Figure. 5.4.18. - 79. Differential function characteristic, biased and non-biased.
In the characteristics the red line presents the allowed differential current in percentage in function of
measured biasing current of the differential protection. In this example the non-biased pick-up is set to
quite low what it would be in normal transformer application. Settings and ranges of the differential
protection are presented in the settings chapter.
The biasing characteristic is formed with following formulas:
,thus forming a straight line from zero current to TP1 (Turn point 1). From there to TP2 (Turn point 2) is
the rst slope which causes the set biasing to be coarser when the measured current amplitude
increases. When measured current is higher than TP2 set value, slope 2 is used.
Differential characteristics settings
To set the characteristics for the transformer application rst needs to be known what these parts of
the characteristics mean.
First part is the Diff
bias<TP1
= I
d>pick-up
the rst 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 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.
AQ-T257
Instruction manual
Version: 2.00
151
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