Application Guide - NP900 Series
321 (504
)
A996A
then current should flow to relay input and if there is enough difference it would cause
pick-up and trip.
This is not the case with NP900 differential relay, which does this
transformation by calculating internally the corrected vectors.
Now the direction of the CTs starpoint on the HV side and LV side has effect on how to
set the differential calculation method. In NP900 differential relay this is possible to set
either “add” or to “subtract” which means that the CTs “current direction” has to be taken
into account. Now in this example the correct setting would be “add” since the CTs in the
main circuit are connected to opposite each other “the starpoints / groundings are
“opposite” thus the measured currents from the CTs are opposite to each other. Now this
is again up to the user which way the signals are wanted to be shown, it is possible also to
negate the CTs currents when the “subtract” mode can be used for differential calculation
and the both sides measurements could be shown as one star connected vector diagram.
As mentioned now the differential algorithm itself, it has calculating formula per each
phase difference:
Subtracting formula:
L1DIFF
Subt
= |IL1
HV
��������
-IL1
LV
�������
|
L2DIFF
Subt
= |IL2
HV
��������
-IL2
LV
�������
|
L3DIFF
Subt
= |IL3
HV
��������
-IL3
LV
�������
|
Or Additive formula:
L1DIFF
Add
= |IL1
HV
��������
+ IL1
LV
�������
|
L2DIFF
Add
= |IL2
HV
��������
+ IL2
LV
�������
|
L3DIFF
Add
= |IL3
HV
��������
+ IL3
LV
�������
|
Can be selected based into the CTs connections.
Differential function has 2 separate stages inbuilt which of the non restraint uses only
these formulas as comparison base. For the restraint characteristics also so called “Bias”
calculation is made per each phase to adjust the sensitivity of the differential stage up to
the measured currents.
For the bias calculation also two separate formulas are available.
Average mode (sensitive biasing):
Содержание NP900 Series
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Страница 49: ...Application Guide NP900 Series 49 504 A996A Figure 3 2 1 3 25 Definite time operating characteristics...
Страница 50: ...Application Guide NP900 Series 50 504 A996A Figure 3 2 1 3 26 IEC predefined characteristics NI VI LTI and EI...
Страница 52: ...Application Guide NP900 Series 52 504 A996A Figure 3 2 1 3 28 IEEE predefined characteristics EI MI and VI...
Страница 243: ...Application Guide NP900 Series 243 504 A996A Figure 3 3 2 1 122 NPS biased thermal trip curves with kNPS values 7 and 10...
Страница 491: ...Application Guide NP900 Series 491 504 A996A 3 7 5 6 SETTING EXAMPLE...
Страница 504: ...Application Guide NP900 Series 504 504 A996A A996A...