"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.

Certainly modern ones likely use thyristors and zero crossing detectors.

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).