ABB Relion 611 Series, Technical Manual

Page: 498 / 536

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
10.1.1.2 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 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
.
Section 10
1MRS757454 D
Requirements for measurement transformers
492 611 series
Technical Manual