstand the high voltage peaks (that is, pulses) which may appear during an internal fault.
Otherwise any flash-over in CT secondary circuits or any other part of the scheme may prevent
correct operation of the high impedance differential relay for an actual internal fault.
Id
IEC05000164-2-en.vsd
R
Metrosil
IEC05000164 V3 EN-US
Figure 32: 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
(
)
max
UR
IF
Rct Rl
>
×
+
EQUATION1531 V1 EN-US
(Equation 14)
where:
IFmax
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.
The minimum operating voltage has to be calculated (all loops) and the IED function is set
higher than the highest achieved value (setting
U>Trip). As the loop resistance is the value to
1MRK 505 343-UEN B
Section 6
Differential protection
95
Application manual
Summary of Contents for Relion 670 series
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