In alt.engineering.electrical Don Kelly wrote:

| Yes -you are shorting a part of the winding but the switching is a bit more
| complex than that so that short circuit currents are limited to reasonable
| values. It is a multistep operation with reactor switching. On-load tap
| changers are expensive and are generally limited to applications where this
| is absolutely needed (I have seen one where the tap changer was nearly as
| large as the transformer).

I was thinking of what I might do to get some fine voltage control within a
very limited range around 120 volts. The obvious option was a 0-140 volt
variable transformer. But I wanted to make sure I had a setup that could
be better limited, for example, to not allow an accidental too low voltage.
I also didn't want to run all the power through the variable. So what I
was going to do was get a smaller variable transformer, and two buck-boost
transformers. One transformer would be wired 120-16 in buck mode to drop
the voltage down to 104. The other transformer would be wired 120-24 and
supplied via the 0-140 variable transformer, giving me a 0-28 variable boost.
The end result is 104-132 over the full range of variable transformer control
(assuming the boost transformer has no issues with being overfed at 140V).

So I might envision a transformer where the taps can be part of a boost
transformer added to the main transformer. The first buck transformer in
my above example would not be needed because the main transformer would be
designed with a 1st secondary at the lowest voltage of the adjustable range.
A 2nd secondary on the same main transformer would have the adjustable taps
and it would feed a separate boost transformer which has a secondary wired
in series with the 1st secondary of the main. So the taps would only be
dealing directly with a fraction of the power (assuming there is no back
feed issue involved) based on the needed adjustment range.

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