1 2 3 4 A - SIPROTEC

Functions
2.19 Monitoring Functions
2.19.1.3 Monitoring of Measured Quantities
Current Symmetry
290
The device detects and signals most of the interruptions, short-circuits, or wrong connections in the secondary
circuits of current or voltage transformers (an important commissioning aid!). The measured quantities are
periodically checked in the background for this purpose, as long as no system fault is present.
In a healthy three-phase system, a certain ssymmetry of the currents is assumed. The monitoring of the
measured values in the device checks this balance for each 3-phase measuring location. For this, the lowest
phase current is set in relation to the highest. An unbalance is detected, e.g. for measuring location 1, when
|I min|/|I max| < BAL. FACT. I M1 as long as I max/I N > BAL. I LIMIT M1/I N.
I max is the highest of the three phase currents and I min the lowest. The symmetry factor BAL. FACT. I M1
represents the allowable asymmetry of the phase currents while the limit value BAL. I LIMIT M1 is the lower
limit of the operating range of this monitoring (see Figure Current Symmetry Monitoring). Both parameters can
be set. The dropout ratio is about 95%.
Current balance monitoring is available separately for each 3-phase measuring location. For single-phase
differential busbar protection this function would not be of any use and is thus disabled. Unsymmetrical
condition is indicated for the corresponding measuring location e.g. with the alarm „Fail balan. IM1“ (No
30110). At the same time, the common annunciation appears: „Fail I balance“ (No 163).
Figure 2-115 Current symmetry monitoring
SIPROTEC, 7UT6x, Manual
C53000-G1176-C230-2, Release date 06.2012