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2.6 Undervoltage protection U1<
(27P)
2 Protection functions
Technical description
56
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VM55.EN005
2.6.
Undervoltage protection U
1
< (27P)
This a special undervoltage protection for generator
applications, where the voltage is measured at the generator
side of the generator circuit breaker. There are special self
blocking features for starting up and shutting down a
generator.
This undervoltage function measures the positive sequence of
fundamental frequency component U
1
of the measured voltages
(for calculation of U
1
see chapter 4.4). By using positive
sequence all the three phases are supervised with one value
and in case the generator looses connection to the network (loss
of mains), the undervoltage situation is detected faster than by
using just the minimum of the three line-to-line voltages.
Whenever the positive sequence voltage U
1
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.
Blocking during VT fuse failure
As all the protection stages the undervoltage function can be
blocked with any internal or external signal using the block
matrix. For example if the secondary voltage of one of the
measuring transformers disappears because of a fuse failure
(See VT supervision function in chapter 3.5). The blocking
signal can also be a signal from the user's logic (see chapter
5.8).
Self blocking at very low voltage
The stages will be blocked when the voltage is below a separate
low voltage blocking setting. With this setting, LVBlk, both
stages are blocked, when the voltage U
1
drops below the given
limit. The idea is to avoid purposeless alarms, when the
generator is not running. The LVBlk setting is common for
both stages. The self blocking can not be disabled.
Initial self blocking
When the voltage U
1
has been below the block limit, the stages
will be blocked until the pick-up setting has been reached.
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