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"Michael Moroney" wrote in message
...
"daestrom" writes:
P.S. In the US, a 'tap-changer' may be built for either for unloaded or
loaded operation. The 'unloaded' type can not be stepped to another tap
while there is load on the unit (although it can still be energized).
It's
switch contacts cannot interrupt load though, so if you try to move it
while
loaded, you can burn up the tap-changer. The classic 'load-tap-changer'
is
actually several switches that are controlled in a precise sequence to
shift
the load from one tap of the transformer to another while not interrupting
the load current.
P.P.S. Load tap changers typically have a significant time-delay built
into
the controls so they do not 'hunt' or respond to short drops in voltage
such
as starting a large load. 15 seconds to several minutes is typical. So
even with load-tap-changers, starting a single load that is a high
percentage of the system capacity will *still* result in a voltage dip.
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. 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).
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Certainly modern ones likely use thyristors and zero crossing detectors.
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Possibly but probably not- I am out of date on this but I would expect that
the old way of good switches plus reactors might still be the better way. It
saves a lot of control wiring plus a lot of money to operate thyristors at
300KV and 500A or more and I doubt whether they would be cost effective or
technically advantageous otherwise. --------------------------
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.
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.
--
Don Kelly
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