Pad mounted transformer wiring reconfiguration

[cuky2000]

It is possible reconfigure in the field a factory installed 120V/240V series secondary winding to 120V parallel connection for a 25 kVA pad mounted transformer?

The reason of this questions is that we do not see any indication on the instruction manuals how this could be accomplishes since we are suspecting the transformer tank is sealed and the winding is not accessible.

The purpose of this field adaptation is to void procure a long lead timme a three phase unit since the site already has three available single phase pad mounted units. If the reconfigurations works the three phase units will be connected in a three phase bank 4Y/2.4 kV primary & 208Y/120 volt secondary to be used as a temp. backup power source while building sequentially a permanent installation.

 

[davidbeach]

If there are four low side bushings you can do it, but if it would mean rewiring in the tank you're pretty much out of luck.

 

[ScottyUK]

It's not impossible to get the lid ground off by an overhaul shop to access the internal winding connections. Don't do this yourself, get the guys who do it for a living to do the work. Before doing this, consider that there may not actually be two discrete windings, but simply a tapping at the midpoint. If this is the case then you're stuck.

 

[PHovnanian]

If the available single phase units have twice the required rating, just use one half of the 240 V winding (120 V to the neutral) to make up your bank.

If you have a second service needing 120/208 Y, you could use the other half of the banks' windings. Just keep each service below 1/2 the padmounds ratings for each phase. And put a warning placcard in the secondary compartments so as not to catch some poor lineman off guard troublesooting these things.

 

[cuky2000]

PHovnanian,

I like your idea. Unfortunatelly we are short in power and the 75kVA full capacity is needed.

Happy new year to all

 

[waross]

This is a common reconnection on pole mounted distribution transformers to form 120:208V banks.
The pole mounted units in North America generally had two windings. Both windings would be terminated on the center bushing.
A couple of anecdotes.
1. Use clean tools and oil safe gloves.
I saw a dirty adjustable wrench used below the oil level to work the bushing nuts. The wrench came out of the oil clean. The transformer failed within a few hours due to contaminated oil.
2. Do a continuity check to verify the winding ends.
The workers got the leads crossed and both windings were shorted to themselves. When the transformer was energized it seemed as if the whole world went HUMMMMMMM.
We were near the small diesel power plant and could hear the old 12 cylinder Cat winding down.
Fortunately no damage. It's a story for another day as to why the transformer was fused at 100 amps rather than the proper 10 Amp or 15 Amp fuses. (Hint> We were converting from a wye/delta to a wye/wye connection.)
My time in the third world was seldom uneventful.
Good luck getting access to the winding ends.

Bill
--------------------
"Why not the best?"
Jimmy Carter