Table of Contents
Advertisement
Undervoltage protection
U< (27)
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.
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 "Voltage
Transformer Supervision" in Chapter 4). The blocking signal can
also be a signal from the user's logic (see "Logic Functions" in
Chapter 6).
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 3.27 shows an example of low voltage self blocking.
A
The maximum of the three line-to-line voltages U
is below the block limit. This is not regarded as an
under voltage situation.
B
The voltage U
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
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.
Protection Functions
is above the block limit but below
LLmin
is under block limit and this is not
LLmin
3-53
LLmax
857-UM001A-EN-P – July 2009
Advertisement
Table of Contents
