what is inrush current in transformer?
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Answer Posted / manish shastri, elect guru
To understand this phenomena, we need to recall one
fundamental that "any situation that calls for
instantaneous change in flux linkage results in large
magnetising inrush current".
When a transformer is connected to a power system with its
secondary ckt open/ no load, assymetrical flux is in
quadrature with voltage (it actually lags the voltage by 90
Degree). The amount of flux produced depends on when
circuit breaker or switch is closed. Also, the assymetry of
flux will correspond to assemtry of voltage wave.
If breaker is closed at point when voltage wave is passing
thro it's ZERO, the assymetrical flux can attain the value
of 2 PHI MAX. If the value of residual flux at this time is
PHI (res), than total flux will be 2 PHI MAX + PHI (res).
If breaker is closed at point when voltage wave is passing
thro it's maximum, flux produced will be PHI MAX.
The linking of flux with the transformer winding will be
limited by core saturation and winding inductance. For flux
of level (2PHI MAX or PHI MAX), core will go into
saturation, meaning flux linking will be thorugh winding.
As flux in winding can not change instantaneously, it will
be linked thro air core inductance.
As air core inductance value is very small, to produce
required amount of flux in winding, the current required is
of very large value.
This can be easily understood by formula I=N*PHI/L. As L is
very less, magnetising inrush is very high.
The magnitude of the inrush current is limited by the air
core inductance of the windings under extreme saturation
conditions. A transformer with concentric windings will
draw a higher magnetising current when energised from the
LV side, since this winding is usually on the inside and
has a lower air core inductance.
Hope this explains the phenomena of magentising inrush
which many times lasts few seconds and goes as high as 6-10
times and consists mainly DC, 2nd Harmonics and 3rd
Harmonics.
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