Use of transformers outside of nameplate voltage.

[FreddyNurk]

Hello all,
we've been brought in to an interesting construction job, for a minerals processing plant. The whole job appears to have been run as a D&C plant, with the appropriate design documentation stored in a hard to access mental archive. Part of our scope is assessing the design intent and ensuring that things get back on track.

One of the plant items that has been purchased is some sort of rectiformer system (I'm not sure of the actual name, only what the locals have referred to it as), consisting of an 11kV feed through two 11kv - 415 transformers and into the rectiformer system itself.

The local supply is at 22kV, and as such its been decided to feed the 11kV side of the transformers with 6k6V, from another transformer. Something was muttered about getting a reduced voltage at the final output by feeding the transformer the reduced input voltage. I understand about transformer turns ratios and so on, but I wasn't aware that operating a transformer out of its nameplate ratings was appropriate. Can anyone shed any light on whether this scheme will work? I'd assume a reduced output voltage from the 415 side as a result, but I'm a bit sceptical as to whether the plant will operate, particularly given the vagueness of the design of the rest of the plant.

I can supply further details if required. Any assistance would be appreciated.

 

[itsmoked]

"Ratio is king."
It will stand.

You can always use a transformer at lower voltages than name-plated. Once in a while you could possibly, maybe, if you're lucky, use one at a higher voltage.

You must not exceed the name-plate current either.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

As long as you don't exceed the transformer nameplate voltages, the currents associated with nameplate voltage and MVA rating, or the design V/Hz value the transformer won't care. Voltage out will differ from voltage in by the turns ratio. So, if 11kV in would give 415V out, 6.6kV in would give 249V out; if the transformer was 10MVA it would now be 6MVA to keep from exceeding the maximum current. That much voltage reduction and it won't even matter if it was a 60Hz transformer being fed at 50Hz. The transformer will be just fine.

Your rectifier on the other hand will only put out 60% as much DC voltage and that will probably be a severe problem.

Find the right voltage source, even if it means buying a 22kV-11kV autotransformer.

 

[FreddyNurk]

Thanks for the replies so far. As I suspected, the turns ratio will dictate the resultant voltage, and as the voltage is lower than rated, there should be no issues with terminal separation etc. But as I also suspected, the reduction in current as well as the unknown of what the DC system requires is another thing entirely.

DavidBeach, I agree completely that the voltage source should be correct, this will be one of my recommendations.

Thanks again to all.

 

[jraef]

Rectiformer...

I'm sorry, but that just sounds painful.