The internal resistance is related to what is the CT rating and how long is the winding length and also
what is the used winding wire dimension. However the internal burden of the ct should be considered
minority in the calculations since majority of the CT burden in typical relay application comes from the
wirings. If the secondary burden is known then of course it should be used (some CT manufacturers
include this information in their end test documentation).
In this example let’s assume for the HV side CT the internal resistance to be 0.05Ω, it is rated 5VA and
for the LV CT internal resistance to be 0.09Ω also rated 5VA. Wiring from the HV side to relay is 10m
and from the LV side 5m, both sides have 30% of wiring made with 6-wire connection and 70% of
wiring with 4-wire connection. Wirings of HV and LV sides are made with 4 mm
2
wires.
HV side: 150/5A 10P10 5VA
LV side: 1200/5A 5P10 5VA
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.
AQ-T216
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
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