Non-Directional Overcurrent Protection - ABB REF615 Applications Manual

Section 11
Requirements for measurement transformers
11.1.1.2
226
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
factor (F ) and is proportional to the ratio of the rated CT burden and the actual CT
n
burden.
The actual accuracy limit factor is calculated using the formula:
S S
+
in n
F F
≈ ×
a n
S S
+
in
A071141 V1 EN
F the accuracy limit factor with the nominal external burden S
n
S the internal secondary burden of the CT
in
S the actual external burden

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
recommended to select a CT with F
The nominal primary current I
dynamic strength of the current measuring input of the relay is not exceeded. This is
always fulfilled when
I > I / 100,
1n kmax
I is the highest fault current.
kmax
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 is the lowest primary current at which the highest set overcurrent stage of the
kmin
relay is to operate, then the start current should be set using the formula:
Current start value < 0.7 x (I
I is the nominal primary current of the CT.
1n
) differs from the rated accuracy limit
a
) of the CTs. It is, however,
a
of at least 20.
a
should be chosen in such a way that the thermal and
1n
/ I )
kmin 1n
1MRS756378 D
n
REF615
Application Manual