ABB RER620, Техническое руководство

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Section 11
1MAC309294-MB F
Requirements for measurement transformers
550 RER620
Technical Manual
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
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
11.1.1.2 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 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 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 x (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 trip time delay caused by the CT saturation is
typically small enough when the overcurrent setting is noticeably lower than F
a
.
When defining the setting values for the low set stages, the saturation of the CT does not
need to be taken into account and the pickup current setting is simply according to the
formula.
F F
S S
S S
a n
in n
in
≈ ×
+
+