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P746/EN CM/F21 Commissioning and Maintenance
(CM) 10-20 MiCOM P746
IMPORTANT: THIS TEST SIMULATES A CURRENT GOING OUT AND A CURRENT
GOING IN THUS, THE CURRENT PHASE COMPARISON ALGORITHM
WILL PREVENT THE TRIP AS LONG AS THE SMALLEST INJECTED
CURRENT IS ABOVE X% OF THE NOMINAL CT CURRENT (IN):
The chosen I
bias
must be lower than 2 x x% x In / (1- k2)*.
X% is the percentage of In current flowing through each CT above which the angle is taken
into account and is defined in the Excel Setting spreadsheet tool.
Example with CT of 2000/1:
For example,if k2 is 60%, x% = 50% and if the CT ratio of the smallest injected current is
1000/1, I
biasmax
is 2500A (so ID>2 up to 1500 A).
For example, if k2 is 60%, x% = 50% and if the CT ratio of the smallest injected current is
500/1, I
biasmax
is 1250A (so ID>2 up to 750 A).
CM
For example,if k2 is 60% x% = 40% and if the CT ratio of the smallest injected current is
1000/1, I
biasmax
is 2000A (so ID>2 up to 1200 A).
For example, if k2 is 60% x% = 40% and if the CT ratio of the smallest injected current is
500/1, I
biasmax
is 1000A (so ID>2 up to 600 A).
To calculate the phase comparison threshold pct, pct = (I
1
– I
2
) / (I
1
+ I
2
)
The differential current will increase twice the value
Δ
I.
Note 1: I
D
>1 alarm timer will be set to 100s during the test.
To test the tripping time, inject 4
×
I
D
>2 at 60Hz and 3.5
×
I
D
>2 at 50Hz whenever possible,
in order to ensure subcycle tripping times
*: for information:
During the injection: I
bias
= I
1
+ I
2
thus I
1
= I
bias
- I
2
and I
2
= x% of In thus I
1
= I
bias
- x% of In
To trip, IDiff must be ≥ k2 x I
bias
, i.e. I
1
- I
2
≥ k2 I
bias
thus I
bias
- x% of In - x% of In ≥ k2 I
bias
Conclusion: I
bias
≥ 2 times x% of In /( 1 - k2)