Autotranformers on 13.8kV or 15kV systems

[bdn2004]

We have (2)10MVA 15kV-4160V transformers that feed essential load of about 9 MVA. There is an independent 13.8kV system on the site.

To increase the reliability, it would be to our advantage replace one of the 15kV feeders and feed from the 13.8kV system.

Instead of buying a new transformer, I wonder if it is feasible to get an autotranformer that could bump the voltage from 13.8kV to 15kV and re-use one of the existing tranformers? Any problems?

 

[dpc]

An autotransformer would definitely work.

If the existing transformers have taps, which they probably do, you might want to look at that option as well.

A standard transformer would have two 2.5% taps above and below the nominal voltage. 95% of 15 kV is 14.25 kV, is only 103.3% of 13.8 kV. So this might work for you, especially in an emergency condition.

 

[dpc]

I should have explained it the other way - on the 14.25 kV tap connected to a 13.8 kV source, your voltage would be about 97% of nominal on the 4160 V side.

 

[magoo2]

You could get a set of voltage regulators since they handle a voltage range of +/- 10%. A 10 MVA transformer would be matched up with a 1 MVA three-phase regulator or 3 - 333 kVA single-phase regulators. A voltage regulator is an autotransformer with a tap changer.

You'd have to see if the available short circuit current exceeds the capability of the voltage regulator. Other than that, a 333 kVA regulator is a standard rating.

Otherwise, the straight autotransformer would work.