The main characteristics of magnetising inrush currents are:
● Higher magnitude than the transformer rated curr
ent magnitude
● Containing harmonics and DC offset
● Much longer time constant than that of the DC offset component of fault current
We can see that inrush current is a regularly occurring phenomenon and should not be considered a fault, as we
do not wish the protection device to issue a trip command whenever a transformer is switched on at an
inconvenient point during the input voltage cycle. This presents a problem to the protection device, because it
should always trip on an internal fault. The problem is that typical internal transformer faults may produce
overcurrents which are not necessarily greater than the inrush current. Furthermore, faults tend to manifest
themselves on switch on, due to the high inrush currents. For this reason, we need to find a mechanism that can
distinguish between fault current and inrush current. Fortunately, this is possible due to the different natures of the
respective currents. An inrush current waveform is rich in harmonics, especially 2nd harmonics, whereas an
internal fault current consists only of the fundamental. We can therefore develop a restraining method based on
the 2nd harmonic content of the inrush current. The mechanism by which this is achieved, is called second
harmonic blocking.
2.7 OVERFLUXING RESTRAINT
Sometimes the protected transformer is subject to overfluxing due to temporary overloading with a voltage in
excess of the nominal v
oltage, or a reduced voltage frequency. For example, when a load is suddenly
disconnected from a power transformer, the voltage at the input terminals of the transformer may rise by 10-20%
of the rated value. Since the voltage increases, the flux also increases. As a result, the transformer steady state
excitation current becomes higher. The resulting excitation current flows in one winding only and therefore
appears as differential current which may rise to a value high enough to operate the differential protection. A
typical differential current waveform during such a condition is as follows.
Figure 39: Typical overflux current waveform
S
uch waveforms have a significant 5th harmonic content. We can therefore develop a restraining method based
on the 5th harmonic content of the inrush current. The mechanism by which this is achieved, is called fifth
harmonic blocking.
Chapter 6 - Transformer Differential Protection P64x
106 P64x-TM-EN-1.3
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