with:
t Tripping time
K Asymmetry factor
Ι
2
Negative sequence current
Ι
NObj
Rated current of the protective object
The asymmetry factor K designates how long a negative sequence current may flow at nominal machine
current. It is therefore the distinctive number of the object to be protected.
If the constantly permissible unbalanced load I2> is exceeded, the summation of the warming negative
sequence system power commences. In this context, the current-time-area is calculated constantly to ensure a
correct consideration of changing load cases. As soon as the current-time-area (Ι
2
/Ι
NObj
)
2
· t) has exceeded the
K asymmetry factor, the thermal characteristic is tripped.
The model of the heating of the object to be protected is limited to 200 % of the thermal tripping limit.
Cool Down, Drop-Out
The “pickup” of the unbalanced load protection falls back, when the allowable unbalanced load I2> is under-
shot. The thermal image maintains its state and an adjustable cool down time T COOL DOWN is started. In this
context, this cool down time is defined as the time required by the thermal replica to cool down from 100 % to
0 %. In synchronous machines this depends on the construction, especially the damper winding. If there is
again an asymmetrical loading during the cool-down phase, the previous history is considered. The tripping
time would then decrease considerably.
Resulting Characteristic
As the thermal replica only works after exceeding the permissible continuous negative sequence current I2>,
this value is the lower limit for resulting tripping characteristic (Figure 2-97). The area of the thermal replica
connects to it with increasing negative sequence current. High negative sequence currents can only be caused
by a phase-to-phase short circuit, which must be covered in accordance with the grading coordination chart.
The thermal characteristics are therefore cut off by the definite time Ι2>>-stage (see above under “Definite
time stage (DT)”). The trigger time of the thermal replica does not fall below the trigger time of the Ι2>>-stage
[resultierende-kennl-therm-schieflastschutz, 1, en_GB]
Figure 2-97 Resulting characteristic of the thermal asymmetrical load protection
Functions
2.8 Unbalanced Load Protection
SIPROTEC 4, 7UT6x, Manual 191
C53000-G1176-C230-5, Edition 09.2016
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