ABB RELION REF615R, Technical Manual

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The accuracy classes 5P and 10P are both suitable for non-directional overcurrent
protection. The 5P class provides a better accuracy. This should be noted also if there are
accuracy requirements for the metering functions (current metering, power metering, and
so on) of the protection relay.
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 relay.
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
13.1.1.2 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 protection relay 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
protection relay. For that reason, in practice, even a few times smaller nominal primary
current can be used than given by the formula.
Section 13
1MRS240050-IB C
Requirements for measurement transformers
622 REF615R
Technical Manual