In the application with a lower threshold setting of about 2.00 %, there is a risk of an overfunction due to the
measuring errors with small values as well as an influence via disturbances.
Parameter: Dropout ratio
•
Default setting (_:17071:4) Dropout ratio = 0.95
The default setting of 0.95 is appropriate for most applications if a higher threshold is used.
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 (_:17071:6) Operate delay = 3.00 s
Specify the Operate delay for the specific application. When using the sensitive setting of the threshold
value that is described in this chapter, the function can be delayed by 3.00 s.
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 6-197 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 6-197 Voltage Phasors during a 2-Phase Local Fault
A 2-phase local fault generates a relatively large negative-sequence voltage of up to 100 % referred to the
positive-sequence 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 % of the nega-
tive-sequence voltage to positive-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 Over-
current-protection stage when a fault occurs.
You can keep the default setting of 0.95 for the dropout ratio. This avoids chattering of the stage.
Set the Negative-sequence voltage stage so that it does not generate a fault when it picks up and does not
initiate tripping. The Overcurrent-protection stage generates a fault indication. The pickup of the Negative-
sequence voltage stage is used as the release criterion because the Short-circuit function must be released
Protection and Automation Functions
6.28 Overvoltage Protection with Negative-Sequence Voltage/Positive-Sequence Voltage
SIPROTEC 5, Overcurrent Protection, Manual 731
C53000-G5040-C017-8, Edition 07.2017
To Next Page
Loading...
