Parameter Value Description
off
In the case of undervoltage, the pickup of the undervoltage-protection
stage persists when the current-flow criterion is not used.
Parameter: Threshold I>
•
Recommended setting value (_:2311:101) Threshold I> = 0.05 A
The Threshold I> parameter makes it possible to detect when the circuit breaker is closed. Siemens recom-
mends setting the Threshold I> parameter to 5% of the rated current. With a secondary rated transformer
current of 1 A, the secondary setting value for Threshold I> would be 0.05 A.
If the sensitivity of the Threshold I> parameter is set too high, compensation processes in the secondary
circuit of the current transformer extend the dropout time when breaking extremely high currents. To speed
up dropout, increase the default setting.
Operation as Supervision Function
If you want the tripping stage to have a reporting effect only, generation of the operate indication and fault
logging can be disabled via the Operate & flt.rec. blocked parameter.
EXAMPLE of a 2-stage undervoltage protection
The undervoltage protection can be used for the protection of motors within the power plant's own power
system. The example describes the possible settings for a two-stage undervoltage-protection function. We will
look at the settings of the parameters Threshold and Operate delay.
1. Stage:
Set the Threshold to approx. 80 % of the rated voltage. In the case of voltage dips down to that value, the
motors can still start up. Depending on the machine characteristic, the Threshold can also be set somewhat
lower.
The following data apply:
Rated motor voltage: V
rated,M
= 6 kV
Minimum startup voltage: 80 % of rated motor voltage
Voltage-transformer ratio:
When setting the threshold value, make sure that the positive-sequence voltage as defined is equal to the
value of one phase-to-ground voltage. Taking into account the rated primary voltage of the motor, the
primary setting value of the first stage is calculated as follows:
[fobglei1-190309-01.tif, 2, en_US]
When setting secondary values, you calculate the secondary setting value as follows, taking into account the
voltage-transformer ratio:
[fobglei3-190309-01.tif, 2, en_US]
Set a value of 3 s for the Operate delay.
2. Stage:
Protection and Automation Functions
6.30 Undervoltage Protection with Positive-Sequence Voltage
754 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
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