2.7 Undervoltage protection U< (27)
2 Protection functions
Technical description
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VM55.EN005
2.7.
Undervoltage protection U< (27)
Enable_U_Under VS_U_Under
This is a basic undervoltage protection. The function measures
the three line-to-line voltages and whenever the smallest of
them drops below the user's pick-up setting of a particular
stage, this stage picks up and a start signal is issued. If the
fault situation remains on longer than the user's operation time
delay setting, a trip signal is issued.
Self blocking at very low voltage
The stages can be blocked with a separate low limit setting.
With this setting, the particular stage will be blocked, when the
biggest of the three line-to-line voltages drops below the given
limit. The idea is to avoid purposeless tripping, when voltage is
switched off. If the operating time is less than 0.08 s, the
blocking level setting should not be less than 15 % to the
blocking action to be enough fast. The self blocking can be
disabled by setting the low voltage block limit equal to zero.
Figure 2.7-1shows an example of low voltage self blocking.
A
The maximum of the three line-to-line voltages U
LLmax
is below the block limit. This is not regarded as an
under voltage situation.
B
The voltage U
LLmin
is above the block limit but below
the pick-up level. This is an undervoltage situation.
C
Voltage is OK, because it is above the pick-up limit.
D
This is an under voltage situation.
E
Voltage is OK.
F
This is an under voltage situation.
G
The voltage U
LLmin
is under block limit and this is not
regarded as an under voltage situation.
H
This is an under voltage situation.
I
Voltage is OK.
J
Same as G
K
Voltage is OK.
