How to reduce transformer output voltage? 1

[oscaril]

Please could anybody help me?

We have a transformer with the following characteristics:
Power: 50KW
Input voltage: 380 V, Delta connected
Output voltage: 208V, Star connected

The input voltage in our power supply system: 388V
And the output voltage: 225V/130V
According to the technical transformer parameters
if input voltage of 380V, output voltage should be of 126.6V and 219V when there is self-current. Unfortunately
these voltages do not meet communications requirements. It must be +/- 3V of output voltage,
it means from 117V up to 123V for 120V.
I want to install a voltage stabilizer to smooth down the output voltage to nominal 120v/208 +/-2.5%. I don't know what kind of voltage stabilizer to choose. And will this voltage stabilizer cope with constant 130V and 225levels? Or are there any other possibilities to solve my problem?
Maybe better to replace this transformer by another one with voltage regulation on its primary coil? But in any case some measures should be taken to protect equipment from unexpected voltage fluctuations.

Thanks

 

[mpparent]

+/- 3% of output seems pretty tight to me, but...

Can you adjust the taps on the transformer? If you have a -2.5% tap, that would get you to 219V.

Mike

 

[RonShap]

As loading of the transformer changes, the output voltage will change dependent on the impedance of the transformer.
To be +/-3V, you need a transformer and UPS, not just a transformer.

 

[ScottyUK]

I doubt the utility can guarantee +/- 3% on their HV system, and the HV system changes show up on the LV side of the transformer, even assuming that your load current does not vary causing load-dependant volt-drop in the transformer.

Why is the tolerance so tight? If this bit of kit is designed to work on the mains supply then it should be designed to work over the normal range of mains voltages including the sags and swells which inevitably occur through routine operation of the distribution network. Are you definitely reading the nameplate correctly and not reading the nominal min and max voltages?

As noted above, you either need a UPS or a constant voltage transformer if your requirements really are as unusual as they seem.


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[nbucska]

50 KW for a regulated supply seems to be a lot.
I think only a small fraction of this load needs so tight
regulation .

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[oscaril]

Thanks guys for immediate replies!
Unfortunately the transformer hasn’t taps.
Yes, the requirements are tight and are intended for sensitive electronic equipment.

 

[davidbeach]

the requirements are tight and are intended for sensitive electronic equipment

Its because people try to build power delivery systems to meet stupid requirements that stupid manufacturers get away with their stupid requirements. If the equipment had been designed by someone who had any real world experience it would be designed for +/- 10% with the hope that it would spend most of its life in a +/- 5% range. Send it back and get something intended for real world applications.

 

[jraef]

How about a Constant Voltage Transformer, like a ferroresonant? They make them that big, however I think it would be more practical to determine exactly which loads absolutely need it and put them there.

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[ScottyUK]

David -

Amen to that.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[oscaril]

So, anyway, we decided to replace this transformer.
Thanks again.