Should secondary protection consider xfmr inrush current?

[Dvhez]

Hi everyone,

I’m setting up protection for a 13.8 kV to 400 V transformer and wondering how much inrush current should influence secondary-side protection. I know primary protection (relays, fuses) is typically set to tolerate inrush, but what about MCCBs or relays on the 400 V side?

At the secondary I have a Micrologic 2.3 which doesn't allow me to add time delay in order to avoid the transformer inrush current. The only way is to increase the Short-Time pickup, but doing this I "cross" all the other devices that are in the primary side.


Thanks!

 

[stevenal]

Do you have a source on the 400 V side?

 

[Dvhez]

Do you have a source on the 400 V side?

No, the source is on the 13.8 kV side.

 

[stevenal]

Then the only inrush you need to cover will be from the down line transformers.

 

[Dvhez]

Then the only inrush you need to cover will be from the down line transformers.

Thank you Stevenal. I'm sorry for asking, but did you mean that I need to set the primary side devices above the inrush curve meanwhile the devices on the secondary side (load) are not required to be above this inrush current in a TCC graph?

If so, do you know which standard (US) talk about this?

 

[stevenal]

Yes. I know of no standard, just physics. Inrush comes from the source side.

 

[waross]

Think about it;
The primary inrush is fed from the grid.
Where would an inrush on the secondary go?

 

[davidbeach]

Hint: It's not through current...

 

[LionelHutz]

Do you actually mean 400V load inrush currents?

 

[waross]

The maximum % inrush of common loads is typically less than the maximum % inrush of the transformer primary.
Thee only time I had an issue was when the undersized fuse on the primary of a 120/24 Volt control transformer would allow the unloaded transformer to energize but would open when the transformer was energized with a load connected.
With the proper protection on the primary, we don't see a problem with secondary loads.