Now to get the HV volts minimum means that the calculation needs to be applied on situation when the
tap changer on secondary side is at maximum output voltage and the output is nominal. In this
example we had maximum of +12.5% increasing effect for the tap changer, and the result will be:
Next is calculated the currents flowing in this situation at HV and LV sides, when the loading of the
transformer is e.g. 1.5 times its rated power.
For LV side currents will be
And for HV side currents will be
By calculating this way these currents now present the worst possible case caused by tap changer
effect into the differential relay measured currents.
Now there is two possible ways to use biasing calculation and in practice one way to calculate
differential current (even there are add and subtract modes, the effect will be the same since the
differential current shall basically always be calculated as:
, thus giving the absolute difference in measured currents, add and subtract just compensate
differently connected CTs when the starpoint is either towards to transformer or away from it)
When more sensitive settings are wanted “Average” mode is selected
If more stabile settings are wanted “Maximum” mode is selected:
With “Average” mode selected the slope is calculated as follows:
AQ-T257
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
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