Parameter: Dropout ratio
•
Default setting (_:271:4) Dropout ratio = 0.95
The default setting of 0.95 is appropriate for most applications.
You can decrease the dropout ratio to avoid chattering of the stage if the threshold value is low. For example,
for the stage with a 2 % setting, you can use a dropout ratio of 0.90.
Parameter: Operate delay
•
Default setting (_:271:6) Operate delay = 3.00 s
Specify the Operate delay for the specific application. 3.00 s is a practicable value.
For a higher threshold value, a shorter tripping delay is required.
Operation as Supervision Function
If you want the 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 1:
Releasing an overcurrent protection stage for unbalanced faults
The following section describes how to set the function to release an Overcurrent-protection stage when
unbalanced faults occur. Set the Overcurrent-protection stage only slightly higher than the load current, that
is very sensitive. To prevent the Overcurrent-protection stage from picking up inadvertently, the Overcur-
rent-protection stage is released when the Negative-sequence voltage stage picks up. The Overcurrent-
protection stage remains blocked as long as the Negative-sequence voltage stage has not picked up.
Figure 6-190 shows the voltage phasors during a 2-phase local fault between phases B and C. The phase-to-
phase voltage V
BC
is virtually 0.
[dwuazeig-020412-01.tif, 1, en_US]
Figure 6-190 Voltage Phasors during a 2-Phase Local Fault
A 2-phase local fault generates a relatively large negative-sequence voltage of up to 50 % referred to the
phase-to-ground voltage. The portion of the negative-sequence decreases in case of a remote fault. The lower
setting limit results from the possible unbalance at full load. If you assume for example 5 % negative-
sequence voltage, the pickup value must be higher. A setting value of 10 % warrants sufficient stability during
unbalanced operating states and sufficient sensitivity to release the Overcurrent-protection stage when a
fault occurs.
For a secondary rated voltage of 100 V, set the following secondary threshold value:
[fo_OVP_V2_Secondary Threshold, 1, en_US]
You can keep the default setting of 0.95 for the dropout ratio. This avoids chattering of the stage.
Protection and Automation Functions
6.26 Overvoltage Protection with Negative-Sequence Voltage
718 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
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