Now when comparing the corrected CT ALF factors to estimated maximum through fault currents can
be seen that the current will not saturate CT:s since they can repeat on HV side 21.6 xIn current while
the calculated HV current in through fault will be maximum of 20.2 xIn. In the LV side also the
maximum output current will be 20.2 xIn when the LV side CT is able to repeat 23.5 xIn current.
From this notation can be expected that the through fault will not be causing problems with this power
transformer and CT combination. Thus this note the non biased differential stage can be set to operate
sensitively in in-zone faults. If the CTs would have possibility to saturate (calculated through fault
current is bigger than the ALF of either side CT) the setting of the instant stage should be set high
enough so that it will not operate on through fault saturation.
For setting of the instant stage should be considered the inrush peak current also, with normal power
transformer the energizing inrush current may be 10 xIn peak, while the measured current is fft- ltered
for fundamental component which is used for differential calculation the found differential current shall
be 50% of the maximum peak current typically. If the setting should be according to theoretical
m margin, then 5 xIn + margin should be considered for the instant stage. For conservative
settings 10 xIn can be used. This value should never cause trips for energizing and yet it will operate
fast on in energisation fault cases (this stage usually is never blocked in application, thus it’s settings
should be considered on absolute differential current possible on normal operation yet it should be
sensitive also for inrush currents especially on energisation cases).
Setting suggestion for this I
di>Pick-up
stage is 6.0 xIn … 10 xIn for sensitive and conservative operation.
Now basic settings for the differential stages are applied and basically the differential protection is
ready to operate. In this example the transformer used is very small, however the formulas presented in
this manual can be applied to any size power transformers. In the TRF module, relay calculates these
settings automatically if so wanted. Relay uses exactly these same formulas for the setting calculations.
AQ-T257
Instruction manual
Version: 2.00
161
© Arcteq Relays Ltd