The CT accuracy primary limit current describes the highest fault current
magnitude at which the CT fulfils the specified accuracy. Beyond this level, the
secondary current of the CT is distorted and it might have severe effects on the
performance of the protection IED.
In practise, the actual accuracy limit factor (F
a
) differs from the rated accuracy
limit factor (F
n
) and is proportional to the ratio of the rated CT burden and the
actual CT burden.
The actual accuracy limit factor is calculated using the formula:
F
F
S
S
S
S
a
n
in
n
in
≈
×
+
+
A071141 V1 EN
F
n
the accuracy limit factor with the nominal external burden S
n
S
in
the internal secondary burden of the CT
S
the actual external burden
4.1.1.2
Non-directional overcurrent protection
The current transformer selection
Non-directional overcurrent protection does not set high requirements on the
accuracy class or on the actual accuracy limit factor (F
a
) of the CTs. It is, however,
recommended to select a CT with F
a
of at least 20.
The nominal primary current I
1n
should be chosen in such a way that the thermal
and dynamic strength of the current measuring input of the IED is not exceeded.
This is always fulfilled when
I
1n
> I
kmax
/ 100,
I
kmax
is the highest fault current.
The saturation of the CT protects the measuring circuit and the current input of the
IED. For that reason, in practice, even a few times smaller nominal primary current
can be used than given by the formula.
Recommended start current settings
If I
kmin
is the lowest primary current at which the highest set overcurrent stage is to
operate, the start current should be set using the formula:
Current start value
< 0.7 x (I
kmin
/ I
1n
)
I
1n
is the nominal primary current of the CT.
Section 4
1MRS757054 A
Requirements for measurement transformers
60
REU615
Application Manual
