Multiple Transformer Inrush

[veman]

HI - We have a customer with 10, 1 MVA transformers connected to our utility system at 13.8 kV three phase. Our current practice for fuse sizing is to assume that the inrush current would be the same as for 1 - 10 MVA transformer. Does anyone know if the total inrush for multiple transformers can be reduced due to the fact that there are multiple connections with smaller transformers?? Our protection people state that the fuse will blow when the feeder auto recloses, however they have had multiple relcoses over the years with no such issues.....
Any information or reference would be appreciated.
Thank you,
Veman.

 

[tinfoil]

The magnitude of the inrush is a direct function of the xmfr's impedance in Ohms. Ten 1-MVA xmfrs will likely have a different Ohmic value than one 10-MVA xmfr.

You'll have to use actual xmfr's %Z values to figure out the actual impact here. Don't forget to allow for the impednaces of the primary cables/wires as well.

 

[jghrist]

tinfoil,
Do you have any references relating inrush to transformer impedance? Usually it is shown as a multiple of full load current.

 

[veman]

thanks tinfoil. jghirst - I don't have any references that relate inrush to total impedance. I've used past practice of 10 x full load current for 0.1 sec. to calculate inrush current. Impedances are typically 4.5 % for the 1 MVA transformers and 5-10 % for the 10 MVA transformer from my recollection....

veman

 

[stevenal]

10X full load for all the transformers on a typical distribution feeder would be huge number. We size transformer fuses for inrush, and upstream protection to coordinate. Hot load inrush has not been a problem. I am not sure how to prove it mathematically.

 

[tctctraining]

Transformer Inrush depends on many variables. if this is a steady state calculation , you need to calculte SC current at transformer bus and consider this as maximum availabe inrush current at that location.
The point on wave form that switching happens, Remanence flux and transformer design can affect all this.

 

[racobb]

For inrush we usually consider it to be 6X full load for .10 to .15 seconds.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[jghrist]

Transformer Inrush depends on many variables. if this is a steady state calculation , you need to calculte SC current at transformer bus and consider this as maximum availabe inrush current at that location.

But if you then design your protection not to trip under inrush, it won't trip for the SC current. Not good.

 

[racobb]

What does short circuit current have to do with transformer magnetizing inrush current?

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[lansford]

If the short circuit current is high (a strong system), then the inrush will be higher than if the system is weak. These terms are all relative to the fault current vs the tranformer size. JIM

 

[veman]

Thank you all for the responses. Stevenal - thanks for the info on coordination - this is how I would have thought to approach a system with multiple transformers instead of adding up the total kVA connected and treating this as one large transformer and trying to assume a total inrush amount.

Thank you again....

Veman.