THE PHYSICS of . . . . MAGNETIZATION-CURRENT INRUSH

[Tony S]

Just to give a practical example, at least once a week I had to perform regular isolation and re-energising of a 1850KVA 11/3.3KV transformer. FLC 97A @11KV, re-energising would send the current to 300A+.

The strange thing is a 2600KVA 11/.66KV transformer caused hardly any ?kick? with inrush. This would also be switched once a week.

All I can put it down to is the core construction and age. The 1850KVA was installed 25 years before the 2600KVA. To say they had a hard life would be an understatement, both worked at near 90% FLC of their time on load.

 

[gar]

150416-2555 EDT

A basic equation to this problem is the differential equation:

v_source = i*R + K*N*dPhi/dt

v is the driving source voltage
i is the magnetizing current
R is the series loop resistance, mostly the transformer coil resistance
K is a scaling constant
N is the number of turns on the transformer coil
dPhi/dt is the rate of change of flux relative to time
Phi is a nonlinear function of the coil current i

For discussion purposes one can drop the i*R term to make the problem easier to understand.

v_source . is the driver of what happens
i has to be whatever is necessary at any time for this equation to balance

As the core goes into saturation the current has to grow at a greater rate than flux to balance the equation, because it takes a greater change of current to produce a given change of flux. Saturation is not one specific point for most ferromagnetic materials, but is a gradually changing slope to the the magnetizing curve.

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