1 2 3 4 A - SIPROTEC

Functions
2.8 Unbalanced Load Protection
222
Example:
Machine I N = 483 A
Current transformer 500 A/5 A
results under address 4033 in the secondary value
I2> = 0.53 [A].
I 2perm = 11 % (salient-pole machine)
This permanently permissible negative system current is simultaneously the pickup threshold for the thermal
asymmetrical load protection and also the limit for the asymmetrical load warning stage. The delay of the
warning indication can be set under address 4033 T WARN. Usually approx. 20 s.
The asymmetry factor FACTOR K (address 4034) is a measure for the thermal stress of the rotor. It indicates
the time for which 100 % asymmetrical load are permissible and corresponds with the permissible thermal
energy loss (K = (I/I N) 2 · t). It is indicated by the machine manufacturer or it can be seen on the asymmetrical
load diagram of the machine.
In example, figure 2-97, the permanently permissible asymmetrical load amounts to 11 % of the machine
internal current and the K-factor K = 20. As the relevant measuring location for asymmetrical load is usually
assigned to the side of the machine to be protected, the setting can be effected directly under address 4034
FACTOR K:
FACTOR K = 20.
Figure 2-97 Example of a pre-defined asymmetrical load diagram
If, however, the asymmetrical load protection must be set in amps secondary during operation, also the K-factor
must be converted as it refers to the machine internal current. The following applies:
SIPROTEC, 7UT6x, Manual
C53000-G1176-C230-2, Release date 06.2012