As can be noted from the graphs the 5
th
harmonic component starts to increase rapidly in comparison
to the fundamental in the start situation when the voltage is about 120% of nominal (this is related
completely to the transformer properties and its saturation characteristics). This behavior is anyway
common to all transformers, when the core starts to saturate there will be heavy amount of 5
th
harmonic in the magnetizing current. When the overvoltage exceeds certain point in the magnetizing
characteristics, 5
th
harmonic is there still but in the other hand the fundamental component of the
current starts to grow very rapidly, which causes that the relation of the 5
th
harmonic to fundamental
will be decreasing rapidly in function of the primary side voltage. Thus the magnetizing current grows it
is seen only in the primary side of the transformer and is seen by differential relay as pure differential
current. In the last graph is seen the differential pick-up setting reached in situation when the voltage is
about 125% of the nominal. This means that in this case the over excitation generated differential
current could trip the transformer in this point, however the fth harmonic component relation ratio to
fundamental magnitude is already decreasing in this point and when considering what would happen if
the overvoltage would be e.g. 130%, it would mean that there is no blocking available even the
differential current would be highly over the setting limit (~40% vs. setting in this case 25%). This
behavior still can be considered to be correct for the power transformer since overvoltage like this can
cause more serious problems and therefore tripping is desired.
Figure. 5.4.10. - 114. Example waveforms of the transformer running with 200% rated voltage with corresponding 5
th
harmonic
to fundamental ratio.
Traditionally the relation of 5
th
harmonic to fundamental has been used for blocking the differential relay
from tripping in overvoltage/over excitation situations. Based into the ratio check this however is not
very failsafe way in order that to set it correctly and so that it could be more of use the magnetizing
properties and hysteresis of the transformer should be completely known.
AQ-T216
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
152
Содержание AQ 200 Series
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