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Converting Existing 7200/4160Y to 12470/7200Y

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mmpower

Electrical
Nov 4, 2007
1
Have a customer presently being served by utility with a 12.47 primary through step downs distributing/metering at 7200/4160 throughout the facility via aerial distribution. The transformers are overloaded and the current thought is to have utility remove transformers and distribute the 12.47/7.2 throughout, which would then be a direct feed from the utility distribution voltage.

I'm told most existing transformers are 4800/8320Y rated, single phase pole mounted cans, connected phase-gnd primary, with 120/240 1-ph secondary. There are a few 2400x4800 transformers also.

They (facilities electrical group) have the thought that they can retain most of the 4800/8320Y transformers and adjust the tap settings for the new 7200 (L-G) service voltage. Has anyone ever done (or heard of) this? Aren't those xfmr's rated for a maximum L-G nominal voltage of 4800?

There are obviously other issues (such as insulator ratings, cutout and surge ratings, fault current ), but the transformer issue concerned me.

Thanks!



 
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First, a minor nit to pick, if you are in the ANSI/IEEE realm, and your use of 12.47kV suggests you are, the preferred nomenclature is L-L voltage with a Y then a slash and the L-N voltage, such as 12470Y/7200 or 7200Y/4160.

The only transformer that could be directly reused would be any single phase transformers presently connected 7200V line-to-line that could be reconnected 7200V line-to-neutral.

A "4800/8320Y" transformer, my rant not withstanding, suggests a three phase transformer that was connected at a lower tap to the 7200V three phase. It would now need to be connected to the 12470V three phase and I doubt there would be a tap at 150% of rated voltage. Diagram the existing connection, the voltage across the individual windings and the rating of the winding. What will you then connect to that winding? As I said, the only thing you will be able to keep will be any single phase transformers presently connected phase-to-phase and they will require new lightning arrestors and possibly new insulators/bushings.
 
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