1 2 3 4 A - SIPROTEC

Functions
2.8 Unbalanced Load Protection
2.8.1 Function Description
Determination of Unbalanced Load
Definite Time Stages
Inverse Time Stage
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The unbalanced load protection of 7UT6x filters fundamental components from applied the phase currents and
dissects them into their symmetrical components. Of this the negative sequence current of the system I 2 is
evaluated. If the largest of the three phase currents lies above the minimum current I-REST of the one
assigned side or measuring location and all phase currents are smaller than 4 times the rated current of the
assigned side or measuring location, then the comparison of negative sequence current and setting value can
take place.
The definite time characteristic is of two-stage design. When the negative sequence current exceeds the set
threshold I2> the timer T I2> is started and a corresponding pickup message is output. When the negative
sequence current exceeds the set threshold I2>> of the high-set stage the timer T I2>> is started and a
corresponding pickup message is output.
When a delay time is expired trip command is issued.
Figure 2-92 Tripping characteristic of the definite time unbalanced load protection
The inverse time overcurrent stage operates with a tripping characteristic either according to the IEC or the
ANSI standard. The characteristics and their equations are given in the „Technical Data“. The definite time
elements I2>> and I2> are superimposed on the inverse time curve.
SIPROTEC, 7UT6x, Manual
C53000-G1176-C230-2, Release date 06.2012