Application Guide - NP900 Series
323 (504
)
A996A
Diff
Bias
TP1
…
TP2
= SL1 × (TP2-TP1) + I
d>Pick
-
up
Diff
Bias>TP2
= SL2 × (Ix-TP2) + SL1 × (TP2-TP1) + I
d>Pick
-
up
, thus forming a straight line from zero current to TP1 (Turn point 1). From there to TP2
(Turn point 2) is the first slope which causes the set biasing to be more coarse when the
measured current amplitude increases. When measured current is higher than TP2 set
value, slope 2 is used.
1.1.1.5
S
ETTINGS OF THE DIFFERENTIAL CHARACTERISTICS
To set the characteristics for the transformer application first needs to be known what
these parts of the characteristics mean.
First part is the
Diff
Bias<TP1
= I
d>Pick
-
up
the first straight line which represents the
transformer normal operation created differential current. This part takes into account
measurement errors, transformer possible tap changer (load side) caused current
variation, and possible other application caused reasons for different load inside the
protected differential zone. In NP900 differential relay this is known as pick-up
current
I
d>Pick
-
up
, a basic sensitivity limit which below measured differential current the
transformer still operates normally and the differential protection should not activate. In
other words
I
d>Pick
-
up
must be higher than all these differential current causing normal
operation factors combined.
When calculating the basic normal situation differential current, following image illustrated
parts should be considered. These transformer components errors need to be taken into
account.
