Autotransformers & Buck/Boost - use in the UK

What's the supply coming from? (presumably not from the public grid as that has a statutory +10% limit).

  - Andy.

Private network, that is going to be difficult to get tapped further up the network (which would be the best solution)

it is done - actually there was a link to one outfit doing it only a few days ago

 Hello, has anyone come across the emerging technology of spintronics within buildings? However their  blurb is so much waffle you'd be hard put to realise that is what they sell as part of their great new wonder cure.

Google searches for

'Whole site Voltage Optimisers' will bring up companies offering more or less what you suggest and yes, they are usually auto transformer based for reasons of size weight and cost..

example

But, if you can possible get the tap settings changed on your own transformer, that would be far more sensible.

120kVA is a big lump of metal, even for an autotransformer doing 10% - in effect the same as a 12kVA double winding with the full current secondary at 24V in anti-series with the supply to buck it down from 240 to not quite 220.

Mike.

I see nothing to prohibit or otherwise prevent use of an autotransformer in the UK. I believe that you will need three transformers, star connected so as to reduce the voltage on each phase by say 5% or 10%. Ensure that the neutral is continuous throughout, and remains nominally at earth potential.

Why is this needed though ? a voltage at the upper end of nominal is often preferred. Older equipment was probably intended for 240/415 volts in any case, rather than for todays 230/400 volts.

If only one or two items are suffering, a local to the load transformer might be cheaper and simpler.

I can see a lot of information on the use of autotransformers and buck-boost transformers in the US, but not much for the UK.

A couple of guesses as to why - firstly our US cousins are much more used to using transformers as they have a hodge-podge of voltages to work with (120V, 208V, 240V, 277V, 480V etc) where as we just use 230/400V for almost everything. Transformers will also add a bit of impedance to the circuit - and we're a lot fussier about Zs than I suspect the Americans are (at 120V the touch voltage for a L-PE fault is likely no more than 60V so fast ADS is far less of a priority).

   - Andy.

Thanks for the comments, I agree it'll be a big lump but not as big as a rated double wound transformer.

it is as a result of equipment failures and if I can get upstream tapped I will, but having a quicker alternative fix is needed as well.  I've seen 3ph auto transformers are available, so I'm assuming with it being a 3P+N supply it can only be YY wound auto transformer.  (Had it been a double wound transformer I guess DY or YY would suit)

it is in effect Y but more like just the one Y with variable length legs ;-)
There are normally either 3 single phase toroids, or a 3 legged core with a phase on each limb, with a thinner winding doing the the neutral up to the lowest tap, and the fatter wire doing the buck windings. There is a very small one that shows the construction on Ebay at  the moment. Its  not anything like large enough for your application but shows the 3 legged core construction nicely.
3 toroids however just look like 3 toroids;-)
 

The voltage optimiser companies sell you something like one or other of these shapes with some contactors for tap selection and extra ADS wrapped around it in a large box. 
The smaller domestic ones sold in countries with really bad voltage control are often motorised  variac designs.

As noted, beware the effect on Zs and ADDS operating times

Mike.

Ha yes, it's just a 'Y'  good point!

Thanks