With “Maximum” mode selected the slope is calculated as follows:
Now to be on safe side for this may be added yet another safety margin if so wished (even the base
sensitivity settings include 5% already) to ensure stability.
One setting is still missing and it is the Slope 2. This setting is used for biasing the differential
characteristics against heavy outside of differential zone faults which can cause heavy saturation on
one or both sides (LV,HV) CTs causing heavy differential current in to measurements even the
transformer itself does not have fault. There is one catch in the settings.
If there is heavy single end in zone fault thus causing the biasing current also to increase, this value
should not be “up to max” since it may mean that the biasing blocks the differential characteristics so
that the trip will not be applicable even there is in zone fault on single end.
Now when thinking the possible situation when feeding is from HV side and the differential current is
directly the fault feeding end current following can be noted for the Slope 2 setting.
If in case of using “Average” mode for biasing (in case of single end fault) the bias current will be
, and the differential current will be directly
and in case of using “Maximum” mode for biasing (in case of single end fault) the bias current will be
same than the differential current so Slope 2 shall be calculated as:
Now the biased characteristic is set. Next should consider settings for the non biased/non restraint
stage I
di>Pick-up
.
The purpose of this stage is to ensure fast and also selective tripping of faults inside differential zone,
yet stabile operation on heavy outside faults. This stage operates only on absolute differential current
amount measured and is not blocked with harmonics or bias restraints.
Setting of the stage should be based into the weakest CT full saturation under worst case through fault
condition (since this causes that only the other side current is measured then and that causes all seen
current to be differential current).
AQ-T257
Instruction manual
Version: 2.00
157
© Arcteq Relays Ltd