Inrush for two transformers in series

[electrofelon]

Is there anything interesting to discuss on this? Transformer A feeds transformer B. Is the inrush additive?

 

[xptpcrewx]

Is there anything interesting to discuss on this? Transformer A feeds transformer B. Is the inrush additive?

Interesting question. From a Steinmetz equivalent circuit model standpoint, you would have the inrush from transformer A in addition to any secondary “load current” reflected over to the primary side. If the secondary“load current”happened to be inrush from transformer B, I could see this as being additive. In reality it’s probably more complicated and dampens out.

 

[gar]

210902-2403 EDT

electrofelon:

An interesting question.

The simple answer is yes. However, transformer inrush current is determined by the residual flux state of the transformer after turn off, and the phase angle of the applied voltage when power is applied to the transformer primary.

So we need to figure out how the second transformer turns off, and thus what its residual flux is likely to be. This may not be as high as would occur if the first transformer was not there. Also note that residual flux is dependent upon what load is on a transformer secondary at turn off.

My guess is that worst case peak inrush won't be as high as 2 x the peak for one transformer for two transformers where one is cascaded from the other.

.

 

[Hv&Lv]

No.

If your question were transformers in parallel I would be closer to saying yes, but not a definite yes..
Typical inrush currents may be 10X rated current. Saying 10X + 10X isn’t going to be the answer.


There are studies that show inrush from energizing one transformer is about 25% less than energizing 10-15 series transformers.
A few variables..
Magnetizing currents in a transformer are nonlinear.

You have to take into account things like the residual flux in the transformer core, what is the variable there?
The characteristics of the transformer core have to be accounted for. This is almost impossible to model..

The phase of the source when energizing the transformer, or point on wave. Inrush mitigation is the reason for point on wave switching, Also used is second harmonic blocking on relays to stop the nuisance tripping on 87 protection.

Another consideration is the impedance and short circuit power of the supplying source. This may be the most difficult to model.


So it comes down to what cable is feeding the first transformer? (Z)
Short circuit of the circuit
core characteristics of the first transformer, with what value of residual flux
Z of the second cable, with the next transformer and those variables, etc…

ALSO a new dynamic is introduced now with a second (or more) transformers added. This first transformer(X1) inrush helps to lessen the inrush on the second transformer(X2) as X1 now becomes a bottleneck for down line inrush currents.

 

[electrofelon]

Interesting. I kinda figured it was a complicated problem and difficult if not impossible to model. From some field experience, I suspected it was less than additive because I've had a few setups where I would expect problems if it was.

 

[electrofelon]

There are studies that show inrush from energizing one transformer is about 25% less than energizing 10-15 series transformers.

Wow interesting.