Non-Directional Overcurrent Protection - ABB Relion 615 series Applications Manual

Section 4
Requirements for measurement transformers
4.1.1.2
40
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
limit factor (F ) and is proportional to the ratio of the rated CT burden and the
n
actual CT 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
and 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
kmin
the 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
a
of at least 20.
a
should be chosen in such a way that the thermal
1n
/ I )
kmin 1n
1MRS756885 A
n
) of the CTs. It is, however,
a
REM615
Application Manual