"Don Kelly" writes:

"Michael Moroney" wrote in message
...

Are the load tap generators configured make-before-break?
Break-before-make would mean a (very short) power outage every activation
but make-before-break would mean a momentarily short-circuited winding and
the break would involve interrupting a large short circuit current.
--------
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. ...

Thanks for your (and esp. daestrom's) explanation on how they work.

When I was a kid living in a rather rural area, there would be a pair of
these on poles every few miles, connected open delta. (all transformer
primaries were connected phase-phase then).

"on load tap changers"? Not likely. These were applied to transformers only
where it was worth the effort.
Definitely transformers in rural areas- typical pole pigs- would have to be
de-energized as the tap changer is a manually operated switch inside the
tank. Some larger transformers did have off-load but live changers operated
from ground level. What you saw could have been somethng else altogether.

I'm not completely sure what these are other than being told that they
were voltage regulators (tapped autotransformers) long ago. These are
large cans with 3 bushings on top, taller and slimmer than most pole pigs,
and they usually have a control box on the pole around eye level. I see
the same style cans in substations between the stepdown transformer and
the distribution system except they sit on the ground and come in sets of
three.

Delta primaries as you indicate were around when you were a kid, would, in
most areas mean that you are now a pensioner. I remember cases of conversion
from delta to star for distribution primaries in small towns being done
about 60 years ago and use of delta for transmission died much before that.

While I'm hardly a kid, I'm no pensioner yet. In fact my father's place
still has delta-connected distribution primaries in the area, at 7200
volts (I have an old fuse/switch holder from there labelled 7200V ??A).

Where I mentioned they had pairs of these "voltage regulators" (or
whatever they were) every several miles was a long run along a state
highway. At some point they upgraded it to a wye configuration, probably
at a higher voltage. However, several side branches haven't been upgraded
yet. On the side branch feeding my father's place there is a bank of 3
transformers connected wye-delta immediately followed by a pair of these
"voltage regulator" cans connected open delta. From that point on the
distribution system is visibly old.