Non-Directional Overcurrent Protection - ABB REF611 Applications Manual

Section 4
Requirements for measurement transformers
4.1.1.2
170
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 protection 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
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 is the lowest primary current at which the highest set overcurrent stage is to
kmin
operate, the start current should be set using the formula:
Current start value < 0.7 × (I
I is the nominal primary current of the CT.
1n
The factor 0.7 takes into account the protection relay inaccuracy, current transformer
errors, and imperfections of the short circuit calculations.
) 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
1MRS757456 D
n
REF611
Application Manual