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[forbanl1-030812-02.tif, 1, en_US]
With:
t
I2Perm
Permissible time of the negative-sequence current
K
Unbalanced-load factor of the machine (parameter
Unbalanced load factor K
)
I
2
/I
rated,machine
Actual unbalanced-load current as a per unit value(negative-sequence current/rated
current of the machine)
[dwunbalo-230913, 2, en_US]
Figure 6-177
Operate Curve of the Unbalanced-Load Protection
The parameter
Unbalanced load factor K
depends on the machine. It also represents the time in
seconds during which the machine can be loaded with a 100 % unbalanced load. This factor is typical in a
range from 5 s to 40 s. The heating of the protected object starts to calculate as soon as the negative-
sequence current I
2
exceeds the parameter
Max. continuously perm. I2
. The operate time is calculated
from the flowing negative-sequence current I
2
with an integrating method of measurement depending on the
thermal characteristic. As soon as the integrate result reaches the parameter
Unbalanced load factor K
,
the thermal characteristic stage operates.
Limitation of Operate Curve
To avoid an overfunction of the thermal characteristic stage in the situation of unbalanced short circuits (for
example, 2-phase short circuit), the input current I
2
is restricted to an upper limit. The limiting value is 10
times of
Max. continuously perm. I2
. If I
2
exceeds the limit, the operate time of the function is
constant.
In addition, the thermal memory is limited to 200 % of
Unbalanced load factor K
. The limitation avoids
a too long cooling after a delayed short-circuit tripping (for example, of external power-system faults).
Protection and Automation Functions
6.22 Unbalanced-Load Protection
688
SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
Summary of Contents for 7SJ82
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