How to size transformer (and feeder/OCP) for short duration pulsing load

[zjust]

Hello,

How would I go about sizing a transformer for a short duration pulse load? I am looking to do a 480V to 4160V, 3P transformer that can handle a 3000 KVA load that is on for 2 seconds every 5 minutes (300 seconds), then repeats itself. There are no constant loads, only the pulsing load on this transformer.

Also, do you size the conductors and over current protection for the size of the transformer or the size of the pulsed load?

Thank you for any possible help!

 

[petersonra]

I think you will probably need to size the transformer for 3000 kVA. Otherwise the pulse load is going to get shortchanged if you bog down the transformer. It does not care much whether the load is on for 2 seconds or 2 hours. It can only deliver the amount of current that it can deliver. If you need a fixed amount of current at a fixed voltage, you will need to accommodate that. If you know the %Z, you should be able to figure out how much you might be able to cheat based on what your real load needs to be.

You might find some dispensation for conductor sizing in the code based on what the load actually is. or maybe, the AHJ would grant some dispensation for this really unusual case. I think you might be able to make a case that you have a special situation.

I think you are going to have to size the conductors at least large enough so the VD does not kill you, even with some dispensation. Personally, I would be looking at who knows how many parallel AL conductors to feed the transformer.

No matter what you do, you will need to size the OCPD so it protects the transformer and the conductors. I don't see an AHJ granting much dispensation on that.

 

[topgone]

Hello,

How would I go about sizing a transformer for a short duration pulse load? I am looking to do a 480V to 4160V, 3P transformer that can handle a 3000 KVA load that is on for 2 seconds every 5 minutes (300 seconds), then repeats itself. There are no constant loads, only the pulsing load on this transformer.

Also, do you size the conductors and over current protection for the size of the transformer or the size of the pulsed load?

Thank you for any possible help!

Your duty cycle is very low at 0.67%! The effective capacity for that pulse load should be roughly 300kVA. Of course, you size your conductors and protection to the max current of the pulse which is 416A at 4160V.

 

[gar]

220805-2052 EDT

zjust:

Your duty cycle is 2/300 = 1/150 = about 0.7 . So 3000 / 150 = 20 kW average load.

The ON duration is short, 2 seconds. Thus, the thermal mass for the size transformer you need is fairly large, and will average temperature rise fairly well.

Using a small transformer for the load current will produce a lot of internal power loss, and this will increase by the square of the current.

Also a small transformer will produce a large internal voltage drop. Going to a closed magnetic path ( a toroid core ) with the appropriate core material will reduce leakage inductance.

How important is the regulation of the output voltage? This may be your determining factor. If this is not important, then consider a smaller transformer. How much smaller will depend upon required voltage regulation. If voltage regulation is of no importance, then ultimately average power dissipation and temperature rise in the transformer will determine how small you can go.

You can solve any current limiting protective device ( fuse size ) by an appropriate circuit design.

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