4.3.6.6
Recommendations for current transformers
High-impedance principle
The sensitivity and reliability of the protection depend on the characteristics
of the current transformers. The CTs must have an identical transformation
ratio. It is recommended that all current transformers have an equal burden and
characteristics and that they are of the same type. This means that they should be
preferably from the same manufacturing batch, that is, an identical construction is
used. If the CT characteristics and the burden values are not equal, the calculation
for each branch in the scheme should be done separately and the worst-case result
is used. If the CT winding resistance and the burden of the branches are not equal,
the maximum burden equal to 3.2 Ω should be used for calculating the stabilized
voltage.
Figure 317: High-impedance differential protection with different CT burden value
on each branch
The stabilizing voltage, that is the voltage appearing across the measuring branch
during an out-of-zone fault, is calculated assuming that one of the CTs connected in
parallel is fully saturated. The stabilizing voltage can be calculated using the formula
U
I
n
R
R
s
k
in
m
=
+
max
(
)
(Equation 106)
I
kmax
The highest through-fault current in primary amps. The highest earth-fault or
short circuit current during the out-of-zone fault.
n
The turns ratio of the CT
R
in
The secondary internal resistance of the CT in ohms
R
m
The resistance (maximum of R
in
+ R
m
) of the CT secondary circuit in ohms
1MRS757644 H
Protection functions
620 series
Technical Manual
601
