ABB Relion REC615, Technical Manual

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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
≈ ×
+
+
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
12.1.1.2 Non-directional overcurrent protection
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.
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 × (I
kmin
/ I
1n
)
I
1n
is the nominal primary current of the CT.
The factor 0.7 takes into account the protection relay inaccuracy, current
transformer errors, and imperfections of the short circuit calculations.
The adequate performance of the CT should be checked when the setting of the
high set stage overcurrent protection is defined. The operate time delay caused
by the CT saturation is typically small enough when the overcurrent setting is
noticeably lower than F
a
.
When defining the setting values for the low set stages, the saturation of the CT
does not need to be taken into account and the start current setting is simply
according to the formula.
Requirements for measurement
transformers
1MRS758755 C
832 REC615 & RER615
Technical Manual