TME: Transformer magnetizing current is the current which flows in the primary winding. Since it is
running only in the primary side this needs to be taken into account of the settings calculation.
Magnetizing current value approximate can be calculated as follows:
This is the case if the primary inductance is known. Magnetizing current is compared to HV side
nominal current and the percentage is directly the TME value. If the transformers primary inductance is
not known then can be used conservative estimate of 3% for the transformer magnetizing effect.
SME: For conservative settings up to 5% safety margin is used typically.
Now we have all necessary data to calculate naturally generated differential current based into the
errors and possible variables known. Firstly the maximum uncertainty needs to be calculated from the
varying magnitudes known inside the transformer. In this case there is tap changer which affects
internal currents and its effect cannot be estimated on-line reliably because it may change any time.
For this reason the currents maximum uncertainty has to be calculated. If there is no tap changer
available just by summing the maximum inaccuracy of the HV and LV side CT is suf cient enough. Let’s
call the measurement uncertainty as IMEAS
UNC
.
Now when looking at how to ll the formula, it is needed to sum in the absolute maximum uncertainty of
the CT errors, tap changer maximum error and also the combined error of the secondary CT and tap
changer maximum error. On the absolute measurement affecting factor is known the expected value
as 1 xIn as well as it is correct that the tap changer is in maximum position thus causing the absolute
measurement to be 1 xIn + TCE
With our example con guration the calculation would be:
Now this value presents the transformer properties maximum caused differential current to nominal.
Other uncertainties known now may be added to this got value and after this operation can be said
that:
This means that in worst case this differential current will flow while the operation is still normal in the
transformer. Therefore normally this got combined result is increased with safety marginal to ensure
stabile operation of the differential protection and to negate possible calculation errors.
With these values following base sensitivity (e.g. the minimum setting for the differential current
required to operate the relay), is given for the differential protection characteristics:
AQ-T257
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
154
