ABB Relion 611 Series User Manual

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
4.1.1.2 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.
Section 4 1MRS757456 D
Requirements for measurement transformers
170 REF611
Application Manual

ABB Relion 611 Series Specifications

General

Brand	ABB
Model	Relion 611 Series
Category	Protection Device
Language	English