[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
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