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Figure. 5.4.18. - 81. Transformer tap changer
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.
AQ-T257
Instruction manual
Version: 2.00
153
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