ABB 615 Series - 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
12.1.1.2 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.
Recommended pickup current settings
If I
kmin
is the lowest primary current at which the highest set overcurrent stage is to trip,
the pickup current should be set using the formula:
Current pickup value < 0.7 × (I
kmin
/ I
1n
)
Section 12 1MAC059074-MB A
Requirements for measurement transformers
1132 615 series ANSI
Technical Manual