Areva MiCOM P441, Technical Manual

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 81/220
It follows that, for an open circuit in a particular part of the system, I2/I1 can be determined
from the ratio of zero sequence to negative sequence impedance. It must be noted however,
that this ratio may vary depending upon the fault location. It is desirable therefore to apply as
sensitive a setting as possible. In practice, this minimum setting is governed by the levels of
standing negative phase sequence current present on the system. This can be determined
from a system study, or by making use of the relay measurement facilities at the
commissioning stage. If the latter method is adopted, it is important to take the
measurements during maximum system load conditions, to ensure that all single phase
loads are accounted for.
Note that a minimum value of 8% negative phase sequence current is required for
successful relay operation.
Since sensitive settings have been employed, it can be expected that the element will
operate for any unbalance condition occurring on the system (for example, during a single
pole autoreclose cycle). Hence, a long time delay is necessary to ensure co-ordination with
other protective devices. A 60 second time delay setting may be typical.
The following table shows the relay menu for the Broken Conductor protection, including the
available setting ranges and factory defaults:-
Menu text Default setting Setting range Step size
Min Max
GROUP 1
BROKEN CONDUCTOR
Broken Conductor Enabled Enabled/Disabled N/A
I2/I1 0.2 0.2 1 0.01
I2/I1 Time Delay 60 0s 100s 1s
I2/I1 Trip Disabled* Enabled Disabled N/A
*
If disabled, only a Broken Conductor Alarm is possible.
2.16.2 Example Setting
The following information was recorded by the relay during commissioning;
I
full load
= 1000A
I
2
= 100A
therefore the quiescent I2/I1 ratio is given by;
I
2
/I
1
= 100/1000 = 0.05
To allow for tolerances and load variations a setting of 200% of this value may be typical:
Therefore set I2/I1 = 0.2
Set I2/I1 Time Delay = 60s to allow adequate time for short circuit fault clearance by time
delayed protections.