76 6LE008146B
Advanced protection functions
5.8 Voltage unbalance protection (UNBV-47)
The voltage unbalance protection (UNBV-47), ANSI 47 permanently monitors the voltage
unbalances of each phase. When the unbalance is detected on one of the phases, the UNBV-
47 protection allows an alarm to be generated or the circuit breaker tripped.
The voltage unbalances in a 3-phase system are caused by the connection of too many
1-phase loads on a single phase. These unbalances can cause major problems for those
administering the electrical grid.
Voltage unbalances also have an influence on all three-phase equipment and in particular on
motors, because they then experience undesirable braking torque which then takes the form
of overheating. Any voltage unbalance greater than 2% results in overheating of equipment,
which makes it necessary for the latter to be overdimensioned to compensate for the
overheating and prevent premature degradation.
Operating principle
Current unbalance protection calculates the voltage unbalances for compound voltages, in
relation to the average current, expressed as a percentage:
U
nba
pg
[%] = 100 avec pg = 12, 23, 31 et
U
avg
*
U
pg
U
avg
_
U
avg
=
U
12 +
U
23+
U
31
3
It compares the percentage of each unbalance to the activation threshold for the protection.
Example of voltage unbalance on phase 2:
U12 U23 U31 Uavg
U23 - Uavg
>0
U12 - Uavg
<0
U31 - Uavg
<0
If the unbalance value of at least one of the voltages is greater than the activation threshold,
the protection trips at the end of the activation time delay.
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