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2.15
Undervoltage Protection (ANSI 27)
Undervoltage protection detects voltage dips in electrical machines and avoids inad-
missible operating states and possible loss of stability. Two-pole short circuits or earth
faults cause asymmetrical voltage collapse. Compared with three single phase mea-
suring systems, the detection of the positive phase-sequence system is not influenced
by these procedures and is particularly advantageous for assessing stability problems.
2.15.1 Functional Description
Mode of Operation
For the above reasons the positive sequence system is calculated from the fundamen-
tal waves of the three phase-earth voltages, and fed to the protection function.
Undervoltage protection consists of two stages. A pickup is signalled as soon as se-
lectable voltage thresholds are undershot. A trip signal is transmitted if a voltage
pickup exists for a selectable time.
In order to ensure that the protection does not accidentally pick up due to a secondary
voltage failure, each stage can be blocked individually or both stages together, via
binary input(s), e.g. using a voltage transformer mcb. In addition to this, the integrated
fuse failure monitor (FFM) blocks both stages (see Section 2.28).
If a pickup occurs as the device changes to operational condition 0 - i.e. no usable
measured quantities are present or the admissible frequency range has been exited -
this pickup is maintained. This ensures tripping even under such conditions. This seal-
in can be retracted only after the measured value has reverted to a value above the
drop-off value or by activation of the blocking input.
If no pickup exists before the device is in operating status 0 (thus e.g. on switchon of
the device without available measured values), no pickup and no tripping occurs. An
immediate tripping may be caused on transition to operating status 1 (i.e. by applica-
tion of measured values). For this reason it is recommended that the blocking input of
the undervoltage protection is activated via the circuit breaker auxiliary contact, thus
for example blocking the protective function after a protection tripping.
The following figure shows the logic diagram for undervoltage protection.
7UM61 Manual
C53000-G1176-C127-3
2.15 Undervoltage Protection (ANSI 27)
103
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