ABB REF615 - Non-Directional Overcurrent Protection

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
11.1.1.2 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 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
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 of the
relay is to operate, then 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 11 1MRS756378 D
Requirements for measurement transformers
226 REF615
Application Manual