Generally tap changer means that the transformer transformation ratio can be adjusted in order to
receive nominal voltage more accurately to the secondary side of the transformer. Reasons for voltage
variations may be many for example heavy or light loading in the high voltage side. In practice this
means that if more or less voltage is needed in the secondary side, more or less winding rounds in
secondary side are utilized. This causes difference into the nominal current condition which can be
noticed as differential current in the relay. Normally tap changer positions are presented as deviation
steps of secondary voltage per step into + and – direction from the center which gives nominal output
voltage.
AUTE: In this example there is 50kVA auxiliary transformer connected to the LV side output before the
CTs so it has to be taken into account for the differential base sensitivity calculations. Same goes if in
the transformer itself is found auxiliary power output and its currents are not measured.
To calculate auxiliary power output effect, calculate the percentage of auxiliary transformer/winding VA
to transformer nominal VA.
This represents the case when the auxiliary load is in nominal.
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
AQ-T216
Instruction manual
Version: 2.00
133
© Arcteq Relays Ltd
