Arcteq Generator Commander, Инструкция по эксплуатации

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The figure above presents the simulated behavior of a power transformer when overvoltage occurs. In
the simulation the transformer was unloaded on the secondary side while the voltage on the primary
side was increased with a ramp. The first graph depicts the excitation current, the 5
th
harmonic
component and their relation (which is used in the blocking); the green lines represents the suggested
setting limits for 5
th
harmonic detection (30 %, 35 %, and 40 %). The second graph depicts the
primary and secondary currents, plotted as a function of the voltage. The third graph depicts the
differential characteristics as well as the differential and bias currents.
As can be noted from the first graph, the 5
th
harmonic component begins increasing rapidly (compared
to the fundamental) in the start situation when the voltage is about 120 % of the nominal (depends
entirely on the transformer properties and its saturation characteristics). This behavior is common to all
transformers: when the core starts to be saturated there is a heavy amount of the 5
th
harmonic in the
magnetizing current. When the overvoltage exceeds a certain point in the magnetizing characteristics,
the 5
th
harmonic remains; however, the fundamental component of the current starts to grow very
rapidly and as a result the relation of the 5
th
harmonic to fundamental decreases rapidly as a function
of the primary side voltage. The growing magnetizing current is only seen on the transformer's primary
side and the differential relay sees it as pure differential current. From the third graph we can see that
the differential pick-up setting is reached when the voltage is approximately 125 % of the nominal
value. This means that the differential current generated by the overexcitation could trip the
transformer, as the ratio between the 5th harmonic and the fundamental magnitude decreases. If the
overvoltage were, for example, 130 % of the nominal value, no blocking would be available; even the
differential current would be greatly over the setting limit (appr. 40 % vs. the set 25 %). Nevertheless,
this behavior can still be considered to be correct for the power transformer because an overvoltage
like this can cause many serious problems and therefore tripping is desired.
The figures below present example waveforms of a transformer that is running with a 200 % rated
voltage with the corresponding ratio between the 5
th
harmonic and the fundamental frequency
component.
Figure. 5.4.27 - 151. Example waveforms.
Traditionally, the ratio between the 5
th
harmonic and the fundamental frequency component has been
used in blocking the differential relay from tripping in overvoltage and overexcitation situations.
However, the ratio is not a reliable method because you need to know the magnetizing properties and
the hysteresis values exactly in order to set it correctly and for it to be of any use.
The figures below present the system voltage and the magnitude of the 5
th
harmonic component (both
in per-unit), absolute and scaled to the transformer nominal.
Genera Generattor Commander or Commander
Instruction manual
Version: 2.07
282 © Arcteq Relays Ltd
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