Xformer inrush current

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electrics

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some questions:
is there an inrush current while switching off like the one occuring during switch on? I guess no but what happens to the residual flux in the core? does it decay with time or always there ?

inrush currents is said to be having second harmonics is it true? what can be said the harmonics level of this currents? , if so (namely it has harmonics in it )is there any current accumulating on neutral wire as an imbalance current coming from phases?
 
Inductors (and transformers have significant inductive effects) tend to try to maintain a constant flow of current.

When you open the circuit on an inductor, it will try to maintain current flow, and cause a _voltage_ spike, which will try to arc across the switch. This is one of the reasons that switches and contactors will have different ratings for resistive and inductive loads.

Since three phase transformer primaries are usually connected line-line, there can never be current added to the neutral.

If you have a single phase transformer connected line to neutral, all of its current shows up on the neutral.

If you had multiple single phase transformer connected line to neutral, and turned them all on at the same time, then I would expect some inrush current to show up on the neutral, but calculating this would not be trivial.

In general, only 3x harmonics add on the neutral.

-Jon
 
Westinghouse ( now ABB ) makes an electro-mechanical protective relay called an HU & HU-1 used on Medium Voltage that takes advantage of the fact that transformer inrush current is rich in second harmonics.

The "HU" has a Harmonic Restraint Unit & it prevents the relay from tripping the breaker feeding the transformer by using the second harmonic content of the inrush current to differentiate between inrush & fault current.
 
what are the harmonics levels in an usual inrush current? if they have triplens so it means this will show up on the neutral of the transformer true?
 
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